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Effects of forest harvesting on snowmelt during rainfall in coastal British Columbia Beaudry, Pierre Guy 1984

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EFFECTS OF FOREST HARVESTING ON SNOWMELT DURING RAINFALL IN COASTAL BRITISH COLUMBIA by PIERRE GUY BEAUDRY B.Sc.A., U n i v e r s i t e L a v a l , 1981 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER IN FORESTRY in THE FACULTY OF GRADUATE STUDIES F o r e s t r y Department We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1984 © P i e r r e Guy Beaudry, 1984 In presenting t h i s thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his or her representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Forestry The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: 29 September 1984 i i A b s t r a c t Rain-on-snow has been recognized as an event with p o t e n t i a l f o r i n c r e a s i n g f l o o d and d e b r i s t o r r e n t hazards. However the e f f e c t s of d e f o r e s t a t i o n on i n c r e a s i n g t h i s hazard are not w e l l understood. To b e t t e r understand t h i s phenomena a study was conducted i n the Jamieson Creek experimental watershed near Vancouver B.C. I t s primary o b j e c t i v e was to determine the e f f e c t s of f o r e s t h a r v e s t i n g on snow melt r a t e s and subsequent runoff d u r i n g rain-on-snow events. The energy balance of a snowcover and the theory of snowmelt are reviewed to b e t t e r understand the processes i n v o l v e d d u r i n g rain-on-snow. Techniques and i n s t r u m e n t a t i o n r e q u i r e d to compute the energy budget are d i s c u s s e d , with the aerodynamic technique r e c e i v i n g g r e a t e r a t t e n t i o n . A l s o reviewed are the U.S. Army Corps of Engineers (1956) snowmelt equations and t h e i r a p p l i c a b i l i t y f o r rain-on-snow s i t u a t i o n s . The experimental set-up c o n s i s t e d of two study p l o t s , one l o c a t e d i n a recent cutover and a second i n an adjacent C o a s t a l Western Hemlock old-growth f o r e s t . Each p l o t was equipped with a l a r g e (22m 2) p l a s t i c sheet l y s i m e t e r r e c o r d i n g through a t i p p i n g bucket arrangement, a l l o w i n g comparison of snowmelt and runoff r a t e s between s i t e s . D i r e c t measurement of snowmelt was a l s o achieved using snow survey techniques. The USACE(1956) snowmelt equations were v e r i f i e d by comparing the computed melt with the l y s i m e t e r and snow survey r e s u l t s . Wind speed, r e l a t i v e humidity and a i r temperatures were measured at 0.6 and 1.5 meters above the snowpack to evaluate l a t e n t and s e n s i b l e heat f l u x e s . Snowpack and ground heat exchanges were measured with a p r o f i l e of f i v e t h e r m i s t o r s , and r a d i a t i o n was monitored with net all-wave radiometers. Three winters of data were c o l l e c t e d (1981-82, 1982-83, 1983-84), with s e v e r a l storms being analysed from each of the l a s t two y e a r s . Peak runo f f i n t e n s i t i e s and t o t a l runoff amounts, d u r i n g rain-on-snow were found to be g r e a t e r at the f o r e s t s i t e when there was presence of snow i n the canopy. When there was no snow in the canopy runoff was always g r e a t e r at the open s i t e . The reasons f o r the g r e a t e r runoff at e i t h e r of the s i t e s are d i s c u s s e d . The use of the USACE (1956) snowmelt equations g e n e r a l l y compared f a v o r a b l y with the snow survey and l y s i m e t e r data, at the open s i t e . However under c e r t a i n s p e c i f i c c o n d i t i o n s these snowmelt equations were shown to be inadequate f o r use at the f o r e s t s i t e . The r o l e of the f o r e s t canopy on snow and r a i n f a l l i n t e r c e p t i o n played a major r o l e i n e x p l a i n i n g the d i f f e r e n c e s i n snowmelt and runoff r a t e s encountered between the two s i t e s . i v Table of Contents A b s t r a c t i i L i s t of Tables v i L i s t of F i g u r e s v i i Acknowledgement ix Chapter I INTRODUCTION 1 Chapter II THEORY OF SNOWMELT 6 2.1 Changes In I n t e r n a l Energy Of The Snowpack 6 2.2 Water Movement Through Show 7 2.3 Energy Budget Of A Snow Cover 9 2.4 Energy Balance Computations 13 2.5 A p p l i c a t i o n s Of Snowmelt Equations 20 Chapter III DESCRIPTION OF STUDY SITE 28 3.1 General 28 3.1.1 L o c a t i o n 28 3.1.2 Climate 28 3.1.3 S o i l s 38 3.1.4 Geology And Topography 42 3.1.5 V e g e t a t i o n 43 3.2 S p e c i f i c S i t e D e s c r i p t i o n 46 3.2.1 Open S i t e 46 3.2.2 F o r e s t S i t e .50 Chapter IV METHODS AND MATERIALS 53 4.1 Winter Of 1981-1982 53 4.2 Winter Of 1982-1983 61 4.3 Winter Of 1983-1984 72 Chapter V RESULTS AND DISCUSSIONS 7 4 5.1 Winter 1 981-1982 R e s u l t s 74 5.2 Winter 1982-1983 R e s u l t s 78 5.2.1 I n t r o d u c t i o n To The A n a l y s i s 78 5.2.2 C a l c u l a t i o n s Of R a d i a t i o n Melt 81 5.2.3 E s t i m a t i n g I n t e r c e p t i o n Losses From The F o r e s t Canopy 84 5.2.4 F i e l d Observations 90 5.2.5 P r e s e n t a t i o n Of The R e s u l t s 92 5.2.6 Event 1: January 25 - 31, 1983 100 5.2.7 Event 2: January 19 - 25, 1983 105 5.2.8 Event 3: January 4 - 1 1 , 1983 111 5.2.9 Event4 : February 8 - 12,1983 120 5.2.10 Event 5: February 12 - 16,1983 125 5.2.11 Summary Of 1982-83 R e s u l t s 128 5.3 Winter 1983-1984 R e s u l t s 130 V 5.3.1 Event 1: February 9 - 14, 1984 (5 Subevents) ...130 5.3.2 Event 1a: February 9, 1 984 137 5.3.3 Event 1b: Evening February 11, 1984 139 5.3.4 Event 1c: Morning Of February 12, 1984 140 5.3.5 Event 1d: Noon February 12 To Noon February 13, 1984 141 5.3.6 Event 1e: Afternoon Of February 13, 1984 142 5.3.7 Event 2: February 18 To 21, 1984 1 42 5.3.8 Event 3: January 22 - 25, 1984 147 5.3.9 Event 3a: Afternoon Of January 22, 1984 148 5.3.10 Event 3b: Evening And Night Of January 22-23, 1984 151 5.3.11 Event 3c: Night Of January 23 - 24, 1984 1 52 5.3.12 Events 3d, 3e And 3f: January 24-25, 1984 1 53 5.3.13 Summary Of 1983-84 R e s u l t s 156 Chapter VI CONCLUSIONS 159 BIBLIOGRAPHY 166 APPENDIX A - LIST OF SYMBOLS 172 APPENDIX B - THE POINT SNOWMELT EQUATIONS FROM USACE (1956) 1 76 APPENDIX C - USACE(1956) BASIN SNOWMELT EQUATIONS 178 APPENDIX D - SNOWMELT CALCULATIONS FROM TOEWS AND WILFORD (1978) 180 v i . L i s t of Tables 1. Comparison of l i t e r a t u r e obtained v a l u e s f o r bulk t r a n s f e r c o e f f i c i e n t s (from Gray and Male 1981) 23 2. B.C. M i n i s t r y of the Environment snow survey measurements around the Greater Vancouver Water D i s t r i c t 38 3. S o i l s of Jamieson Creek watershed 41 4. F o r e s t cover Jamieson Creek watershed 48 5. F o r e s t stand c h a r a c t e r i s t i c s f o r the St u n i t i n the MHa subzone (adapted from B r i e r e 1979) 49 6. F o r e s t stand c h a r a c t e r i s t i c s f o r the SM u n i t i n the CWHb subzone (adapted from B r i e r e 1979) 49 7. F o r e s t cover around f o r e s t l y s i m e t e r 51 8. Snow survey measurements winter 1981-82 60 9. Snow survey measurements winter 1981-82 75 10. Measured runoff r a t e s f o r open and f o r e s t e d s i t e s , f r o m May 18 to May 26, 1982 76 11. E v o l u t i o n of the snowpack 1982-83 79 12. F i e l d o b s e r v a t i o n s 91 13. Data Table, January 25-31, 1983 102 14. Data Table, January 19-25,1983 106 15. Data T a b l e , January 4-11, 1 983 115 16. Data T a b l e , February 8-12, 1983 124 17. Data T a b l e , February 12-16,1983 126 18. Summary of 1983 r e s u l t s 129 19. Summary of the 1984 r e s u l t s 158 VI 1 L i s t of F i g u r e s 1. Annual hydrograph of Jamieson Creek (average of 1973-1979) 2 2. General l o c a t i o n of study area 29 3. L o c a t i o n of Jamieson Creek watershed 30 4. D i s t r i b u t i o n of annual p r e c i p i t a t i o n (inches) (A.E.S., Vancouver B.C.) 33 5. D i s t r i b u t i o n of annual s n o w f a l l (inches) A.E.S., (Vancouver, B.C.) .....34 6. Mean monthly d i s t r i b u t i o n of temperature at Elbow Creek 36 7. Mean monthly d i s t r i b u t i o n of p r e c i p i t a t i o n at Jamieson Creek 37 8. D i s t r i b u t i o n of M.O.E. snow courses around the Greater Vancouver Water D i s t r i c t 39 9. S o i l s of Jamieson Creek watershed 40 10. Area e l e v a t i o n curve f o r Jamieson Creek watershed (Cheng 1975) 44 11. Area-slope percentage curve f o r Jamieson Creek watershed (Cheng 1975) 45 12. V e g e t a t i o n of the Jamieson Creek watershed 47 13. D i s t r i b u t i o n of t r e e s around f o r e s t l y s i m e t e r 52 14. Design of the l y s i m e t e r and flow system 54 15. General view of open s i t e 56 16. General view of f o r e s t e d s i t e 56 17. Snow accumulation m i d - A p r i l 1982 60 18. Snow runo f f through p r e f e r e n t i a l pathways 61 19. 22m2 t a r p l y s i m e t e r , f o r e s t e d s i t e 62 20. 22m2 t a r p l y s i m e t e r , open s i t e 62 21. Snow sampling l o c a t i o n s at the f o r e s t s i t e 64 v i i i 22. Snow sampling l o c a t i o n s at the open s i t e 64 23. Instrumentation and snowpack at f o r e s t e d s i t e , mid-winter 68 24. Instrumentation and snowpack at f o r e s t e d s i t e , l a t e winter 68 25. Instrumentation and snowpack at open s i t e , mid-winter 69 26. Instrumentation and snowpack at open s i t e , l a t e winter 69 27. I n s u l a t e d c o o l e r , heater and data logger 71 28. Lysimeter outflow, funnel and t i p p i n g bucket 71 29. Measured & computed variables,Jan.25-27,1983 101 30. Measured & computed variables,Jan.22-24,1983 107 31. Measured & computed variables,Jan.4-7,1983 113 32. Measured & computed variables,Jan.8-10,1983 114 33. Measured & computed variables,Feb.8-12,1983 121 34. Measured & computed v a r i a b l e s , F e b . 9-10,1984 132 35. P r e c i p i t a t i o n and r u n o f f , Feb. 9-10,1984 133 36. Measured & computed variables,Feb.11-14, 1 984 134 37. P r e c i p i t a t i o n and r u n o f f , Feb. 1 1 -1 4,1984 135 38. Measured & computed v a r i a b l e s , F e b . 18-22,1984 144 39. P r e c i p i t a t i o n and r u n o f f , Feb.18-22,1984 145 40. Measured & computed variables,Jan.22-25,1984 149 41. P r e c i p i t a t i o n and r u n o f f , Jan.22-25,1984 150 ix Acknowledgement I would f i r s t l i k e to thank my t h e s i s s u p e r v i s o r Dr. D. Golding, who through h i s encouragements and advic e was ab l e to provide guidance, while l e a v i n g the author the freedom that i s necessary to make such a l e a r n i n g experience a s u c c e s s f u l one. I would l i k e to extend my s i n c e r e thanks to Dr. T.A. Black f o r h i s h e l p , a d v i c e and suggestions concerning the many matters r e l a t e d to micrometeorology that I encountered d u r i n g the course of t h i s work. The f i e l d a s s i s t a n c e p r o v i d e d by Mr. Kuochi Rae i s a p p r e c i a t e d with g r a t i t u d e , as i s the support p r o v i d e d by the Greater Vancouver Water Board. F i n a n c i a l suppport f o r t h i s study was p r o v i d e d by the N a t i o n a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l Canada, grant no. A6957, and a Canadian F o r e s t r y S e r v i c e bloc g r a n t . Much of the i n s t r u m e n t a t i o n was loaned by Dr. R.H. Swanson, Northern F o r e s t Research Centre, Canadian F o r e s t r y S e r v i c e , Edmonton, A l b e r t a . 1 I . INTRODUCTION In Southwestern B r i t i s h Columbia the autumn and winter weather p a t t e r n s are dominated by a low pressure system that b r i n g s l a r g e amounts of p r e c i p i t a t i o n to the C o a s t a l Mountains. T h i s mountainous region can be d i v i d e d i n t o three broad e l e v a t i o n zones c h a r a c t e r i z e d by the form i n which t h i s p r e c i p i t a t i o n reaches the ground. 1) Low e l e v a t i o n where the p r e c i p i t a t i o n i s almost e x c l u s i v e l y i n the form of r a i n . 2) Mid e l e v a t i o n , where snow w i l l accumulate and melt throughout the winter responding to continuous temperature f l u c t u a t i o n s g e n e r a l l y between - 5°C and +5°C, t h i s zone i s known as the t r a n s i e n t snow zone. 3) High e l e v a t i o n , where snow w i l l begin i n e a r l y October and the snowpack can accumulate to depths of up to 6 or 7 meters before continuous melt occurs i n l a t e March. Two p e r i o d s i n the year can be a s s o c i a t e d with high streamflows or peakflows, the f i r s t o c c u r r i n g d u r i n g these high frequency long d u r a t i o n winter p r e c i p i t a t i o n events and the second o c c u r r i n g d u r i n g s p r i n g snowmelt. The winter peak flows experienced from November to February are generated from the runoff o c c u r r i n g i n the low and mid e l e v a t i o n zones, while the higher e l e v a t i o n s generate the peak snowmelt r u n o f f s observed i n s p r i n g ( F i g u r e 1). Because the g r e a t e s t amounts of p r e c i p i t a t i o n c o i n c i d e with the presence of snow in the t r a n s i e n t snow zone, f l o o d and Means over years of Daily Observations Jamieson Creek 5 0 T ; i i i 1 i i JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC J A N Figure 1 - Annual hydrograph of Jamieson Creek (average of 1973-1979) 3 d e b r i s t o r r e n t producing events and headwater e r o s i o n may occur from the combination of l a r g e amounts of r a i n f a l l coupled with r a p i d snowmelt. Harr (1981) has w e l l documented the p o t e n t i a l hazards n a t u r a l l y o c c u r r i n g from t h i s type of event and i n d i c a t e s t hat 85% of l a n d s l i d e s s t u d i e d i n Western Oregon were a s s o c i a t e d with snowmelt d u r i n g r a i n f a l l . Three major d e b r i s t o r r e n t s causing l o s s of human l i v e s and ex t e n s i v e p r o p e r t y damage have o c c u r r e d d u r i n g the l a s t three winters (1981-82, 1982-83, 1983-84) i n the southwestern c o a s t a l area of B r i t i s h Columbia. A l l of these h y d r o l o g i c events o c c u r r e d as a r e s u l t of long d u r a t i o n r a i n f a l l at p e r i o d s when snow was present i n the t r a n s i e n t snow zone. During these events the f r e e z i n g e l e v a t i o n s were high and i n a l l p r o b a b i l i t y r a i n was f a l l i n g on these warm t r a n s i e n t zone snowpacks. Each one of these h y d r o l o g i c d i s a s t e r s was q u i c k l y blamed on f o r e s t h a r v e s t i n g a c t i v i t i e s by much of the general p u b l i c . The most recent of these was the H a t z i c V a l l e y d i s a s t e r i n e a r l y January of 1984 where n e a r l y one m i l l i o n d o l l a r s of pro p e r t y damage was caused by f l o o d waters of McConnell Creek. In an e f f o r t to assess the damage, two m i n i s t e r s of the B r i t i s h Columbia p r o v i n c i a l government toured the area. T h e i r c o n c l u s i o n s d i f f e r e d d r a m a t i c a l l y as to the p o s s i b l e i n f l u e n c e s that l o g g i n g might have had on the i n i t i a t i o n of t h i s f l o o d . Stephen Rogers, M i n i s t e r of Mines and Resources co u l d n ' t accept that nature was s o l e l y to blame f o r the damage, and was convinced that l o g g i n g was at l e a s t i n part r e s p o n s i b l e . Tom Waterland, M i n i s t e r of F o r e s t s was of the o p i n i o n that i t was a n a t u r a l l y caused 4 d i s a s t e r as he had never seen evidence that l o g g i n g had been p o s i t i v e l y l i n k e d to f l o o d problems i n the p r o v i n c e . The t r a n s i e n t snow zone f o r Southwestern C o a s t a l B r i t i s h Columbia i s approximately l o c a t e d between 400 and 1000 meters above m.s.l., an e l e v a t i o n t h a t c o i n c i d e s with much of the present f o r e s t h a r v e s t i n g a c t i v i t i e s . I t i s thus important to know what e f f e c t s c l e a r c u t l o g g i n g may have on i n c r e a s i n g downstream f l o o d hazards. Does c l e a r c u t l o g g i n g i n c r e a s e snowmelt d u r i n g r a i n f a l l and a f f e c t subsequent runoff to a degree that may be d e t r i m e n t a l to e i t h e r i n h a b i t a n t s of the downstream f l o o d p l a i n or the f i s h h a b i t a t ? T h i s q u e s t i o n i s of prime concern when c o n s i d e r i n g the r a t e of c u t , which i s the f r a c t i o n of a watershed to be h a r v e s t e d f o r i t s timber in the f i r s t pass. G e n e r a l l y , the g r e a t e r the r a t e of cut the cheaper i s the per u n i t c o s t of l o g s . Thus i t becomes evident that the r a t e of cut d e c i s i o n may be of great economical importance, j u s t i f y i n g i t as a p r e s s i n g concern. The use of the s e m i - e m p i r i c a l snowmelt equations produced by the U n i t e d S t a t e s Army Corps of Engineers i n the mid 1950's suggests that c l e a r c u t l o g g i n g c o u l d s i g n i f i c a n t l y i n c r e a s e snowmelt r a t e s and subsequent r u n o f f . T h i s i n c r e a s e d melt would be mostly due to i n c r e a s e d t u r b u l e n t t r a n s f e r of s e n s i b l e and l a t e n t heat to the snow s u r f a c e . However experimental data from watershed s t u d i e s (Harr et a l . 1979, Harr and McCorison 1979, Rothatcher 1973, Anderson 1970) have produced mixed r e s u l t s and the q u e s t i o n of how h a r v e s t i n g i n f l u e n c e s snowmelt duri n g r a i n f a l l and u l t i m a t e l y peakflows remained u n c e r t a i n . 5 U n t i l the s t a r t of the present work no r e s e a r c h had been done to a c c u r a t e l y q u a n t i f y the e f f e c t s of f o r e s t h a r v e s t i n g on snowmelt r a t e s i n e i t h e r B r i t i s h Columbia or the P a c i f i c Northwest r e g i o n of the U n i t e d S t a t e s , where t h i s q u e s t i o n had a l r e a d y been i d e n t i f i e d as a s e r i o u s concern. As the development and use of computer h y d r o l o g i c modeling i n c r e a s e s i t becomes i n c r e a s i n g l y important to understand the p h y s i c a l processes i f we are to p r o p e r l y model the h y d r o l o g i c e f f e c t s of s i l v i c u l t u r a l a c t i v i t i e s and make i n t e l l i g e n t d e c i s i o n s . In an e f f o r t to b r i d g e t h i s i n f o r m a t i o n gap and supply some much needed answers, t h i s "rain-on-snow" study was i n i t i a t e d . The o b j e c t i v e s of the study are as f o l l o w s : 1) To compare water balances of a m e l t i n g snowpack i n a f o r e s t e d and a c l e a r c u t s i t e d u r i n g rain-on-snow events. 2) To d e s c r i b e the p h y s i c a l processes i n v o l v e d d u r i n g r a i n -on-snow and how i n turn they are a f f e c t e d by the removal of the f o r e s t canopy. 3) V e r i f i c a t i o n of the u s e f u l n e s s of the U n i t e d S t a t e s Army Corps of Engineers snowmelt equations as a t o o l f o r p r e d i c t i n g the e f f e c t s of f o r e s t h a r v e s t i n g on snowmelt r a t e s d u r i n g r a i n -on-snow i n the t r a n s i e n t snow zone. 6 I I . THEORY OF SNOWMELT The accurate d e s c r i p t i o n of a m e l t i n g snowpack i n v o l v e s q u a n t i f i c a t i o n of the energy balances at the snow-air i n t e r f a c e , the d e s c r i p t i o n of the i n t e r n a l energy changes o c c u r r i n g i n the snowpack along with the d e s c r i p t i o n of the i n t e r n a l water movements i n the pack. The energy exchange at the snow s u r f a c e i s r e c o g n i z e d as the major f a c t o r governing the p r o d u c t i o n of melt water (Male and Granger 1981). However the i n t e r n a l energy exchanges and water r o u t i n g through the snowpack must be c o n s i d e r e d c a r e f u l l y , e s p e c i a l l y f o r p r e d i c t i n g peak flows and f l o o d hazards. 2.1 Changes In I n t e r n a l Energy Of The Snowpack To measure p r e c i s e l y the i n t e r n a l energy changes of the snowpack s e v e r a l parameters must be known a c c u r a t e l y . These i n c l u d e the continuous f l u c t u a t i o n s i n d e n s i t y and temperature of the s o l i d p o r t i o n of the pack and the l i q u i d water content (McKay 1978). The i n t e r n a l energy changes are the r e s u l t of heat l o s s e s and gains at the snow-air i n t e r f a c e and a l s o the ground-snow i n t e r f a c e . They are a l s o caused by phase changes w i t h i n the pack which i n v o l v e s energy uptake or r e l e a s e at that l o c a t i o n . However the e v a l u a t i o n of snowmelt i s s i m p l i f i e d when the snowpack becomes isothermal at 0°C. Under these c o n d i t i o n s the snowpack can no longer conduct heat, and consequently i n t e r n a l energy changes become n i l . Once t h i s c o n d i t i o n has occurred any net energy input w i l l r e s u l t i n melt. T h i s melt takes plac e 7 mostly at or near the upper surface of the pack. Although warm rain can carry energy a short distance into the snow the melting that occurs i s s t i l l very near the upper surface (Gray and Male 1981). When the s o i l i s not deeply frozen, i t may also provide some energy for melt. However, th i s melt w i l l take place at the soil-snow interface instead of at the upper surface. The change from a loose, dry and subfreezing snowpack of low density to a coarse, granular, and moist snowpack of high density i s termed "ripening" of the pack. A ripe snowpack i s said to be "primed" to produce runoff when i t s l i q u i d water holding capacity has been reached (USACE 1960). 2.2 Water Movement Through Snow When considering delays between positive net energy inputs to the snowpack and subsequent runoff one must take into account the "cold content" of the snowpack and i t s l i q u i d water deficiency. Generally runoff cannot occur before the cold content of the snowpack and the l i q u i d water d e f i c i e n c i e s have been s a t i s f i e d . The USACE (1960) defines the "cold content" (W ) as the heat required per unit area to raise the temperature c of the snowpack to 0°C. The l i q u i d water holding capacity (F'') P i s defined as the maximum amount of hygroscopic and c a p i l l a r y water the pack can hold against gravity. This capacity i s a function of snowpack conditions, but i t generally f a l l s within the range of 2 to 5 percent of the t o t a l water equivalent of the snowpack (USACE 1960), although some investigators have measured values as high as 25% (deQuervain 1948). The cold content (W ) c 8 can be expressed i n mm of l i q u i d water, at 0°C, produced at the s u r f a c e by e i t h e r r a i n or melt, which upon f r e e z i n g w i l l warm the pack to 0°C. Because of t h i s W can be added d i r e c t l y to c F'' to produce what i s known as the l i q u i d water requirement of P the snowpack (S ). Once S has been s a t i s f i e d any a d d i t i o n a l P P energy input produces melt water which subsequently d r a i n s to the ground. When melt r a t e s are at t h e i r h i g h e s t , 20% or more of the pack may be l i q u i d water, most of which i s i n t r a n s i t through the snow under the i n f l u e n c e of g r a v i t y (USACE 1956). The movement of water through a snowpack i s a h i g h l y c omplicated process because of the h e t e r o g e n i e t y and a n i s o t r o p y of the snow cover. Most snowpacks develop a l a y e r e d s t r u c t u r e with i c e l a y e r s , f i n e - t e x t u r e d high d e n s i t y l a y e r s a l t e r n a t i n g with coarse t e x t u r e d , low d e n s i t y and h i g h l y permeable l a y e r s . Because i n t e r n a l s t r u c t u r e s i g n i f i c a n t l y i n f l u e n c e s the r e t e n t i o n and movement of melt water through the snow, d e t a i l e d a n a l y s i s of the t r a n s m i s s i o n process i s d i f f i c u l t . The flow of water around snow g r a i n s occurs g e n e r a l l y by two mechanisms. A t h i n f i l m of water can form around i n d i v i d u a l g r a i n s or water can flow through i s o l a t e d s a t u r a t e d pores. A l s o v e r t i c a l channels of coarse g r a i n snow have been observed and are o f t e n a s s o c i a t e d with r a i n f a l l on snow. These channels form a p r e f e r r e d pathway f o r meltwater and r e s u l t from the r a p i d g r a i n growth caused by the water s a t u r a t i o n i n the channels (Gerdel 1949). 9 The wide range i n v e l o c i t i e s of the l i q u i d water i n a snowpack can be a t t r i b u t e d to the i n t e r n a l s t r u c t u r e of the snow, the c o n d i t i o n of the snowcover p r i o r to the i n t r o d u c t i o n of water and the amount of a v a i l a b l e water at the snow s u r f a c e (Gray and Male 1981). If the i n f i l t r a t i o n c a p a c i t y of the s o i l below the snow i s exceeded some of the water w i l l flow o v e r l a n d in a s a t u r a t e d snow l a y e r . C a l c u l a t i o n s based on the assumption of homogeneous flow through an isothermal snowpack at 0°C give t r a v e l times of one to s e v e r a l hours, depending on the melt rate and depth of snow. Ice l a y e r s t h a t form i n the snowpack may i n c r e a s e the time and even b r i n g flow to a h a l t (Langham 1974); the r e t a i n e d water i s of t e n r e l e a s e d rather suddenly. A l l these f a c t o r s make i t d i f f i c u l t to p r e d i c t r u n o f f r a t e s from c a l c u l a t e d melt r a t e s . 2.3 Energy Budget Of A Snow Cover The e v a l u a t i o n of the energy exchange between the snow cover and the a i r above i t r e q u i r e s the f o r m u l a t i o n of i t s energy budget. T h i s permits e s t i m a t i o n s of the i n t e r n a l energy changes i n the snowpack, or i n the case of an iso t h e r m a l pack at 0°C w i l l a l l o w c a l c u l a t i o n s of melt r a t e s of the snowpack. Knowledge of the melt r a t e s w i l l i n turn permit estimates of the tim i n g and i n t e n s i t y of the snowmelt r u n o f f . Male and Granger (1978) s t a t e that two methods are used when w r i t i n g the energy balance f o r snow: 1) The f i r s t approach c o n s i d e r s the v e r t i c a l components of the energy and mass f l u x e s at the snow-air i n t e r f a c e and has the f o l l o w i n g g e n e r a l form d e s c r i b e d by Kraus (1972). 10 Z Q + 1 (mh) = 0 i i Q = energy f l u x due to r a d i a t i o n , conduction or c o n v e c t i o n , i (mh) = energy t r a n s f e r due to p r e c i p i t a t i o n (m), a s s o c i a t e d i w ith a s p e c i f i c enthalpy ( h ) . 2) In the second approach the snowpack i s c o n s i d e r e d a c o n t r o l volume to which energy can be t r a n s f e r r e d by r a d i a t i o n , c o n v e c t i o n and conduction, and a c r o s s whose boudaries (snow-air and snow-ground i n t e r f a c e ) mass f l u x e s i n the s o l i d , l i q u i d , or vapour phase are p o s s i b l e . In t h i s case the equation takes the form : dv/dt = L Q + L (mh) (2) i i Now Q i n c l u d e s heat t r a n s f e r between the ground and the snow i and mh i n c l u d e s melt water d r a i n i n g from the bottom of the pack. The new term dv/dt equals to the change i n i n t e r n a l energy. Equation 1 i s u s u a l l y a p p l i e d to deeper mountain snowpacks and on g l a c i e r s s i n c e i t onl y r e q u i r e s measurements at or near the upper snow s u r f a c e . Equation 2 i s r e q u i r e d to d e s c r i b e completely the thermal regime of a snowpack and i s of p r a c t i c a l use f o r the shallow snow cover ( l e s s than 40 cm i n depth). Using the f i r s t approach a complete energy balance equation f o r a snow cover can be expressed as Q=Q + Q + Q + Q + Q - Q 6 n e h g r m where 11 Q = change of energy s t o r e d i n the snowpack 6 Q = net r a d i a t i o n t r a n s f e r n Q = l a t e n t heat t r a n s f e r e Q = s e n s i b l e heat t r a n s f e r h Q = t r a n s f e r of heat from r a i n water r Q = heat t r a n s f e r a c r o s s the snow s o i l i n t e r f a c e g Q = Energy used to melt snow m The t r a n s f e r of both s e n s i b l e and l a t e n t heat r e s u l t from the turbulence i n the boundary l a y e r immediately above the snow s u r f a c e . The r a d i a t i o n exchange (Q ) i s the net sum of the n longwave and the shortwave energy f l u x e s . Because f u s i o n of the snow cannot occur before the pack reaches 0°C, and no a d d i t i o n a l amounts of thermal energy can be s t o r e d i n the pack once i t has reached 0°C, e i t h e r Q or Q must be 0 at a l l times. e m Before a n a l y s i n g the energy balance, three p a r t i c u l a r c h a r a c t e r i s t i c s of snow and i c e must be c o n s i d e r e d . 1) Contrary to most s u r f a c e s snow and i c e both allow some t r a n s m i s s i o n of shortwave r a d i a t i o n . Thus the shortwave r a d i a t i o n i n c i d e n t at any depth can be t r a n s m i t t e d , r e f l e c t e d or absorbed, r e s u l t i n g i n r a d i a t i o n a b s o r p t i o n w i t h i n a volume rather than at a plane. The amount of shortwave r a d i a t i o n r e c e i v e d at the s u r f a c e i s g r e a t e r than that found at any depth below. T h i s amount of shortwave r a d i a t i o n (K) reaching a depth 12 -Az z i s given by K(z)=K e (K =short wave r a d i a t i o n at the s u r f a c e , A = e x t i n c t i o n c o e f f i c i e n t dependent on the nature of the t r a n s m i t t i n g medium and the wavelength of the r a d i a t i o n ) . The i n t e r n a l t r a n s m i s s i o n of r a d i a t i o n through snow and i c e c r e a t e s problems i n f o r m u l a t i n g the s u r f a c e balance and i n o b s e r v a t i o n . The c a l c u l a t e d albedo i s thus a volume not a s u r f a c e v a l u e . Another problem c r e a t e d by the t r a n s m i s s i o n of shortwave r a d i a t i o n i s i n measuring sub-surface snow temperatures. The measuring instrument i s l i k e l y to absorb t r a n s m i t t e d r a d i a t i o n c a u s i n g i t to warm up and become an anomalous thermal f e a t u r e . I t t h e r e f o r e records i t s own response and not that of the surrounding environment. 2) One of the most important c h a r a c t e r i s t i c s i s the high albedo of snow and i c e . T h e i r r e f l e c t i o n of l a r g e amounts of incoming shortwave r a d i a t i o n i s of primary importance when c o n s i d e r i n g t h e i r energy budgets. U n l i k e most s o i l and v e g e t a t i o n covers the albedo of snow i s h i g h e s t f o r the s h o r t e s t wavelengths, d e c r e a s i n g to q u i t e low valu e s i n the near i n f r a -red. 3) Ice and snow behave as almost i d e a l b l a c k - b o d i e s as t h e i r r e f l e c t i v i t i e s f o r longwave r a d i a t i o n are l e s s than 0.5% t h e r e f o r e t h e i r longwave e m i s s i v i t i e s are almost 1. Although the e m i s s i v i t y i s high the a b s o l u t e magnitude of emitted longwave r a d i a t i o n i s u s u a l l y r e l a t i v e l y small because temperatures are low. One h e l p f u l s i m p l i f i c a t i o n occurs i f the su r f a c e i s m e l t i n g ; then the value of the emitted longwave r a d i a t i o n becomes a constant because s u r f a c e temperature i s 0°C 1 3 and i t i s assumed that the emissivity equals one. 2 . 4 Energy Balance Computations Computing an accurate energy balance at a point i s quite complex with the greatest d i f f i c u l t i e s l y ing in the estimation of the turbulent fluxes. Instrumentation exists that can measure with adequate precision the net radiation and ground heat flux at a point. However only a few papers have reported dire c t measurement of the turbulent fluxes of sensible and latent heat over a snowpack (McKay and T h u r t e l l 1978, Hicks and Martin 1972). The r e l a t i v e importance of the turbulent fluxes in the energy balance i s quite variable as i t depends on atmospheric and snowpack conditions. For example the value of Q for a cold e pack for cold atmospheric conditions w i l l be quite small, while Q for a warm wet snowpack can be quite important. This i s e explained by the fact that in the cold snowpack scenario there is no l i q u i d water for evaporation and there i s l i t t l e atmospheric vapour for condensation. During favorable atmospheric conditions such as high wind speeds and vapour pressure gradients towards the snow, Q can become an important e energy source for melt. The a i r to surface vapour pressure gradient would result in a downward flux of moisture and condensation on the surface. Since the latent heat of vaporization is 7.5 times greater than the latent heat of fusion, for every one gram of water condensed s u f f i c i e n t energy 14 i s s u p p l i e d to melt a f u r t h e r 7.5 grams of snow. Three techniques are a v a i l a b l e f o r e s t i m a t i n g the t u r b u l e n t f l u x e s of s e n s i b l e and l a t e n t heat i n the constant f l u x boundary l a y e r . These are : 1) Eddy c o r r e l a t i o n 2) Bowen's r a t i o 3) Aerodynamic method The most ac c u r a t e and only d i r e c t method i s eddy c o r r e l a t i o n , where Q and Q are determined d i r e c t l y from the h e eddy f l u x r e l a t i o n s h i p s . : Q = c 'pTTwT~ h p Q = -L pq^P" e v where c i s the s p e c i f i c heat of a i r at constant pressure p P i s the a i r d e n s i t y , L i s the l a t e n t heat of v a p o u r i z a t i o n . The v q', T', w' represent the f l u c t u a t i n g components of the s p e c i f i c humidity, the a i r temperature and the v e r t i c a l component of the wind r e s p e c t i v e l y . The overbar i n d i c a t e s a time average. U n f o r t u n a t e l y , however because of very expensive i n s t r u m e n t a t i o n requirements t h i s method i s g e n e r a l l y i m p r a c t i c a l . 1 5 In many i n v e s t i g a t i o n s of the energy exchange at the e a r t h ' s s u r f a c e Qh and Qe are determined from the Bowen r a t i o (0) where 0 = Q /P c \ [T - T^l h = a p M z o ' -Q— \ .622 L / /e - e \ e s ' z o) L = l a t e n t heat of s u b l i m a t i o n , P i s the a i r p r e s s u r e , T and s a z e are the a i r temperature and vapour pressure r e s p e c t i v e l y at z height z and T and e are the s u r f a c e a i r temperature and o o s u r f a c e vapour pressure r e s p e c t i v e l y . The Bowen's r a t i o method i n v o l v e s measuring a temperature and vapour pressure g r a d i e n t above the energy exchange s u r f a c e . McKay and T h u r t e l l (1978) and Male and Granger (1978) both showed the absence of a good c o r r e l a t i o n between measured and c a l c u l a t e d Bowen r a t i o s over m e l t i n g snow. Reasons given by the authors to e x p l a i n t h i s poor c o r r e l a t i o n were many. Among these was that s i n c e the t u r b u l e n t f l u x e s over snow were small (much sm a l l e r than summer), the small e r r o r s i n measurements can l e a d to wide f l u c t u a t i o n s i n /3. To use the Bowen r a t i o approach Q m must be known. However, i n most p r a c t i c a l s i t u a t i o n s Q i s m determined as the r e s i d u a l term from the energy balance due to the d i f f i c u l t y i n t r y i n g to measure i t . McKay and T h u r t e l l (1978) concluded that the Bowen r a t i o energy balance approach was inadequate f o r energy balance c a l c u l a t i o n s over m e l t i n g 16 snow. The p r o f i l e method has been the most widely used method f o r measuring the t u r b u l e n t f l u x e s over a m e l t i n g snowpack. T h i s method i s based on a set of f l u x g r a d i e n t equations that are used to d e s c r i b e the s e n s i b l e and c o n v e c t i v e heat t r a n s f e r s . The b a s i c equations are a f u n c t i o n of 1) the mean g r a d i e n t of the c o n c e n t r a t i o n of the e n t i t y ( d i / d z ) 2 ) an eddy d i f f u s i o n c o e f f i c i e n t (K) The b a s i c equations a r e : T= pK du/dz m Q = -pc K dT/dz h p h Q = -L K dq/dz e v e where du/dz, dT/dz, dq/dz are the mean g r a d i e n t s of the wind speed, the temperature and the s p e c i f i c humidity. K , K and K m h e are the eddy d i f f u s i o n c o e f f i c i e n t s f o r momentum, s e n s i b l e heat and water vapour r e s p e c t i v e l y . The s u r f a c e s h e a r i n g s t r e s s i s termed r . The mean c o n c e n t r a t i o n g r a d i e n t i s r e l a t i v e l y e a s i l y o btained, however, the eddy d i f f u s i o n c o e f f i c i e n t s are very d i f f i c u l t to determine. I t i s p o s s i b l e to s i m p l i f y the problem by invoking a theory that enables us to a v o i d measuring K. T h i s theory i s based on the p r i n c i p l e s that 1) r can be determined s o l e l y by the use of the wind prof i l e 17 2) the assumption that t h i s wind p r o f i l e i s l o g a r i t h m i c 3) that an eddy i s n o n - d i s c r i m i n a t o r y with regard to the prope r t y being t r a n s p o r t e d , thus a l l d i f f u s i o n c o e f f i c i e n t s are equal K =K =K . T h i s i s u s u a l l y r e f e r r e d to as the p r i n c i p l e of m e h s i m i l a r i t y . Now with a measurement of one f l u x ( r ) and inv o k i n g the p r i n c i p l e of s i m i l a r i t y we can use r a t i o s of f l u x e s invoking T to o b t a i n s o l u t i o n s f o r Q and Q without the use of t r a n s f e r h e coef f i c i e n t s . Q /r =-[pc K ( d T / d z ) ] / [ p K (du/dz)] h p h m Q A = c * K /K * (dT/du) h p h m Q = -TC (K /K (dT/du) h p h m It has been shown that the shearing s t r e s s (r) i s p r o p o r t i o n a l to the square of the wind v e l o c i t y , thus : u* 2 =r/p, where u* i s termed the f r i c t i o n v e l o c i t y . When the wind speed i s p l o t t e d a g a i n s t the lo g a r i t h m of the height z, a l i n e a r f u n c t i o n r e s u l t s . The slope of t h i s curve has been shown to be equal to u*/k, where k i s Von Kantian's constant = 0.4. Seeing that the equation f o r u, f o r a l o g a r i t h m i c wind p r o f i l e , i s d e f i n e d as u=(u*/k) In (z/z ) we can s t a t e that o du/dz = u*/kz. When u* i s i s o l a t e d we o b t a i n u*=kz(du/dz). Since r=u* 2 p we can o b t a i n r from knowing only the wind p r o f i l e : r = p k 2 z 2 ( d u / d z ) 2 , as long as the wind p r o f i l e i s l o g a r i t h m i c . F o l l o w i n g t h i s Q and Q become h e 18 Q =-pc k 2 z 2 K /K (du/dz.dT/dz) h p h m Q =-pL k 2 z 2 K /K (du/dz.dq/dz) e s e m Assuming K =K =K we have a s o l u t i o n f o r Q and Q . h e m h e To a c c u r a t e l y measure the t u r b u l e n t f l u x e s of s e n s i b l e and l a t e n t heat simultanuous measurements of windspeed, temperature and vapour pressure must be made at s e v e r a l h e i g h t s above the exchange s u r f a c e . T h i s u s u a l l y i n v o l v e s about seven measurement he i g h t s d i s t r i b u t e d i n a l o g a r i t h m i c f a s h i o n above the s u r f a c e . T h i s a l l o w s f o r a g r e a t e r d e n s i t y of measurements near the exchange s u r f a c e where the measured g r a d i e n t s vary more r a p i d l y . In the s u r f a c e boundary l a y e r the f l u x d e n s i t i e s T , Q and Q h e are assumed to be independent of height (steady s t a t e ) . The K c o e f f i c i e n t s which i n c r e a s e with height above the s u r f a c e w i l l t h e r e f o r e be balanced by c o r r e s p o n d i n g decreases in the g r a d i e n t . Having obtained the l o g a r i t h m i c p r o f i l e s f o r wind, temperature and humidity one can e a s i l y o b t a i n v a l u e s f o r du/dz, dT/dz and dq/dz f o r a s p e c i f i c height z e i t h e r through g r a p h i c a l or mathematical techniques. Although the b a s i c aerodynamic approach i s only a p p l i c a b l e i n n e u t r a l c o n d i t i o n s , s e m i - e m p i r i c a l r e l a t i o n s h i p s can be used to extend i t s u s e f u l n e s s to a wide range of s t a b i l i t y regimes. However the problem of s i m i l a r i t y remains. It has been suggested by Dyer and Hicks (1970) that f o r s t a b l e and n e u t r a l c o n d i t i o n s K /K =K /K =1 and that f o r h m e m unstable c o n d i t i o n s K /K = (1 - 1 6 z / L ) 0 - 2 5 , where z/L i s a h m 19 s t a b i l i t y parameter. Bussinger and others (1971) have suggested a value of k=.35 and K /K =1.35 for stable conditions, while h m Anderson (1976) suggests K /K =K /K =1.0 under stable h m e m conditions. This point remains controversial and there i s s t i l l a question as to which i s correct. Although Male and Granger (1981) suggests that there i s some evidence to indicate that over melting snow K <K , the state of the art i s such that the e h choice of any pa r t i c u l a r expression for K /K or K /K s t i l l e m h m ca r r i e s with i t a large uncertainty. Several equations have been formulated to determine Q and h Q from the use of only 2 measurement heights such as the one e developed and presented in P r i e s t l y (1959) and Kraus (1972), where : Q = -pc (K /K.)k 2(u -u )(T -T )ln 2(b/a) h p h m b a b a Q = -pL (K /K )k 2(u -u )q -q )ln 2(b/a) e s h m b a b a where the a and b denote measurement heights, u i s the windspeed and T i s the a i r temperature. However t h i s does not allow the v e r i f i c a t i o n of the logarithmic p r o f i l e s for wind, temperature and vapour pressure. Thus when using such equations one must be already assured that the logarithmic functions hold true for the measurement s i t e . 20 2.5 A p p l i c a t i o n s Of Snowmelt Equations In g e n e r a l the o b j e c t i v e of p o i n t snowmelt s t u d i e s i s to gain knowledge of the processes i n v o l v e d and to q u a n t i f y each of the components of the energy balance. Numerous s t u d i e s have d e s c r i b e d the energy balance of a m e l t i n g snowpack (Gold and W i l l i a m s 1961, Makkonnen et a l . 1981, Obled and Harder 1979, P r i c e and Dunne 1 976, Male and Granger 1978, Hendrie .and P r i c e 1979, de l a C a s i n i e r e 1974 ) f o r a small instrumented s i t e . T h i s knowledge can thus be t r a n s f e r r e d to the development of g e n e r a l i z e d b a s i n snowmelt equations and models usable at a management l e v e l , without the need f o r h i g h l y s o p h i s t i c a t e d equipment. The simplest and most common method i s to r e l a t e observed d a i l y melts to accumulated degree days, but t h i s i s g e n e r a l l y too v a r i a b l e . Both McKay (1978) and Anderson (1976) have noted that the v a r i a b i l i t y i n m e t e o r o l o g i c a l c o n d i t i o n s o c c u r r i n g d u r i n g snowmelt i s not always d e t e c t e d by the temperature index approach. These models are formulated on the concept that s e n s i b l e heat exchange i s the primary source of energy f o r melt. Thus when other terms dominate such as condensation melt, which may be the case d u r i n g rain-on-snow, these temperature models may do p o o r l y i n p r e d i c t i n g snowmelt r a t e s . A more p r e c i s e method i s to use energy balance equations that have been developed from these p o i n t snowmelt s t u d i e s , u sing i n d i c e s that are a p p l i c a b l e to the d e s i r e d s i t u a t i o n . One of the most e x t e n s i v e s t u d i e s was undertaken by the United S t a t e s Army Corps of Engineers i n the mid 1940's producing a 21 volume e n t i t l e d "Snow Hydrology" (USACE 1956). A s e r i e s of poin t snowmelt equations was d e r i v e d that p e r m i t t e d an e s t i m a t i o n of the energy s u p p l i e d to the snowpack from each component of the snowmelt equation(Appendix B). From t h e i r p l o t s t u d i e s they a l s o d e r i v e d snowmelt equations f o r use on e n t i r e b a s i n s , t h i s being t h e i r main concern as f l o o d c o n t r o l and r e s e r v o i r management was t h e i r mandate. These b a s i n snowmelt equations take i n t o account f o r e s t and c l o u d cover by v a r y i n g c e r t a i n i n d i c e s (Appendix C) In the 1956 r e p o r t "Snow Hydrology", the U n i t e d S t a t e s Army Corps of Engineers r e c o g n i z e d the problem of rain-on-snow and c i t e d a few examples of f l o o d s r e s u l t i n g from such events, such as the 1950 November f l o o d where intense r a i n s f a l l i n g on a r e l a t i v e l y shallow snowpack were ab e t t e d by m e l t i n g snow. A re c o r d peak d i s c h a r g e was observed from a small four square mile b a s i n . The snow hydrology r e p o r t was a major step i n t h i s f i e l d and i s s t i l l r e c o g n i z e d as being e s s e n t i a l l y v a l i d . The s i m p l i f i e d equations suggested f o r c a l c u l a t i o n of b a s i n snowmelt are d e s c r i b e d i n Appendix C. P r i c e and Dunne (1976) have s t a t e d that the p h y s i c a l b a s i s of the work done by the U n i t e d S t a t e s Army Corps of Engineers (1956) i s sound. However they suggest that some of the assumptions made i n the development of the equations governing the t u r b u l e n t exchanges are u n r e a l i s t i c , p a r t i c u l a r l y the adoption of the e x p o n e n t i a l wind p r o f i l e and the i g n o r i n g of the e f f e c t s of s t a b i l i t y . As can be seen i n Appendix B the equations used to compute 22 the t r a n s f e r of s e n s i b l e and l a t e n t heat to the snowpack i n c l u d e a f u n c t i o n of s e v e r a l parameters. I f we c o n s i d e r p/p and Z .Z o a b to be constant (terms d e f i n e d i n Appendix A and B) f o r a c e r t a i n e l e v a t i o n and instrument h e i g h t , then the l a t e n t and s e n s i b l e heat terms become a f u n c t i o n of the temperature or vapour pressure g r a d i e n t and the windspeed. The constant l o c a t e d at the f r o n t of the equation i s termed the bulk t r a n s f e r c o e f i c i e n t and was determined with no c o n s i d e r a t i o n s made f o r the p o s s i b l e v a r i a t i o n s i n the s t a b i l i t y of the atmosphere. S e v e r a l other authors have d e f i n e d s i m i l a r bulk t r a n s f e r c o e f f i c i e n t s and as can be seen i n Table 1, taken from Gray and Male (1981), no values have been agreed upon. To o b t a i n these c o e f f i c i e n t s the USACE used l y s i m e t r y techniques and the snowmelt energy balance equation : Q =Q +Q +Q +Q +Q • Q , the t o t a l melt, was measured as the m n h e g r m outflow from a l a r g e 1300 f t 2 cement snowmelt l y s i m e t e r . Q was n measured d i r e c t l y with the use of net radiometers, Q was 9 obtained with the use of heat f l u x p l a t e s and Q was equated to r zero as no r a i n f e l l d u r i n g the c a l i b r a t i o n p e r i o d . Q was e obtained by p e r i o d i c weight measurements of a block of snow to eval u a t e gains or l o s s e s of water through e v a p o r a t i o n or condensation. Q was obtained as the r e s i d u a l of the energy h balance equation. Knowing Q and Q , valu e s f o r the bulk e h Table 1 - Comparison of l i t e r a t u r e obtained values f o r bulk t r a n s f e r c o e f f i c i e n t s (adapted from Gray and Male 1981) Author D h x 10' Measurement D e x IO 3 Height (m) Comments kJ/m' • °C kJ/m' • mbar U z Ta ea Hicks and Martin (1972) 1.06 5.15 3.2 2 2 Values based on eddy correlation measurements made in four l-h periods under highly stable conditions with wind speeds less than 3.5 m/s. Gold and Williams (1961) 15 25 2 1.2 • ea measured 1.6 km from site. Dh and D e are average values for a two-week period. Assumed Dh/De = 0.6 (Bowen ratio) Yoshida (1962) 3.56 6.62 0.7 1.2 1.2 Values apply only for wind speeds greater than 2 m/s. U.S. Army Corps of Engineers (I9S6) 1.68 8.0 I I I Coefficients corrected to 1 m using 1/6 power law. Sverdrup (1936) as quoted by U.S. Army Corps of Engineers (1956) 5.74 10.0 1 1 1 Coefficients corrected to 1 m using 1 /6 power law. de Quervain (1952) as quoted by U.S. Army Corps of Engineers (1956) - 11.4 I I I Coefficients corrected to 1 m using 1/6 power law. Granger (1977) 6.69 2.17 1 1 1 Measurements taken above melting prairie snowpack. Dh - average for 6 days; D e - average for 8 days, based on lysimeter results. 24 t r a n s f e r c o e f f i c i e n t s were determined by performing r e g r e s s i o n type a n a l y s i s with windspeed, temperature and vapour pressure data. It has been shown by Male and Granger (1978) from Anderson (1976) that the use of bulk t r a n s f e r c o e f f i c i e n t s (D) of the form : Q = D u (e -e ) e e a z o Q = D u (T -T ) h h a z o can l e a d to s u b s t a n t i a l e r r o r s . T h i s i s probably because of the assumed e q u a l i t y of t r a n s f e r mechanisms f o r l a t e n t and s e n s i b l e heat. Since l a t e n t heat t r a n s f e r i s g e n e r a l l y l e s s e f f i c i e n t than the s e n s i b l e heat t r a n s f e r , c o e f f i c i e n t s overestimate the evaporation (Male and Granger 1978). Male and Gray (1975) s t a t e that the accuracy of such e x p r e s s i o n s i s d i f f i c u l t to determine and depends l a r g e l y on the t r a n s f e r c o e f f i c i e n t D. Nonetheless these types of equations have been used i n a v a r i e t y of s i t u a t i o n s and f o r s e v e r a l purposes. A recent r e p o r t p u b l i s h e d by F i s h e r i e s and Environment Canada used these equations to e v a l u a t e the p o t e n t i a l impacts of timber h a r v e s t i n g on the hydrology of the Queen C h a r l o t t e I s l a n d s (Toews and W i l f o r d 1978). T h e i r concern was to determine a f e a s i b l e r a t e of cut that would not a l t e r s i g n i f i c a n t l y the hydrology of the a r e a . They wanted to a v o i d slope f a i l u r e , bank and channel e r o s i o n and d e s t r u c t i o n of the f i s h h a b i t a t . One of the major h y d r o l o g i c problems that they encountered was the e f f e c t s of h a r v e s t i n g on snowmelt r a t e s 25 during rain events. The snowmelt calculations were done using the United States Army Corps of Engineers equations with assumed winter storm conditions. The detailed calculations can be found in Appendix D. The results from the purely hypothetical calculations using a 2.50 cm r a i n f a l l in 24 hours are as follows. runoff from the forested plot : 5.47 cm runoff from the clearcut plot : 8.79 cm These results were included in the report that recomends that rate of cut in the Queen Charlotte Islands be limited to 33% of a watershed in 25 years. This report was quick to i n i t i a t e some controversy in B r i t i s h Columbia. This was because the USACE point snowmelt equations were derived from lysimeter studies at an open s i t e , and i t was cautioned that the bulk transfer c o e f f i c i e n t s would probably not be v a l i d for a forested s i t u a t i o n . Some s p e c i f i c plot studies were necessary to evaluate the v a l i d i t y of these equations for the north P a c i f i c coast. Harr (1981) has discussed the potential hazards and r e l a t i v e l y high frequency of flood events caused by rain-on-snow storms, and has suggested the need for more s p e c i f i c plot studies. Rain on snow storms would seem to be a problem elsewhere than on the northern P a c i f i c coast of America. "Rapid melt of the Central Otaga mountain snowpack produced, in three days, 97 mm of water compared with 150 mm of r a i n f a l l . The energy sources for melt were dominated by convective fluxes. The contribution of snowmelt to the major October 1978 flood was over one t h i r d of the flow of the Fraser River (New Zealand)." ( F i t z h a r r i s et a l . 1980). 26 Researchers on the East coast of America (Pysklywec 1966) have found the USACE (1956) equations not to be v a l i d , and have derived their own equations. However, the o r i g i n a l USACE (1956) equations have not yet been v e r i f i e d for the west coast mountainous regions, where rain on snow i s an important and reoccurring event. The questions remain; i s runoff s i g n i f i c a n t l y increased after harvesting during these types of events and i f so why and for how long? Is there a p o s s i b i l i t y of managing in such a way as to minimize these effects? Can the USACE (1956) equations be used on the west coast for rain on snow situations? To v e r i f y t h i s a snowpack energy and water balance study must be performed in a forested and unforested area. Each of the energy terms must be evaluated pre c i s e l y and compared with results obtained using the USACE turbulent flux transfer equations. As has been previously mentioned t h i s may be achieved by using one of three methods. The eddy co r r e l a t i o n technique was discarded for t h i s study because of equipment l i m i t a t i o n s . Bowen's r a t i o method was also discarded because, as suggested by several authors, i t i s not applicable over melting snow. Thus the aerodynamic approach was essentialy the only method available. Because the aerodynamic approach assumes s i m i l a r i t y among the transfer c o e f f i c i e n t s K , K and K i t was necessary that m h m this condition p r e v a i l during our measurement period. It i s generally accepted that stable conditions p r e v a i l during cloudy 27 days over a m e l t i n g snowpack (Hendrie and P r i c e 1978, Anderson 1976). Dyer and Hicks (1970) suggest that d u r i n g these p e r i o d s the r a t i o s of K /K and K /K are c l o s e to one. However as h m e m suggested by Federer and Leonard (1971), the t u r b u l e n t t r a n s f e r theory developed f o r a uniform s u r f a c e , and the assumption of an e x p o n e n t i a l wind p r o f i l e may not h o l d under a f o r e s t canopy. T h i s would render the use of the aerodynamic equations inadequate f o r the f o r e s t s i t e . Because l i t t l e work has been done to e v a l u a t e t u r b u l e n t exchanges of heat and water vapour under f o r e s t cover, the l i t e r a t u r e o f f e r s no reasonable s u b s t i t u t e to the use of the aerodynamic formulae. 28 I I I . DESCRIPTION OF STUDY SITE 3.1 General 3.1.1 Locat ion T h i s study was c a r r i e d out on the Jamieson Creek experimental watershed. T h i s f i r s t order mountainous watershed extending from 300m to 1300m above sea l e v e l i s l o c a t e d at the north end of the Seymour River B a s i n . The Seymour b a s i n i s one of three major catchements that p r o v i d e s water to the c i t y of Vancouver, B r i t i s h Columbia. I t i s l o c a t e d d i r e c t l y to the north of the c i t y and n e s t l e d amongst the southern mountains of B r i t i s h Columbia's c o a s t a l range ( F i g u r e s 2 and 3). Much has been w r i t t e n about the p h y s i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s of the Seymour watershed. The Seymour b a s i n i s one of the e a r l i e s t to be monitored with hydrometric s t a t i o n s i n the p r o v i n c e of B r i t i s h Columbia, where d i s c h a r g e data goes back to 1913. Most of the v a r i o u s p h y s i c a l and b i o l o g i c a l d e s c r i p t i o n s of the Jamieson Creek experimental watershed found i n t h i s chapter, were obtained from theses, r e p o r t s and s c i e n t i f i c papers w r i t t e n f o r the Greater Vancouver Regional D i s t r i c t or the U n i v e r s i t y of B r i t i s h Columbia. 3.1.2 Climate The o u t s t a n d i n g f e a t u r e s of the c l i m a t e along the southern coast of B r i t i s h Columbia a r e : 1) The mildness and humidity of the winters f o r t h i s l a t i t u d e . Figure 2 - General location of study area 30 Figure 3 - Location of Jamieson Creek Watershed 31 2) The very heavy p r e c i p i t a t i o n and c l o u d i n e s s . 3) The l a r g e accumulation of snow at hig h e l e v a t i o n s , which g e n e r a l l y l a s t s i n t o the summer. 4) The s t r i k i n g d i f f e r e n c e s i n temperature and p r e c i p i t a t i o n as a r e s u l t of changes i n e l e v a t i o n , d i s t a n c e from the coast, and complex mountain topography. These f e a t u r e s are l a r g e l y the r e s u l t of the moderating i n f l u e n c e of the ocean and ocean c u r r e n t s , the p r e s s u r e p a t t e r n s which cause a west to east movement of a i r , and the high mountain b a r r i e r of the Coast Mountains (Brooke 1966). The Jamieson Creek watershed has w i t h i n i t s boundaries two of K r a j i n a ' s (1965) b i o g e o c l i m a t i c zones of B r i t i s h Columbia. These are 1) C o a s t a l Western Hemlock Zone - An equable (marine) humid to r a i n y c l i m a t e with a t o t a l p r e c i p i t a t i o n of 65 to 262 inches. 2) Mountain Hemlock Zone -A subalpine c l i m a t e with heavy snow cover over unfrozen ground, winter not severe, and t o t a l annual p r e c i p i t a t o n of 70 to 170 inches. The Coast Mountains, a system of h i g h l y d i s s e c t e d highlands ranging up to 2,200 m, form a wide range and an almost complete b a r r i e r between the ocean and the i n t e r i o r of the p r o v i n c e . These mountains h e l p p r o t e c t the coast from extremes of C o n t i n e n t a l a i r (Chapman 1952). When Pol a r Maritime a i r masses reach the Coast Mountains they are impeded. Great masses of c l o u d and heavy p r e c i p i t a t i o n occur i n the uns t a b l e a i r and f r o n t a l a c t i v i t y of the many passing d e p r e s s i o n s (Kendrew and Kerr 1955). In general there i s an i n c r e a s e i n p r e c i p i t a t i o n 32 with i n c r e a s i n g d i s t a n c e from the coast l i n e and i n c r e a s i n g with a l t i t u d e ( B r o o k e 1966) (Figure 4). However the i n c r e a s e i n p r e c i p i t a t i o n with i n c r e a s i n g a l t i t u d e does not i n c r e a s e i n d e f i n i t e l y s i n c e the c o o l e r a i r has a maximum p o s s i b l e moisture content, so that at a c e r t a i n l e v e l a maximum p r e c i p i t a t i o n i s noted ( G r i f f i t h s 1966). O r l o c i (1964) found an almost l i n e a r c o r r e l a t i o n of t o t a l annual s n o w f a l l with a l t i t u d e . The average lapse r a t e due to c o n v e c t i o n and r a d i a t i o n d i f f e r e n c e s i s about 5.5° C/l000m, however t h i s must be viewed with c a u t i o n because of l o c a l topographic c o n d i t i o n s , p r e v a i l i n g a i r masses and other f a c t o r s which may cause s i g n i f i c a n t v a r i a t i o n s (Chapman 1952). The topography in the southern Coast Mountains of B r i t i s h Columbia i s very complex, and the e f f e c t s of slope, a s p e c t , a l t i t u d e , landform and exposure are of the utmost importance when c o n s i d e r i n g m i c r o - c l i m a t i c c o n d i t i o n s . The a i r and s o i l temperature, d u r a t i o n of snow cover, amount and d u r a t i o n of s o l a r r a d i a t i o n , p r e c i p i t a t i o n and numerous other f a c t o r s can vary s h a r p l y over short d i s t a n c e s . S p e c i f i c c l i m a t i c c o n d i t i o n s f o r Jamieson Creek watershed The c l i m a t i c c o n d i t i o n s f o r Jamieson Creek watershed are t y p i c a l of the south coast region of B r i t i s h Columbia, being wet and m i l d i n winter and dry and warm in summer. The average y e a r l y temperature measured j u s t below the watershed o u t l e t (305m) i s 4.9°C with a maximum recorded temperature of 31.7°C and a minimum -12°C . However, as has been mentioned, 00 Figure 4 - D i S^m6s b D Uheric n Fnvi rnnmnf* 1 J? r eP 1 t a t i on ( i ncheS ) (adapted from a map published by the A-mospneric Environmnet Service, Vancouver T..C. date unknown) 35 temperatures vary with e l e v a t i o n , thus at the top of the watershed (1188m) average temperatures would c e r t a i n l y r e g i s t e r s e v e r a l degrees c o o l e r . F i g u r e 6 shows the d i s t r i b u t i o n of monthly average temperatures with the July-September mean being around 12 to 15°C and the mean November-April temperature v a r i e s from -4.5 to 1.1°C (Golding and Rae 1981). S e v e r a l B e l f o r t type r e c o r d i n g r a i n gauges have been in use i n the Jamieson Creek watershed s i n c e 1969 to monitor hourly p r e c i p i t a t i o n . The records have shown that East f a c i n g slopes u s u a l l y c a t c h more r a i n than do the west. The average annual p r e c i p i t a t i o n (temporal and s p a t i a l ) f o r the p e r i o d between 1969 and 1980 i s approximately 3300 mm. The mean monthly d i s t r i b u t i o n i s shown i n F i g u r e 7. The June-August p e r i o d i s the d r i e s t q u a r t e r of the year. Approximately 70% of annual p r e c i p i t a t i o n occurs dur i n g the 6 month p e r i o d October-March, most of i t as snow. November and December are the two wettest months, accounting f o r 31% of annual p r e c i p i t a t i o n . The monitoring of snow depths and water e q u i v a l e n t s on the Jamieson Creek experimental watershed has been l i m i t e d to one l o c a t i o n up u n t i l the s t a r t of t h i s rain-on-snow r e s e a r c h . T h i s s t a t i o n i s l o c a t e d at 1189m where maximum s n o w f a l l i s l i k e l y to occur, thus not r e p r e s e n t a t i v e of average s n o w f a l l s f o r the watershed as a' whole. The average annual snow accumulation at t h i s high e l e v a t i o n s i t e i s 2611mm water e q u i v a l e n t , with a maximum accumulation of 3721mm and a minimum of 1161mm(Province A V E R A G E T E M P E R A T U R E D I S T R I B U T I O N AT E L B O W C R E E K ( 1 9 7 2 - 1 9 7 6 ) 25-r U J Or: u U J 5 < 20 A 15 H io A ^ f # ^ tf* ^ ^ ^ V ^ ° stf 0 ^ ^ tfP MONTH L e g e n d AVERAGE: MAXIMUM MINIMUM Figure 6 - Mean monthly d i s t r i b u t i o n of temperature at Elbow Creek M E A N M O N T H L Y D I S T R I B U T I O N O F P R E C I P I T A T I O N ( 1 9 6 9 - 1 9 8 0 ) 600-1 5 5 0 : 5 0 0 : 450: • 400: 2 , 350-O i? 300: Q_ 250: O U J -oc 200-Q_ 150: 100: 50-MONTH Figu^e" l/^'^Mear^rion^ilyd^3 38 T a b l e 2 - B.C. M i n i s t r y of the Environment snow survey measurements around the G r e a t e r Vancouver Water D i s t r i c t S t a t i o n * Name E l e v a t i o n ( m ) L o c a t i o n on map 3A07 B u r w e l l Lake 880 C 3A08 H o l l y b u r n 1 100 E 3A1 1 Loch Lemond 900 A 3A1 5 Mt Seymour 1070 D 3A1 9 O r c h i d Lake 1 190 B A p r i l 1 snow c o u r s e d a t a S t a t i o n * Y ears Number of Ave. snow Average samples depth(cm) w.e.(mm) 3A07 1945-80 28 267 1014 3A08 1945-80 36 382 1 556 3A1 1 1945-80 34 305 1 261 3A1 5 1960-80 21 362 1605 3A19 1972-80 9 464 2093 of B r i t i s h Columbia 1980). F i g u r e 8 and T a b l e 2 show the v a r i a b i l i t y of snow depths w i t h l o c a t i o n and e l e v a t i o n i n and around the G r e a t e r Vancouver Water D i s t r i c t . 3.1.3 S o i l s The s o i l s of Jamieson Creek e x p e r i m e n t a l watershed have been mapped by L a v k u l i c h (1973) and mapped and c l a s s i f i e d by Lewi s (1973) ( F i g u r e 9 and Ta b l e 3 ) . They have found l a r g e v a r i a t i o n s i n s o i l c h a r a c t e r i s t i c s i n d i f f e r e n t p a r t s of the watershed. Zeman (1973) c l a s s i f i e d the s o i l s i n t o t h r e e groups as d e s c r i b e d from a h y d r o l o g i c a l p o i n t of view. 1) The upper p a r t of the watershed has w e l l - d r a i n e d s o i l s 39 o Figure 9 - Soils of Jamieson Creek Watershed 41 Table 3 - S o i l s of Jamieson Creek watershed Map u n i t S o i l s e r i e s Mater i a i Slope TA t a l u s misc. RO rock outcrop misc. -LS L i o n s c o l l u v i u m 50-90 HB H o l l y b u r n g l a c . t i l l <2' 20-60 DE Dennet s h a l l . o r g / b d r k . -20-90 LS L i o n s c o l l u v i u m 20-60 DE Dennet s h a l l . o r g / b d r k . 20-90 PA P a l i s a d e c o l l u v i u m 50-90 RO rock outcrop misc. — SN Strachan g l a c . t i l l 20-50 BW Burwell g l a c . t i l l 10-40 CE Cannel g l a c . t i l l 20-60 Map Unit C l a s s i f i c a t i o n Drainage TA RO - -LS mini-humo-ferric-pod. w e l l - r a p i d HB orthic-humo-fer-pod. well-mod.well DE l i t h i c f o l i s o l well-mod.well LS mini-humo-ferric-pod. w e l l - r a p i d DE l i t h i c f o l i s o l well-mod.well PA mini-ferro-humic-pod. well-mod.well RO — — SN O r t h i c ferro-humic-pod. IT!od. w e l l BW g l e y . o r t h o fer.hum.pod. imperfect CE l i t h . m i n i hum-fer pod. w e l l - r a p i d 42 de v e l o p i n g on bedrock and dominated by p o o r l y to w e l l decomposed organic matter h o r i z o n s . These s o i l s belong to the category of l i t h i c F o l i s o l s and l i t h i c P odzols. 2) The second group are s o i l s that have formed i n a b l a t i o n and weathered b a s a l t i l l or bedrock, i n i m p e r f e c t l y d r a i n e d s i t e s . These s o i l s belong to the category of Gleyed Ferro-humic Podzols. 3) The t h i r d category comprises moderately w e l l d r a i n e d s o i l s developed i n t i l l and/or c o l l u v i u m . These s o i l s show good Ae h o r i z o n s o v e r l y i n g strong P o d z o l i c Bfh and Bhf h o r i z o n s . An important c h a r a c t e r i s t i c of the Jamieson Creek watershed i s the almost t o t a l absence of s u r f a c e r u n o f f . T h i s i s du to the h i g h p e r m e a b i l i t y of the m i n e r a l s o i l , the low i n t e n s i t i e s of r a i n f a l l and n e g l i g i b l e presence of f r o z e n s o i l s . Thus, e s s e n t i a l l y , a l l r a i n f a l l and snowmelt water undergoes p e r c o l a t i o n through the s o i l to stream channels. 3.1.4 Geology And Topography Roddick (1965) i n d i c a t e s t hat the rock mantle of about 80% of the Coast Mountains i s p l u t o n i c , the remainder being composed of sedimentary, v o l c a n i c and metamorphic roc k s . The rock i n the upper Seymour i s mainly a medium-grained q u a r t z d i o r i t e with more hornblende than b i o t i t e (Roddick 1965). G l a c i a l d e p o s i t s cover the e n t i r e area of which g l a c i a l t i l l of v a r i o u s depths predominate. R e l a t i v e l y impervious b a s a l or lodgement t i l l i s commonly encountered at 0.5 to 1.2 m below the ground s u r f a c e . Above t h i s l i e s a mixture of loose a b l a t i o n t i l l and weathered b a s a l t i l l (Zeman 1973). 43 Jamieson Creek watershed has a w e l l d e f i n e d topographic boundary with a watershed area of 299 ha.. The e l e v a t i o n of the watershed ranges from 305m at the o u t l e t to 1, 31Om at the highest p o i n t , with more than 30% of the watershed area l y i n g above 900m. The land s l o p e s of the watershed are g e n e r a l l y steep with a c o n s i d e r a b l e area of shallow s o i l s and o c c a s i o n a l outcrops (Cheng 1975). F i g u r e s 10 and 11 (from Cheng 1975) show the area e l e v a t i o n curves and slope percentages of t h i s watershed. The m a j o r i t y of the watershed faces n o r t h e a s t and southwest as the main channel has a g e n e r a l southeast o r i e n t a t i o n . The g r a d i e n t of the stream channel averages about 20%, however, some short reaches, p a r t i c u l a r i l y i n the upper p o r t i o n of the watershed, have g r a d i e n t s g r e a t e r than 100%. 3.1.5 V e g e t a t i o n The f o r e s t on Jamieson Creek watershed i s composed e n t i r e l y of mature and over mature stands. Two b i o g e o c l i m a t i c zones are represented: C o a s t a l Western Hemlock below 900m e l e v a t i o n c o n s i s t i n g of Douglas F i r ( Pseudotsuga menziezi i ( M i r b . ) F r a n c o ) , western hemlock(Tsuga  h e t e r o p h y l l a (Rafn.)Sarg) Western red cedar (Thuja p l i c a t a Donn), s i t k a spruce ( P i c e a s i t c h e n s i s (Bong.)Carr.) and subalpine Mountain Hemlock above 900m, c o n s i s t i n g of mountain hemlock (Tsuga mertensiana (Bong.) C a r r . ) , yellow cedar ( Chaemacyparis n o o t k a t e n s i s (D.Don) Spach), a m a b i l i s f i r (Abies  Amabilis (Dougl.) Forbes) with d e c r e a s i n g amounts of western red cedar and western hemlock. F i g u r e 12 and Table 4 d e s c r i b e the d i s t r i b u t i o n of the f o r e s t 44 watershed (Cheng 1975) 45 watershed (Cheng 1975) 46 cover on the Jamieson Creek watershed. B r i e r e ( l 9 7 9 ) d e s c r i b e d i n d e t a i l the v e g e t a t i o n on the Jamieson Creek and Seymour r i v e r b a s i n s , based on h i s own Aqua-Te r r a c l a s s i f i c a t i o n system. S e v e r a l d i s t i n c t landscape u n i t s were used to c l a s s i f y the p h y s i c a l and b i o l o g i c a l f e a t u r e s of the Jamieson Creek watershed, however the stand c h a r a c t e r i s t i c s c o u l d be d e s c r i b e d adequately with the use of two major u n i t s . The lower e l e v a t i o n would be represented by B r i e r e ' s seepage zone middle slope u n i t (SM) i n the c o a s t a l western hemlock subzone (CWMb) while the higher e l e v a t i o n would f a l l i n t o the c a t e g o r i e s of the seepage zone top slope u n i t (ST) i n the mountain hemlock a subzone (MHa). These stand c h a r a c t e r i s t i c s are given i n t a b l e s 5 and 6 from B r i e r e (1979). 3.2 S p e c i f i c S i t e D e s c r i p t i o n Two s i t e s were chosen to measure snowmelt and v a r i o u s m i c r o m e t e o r o l o g i c a l v a r i a b l e s f o r comparative purposes, one was s i t u a t e d i n the mature f o r e s t and the second in a r e c e n t l y c l e a r c u t s i t e . 3.2.1 Open S i t e The n o n - f o r e s t e d snowmelt p l o t i s l o c a t e d i n a 16 hectare c l e a r c u t t hat was harvested i n 1979. The p l o t has a southwestern aspect with an average slope of 29° at an e l e v a t i o n of 730 meters. The c l o s e s t f o r e s t e d edge i s 142 meters from the center of the p l o t and the height of the canopy at t h i s edge i s 44 meters. The c l e a r c u t was p l a n t e d with D o u g l a s - f i r i n 1981, 48 Table 4 - F o r e s t cover Jamieson Creek watershed Map u n i t Spec i e s Age Height (m) S i t e r a t i n g a l p i n e - - - -HC(CY) 941-P hem, w.r.cedar yellow cedar 251 + 30-38 poor HB(CY) 941-P hem,bal.,yel.ced. 251 + 30-38 poor HBC(S) 951-M hem,bal.,w.r.cedar spruce 251 + 38-47 medium HBC 951-M hem,bal. fw.r.cedar 251 + 38-47 medium BHC 951-M bal.,hem,w.r.cedar 251 + 48-56 good hem=western hemlock, w.r. cedar=western red cedar, y e l . ced.=yellow cedar, b a l = a m a b i l i s f i r , spruce=Sitka spruce which has reached a height of approximately 1 meter i n 1983. There i s an abundance of s l a s h s c a t t e r e d over the area as no burning was done on the s i t e a f t e r h a r v e s t i n g . 49 Table 5 - Forest stand c h a r a c t e r i s t i c s for the St unit in the MHa subzone ( a d a p t e d f r o m B r i e r e 1979) Forest Stand Tsuga mertenalana Abies Mensuration + Tsuga h e t e r o p K y l l a a m a b i l i s T o t a l Volume/Acre i n cu. feet 4536.0 8655.5 13191.5 Number of Stem/Acre 15.0 84.3 99.3 Average Volume/Tree i n 3 Q 2 t 1 Q 2 6 , 3 2 g cu. feet Average D.B.H. (inches) 40.7 20.7 23.7 Average Height (feet) 100.4 83.7 86.2 Table 6 - Forest stand c h a r a c t e r i s t i c s for the SM unit in the CWHb subzone ( a d a p t e d f r o m B r i e r e 1979) Forest Stand Pseudotsuqa Thuja Tsuga Abies Mensuration m e n z i e s l i p l i c a t a heterophylla amabilis T o t a l Volume/Acre i n cu. feet 113.7 8386.8 4188.9 1501.6 14191.0 number of Stem/Acre 0.14 21.2 64.9 64.1 150.3 Average Volume/Tree i n ^ cu. feet Average D.B.H. (inches) 66.0 41.1 16.0 10.5 17.2 Average Height (feet) 150.0 97.9 64.6 50.3 63.3 50 3.2.2 F o r e s t S i t e The f o r e s t e d snowmelt p l o t i s s i t u a t e d at an e l e v a t i o n of 655 metres with an average slope of 26° and a southwestern a s p e c t . The f o r e s t surrounding the p l o t i s composed of mature and over-mature western red cedar and western hemlock with o c c a s i o n a l young D o u g l a s - f i r i n the understory r e a c h i n g an average height of 3.6 meters. The b a s a l area at t h i s p l o t i s 48m2/ha with a crown cover of about 70%. The crown cover d i r e c t l y over the the 22m2 l y s i m e t e r has been estimated at 90%. F i g u r e 13 and Table 7 p r o v i d e a d e t a i l e d d e s c r i p t i o n of the f o r e s t cover and l o c a t i o n of t r e e s around the l y s i m e t e r . 51 Table 7 - F o r e s t cover around f o r e s t l y s i m e t e r TREE SPECIES DBH(cm) DISTANCE AZIMUTH from (from o u t l e t ) o u t l e t (m) A RED CEDAR SNAG 230.0 4.70 276° B HEMLOCK 52.0 4.35 290° C HEMLOCK 88.3 3.40 0° D CEDAR 250.0 5.00 74° E HEMLOCK 28.9 5.30 85° F RED CEDAR SNAG 275.0 8.70 1 10° F.A HEMLOCK 37.0 1 1 .30 1 29° G AMABILIS-FIR 26, 1 6.85 130° H DOUG-FIR 5.3 4.90 148° I DOUG-FIR 5.0 4.80 168° J DOUG-FIR 5.6 2.60 184° K HEMLOCK 86.5 8.60 213° L HEMLOCK 101.3 5.80 228° M HEMLOCK 55.9 8.80 263° 53 IV. METHODS AND MATERIALS 4.1 Winter Of 1981-1982 In the summer of 1981 each of the f o r e s t e d and open s i t e s were intrumented with two small l y s i m e t e r s p r o v i d i n g a t o t a l s u r f a c e area of 0.5m2 at each s i t e . These l y s i m e t e r s were c o n s t r u c t e d using the lower p a r t of a 45 g a l l o n b a r r e l cut at an angle s i m i l a r to that of the sl o p e . They were b u r i e d i n such a way that a l i p of s e v e r a l c e n t i m e t e r s protruded above the s o i l s u r f a c e thus p r e v e n t i n g s u r f a c e r u n o f f from e n t e r i n g the l y s i m e t e r . These b a r r e l s were l o o s e l y f i l l e d with g r a v e l to f i l t e r out s o i l and d i r t from the l y s i m e t e r o u t l e t . Two independent 2.5 cm diameter p.v.c. p i p e s were used to d i r e c t the c o l l e c t e d water to the instrument shed where measuring instruments were l o c a t e d . The flow from the two l y s i m e t e r s was channeled together p r i o r to measurement i n a t i p p i n g bucket type arrangement (Figure 14). Each of the aluminum t i p p i n g buckets had a c a p a c i t y of 200ml thus p r o v i d i n g 400 m l / c y c l e . An E s t e r l i n e Angus 20 channel s t r i p c h a r t event recorder powered by two s i x v o l t b a t t e r i e s was used to monitor the l y s i m e t e r flow at each of the s i t e s . Each c y c l e of the t i p p i n g bucket was det e c t e d by an e l e c t r i c impulse to the recorder with the use of a mercury switch p l a c e d on the underside of the t i p p i n g bucket. A l s o i n c l u d e d at the open s i t e was a B e l f o r t type r e c o r d i n g weighing r a i n gauge p r o v i d i n g the c a p a b i l i t y of e v a l u a t i n g one h a l f hour r a i n i n t e n s i t i e s . As has been d e s c r i b e d e a r l i e r temperatures during the winter months at t h i s s i t e do not o f t e n cn Figure 14 - Design of the lysimeter and flow system 55 reach values much lower than -5° C. The f a l l i n g snow i s generally r e l a t i v e l y dense, wet and accompanied by low wind v e l o c i t i e s . The use of the Belfort rain gauges during the winter is thus possible i f about 250 ml of antifreeze is added to the c o l l e c t i n g bucket before the start of the recording period. This method has been used for several years on the Jamieson Creek experimental watershed and has provided quite adequate results (Figures 15 and 16). Along with lysimeter runoff and p r e c i p i t a t i o n measurements, snow surveys were done on a bi-weekly basis at both the forested and open s i t e s . Also, continuous temperature variations were monitored with a Cassela-London hygrothermograph. The study s i t e s were v i s i t e d weekly, during which the instruments were checked for proper functioning, the charts were c o l l e c t e d and replaced, and measurements of snow depths and density were made. The objective of the f i r s t winter study was to use thi s l i m i t e d instrumentation to d i r e c t l y measure and comparatively analyse runoff from the snowpack in the open and forested s i t e . To quantitatively evaluate snowmelt rates the use of p r e c i p i t a t i o n and snowpack data was necessary. Under favorable conditions snowmelt could be estimated simply by a water balance routine : snowmelt = runoff + snowpack l i q u i d water deficiency + cold content - r a i n f a l l . This value could be compared to the difference between two successive water equivalency measurements of the snowpack, thus providing a means of v e r i f i c a t i o n of the water balance. If these two values were s i g n i f i c a n t l y d i f f e r e n t that would indicate faulty measurements of either the runoff, 56 Figure 15 - General view of open s i t e Figure 16 - General view of forested s i t e 57 the p r e c i p i t a t i o n , or the water equivalency of the snowpack. However a major problem i n snowmelt a n a l y s i s l i e s i n the d i f f i c u l t y of determining e x a c t l y the type of p r e c i p i t a t i o n o c c u r r i n g when temperatures are i n the v i c i n i t y of 0° C, i n the absence of continuous f i e l d o b s e r v a t i o n s . Four c l i m a t i c s i t u a t i o n s , or any combinations of the four , can p r e v a i l between f i e l d t r i p s and s u c c e s s i v e snow measurements. These s i t u a t i o n s can be i d e n t i f i e d and separated i n t o i n d i v i d u a l events using p r e c i p i t a t i o n and a i r temperature data, and i t i s important to i d e n t i f y them i n d i v i d u a l l y as d i f f e r e n t c o n s i d e r a t i o n s must be made when a n a l y s i n g each one. 1) The f i r s t s i t u a t i o n i f not combined with others i s the i d e a l snowmelt s i t u a t i o n d e s c r i b e d above, and prese n t s no a n a l y t i c a l problems. T h i s i s when a i r temperatures remain above 1.5° C, as i t can s a f e l y be assumed that a l l p r e c i p i t a t i o n w i l l f a l l as r a i n . 2) S i t u a t i o n s when a i r temperatures remain below 0.5°C are a l s o easy to analyse because any p r e c i p i t a t i o n that occurs should be i n the form of snow and dete c t e d d i r e c t l y by the snow survey measurements. During these p e r i o d s s u b l i m a t i o n and snowmelt should remain q u i t e s m a l l . 3) During c l e a r sky p e r i o d s when a i r temperatures remain between 0°C and 2°C, and the r e l a t i v e humidity i s low, a l o s s of water from the snowpack occurs because of very f a v o r a b l e c o n d i t i o n s f o r s u b l i m a t i o n . T h i s must be taken i n t o c o n s i d e r a t i o n because although no runoff w i l l occur y et, the snow pack may l o s e some water. 58 4) The most d i f f i c u l t s i t u a t i o n s to analyse are p e r i o d s when p r e c i p i t a t i o n occurs while a i r temperatures are i n the 0 to 1.5° C range. During these c o n d i t i o n s i t i s d i f f i c u l t t o determine how much of the p r e c i p i t a t i o n f e l l as r a i n , snow or a mixture of both. Although, the comparison between the water balance and the v a r i a t i o n i n snowpack water e q u i v a l e n c y p r o v i d e s a v e r i f i c a t i o n of measurements, i t i s q u i t e d i f f i c u l t t o a p p o r t i o n the measured run o f f i n t o snowmelt and p e r c o l a t e d r a i n . T h i s l a s t c o n d i t i o n was the one most o f t e n encountered at our study s i t e , and i t s a n a l y s i s n e c e s s i t a t e s a c e r t a i n amount of s u b j e c t i v i t y . The USACE s t a t e that many methods that have been used t o attempt to determine the nature of f a l l i n g p r e c i p i t a t i o n , i n the absence of a c t u a l o b s e r v a t i o n . They suggest that s u r f a c e a i r temperatures (approx. 1.5 m i n height) i s as r e l i a b l e as any of the other v a r i a b l e s t e s t e d . Included i n the USACE(1956) r e p o r t were the r e s u l t s of f i v e years of o b s e r v a t i o n s of a i r temperature and p r e c i p i t a t i o n type. Some 2400 occurrences of p r e c i p i t a t i o n at a i r temperatures ranging from 29°F to 40°F were an a l y s e d to o b t a i n the d i s t r i b u t i o n of occurrences of r a i n , snow or mixed r a i n and snow. On the b a s i s of t h i s a n a l y s i s the USACE de s i g n a t e d 34-35°F (1°C - 1.5°C) as the d i v i d i n g l i n e between r a i n and snow, and showed that 90% of the cases would be c o r r e c t l y determined with the use of t h i s d i v i d i n g l i n e During the f i r s t season, measurements were not s t a r t e d t i l l the end of January 1982. The atmospheric c o n d i t i o n s that winter r e s u l t e d i n constant accumulation of snow at the study s i t e . 59 Because a l l p r e c i p i t a t i o n occurred in the form of snow very l i t t l e melt occurred before the onset of warmer spring conditions. During t h i s period (mid-April) over two metres of snow had accumulated at the open s i t e (Table 8 and Figure 17). During A p r i l and May l i t t l e p r e c i p i t a t i o n occurred on the Jamieson Creek watershed, and most of i t was in the form of snow. Melt did occur however under the influence of the radiative and turbulent energy sources. No rain-on-snow events occurred during our spring 1982 monitoring period. However, lysimeter runoff rates from radiative and turbulent energy melt were measured for both the open and forested s i t e s . As has been described, snow surveys were done to estimate the v a l i d i t y of our lysimeter r e s u l t s . As w i l l be described in the following chapter under the heading 1981-82 winter r e s u l t s , the lysimeter results did not agree with the snow survey measurements and p r e c i p i t a t i o n and a i r temperature data. After v e r i f i c a t i o n of the mechanical and st r u c t u r a l soundness of the lysimeter and i t s associated measuring and monitoring equipment, i t was concluded that something other than the equipment was responsible for the erroneous r e s u l t s . The basic problem with our experimental set-up, i t was decided, was the smallness of the lysimeter area and the consequent problems of not being representative of larger areal snowmelt conditions. As can be seen in Figure 18 there i s a d e f i n i t e concentration of the water as i t flows through the snowpack, in p r e f e r e n t i a l pathways as described by Smith (1974). If a small 60 Table 8 - Snow survey measurements winter 1981-82 DATE OPEN FOREST DEPTH (cm) W.E. (cm) DENSITY (g/cm3) DEPTH (cm) W.E. (cm) DENSITY (g/cm3) MARCH 11/82 218 73 0.33 135 48 0.36 APRIL 20/82 226 79 0.33 163 55 0.32 MAY 18/82 72 32 0.44 65 28 0.44 MAY 26/82 00 00 00 00 00 00 Figure 17 - Snow accumulation mid-April 1982 lysimeter is situated d i r e c t l y under one of these channels the resulting runoff flowing through the lysimeter would be much 61 Figure 18 - Snow runoff through p r e f e r e n t i a l pathways greater than that calculated for the area i t represents. I f , on the other hand, pathways completely avoid the lysimeter, the measured runoff could be less than expected. Another problem was the formation of ice lenses causing an impeding layer to the free flow of water to the underlying lysimeter. Thus to overcome these problems the use of a much larger lysimeter was advocated, to average any e f f e c t s of inconsistent meltwater routing. 4.2 Winter Of 1982-1983 During the summer of 1982 both s i t e s were prepared for a much more intensive data c o l l e c t i n g programme. New lysimeters were i n s t a l l e d . These were constructed out of a 22m2 triangular shaped tarpaulin of fiberglass-reinforced p l a s t i c . In the 62 f o r e s t s i t e the t a r p was p l a c e d d i r e c t l y on the f o r e s t f l o o r with i t s edges supported and h e l d i n place by a 20 to 30 cm high fence, c o n s t r u c t e d of 2x4 i n c h dimensional lumber, surrounding i t s e n t i r e p erimeter. In the open s i t e i t was necessary to cut and remove some of the s l a s h to obt a i n a l a r g e enough area to la y down the t a r p . The s l a s h was p l a c e d around the perimeter of the t a r p , t h i s was s u f f i c i e n t to hol d the t a r p i n p l a c e once n a i l e d to i t . The s l o p e s from the o u t l e t s of the t a r p s to the entrance of the instrument sheds were c o n s t r u c t e d to p r o v i d e a constant downward g r a d i e n t ( F i g u r e s 19 and 20 ). T h i s was to prevent accumulation of water i n the pipe where f r e e z i n g c o u l d occur d u r i n g p e r i o d s of c o l d weather. The 7.6cm diameter p.v.c. pipe fed the r u n o f f i n t o t i p p i n g buckets of 750 ml per c y c l e . Thus for the 1982-83 season two independent l y s i m e t e r s , one of 0.50m2 ( b a r r e l s ) and the other of 22m2 ( t a r p ) , and t h e i r a s s o c i a t e d t i p p i n g buckets would be used to monitor p e r c o l a t e d water through the snowpack and es t i m a t e snowmelt. A permanent snow course was set up with 12 foot aluminum stakes i n s e r t e d i n t o the ground. At each s i t e 15 stakes were p l a c e d i n two c o n c e n t r i c c i r c l e s around the l y s i m e t e r s i n d i c a t i n g permanent snow sampling l o c a t i o n s ( F i g u r e 21 and 22). F i f t e e n other snow samples were taken halfway between s t a t i o n s thus p r o v i d i n g a t o t a l of 30 measurements of snow depths, d e n s i t y and water e q u i v a l e n c y f o r each s i t e on a weekly b a s i s . The measurements were taken with a standard USDA snow tube, c a l i b r a t e d i n Imperial u n i t s . 63 64 Figure 21 - Snow sampling locations at the forest s i t e Figure 22 - Snow sampling locations at the open s i t e 65 To determine the energy balance over the melting snowpack, providing us with a t h i r d estimator of snowmelt, measurements of ground, snow and a i r temperatures, net radiation, r e l a t i v e humidity and wind ve l o c i t y were needed. The radiation component of the energy balance was measured d i r e c t l y with the use of Fritchen-type net radiometers. To keep the inner surface of the domes dry and to prevent collapsing of the domes a constant flow of dry nitrogen was used. Before reaching the radiometers the nitrogen was forced through s i l i c a gel to ensure dryness of the gas. A small nitrogen container approximately the size of a large scuba tank provided enough gas for the entire season, once a l l the leaks in the system of tubes and connectors were stopped. Assuming an even d i s t r i b u t i o n of net radiation in the open s i t e , the use of one radiometer was deemed sa t i s f a c t o r y . However because of large v a r i a b i l i t y of net radiation under the forest canopy, the forested s i t e was equipped with three radiometers d i s t r i b u t e d in such a way as to obtain an average representative value. A l l radiometers were connected in series to the flow of dry nitrogen gas. The radiometers were mounted on a 1 m steel rod attached to a handy-angle frame providing the p o s s i b i l i t y of r a i s i n g and lowering the instruments with variations in the depth of the snowpack. The objective was to maintain the radiometers at height of 1.5 m above the snowpack p a r a l l e l to the ground surface. To obtain an estimate of the energy available from the ground for snowmelt, three Campbell S c i e n t i f i c model 101 66 t h e r m i s t o r probes were i n s e r t e d i n t o the ground at depths of 0.5, 5 and 10 cm. Although t h i s arrangement cannot p r o v i d e accurate heat f l u x v a l u e s , i t w i l l g ive the d i r e c t i o n of the heat flow, an e s t i m a t i o n of the g r a d i e n t and i n d i c a t e whether or not the s o i l i s f r o z e n . T h i s knowledge i s of prime importance when c o n s i d e r i n g the r o u t i n g of the p e r c o l a t e d r a i n f a l l and meltwater through the s o i l . Two other t h e r m i s t o r s of s i m i l a r f a b r i c a t i o n were used to estimate the c o l d content and temperature g r a d i e n t of the snowpack. To measure the t u r b u l e n t energy f l u x e s above the snowpack, g r a d i e n t s of a i r temperature, humidity and wind v e l o c i t i e s are necessary as d e s c r i b e d i n Chapter I I . A l a r g e tower and numerous l o w - t h r e s h o l d anemometers would be d e s i r a b l e to c a l c u l a t e p r e c i s e l y the wind p r o f i l e above the snowpack, because wind v e l o c i t y readings at s e v e r a l l e v e l s would be r e q u i r e d . As t h i s was economically u n f e a s i b l e f o r t h i s p r o j e c t , measurements of the wind v e l o c i t y at two l e v e l s along with a i r temperature and r e l a t i v e humidity readings were thought to be s a t i s f a c t o r y to estimate Q and Q , and v e r i f y the U n i t e d S t a t e s Army Corps e h of Engineers snowmelt equations. The R.M. Young model No. 6001 anemometers, with a t h r e s h h o l d v e l o c i t y of 0.7 meters/second were mounted on handy angle b u i l t stands, i d e n t i c a l to those used f o r the radiometers, at l e v e l s of 60 cm and 150 cm above the snowpack. The a i r temperature and r e l a t i v e humidity were measured at 60 and 150cm with the use of Campbell S c i e n t i f i c model 201 temperature 67 compensated humidity probes. These probes were covered with a s t i f f p l a s t i c , open ended c y l i n d e r to p r o t e c t them from the environment. The anemometers and humidity sensors were a l l p l a c e d on a d j u s t a b l e rods to allow height adjustements with the v a r y i n g snowpack depths. The energy s u p p l i e d by heat t r a n s f e r of r a i n c o u l d be estimated with the use of dew p o i n t temperatures c a l c u l a t e d from a i r temperature and r e l a t i v e humidity measurements. F i g u r e s 23 to 26 show the i n s t r u m e n t a t i o n set-up of both s i t e s at v a r i o u s times d u r i n g the study. A l l data were monitored with two Campbell S c i e n t i f i c CR-21 data l o g g e r s at each s i t e . Each logger scanned a l l nine input channels once every minute and s t o r e d the readings i n a temporary memory. The data l o g g e r s were programmed to output r e s u l t s every 30 or 180 minutes , depending on the sensor being read. The outputs are e i t h e r i n the form of averages, maximums, minimums, t o t a l s or samples. Appendix E g i v e s a sample of an input and output programme f o r one of the CR-21s. Thus, f o r example, a i r temperature at 60 cm was c o l l e c t e d e l e c t r o n i c a l l y once per minute and the maximum, minimum and average f o r every 30 readings was s t o r e d i n memory and l a t e r dumped a u t o m a t i c a l l y on to a c a s s e t t e tape. Output data a l s o i n c l u d e d the J u l i a n date and the time of output. The Panasonic tape r e c o r d e r s used f o r t h i s process are equipped with a patch c o r d connected to the data l o g g e r . They are s p e c i a l l y a l t e r e d by Campbell S c i e n t i f i c to permit c o n t r o l of the power switch by the data logger thus a l l o w i n g a l a r g e r e d u c t i o n i n the energy requirements from the 68 Figure 23 - Instrumentation and snowpack at forested s i t e , mid-winter Figure 24 - Instrumentation and snowpack at forested s i t e , late winter Figure 26 - Instrumentation and snowpack at open s i t e , late winter 70 small b a t t e r y supply. I t was found that at a r a t e of 30 minutes of recorded data per week, these small c a s s e t t e r e c o r d e r s c o u l d l a s t over four months on the same b a t t e r y supply. The data l o g g e r s are powered by e i g h t , 1.5 v o l t , s i z e D b a t t e r i e s and supply enough power to permit s i x months of continuous o p e r a t i o n . The data l o g g e r s along with the tape r e c o r d e r s were housed in i n s u l a t e d Coleman c o o l e r s , heated with a s m a l l propane heat exchanger, manufactured by Cis-Can of A l b e r t a , and kept dry with a weekly supply of s i l i c a g e l ( F i g u r e 27 and 28). Other than the number of radiometers, the i n s t r u m e n t a t i o n was i d e n t i c a l f o r both the open and f o r e s t e d s i t e s . P r e c i p i t a t i o n was monitored as i n the p r e v i o u s year with the same B e l f o r t r e c o r d i n g weighing r a i n gauge, s i t u a t e d i n the c l e a r c u t s i t e . Data c o l l e c t i o n at both s i t e s s t a r t e d i n e a r l y November 1982, although no i n t e r e s t i n g events o c c u r r e d u n t i l two or three weeks l a t e r . The months of November and December saw s e v e r a l problems i n o b t a i n i n g the proper f u n c t i o n i n g of a l l instruments s i m u l t a n e o u s l y , e s p e c i a l l y with the net radiometers and the new t i p p i n g buckets. The s i t e s were v i s i t e d once or twice per week and o c c a s i o n a l l y more o f t e n d u r i n g important events or f o l l o w i n g equipment f a i l u r e s . During these f i e l d v i s i t s d e s c r i p t i o n of equipment performance and weather c o n d i t i o n s were made and o b s e r v a t i o n s of snow d i s t r i b u t i o n , melt c o n d i t i o n s and any other i n t e r e s t i n g changes in the snowpack were noted. A l s o the c h a r t s , tapes and s i l i c a g e l were changed, a s h o r t data logger t e s t was performed to v e r i f y the proper readings of a l l 71 Figure 27 - Insulated cooler, heater and data logger Figure 28 - Lysimeter outflow, funnel and tipping bucket 72 instruments and the power supply, and the snow survey was accomplished. 4.3 Winter Of 1983-1984 During the f a l l of 1983 s e v e r a l changes and improvements were made at the study s i t e in p r e p a r a t i o n f o r one more winter of data c o l l e c t i o n . Since the measurement of an a c c u r a t e energy balance c o u l d not be achieved with our l i m i t e d i n s t r u m e n t a t i o n , i t was d e c i d e d to put our e f f o r t s i n t o a c c u r a t e l y measuring l y s i m e t e r r u n o f f . S e v e r a l improvements were made to the l a r g e l y s i m e t e r and the t i p p i n g bucket arrangement. The perimeter around both the open and f o r e s t l y s i m e t e r s were r e b u i l t to o f f e r g r e a t e r support with a higher " l i p " . I t was hoped that t h i s would prevent water from running over the top of the edges of the l y s i m e t e r . A system of funnels was p l a c e d at the o r i f i c e of both the open and f o r e s t l y s i m e t e r s to prevent any leakage from the o r i f i c e a r e a . These s t a i n l e s s s t e e l funnels were c o n s t r u c t e d of s p e c i f i e d dimensions and i n s t a l l e d i n a way that overflow from the s m a l l e r primary funnel would be caught by a much l a r g e r secondary funnel and d i r e c t e d towards the t i p p i n g bucket. A l s o , i n the l i g h t of the p r e v i o u s years experience, metal s c r e e n i n g was p l a c e d on the o r i f i c e of the l y s i m e t e r to prevent blockage caused by l i t t e r accumulation at the o r i f i c e . The s c r e e n i n g c o u l d be e a s i l y accessed and was f r e q u e n t l y c l e a n e d . Included i n t h i s system overhaul were four new s t a i n l e s s s t e e l t i p p i n g buckets of s u p e r i o r c o n s t r u c t i o n and g r e a t e r measuring accuracy. I t was hoped that with these improvements most of the 73 l y s i m e t e r problems encountered the p r e v i o u s year would be avoided. The measuring of m e t e o r o l o g i c a l data was s i m i l a r to the p r e v i o u s year although not q u i t e so i n t e n s i v e . R e l a t i v e humidity and windspeed were measured only at the 150 cm l e v e l , a i r temperature was measured at the 150 and 60 cm l e v e l s , snow temperature was measured at 30 cm above the ground and s o i l temperature was measured at 10 cm below the s u r f a c e . Net radiometers were not i n s t a l l e d f o r the 1983-84 season. A l l the above mentioned sensors were again monitored with the use of the Campbell S c i e n t i f i c CR-21 data l o g g e r s . The 30 p o i n t snow surveys were performed weekly at both the f o r e s t and open s i t e s at the exact same l o c a t i o n as the p r e v i o u s year. 74 v « RESULTS AND DISCUSSIONS 5.1 Winter 1981-1982 R e s u l t s As has been mentioned the winter of 1981-82 saw continuous i n c r e a s e s i n depth of the snowpack w e l l i n t o the middle of March. T h i s being the f i r s t winter of measurement, one of the o b j e c t i v e s was to simply observe the snow c o n d i t i o n s at that e l e v a t i o n i n an e f f o r t to become f a m i l i a r with the rain-on-snow problem. Since no rain-on-snow events o c c u r r e d , and no s i g n i f i c a n t amounts of melt were observed u n t i l e a r l y A p r i l , snow surveys were not i n i t i a t e d u n t i l March 11 (Table 9) and l y s i m e t e r m o n i t o r i n g only began on A p r i l 17 1982. The l y s i m e t e r r e s u l t s showed spor a d i c snowmelt p a t t e r n s throughout A p r i l and e a r l y May. However, as the snow survey i n d i c a t e s , approximately 40% of the snowpack melted i n the short p e r i o d between May 18 and 25. A f i r s t glance at the data r e v e a l s what the theory suggests i . e . , more r a p i d snowmelt i n the open than i n the f o r e s t d u r i n g c l e a r sky c o n d i t i o n s . However, p r e l i m i n a r y computations showed that these r e s u l t s might be exaggerated somehow as d a i l y snowmelt r a t e s i n the open reached r a t e s w e l l over 30 times those measured i n the f o r e s t . T h i s type of event where runoff in the open was g r e a t e r than i n the f o r e s t was a r e g u l a r occurrence d u r i n g our monitoring p e r i o d . To v e r i f y the accuracy of these measurements, runoff r a t e s measured from May 18 t i l l the d e p l e t i o n of the snowpack May 22, were compared to the water e q u i v a l e n t (w.e.) of the snowpack 75 Table 9 - Snow survey measurements winter 1981-82 DATE OPEN FOREST DEPTH (cm) W.E. (cm) DENSITY (g/cm 3) DEPTH (cm) W.E. (cm) DENSITY (g/cm 3) MARCH 11/82 218 73 0.33 1 35 48 0.36 APRIL 20/82 226 79 0.33 1 63 55 0.32 MAY 18/82 72 32 0.44 65 28 0.44 MAY 26/82 00 00 0.00 00 00 0.00 measured on May 18. During t h i s p e r i o d no p r e c i p i t a t i o n o c c u r r e d , thus a l l runoff c o u l d be equated to snowmelt. Table 10 shows the l y s i m e t e r measured snowmelt f o r each day of the d e p l e t i o n p e r i o d f o r both the open and f o r e s t e d s i t e s . On May 18, 1982 the measured water eq u i v a l e n c y (w.e.) of the snowpack was 32 cm i n the open and 28 cm i n the f o r e s t . The b a r r e l l y s i m e t e r s have a diameter of 57.2 cm, corresponding to a s u r f a c e area of 2569 cm 2. Because each s i t e i s equipped with two, the t o t a l l y s i m e t e r area i s 5139 cm 2 at each s i t e . I f we assume that the snow survey measurements g i v e a r e p r e s e n t a t i v e value and can be equated to the snow water e q u i v a l e n t (s.w.e.) above the l y s i m e t e r s , than we can e a s i l y compute the amount of water that should flow i n t o the l y s i m e t e r and subsequently measured i n the t i p p i n g bucket. 76 Table 10 - Measured runoff r a t e s f o r open and f o r e s t e d s i t e s , f r o m May 18 to May 26, 1982. DATE OPEN RUNOFF(cm 3) FOREST RUNOFF(cm 3) 18/05/82 6,975 754 19/05/82 110,623 1 ,962 20/05/82 142,290 2,113 21/05/82 114,390 4,227 22/05/82 16,042 302 23/05/82 2,363 1 ,509 24/05/82 0 7,548 25/05/82 0 6,038 26/05/82 0 1 , 164 TOTAL 392,783 25,617 T h i s computed amount of snowmelt i s c a l c u l a t e d as : Volume of water from snowmelt = area of l y s i m e t e r * water e q u i v a l e n t of snowpack Open s i t e = 5139 cm 2 * 31.8cm = 163,433cm3 F o r e s t s i t e = 5139 cm 2 * 28.0 cm = 143,892 cm 3 77 If these v a l u e s are compared with those obtained i n Table 10 i t becomes evident that our l y s i m e t e r s were not p r o p e r l y measuring snowmelt. computed snowmelt l y s i m e t e r snowmelt percentage d i f ference open 163,433 cm 3 392,783 cm: 58% f o r e s t 143,892 cm 3 25,617 cm 3 -462% To remedy t h i s problem a l a r g e r l y s i m e t e r was c o n s t r u c t e d as has been d e s c r i b e d i n the p r e v i o u s chapter. Although i t would seem t h a t the f i r s t winter was t o t a l l y u n s u c c e s s f u l , i n e f f e c t t h i s i s q u i t e untrue. I t i s c o r r e c t that the data c o l l e c t e d d u r i n g the s p r i n g of 1982 were of no use fo r q u a n t i f y i n g snowmelt and f o r comparative a n a l y s i s between the f o r e s t and the open, however they d i d pr o v i d e some very u s e f u l i n f o r m a t i o n f o r the p r e p a r a t i o n of the next year's r e s e a r c h . T h i s i n c l u d e d the d i s c o v e r y of the need f o r a l a r g e r l y s i m e t e r , and the p o t e n t i a l depths of snowpack thus p r o v i d i n g g u i d e l i n e s f o r the c o n s t r u c t i o n of instrument towers. 78 5.2 Winter 1982-1983 R e s u l t s 5.2.1 I n t r o d u c t i o n To The A n a l y s i s Snowpack c o n d i t i o n s d u r i n g the winter of 1982-83 (Table 11) were such that only a few rain-on-snow events o c c u r r e d when both c l e a r c u t and f o r e s t s i t e s had s i g n i f i c a n t snow cover. U n f o r t u n a t e l y the f a l l 1982 rain-on-snow events c o u l d not be p r o p e r l y a n a l y s e d because of i n i t i a l i n s t r u m e n t a t i o n problems at one or both s i t e s . A f t e r the major i n s t r u m e n t a t i o n problems were overcome and the events with i n s u f f i c i e n t snow were d i s c a r d e d , s e v e r a l events were l e f t that p r o v i d e d some i n t e r e s t i n g a n a l y s i s . As a n a l y s i s began i t soon became evident that the computing of an a c c u r a t e energy balance over snow i n a f o r e s t e d mountainous environment was i m p o s s i b l e with our l i m i t e d i n s t r u m e n t a t i o n . I t was thus decided that i t would be more advantageous, and would conform more with our goals to compute the condensation and c o n v e c t i v e melts with the USACE (1956) d e r i v e d equations and compare them with l y s i m e t e r and snow survey measurements f o r accuracy. The o r i g i n a l USACE (1956) equations f o r computing c o n v e c t i v e ( s e n s i b l e heat) (M ), condensation (M ) and r a i n (M ) c e r snowmelt at a p o i n t i n inches/day are as f o l l o w s : M =.00629(P/P )(Z Z ) - / 6 (T -T )v c o a b a s b (5.1) 79 T a b l e 11 - E v o l u t i o n of the snowpack 1982-83 DATES OPEN FOREST DEPTH cm W.E. cm %SNOW COVER DEPTH cm W.E. cm %SNOW COVER 18/1 1 20 100 1 6 100 23/1 1 42 100 17 100 25/1 1 25 100 50 30/1 1 30 100 15 50 07/1 2 1 5 100 PATCHY 0 14/12 28 100 PATCHY 0 23/1 2 88 100 34 100 04/01 1 14 28.3 1 00 46.7 10.6 100 11/01 91 28.8 100 50.4 13.7 1 00 19/01 72.3 24.3 1 00 24.3 7.4 80 25/01 66 22.0 1 00 25.2 7.2 70 01/02 57 18.5 1 00 PATCHY 5.5 40 08/02 53 18.8 100 PATCHY - 15 10/02 88 - 100 33 - 90 12/02 63 - 100 PATCHY - 50 1 6/02 61 22.0 1 00 PATCHY - 25 22/02 62 22.0 100 PATCHY - 10 01/03 57 20.0 95 PATCHY - 0 80 M =.054(Z Z ) - l / s (e -e )v (5.2) e a b a s b M =.007 P (T -32) (5.3) r r a where P and P are the a i r p r e s s u r e s at the l o c a t i o n and at sea o l e v e l , r e s p e c t i v e l y , Z and Z are the h e i g h t s of measurement i n a b f e e t above the snow s u r f a c e , of a i r temperature and wind speed r e s p e c t i v e l y , T i s the a i r temperature i n °F, T i s the snow a s s u r f a c e temperature i n °F (32 °F when m e l t i n g ) , v i s the wind b speed i n m i l e s per hour, e i s the vapour p r e s s u r e i n mb, e i s a s the snow s u r f a c e vapour pressure i n mb (6.11 mb f o r a m e l t i n g snow s u r f a c e ) and P i s the d a i l y r a i n f a l l i n inches. These r equations were transformed to allow computation of snowmelt i n cm/day using e x c l u s i v e l y m etric input data as s u p p l i e d by our instruments. M =.957 (P/P ) (Z Z (T -T )v (5.4) c o a b a s b M =.2007 (Z Z )" 1 / 6 (e -e )v (5.5) e a b a s b M =.00125*T *P (5.6) r r r In these equations Z i s measured i n c e n t i m e t e r s , T i n °C, v i n meters per second, e and e i n mb, P i n m i l l i m e t e r s , b a s r As d e s c r i b e d i n the p r e v i o u s chapter these v a r i a b l e s were monitored on a h a l f h o u r l y b a s i s , thus i n order to enable the 81 use of our data d i r e c t l y , melt was also computed on a half hourly basis and summed over the 48 half-hour periods to give mm/day. Considering that the values for Z and Z remain a b constant at 150 cm, and the study s i t e elevation of 730 m, the above equations reduce to the following in mm/half-hour. M =.00707(T .v ) (5.7) c a b M =.037(e -e )v (5.8) e a s b M =.01252*P *T (5.9) r r a The vapour pressure of the a i r , e , was evaluated for each half a hour using Teten's (1930) empirical formula. e'=(.6l08) an t i l o g [7.5*T /(T +237.3)] (5.10) 10 a a and the relationship : I00(e /e')=R.H. (5.11) a where e' i s the saturated vapour pressure (kPa) at T and R.H. a i s the r e l a t i v e humidity(%) at the study s i t e . 5.2.2 Calculations Of Radiation Melt The radiation melt was calculated d i r e c t l y from the radiation data(kj/(m 2 30min)) and the following conversions: 1kJ/(m2 30min)=0.55 Watts/m2 (5.12) 1kJ/(m2 30min)=7.88*10-" cal/(cm 2 min) (5.13) M=Q /80p (5.14) n where M i s the snowmelt rate in mm/30 min net and Q ='radiation energy in kJ/(m 2 30min) n 82 p= d e n s i t y of water 80=latent heat of f u s i o n of. i c e (ca l / g ) M = 7.88*l0- f tcal*(m 230min)*30min*1g*1cm 3*l0mm*Q ( 5 . 1 5 ) n (cm 2 min) kJ 80cal g cm The f i n a l equation i s thus: R a d i a t i o n melt = 0.00295Q (5.16) n During p r e l i m i n a r y a n a l y s i s i t was observed that c e r t a i n v alues of the net r a d i a t i o n exchange over the m e l t i n g snowpack, as measured with the net radiometer, p r o v i d e d some u n r e a l i s t i c r e s u l t s when compared with the l i t e r a t u r e . The accuracy of these measurements was que s t i o n e d and, to v e r i f y the p r e c i s i o n of the instruments, c a l i b r a t i o n was done under c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s . I t was noted that the radiometer performed w e l l under high i n t e n s i t y r a d i a t i o n , however e r r o r s became q u i t e s i g n i f i c a n t under low i n t e n s i t y c o n d i t i o n s . T h i s was a t t r i b u t e d to the c o n s t r u c t i o n of the radiometers, p r i n c i p a l l y to t h e i r low s e n s i t i v i t i e s , and the d i f f i c u l t y of the CR-21 to monitor such low e l e c t r i c a l s i g n a l s . U n f o r t u n a t e l y the radiometers were not c a l i b r a t e d under r a i n c o n d i t i o n s d u r i n g which time the f i e l d measurements seem to be the most erroneous. These p e r i o d s of high r e l a t i v e h u m i d i t i e s , low c l o u d s and r a i n f a l l over wet snow correspond to low net r a d i a t i o n and consequently very low s i g n a l s to the CR-21. The water on the radiometer domes has an unknown e f f e c t but i t would be reasonable to assume that i t c o n t r i b u t e s at l e a s t p a r t i a l l y to the erroneous r e s u l t s . Thus i t was decided t h a t , d u r i n g 83 p r e c i p i t a t i o n events, an a l t e r n a t e method of e s t i m a t i n g net r a d i a t i o n at the snow s u r f a c e would be used. Two recent r a i n -on-snow s t u d i e s (Harr and B e r r i s 1983, Zuzel et a l . 1983) used e m p i r i c a l equations to eva l u a t e the net longwave r a d i a t i o n exchange at the snow s u r f a c e and obtained good r e s u l t s . S e v e r a l authors (USACE 1956, Gray and Male 1981) have suggested that under 100% low c l o u d cover the net longwave r a d i a t i o n exchange i s simply a f u n c t i o n of a i r and snow temperature and can be estimated as: R *=a(T ft-T ") (5.17) lw a s where a i s the Stephan-Boltzmann constant (5.67*10" 8 W / ( m 2 ° K a ) ) and T and T are a i r and snow temperatures r e s p e c t i v e l y , a s The USACE (1956) have suggested that the net shortwave r a d i a t i o n exchange dur i n g rain-on-snow i s r e l a t i v e l y s m a l l . Using the p l o t t e d data presented i n Gray and Male (1981) and assuming an albedo of 0.6 f o r wet snow s e v e r a l days o l d , a l a t i t u d e of 50° north and a southwest f a c i n g slope of 25 degrees, an average 24 hour net shortwave r a d i a t i o n value f o r r a i n events o c c u r i n g i n January was estimated at 1.8 mm snowmelt per day. The t o t a l net r a d i a t i o n , a v a i l a b l e f o r snowmelt at the su r f a c e of the snowpack, i s the sum of the net longwave r a d i a t i o n p l u s the net shortwave r a d i a t i o n . During non r a i n events t h i s t o t a l net r a d i a t i o n was obtained d i r e c t l y with the 84 use of the net radiometers. However, because of the reasons e x p l a i n e d above, the t o t a l net r a d i a t i o n d u r i n g r a i n events was c a l c u l a t e d as the sum of equation 5.17 p l u s the constant value c i t e d f o r shortwave r a d i a t i o n . 5.2.3 E s t i m a t i n g I n t e r c e p t i o n Losses From The F o r e s t Canopy One of the important elements of the f o r e s t water balance that d i f f e r e d from the open s i t e water balance was the presence of canopy i n t e r c e p t i o n l o s s . I n t e r c e p t i o n l o s s as d e f i n e d by Zinke (1966) i s the p o r t i o n of the p r e c i p i t a t i o n r e t a i n e d by the a r e a l p o r t i o n of the v e g e t a t i o n and i s e i t h e r absorbed by i t or i s returned to the atmosphere by e v a p o r a t i o n . S e v e r a l authors ( P a t r i c 1966, Layton et a l • 1967) have s t a t e d that s t u d i e s of i n t e r c e p t i o n probably outnumber those of any other aspect of the f o r e s t water balance. However, because canopy i n t e r c e p t i o n l o s s i s a f u n c t i o n of s e v e r a l parameters such as t r e e s p e c i e s , crown c l o s u r e , stand d e n s i t y and type, i n t e n s i t y and d u r a t i o n of p r e c i p i t a t i o n and l o c a l c l i m a t i c c o n d i t i o n s , no s i n g l e i n t e r c e p t i o n value can be used f o r a l l c a s e s . P r e c i p i t a t i o n f a l l i n g at the upper s u r f a c e of the canopy may f a l l d i r e c t l y to the ground through openings i n the canopy or may be i n t e r c e p t e d by the l e a v e s , branches or trunks of the f o r e s t stand or by the understory v e g e t a t i o n . The drops of water on the leaves grow i n t o f i l m s as more ra i n d r o p s j o i n them. When these are t h i c k enough to overcome i n t e r n a l f r i c t i o n and s u r f a c e t e n s i o n , bulk flow begins. T h i s flow may c o l l e c t at the 85 l e a f t i p and d r i p o f f , or, run as r i v u l e t s down the branches and the t r e e trunk. The storage c a p a c i t y of a canopy i s the amount of p r e c i p i t a t i o n t h a t may accumulate on the canopy before the occurrence of bulk flow. T h i s storage c a p a c i t y , charged by snow or r a i n f a l l w i l l be d i s c h a r g e d by eva p o r a t i o n or drainage (Rutter et a l . 1975). Zinke (1966) i n a review of the l i t e r a t u r e on canpopy i n t e r c e p t i o n found that the values c i t e d f o r canopy storage v a r i e d from 0.25 mm to 9.14 mm of r a i n . Because of the numerous routes that i n t e r c e p t e d p r e c i p i t a t i o n may take before r e a c h i n g the ground, the acc u r a t e measure of t h r o u g h f a l l i s d i f f i c u l t . Kimmins (1974) i n a f o r e s t ecosystem study i n B r i t i s h Columbia found that to measure mean t h r o u g h f a l l f o r a 30 X 30 m p l o t with a high degree of confi d e n c e 272 gauges would be needed. Because of the measurement requirements s t a t e d above mean t h r o u g h f a l l measurements were not attempted at our f o r e s t s i t e . However to o b t a i n a measure of snowmelt from the f o r e s t l y s i m e t e r , the t h r o u g h f a l l must be s u b t r a c t e d from the measured r u n o f f . Since p r e c i p i t a t i o n i s only measured i n the open, a value f o r the i n t e r c e p t i o n l o s s e s o c c u r r i n g at our f o r e s t s i t e would be e s s e n t i a l to o b t a i n an estimate of t h r o u g h f a l l . The l i t e r a t u r e o f f e r s some p o s s i b l e values of i n t e r c e p t i o n l o s s d u r i n g r a i n events f o r c o a s t a l c o n i f e r o u s f o r e s t s . However these values are g e n e r a l l y f o r the growing season when gr e a t e r amounts of energy are a v a i l a b l e f o r e v a p o r a t i o n . P a t r i c (1966) i n a study of o l d growth western hemlock and S i t k a spruce found 86 an average interception loss of 25% with values varying from 21 to 36%. In his research work with mature coniferous forests of southeast Alaska he found that d a i l y interception losses were up to nine times the t h e o r e t i c a l l y calculated potential evaporation. Winter interception losses exceeding potential evaporation were also reported by Rutter (1963) in studies of Scot's pine in England. It was speculated in Patric (1966) that as December temperatures hovered near freezing, the r e l a t i v e l y warm sea water off the Alaskan coast may have provided the heat source for most of the energy causing interception loss during cool winter months. In the transient snow zone winter p r e c i p i t a t i o n f a l l s either as snow or rain, and both must be considered when trying to quantify evaporative losses. Several authors (Mi l l e r 1977, 1966, Denmead 1964, Hoover and Leaf 1966) have suggested that evaporation losses from snow in a forest canopy are generally small. This i s because the low vapour pressure of snow does not promote a large vapour flux from i t , p a r t i c u l a r l y when the vapour pressure in the a i r i s nearly as large as that of the snow surface, as i s true during storms. Cold humid a i r does not provide a large supply of sensible heat for the evaporation of intercepted snow, and a i r f u l l of f a l l i n g snowflakes cannot be other than cold and humid; both conductive and radiative heat sources are small. In many conditions condensation to the snow is greater than evaporative lo s s . However at the Jamieson Creek study s i t e much of the winter p r e c i p i t a t i o n f a l l s as rain, providing conditions for 87 evaporative losses, from the forest canopy, that are more favorable than during snow events. During these rain events the a i r temperature i s several degrees above 0°C and windspeeds tend to be greater than during non-precipitation periods. These conditions are similar to those that occur in southeast Alaska where measured winter evaporation losses from the forest canopy were greater than the t h e o r e t i c a l l y calculated potential evapotranspiration (Patric 1966). Because the Jamieson Creek snowmelt study s i t e is also located in the coastal mountains adjacent to the P a c i f i c ocean these sources of advected heat may also be prevalent in this study, as was the case for the Alaskan study. Thorn and Oliver (1977), working in the B r i t i s h I s l e s , where winter cl i m a t i c conditions are similar to those in southwestern B r i t i s h Columbia, demonstrated that winter evaporation was much greater than what had been t r a d i t i o n a l l y calculated with the use of Penman's evaporation equation. Evaporative loss from a vegetated surface with dry leaves is dependent on vapour pressure d e f i c i t (e'-e), the surface roughness of the element, and the stomatal resistance. For a wet old growth forest canopy the surface roughness w i l l be large and the stomatal resistance small thus greatly favoring evaporation. The vapour pressure d e f i c i t w i l l tend to be small during a rainstorm but may increase af t e r the storm favoring evaporation. Thorn and Oliver (1977) found that for a thoroughly wet, extremely rough, surface behaving as a perfect wet-bulb thermometer, evaporation (E ) tends to occur at the rate of : wet 88 E = 65 7 . e'-e wet A +7 r a where E i s i n mm/d,7 i s the thermodynamic value of the wet psychrometric constant, equal to 0.66 mb K~ 1, A i s the slope of the s a t u r a t i o n - v a p o u r pressure versus temperature curve f o r water at a i r temperature (mb/K), r i s the aerodynamic a r e s i s t a n c e (s/m) and e and e' (mb) are the a c t u a l and s a t u r a t i o n v a l u e s of vapour pressure 2m above the s u r f a c e . Thorn and O l i v e r i l l u s t r a t e d t h i s equation by using t y p i c a l E n g l i s h November to February mean value s of 1.0 mb f o r (e'-e) and u n i t y f o r A/7. With r =6 s/m f o r a f o r e s t the above equation suggests that Ewet a i s about 5 mm/day. However t h i s value i s about twice as l a r g e as the i n t e r c e p t i o n c a p a c i t y of many f o r e s t s , but Thorn and O l i v e r suggest that even a l l o w i n g f o r s u b s t a n t i a l negative feedback e f f e c t s on ( e ' - e ) , e vaporation r a t e s from the f o r e s t s may w e l l approach 2 or 3 mm/day, even i n wi n t e r . These authors s t a t e that : " such l a r g e e v a p o r a t i o n r a t e s are l i k e l y to occur r e g u l a r l y only from a mesoscale (<10 2 km) f o r e s t s exposed to the sy n o p t i c s c a l e (>10 3 km) adv e c t i o n of unsatured a i r , or i n mountainous regions as a whole where ' a d d i t i o n a l ' e v a p o r a t i v e power ( i n the form of enhanced s a t u r a t i o n d e f i c i t s ) may p o s s i b l y be a v a i l a b l e from the l a t e n t heat r e l e a s e d d u r i n g orographic r a i n f a l l events." The l a t t e r c o n d i t i o n s p r e v a i l at the Jamieson Creek snowmelt study s i t e suggesting that e v a p o r a t i v e l o s s e s of i n t e r c e p t e d r a i n f a l l c o u l d be q u i t e s i g n i f i c a n t d u r i n g winter rain-on-snow events i n a f o r e s t e d area. Thus i n summary even though the 89 vapour pressure d e f i c i t over a wet forest i s small during the winter, evaporative loss can be s i g n i f i c a n t because of advected heat, and small aerodynamic resistances r e s u l t i n g from a large surface roughness. McMinn (1960) in a interception study that lasted for 5.5 years on Vancouver Island obtained mean annual and mean summertime values of interception loss for several forest tree species. His results are as follows: SPECIES Douglas f i r Douglas f i r , western hemlock western red cedar INTERCEPTION LOSSES MEAN ANNUAL 44% 34! 33% MEAN SUMMER 57% 51% 40% This would suggest mean wintertime values (October to March) of 31%, 17%, 26% for Douglas-fir, Douglas-fir and western hemlock and western red-cedar respectively. These results at f i r s t glance seem quite high but are quite reasonable in l i g h t of the considerations offered by Thorn and Oliver (1977). Since the forest at the Jamieson Creek snowmelt study s i t e i s over-mature with a highly irregular and very rough canopy structure composed of Douglas-fir, western hemlock and western red cedar i t would seem reasonable that the winter time interception 90 losses would be quite high. Although as has been mentioned, no one interception value can be used for a l l situations, I w i l l nonetheless use an interception value of 25% to make a rough estimation of the amount of p r e c i p i t a t i o n reaching the forest f l o o r . 5.2.4 F i e l d Observations Among the knowledge gained by t h i s research perhaps some of the most important was that acquired through f i e l d observations rather than measurement. Site v i s i t s were performed under numerous f i e l d conditions enabling dir e c t observations of some interesting phenomena associated with melting snow during r a i n f a l l and the difference the forest canopy made to the snow melting process. Bas i c a l l y the f i e l d observations can be divided into f i v e categories based on micro-climatic conditions. These are presented in tabular form in Table 12. For a given set of climatic conditions the snowmelt and subsequent runoff process may d i f f e r depending on the presence or absence of a forest cover. In most cases the presence of trees w i l l cause melt to start sooner and end later than in open conditions, thus making the runoff more evenly d i s t r i b u t e d over time. An important influence of the forest canopy on runoff rates, as observed at the study s i t e , i s the interception of the p r e c i p i t a t i o n , either as snow or r a i n . Interception w i l l often a l t e r s i g n i f i c a n t l y the timing of runoff compared to that in the clearcut s i t e as described in conditions 1 through 4, Table 12. The importance of direct f i e l d observations, for this 91 Table 12 - F i e l d o b s e r v a t i o n s CONDITION OPEN FOREST 1)SNOWING T°=-.5 TO 1.5°C VERY WET SNOW -ACCUMULATION OF SNOW -NO RUNOFF -SNOW TRAPPED IN CANOPY,FALLS THROUGH AS RAIN. -SOME RUNOFF 2)LIGHT RAIN WHEN SNOW IN CANOPY T°>1.5°C -RAIN PRIMES SNOWPACK -RUNOFF MINIMAL -RAIN ON WET WARM SNOWPACK -DRIP FROM CANOPY -RUNOFF GREATER IN FOREST 3)LIGHT RAIN NO SNOW IN CANOPY T°>1.5°C -RAIN PRIMES PACK OR GETS STORED AS FREE WATER DEPENDS ON SNOW QUALITY -RAIN CAUGHT IN CANOPY; LOST BY EVAPORATION AND DRIP 4)HEAVY CONSTANT RAIN NO SNOW IN CANOPY T°>1.5°C -DEEPER SNOW MORE STORED WATER -NO INTERCEPTION -AIR T° > FOREST -RUNOFF IS IMPORTANT -ABSENCE OF TURBULENT ENERGY -AIR T° < OPEN -W.E. < OPEN -CANOPY INTERCEPTION 5)SNOWING T°<.5°C SNOWFALL IS STORED WATER COLDER SNOW, GREATER ENERGY DEFICIT -SNOW STORED IN TREES,WILL LEAVE AS: 1) EVAPORATION 2) CANOPY DRIP 3) FALL IN HEAVY BALLS study, cannot be overemphasized. I t not only p e r m i t t e d understanding of the snowmelt process t a k i n g p l a c e d u r i n g the f i e l d v i s i t , but i t a l s o permitted g r e a t e r ease of a n a l y s i s of a l l the data gathered e l e c t r o n i c a l l y . As the a n a l y s i s of the f i v e chosen events proceeds, frequent r e f e r e n c e w i l l be made to Table 12 to permit g r e a t e r understanding of the p l o t t e d and t a b u l a t e d data. 92 5.2.5 Presentation Of The Results The 1982-83 results include the analysis of four important rain-on-snow events. Snowmelt rates in the open (calculated and measured) are compared to calculated and measured snowmelt rates in the fores t . Whenever possible, the analysis of snowmelt rates at each s i t e i s based on three r e s u l t s : ; the snow survey, the lysimeter measured melt, and the calculated melt. A fourth, and possibly a f i f t h , value may appear in the r e s u l t s , those of measured and/or calculated runoff. The measured runoff i s the t o t a l measured runoff from the lysimeter, while the calculated runoff i s the sum of the p r e c i p i t a t i o n plus the calculated melt. As was explained in the previous chapter most data were co l l e c t e d on a half hourly basis. Thus a l l c a l c u l a t i o n s using the USACE (1956) equations are i n i t i a l l y performed on a half hourly basis. To f a c i l i t a t e analysis, the results of these calc u l a t i o n s are summed up and presented in a tabular form on a da i l y basis. Because the snow survey i s only performed on a weekly basis, the three methods for estimating snowmelt can only be compared in th i s time frame (usually 7days). However, for some events, lysimeter runoff and calculated runoff are compared on a shorter time scale. Graphs of variations in a i r temperature, net radiation and lysimeter runoff are presented for each event, for both forest and open s i t e s . Windspeeds are only presented for the open s i t e . This i s simply because wind at our forest s i t e was rarely greater than the s t a l l i n g speed of the anemometers. Pr e c i p i t a t i o n i n t e n s i t i e s are only presented for the open s i t e 93 because no p r e c i p i t a t i o n measurements were taken at our forest s i t e . Also presented in a graphical form are the calculated open s i t e melt rates. These include sensible heat melt, condensation melt, rain melt, radiation melt, t o t a l melt and evaporation from the snowpack. Included with the analysis of each of the events i s a data table which, presents the results of most measurements and calculations on a da i l y basis. The description of each of the variables in the data tables i s as follows: MIN.DATE : refers to the beginning of the analysed period, expressed in a Julian date form to the l e f t of the decimal point and a fra c t i o n of a 24. hour period on the right of the decimal point(e.g 4.25 = Jan.4 at 6 a.m., 34.667 = Feb. 3 at 4 p.m.) MAX.DATE : refers to the end of the analysed period CASES : The number of 1/2-hour cases used to compute the summed or average values of the desired variables HOURS : The time period in hours that t h i s i n t e r v a l represents. The values for each of the f i e l d measured and the computed variables are the sum of a l l the cases that occurred in the time int e r v a l described by MIN.DATE and MAX.DATE. MMPREC : The t o t a l amount of p r e c i p i t a t i o n in mm measured at the open s i t e 94 The t o t a l amount of r a i n i n mm measured at the open s i t e The t o t a l amount of s n o w f a l l i n mm w.e. measured at the open s i t e T o t a l energy a v a i l a b l e from the t r a n s f e r of r a i n heat to the snow pack i n the c l e a r c u t , expressed i n mm of snowmelt of water e q u i v a l e n t (s.m.w.e.). The t o t a l energy a v a i l a b l e from condensation of atmospheric vapour to the snowpack at the open s i t e , e x p r e s s e d i n mm s.m.w.e. The t o t a l energy a v a i l a b l e from the t u r b u l e n t t r a n s f e r of s e n s i b l e heat to the snowpack i n the c l e a r c u t , expressed i n mm s.m.w.e. Evaporation from the snowpack i n mm of water at the open s i t e . A l s o corresponds to a l o s s of energy from the snowpack due to the t u r b u l e n t exchange -of l a t e n t heat from the snowpack to the atmosphere. T o t a l amount of runoff i n mm, measured from the la r g e l y s i m e t e r at the open s i t e . T o t a l energy a v a i l a b l e from net r a d i a t i o n at the c l e a r c u t s i t e , expressed i n mm s.m.w.e. T o t a l a v a i l a b l e amount of net longwave r a d i a t i o n i n the c l e a r c u t , expressed i n mm s.m.w.e., as c a l c u l a t e d by the emperical equation. T o t a l net shortwave r a d i a t i o n energy expressed as a constant value per ha l f - h o u r i n mm s.m.w.e. 95 MTOT : Sum of MC,MR,MCOND,MRN,EVAP f o r each h a l f hour p e r i o d summed up over the s p e c i f i e d time i n t e r v a l . MTOT' : Sum of MC,MR,MCOND,MMRLN,MSWR,EVAP f o r each h a l f hour p e r i o d summed over the s p e c i f i e d time i n t e r v a l . MTOT'' : Summation of MTOT and/or MTOT' f o r the time i n t e r v a l s p e c i f i e d , where MTOT i s used f o r non-r a i n events and MTOT' i s used f o r r a i n events. L.TOT : Summation of MTOT and |EVAP|. Corresponds to the t o t a l d e p l e t i o n of water equivalency of the snowpack. L.TOT' : Summation of MTOT' and |EVAP|. LTOT'' : Summation of MTOT'' and |EVAP|. MMPRECF : T o t a l amount of p r e c i p i t a t i o n estimated f o r the f o r e s t s i t e . MRF : T o t a l energy a v a i l a b l e from the t r a n s f e r of r a i n heat to the snow pack i n the f o r e s t , expressed i n mm of snowmelt of water e q u i v a l e n t (s.m.w.e.). BFMMRO : T o t a l amount of runof f i n mm, measured from the l a r g e l y s i m e t e r at the f o r e s t s i t e . MRNF : T o t a l energy a v a i l a b l e from net r a d i a t i o n at the f o r e s t s i t e , expressed i n mm s.m.w.e. MMRLNF : T o t a l a v a i l a b l e amount of net longwave r a d i a t i o n i n the f o r e s t , expressed i n mm s.m.w.e., as c a l c u l a t e d by the emperical equation. MTOTF : Sum of MRF and MRN f o r each h a l f hour p e r i o d 96 summed up over the s p e c i f i e d time i n t e r v a l . MTOTF' : Sum of MRF and MMRLN f o r each h a l f hour p e r i o d summed over the s p e c i f i e d time i n t e r v a l . MTOTF'' : Summation of MTOTF and/or MTOTF' f o r the time i n t e r v a l s p e c i f i e d , where MTOTF i s used f o r non-r a i n events and MTOTF' i s used f o r r a i n events. HTEMPO : Average a i r temperature measured at 150 cm above the snowpack i n the open s i t e . HR.H.O : Average r e l a t i v e humidity measured at 150 cm above the snowpack i n the open s i t e . HTEMPF : Average a i r temperature measured at 150 cm above the snowpack at the f o r e s t s i t e HR.H.F : Average r e l a t i v e humidity measured at 150 cm above the snowpack at the f o r e s t s i t e CALCULATING MELT IN THE OPEN 1) Snow survey: NWSM=(SNOW -SNOW ) + s n o w f a l l bw ew Where NSWM r e f e r s to the net weekly snowmelt, SNOW i s the bw measured w.e. of the snowpack at the beginning of the week, SNOW i s the measured w.e. of the snowpack at the end of the ew week and the s n o w f a l l i s the t o t a l estimated s n o w f a l l d u r i n g the week. 2) Lysimeter: DSM=R.O. - r a i n f a l l m 97 Where DSM i s the dail y snowmelt, R.O. i s the dail y measured m runoff and r a i n f a l l i s the estimated da i l y r a i n f a l l . 3) Energy balance: For every half hour, a value for each of the USACE melt components i s computed. A l l of these components are summed up to y i e l d a t o t a l calculated half hourly melt. Radiation melt i s computed by two methods. The f i r s t method uses the results from the net radiometer (MRN), while the second computes i t as the sum of the calculated longwave radiation melt ( equation 5.17) (MMRLN) plus shortwave radiation melt (MSWR). Thus, for every half hour, calculations for two t o t a l melts are generated (MTOT, MTOT'). Depending on the occurrence or not of p r e c i p i t a t i o n ; only one of these two results w i l l be used. Half-hourly melt in the open i s thus computed as such: ( variables used are the same as those used in the data t a b l e s ) . Non p r e c i p i t a t i o n event: MTOT=MC+MCOND+MR+MRN+EVAP Pre c i p i t a t i o n event: MTOT'=MC+MCOND+MR+MMRLN+MSWR+EVAP To obtain the calculated d a i l y t o t a l s , 48 half hourly values of MTOT and MTOT' are summed up. At each half hour i n t e r v a l , one of the two values, MTOT or MTOT' is chosen depending on the occurrence or not of p r e c i p i t a t i o n . This choice i s then placed under a variable named MTOT'', which is the value used in the analysis. Ground heat to the snowpack was not measured, however, 9 8 ground temperature data suggests that t h i s heat input would be re l a t i v e l y small because ground temperatures close to the snow remained close to 1 °C. The USACE (1956) suggests that under these conditions the melt caused by ground heat (M ) can be 9 taken as a constant of 0.5mm of s.m.w.e. per day. This suggested value of 0.5 mm/day i s added to MTOT' before comparing i t to the lysimeter calculated melt. When evaporation occurs from the snowpack there i s both the loss of water and loss of energy from the pack. EVAP appears in the MTOT and MTOT' equations as a negative term thus representing a loss of energy. When the absolute value of EVAP is added to MTOT1' a value for the t o t a l loss in snowpack w.e. is generated (LTOT''). Thus: LTOT'' = MTOT''+ |EVAP| CALCULATING SNOWMELT IN THE FOREST To be able to estimate melt, either with the snow survey or lysimeter runoff data, one must know how much p r e c i p i t a t i o n occurred on the snowpack. Since t h i s data i s not available for the forest s i t e , a throughfall value must be assumed. For the 1982-83 analysis a value of 75% throughfall was chosen for rain events, the reasons being explained in section 5.2.3. 1) Snow survey: NWSM = (SNOW - SNOW ) + throughfall(snow) bw ew As w i l l be discussed in the analysis, estimates of snow throughfall are often d i f f i c u l t to make. 99 2) Lysimeter: DSM = R.O. - t h r o u g h f a l l ( r a i n ) m 3) Energy balance: Rare were the oc c a s i o n s at the f o r e s t s i t e when there was enough wind to be de t e c t e d by our instruments. Because the c a l c u l a t i o n s of the t u r b u l e n t t r a n s f e r of s e n s i b l e and l a t e n t heat are a d i r e c t f u n c t i o n of windspeed, the absence of measurable wind r e s u l t e d i n the c a l c u l a t i o n of these two terms to z e r o . However, t h i s assumption of n e g l i g i b l e t r a n s f e r of s e n s i b l e and l a t e n t heat was not t o t a l l y erroneous as Hendrie and P r i c e (1978) demonstrated. They showed that the t u r b u l e n t exchanges over a me l t i n g snowpack under a l e a f l e s s deciduous hardwood canopy were so damped th a t they c o u l d be c o n s i d e r e d q u a n t i t a t i v e l y i n s i g n i f i c a n t . The energy a v a i l a b l e f o r snowmelt i n the f o r e s t was computed simply as the sum of the heat t r a n s f e r of r a i n p l u s the energy a v a i l a b l e from net r a d i a t i o n . Because of the reasons s t a t e d above the c o n v e c t i v e and condensation melts were co n s i d e r e d n e g l i g i b l e . Thus: MTOTF = MRF + MRN MTOTF' = MRF + MMRLNF + .20MSWR Under the f o r e s t canopy the amount of shortwave r a d i a t i o n r e a c h i n g the f o r e s t f l o o r was assumed to be 20% of that i n the open. M i n the f o r e s t was, l i k e the open s i t e , assumed to be 9 0.5 mm/day and was added to MTOTF'' before any comparative a n a l y s i s . 100 5.2.6 Event 1: January 2 5 - 3 1 , 1983  OPEN SITE During the f i r s t three days of thi s i n t e r v a l two main rain on snow events occurred, one of 15 hours duration with 30.4 mm of rain and the other of 17 hours with 40.6 mm of rain (Figure 29). This sequence of events i s a good example of the f i e l d observations described as condition 4, Table 12. The i n i t i a l snow water equivalent for the open s i t e was 220 mm (Table 11), and i t i s assumed here that the l i q u i d water requirements had been s a t i s f i e d by the previous days' r a i n . A l l pr e c i p i t a t i o n must have f a l l e n as rain because a i r temperatures remained well above 1.5° C for both events (Figure 29). Later in the week, on January 30, a small p r e c i p i t a t i o n event occurred during which a i r temperatures in the open dropped below 1°C suggesting the occurrence of snow. However at the same time a i r temperatures were several tenths of a degree centigrade warmer in the forest and as described in Table 12 condition 1, thi s p r e c i p i t a t i o n was in a l l p r o b a b i l i t y f a l l i n g as rain on the forest lysimeter. The differences in the two consecutive snow survey results added to the snowfall in the open indicate a t o t a l snow ablation of 39 mm ((220-185)+4). The lysimeter runoff measurements show a t o t a l snowmelt of 38 mm, obtained by subtracting 73 mm of rain (MMRAIN, Table 13) from a measured runoff cf 110.9 mm (BOMMRO ,Table 13). Although the seven day t o t a l of the energy balance computations provide results that agree very c l o s e l y with the 101 A I R T E M P E R A T U R E Legend OPEN 150 < FOREST 150 cm 5 -c £ 4.5-o m 4-\ £ 3 . 5 -£ 3 -ION 2 . 5 -» b 2 -,< h - 1 . 5 -o. u 1 -LU (X 0 . 5 -a. P R E C I P I T A T I O N I N T E N S I T Y Legend R A I N S N O W 3.5 ^ 3 £ 2.5 O 2 1.5 * 1 0 . 5 ^ ^ 0 i - 5 O + ^ 3.5 E 2.5-| t 2 O 1-5 3 cr 0.5 1-i LW n ™'»''™ ,nwwwmmui|MiiiiniTmnTTTmifT WIND AND NET RADIATION q R E S T R A D I A T I O N / iiiiiiiiiiuiimmnmmfmnww LYSIMETER RUNOFFl RUNOFF OPEN SITE FOREST SITE l|ll|llllll I m -r 4 0 i - 3 0 JE - 2 0 O -10 < - 0 Q < or - - 1 0 LU - - 2 0 2 •^mrnrmrrmrr TfffMfll'lfflllll TURBULENT ENERGY MELT IN OPEN SENSIBLE HEAT CONDENSATION ^ 0.45 -I c E O 0.35 cn \ E E 0.25 b 0.15 -\ LU 0.05 H -0 .05 |'|ihii| \i\inm^m^mmmrmnmmymmw RADIATION, RAIN & TOTAL MELT 0.25 ^  C ' E ° f-o.t 5 cn \ £ £ 0 . 0 5 X —' b LU - 0 . 0 5 5 25 " " ' " 1 "'I I I I I I I I I I I i 26 27 JANUARY 1983 MELT IN OPEN RAIN  RADIATION TOTAL Figure 29 - Measured & computed variables,Jan.25-27,1983 1 02 Table 13 - Data Table, January 25-31,1983 M I N . D A T E M A X D A T E C A 5 E S H O U R S M M P R E C M M R A I N M M S N O W B O M M R O ' M C O N D E V A P 2 5 . 5 2 1 2 6 . 0 0 0 2 4 1 2 . 0 0 0 1 8 . 2 6 3 1 8 . 2 6 3 0 . 2 2 . 2 7 7 . 8 7 0 8 0 0 2 6 . 0 2 1 2 7 . 0 0 0 4 8 2 4 . 0 0 0 4 3 . 3 8 3 4 3 . 3 8 3 0 . 6 8 . 7 6 2 2 . 1 4 1 6 0 2 7 . 0 2 1 2 8 . 0 0 0 4 8 2 4 O O O 9 . 3 4 7 2 9 . 3 4 7 2 0 . 1 8 . 7 0 3 1 . 0 2 4 3 0 . 2 8 . 0 2 1 2 9 . 0 0 0 4 8 2 4 . O O O 0 . 0 . 0 . . 5 3 8 5 6 . 4 9 6 2 9 - 1 . 3 8 5 2 4 2 9 . 0 2 1 3 0 . 0 0 0 4 8 2 4 . 0 0 0 0 . 0 . 0 . 0 . 0 . 2 . 1 5 8 5 3 0 . 0 2 1 3 1 . 0 0 0 4 8 2 4 . 0 0 0 6 . 0 9 6 0 2 . 0 3 2 0 4 . 0 6 4 0 . 5 1 4 0 8 . 3 7 9 2 9 . 2 2 8 1 9 - 1 3 1 . 0 1 3 3 1 . 8 7 5 4 2 2 1 . 0 0 0 0 . 0 . 0 . . 1 2 2 4 0 . 2 1 2 6 4 . 9 9 9 7 3 - 1 **.**«»«««« . . . . . . . W E E K L Y T O T A L S 7 7 . 0 8 9 7 3 . 0 2 5 4 . 0 6 4 0 1 1 0 . 9 2 4 . 6 7 8 2 2 . 6 6 6 5 M I N . D A T E M A X . D A T E M C M R M T O T M T O T ' L T O T L T O T ' M T O T ' ' L T O T ' ' 2 5 . 5 2 1 2 6 . 0 0 0 . 4 0 3 4 6 . 8 2 9 5 0 3 . 5 1 8 4 4 . 8 8 2 8 3 . 5 1 8 4 4 . 8 8 2 8 4 8 8 2 8 4 . 8 8 2 8 2 6 . 0 2 1 2 7 . O O O . 8 7 2 2 5 2 . 1 8 7 2 7 . 3 5 9 8 1 1 . 3 B 6 7 . 3 5 9 8 1 1 . 3 8 6 1 1 . 3 8 6 1 1 . 3 8 6 2 7 . 0 2 1 2 8 . O O O . 4 1 9 6 6 . 4 8 9 7 2 3 . 9 0 1 0 7 . 2 5 4 3 3 . 9 0 1 0 7 . 2 5 4 3 7 . 2 5 4 3 7 . 2 5 4 3 2 8 . 0 2 1 2 9 . 0 0 0 . 3 3 5 0 1 0 . 2 . 8 6 2 2 5 . 8 9 6 1 3 . 2 4 7 5 6 . 2 8 1 4 4 . 4 2 8 9 4 . 8 1 4 1 2 9 . 0 2 1 3 0 . 0 0 0 1 . 0 0 8 6 0 . . 3 9 5 6 9 6 . 5 8 4 2 2 . 5 5 4 1 8 . 7 4 2 7 . 3 9 5 6 9 2 . 5 5 4 1 3 0 . 0 2 1 3 1 . 0 0 0 . 2 1 8 0 2 . 6 7 6 0 9 - . 7 3 4 1 4 3 . 6 0 5 9 . 7 5 6 9 6 3 . 6 2 8 7 3 . 6 0 5 9 3 . 6 2 8 7 3 1 . 0 1 3 3 1 . 8 7 5 . 4 0 9 9 0 0 . 2 . 2 2 4 5 5 . 0 7 9 4 2 . 3 2 4 4 5 . 1 7 9 4 5 . 9 9 2 4 6 . 0 9 2 4 • • ' * • W E E K L Y T O T A L S : 3 . 6 6 6 9 3 . 5 7 4 0 2 0 . 9 9 6 4 4 . 6 8 8 2 3 . 6 6 2 4 7 . 3 5 5 3 7 . 9 4 5 4 0 . 6 1 2 M I N . D A T E M A X D A T E M R N M M R L N M S W R M M R A I N F B F M M R O ' M R F M M R L N F M R N F 2 5 . 5 2 1 2 6 . O O O 1 . 4 1 4 6 1 9 6 3 0 8 1 G O O 8 5 8 3 4 9 . 6 3 0 6 . 3 9 3 5 4 1 . 8 8 4 2 - 1 . 7 5 5 5 2 6 . 0 2 1 2 7 O O O 2 . 1 5 8 8 4 . 5 5 2 6 1 . 6 3 2 0 2 0 . 3 9 0 2 6 . 7 0 1 . 9 5 4 5 0 4 . 1 9 0 7 - 3 . 3 5 9 3 2 7 . 0 2 1 2 8 . 0 0 0 1 . 9 6 7 3 3 . 6 8 8 6 1 . 6 3 2 0 4 . 3 9 3 2 1 2 . 3 5 9 . 2 0 5 4 6 3 . 4 1 1 8 - 5 . 9 1 8 3 2 8 . 0 2 1 2 9 O O O 2 . 8 6 2 8 4 . 2 6 4 7 1 . 6 3 2 0 0 . 1 . 5 2 9 3 0 . 1 . 9 6 5 2 - 9 9 6 9 9 2 9 . 0 2 1 3 0 . O O O 1 . 5 4 5 5 6 . 1 0 2 0 1 . 6 3 2 0 0 . 2 . 3 1 4 7 0 . 5 . 0 7 7 1 - . 4 3 2 1 0 3 0 . 0 2 1 3 1 . 0 0 0 . 9 2 0 4 4 - 1 . 3 3 1 8 1 . 6 3 2 0 2 . 8 6 5 1 2 . 3 9 7 3 . 3 7 1 5 7 - 1 1 . 8 2 5 7 - 2 . 9 6 6 3 3 1 . 0 1 3 3 1 . 8 7 5 1 . 7 0 1 9 3 . 1 2 8 9 1 . 4 2 8 0 0 . 1 . 7 7 7 3 0 . 2 . 6 4 2 2 - . 9 8 2 4 1 . . . . W E E K L Y T O T A L S : 1 1 . 7 4 3 2 5 . 0 3 2 1 0 . 4 0 4 3 6 . 2 3 2 5 6 . 7 0 9 1 . 5 9 0 7 2 0 . 9 9 7 - 1 6 . 4 1 1 M I N . D A T E M A X . D A T E M T O T F M T O T F ' M T O T F ' ' H T E M P O H R . H . 0 H T E M P F H R . H . F 2 5 . 5 2 1 2 6 . 0 0 0 - 1 . 3 6 2 0 2 . 2 7 7 8 2 . 2 7 7 8 3 . 2 4 5 0 9 5 . 0 3 3 3 . 1 1 8 3 8 9 . 3 3 3 2 6 . 0 2 1 2 7 . 0 0 0 - 2 . 4 0 4 8 5 . 1 4 5 2 5 . 1 4 5 2 3 . 7 5 4 0 9 7 . 6 6 9 3 . 4 6 1 9 9 0 . 0 5 8 2 7 . 0 2 1 2 8 . 0 0 0 - 5 . 7 1 2 8 3 . 6 1 7 2 3 . 6 1 7 2 3 . 0 5 0 2 9 8 . 5 9 4 2 . 8 2 6 9 9 0 . 1 8 5 2 8 . 0 2 1 2 9 . 0 0 0 - . 9 9 6 9 9 1 . 9 6 5 2 1 . 1 7 0 5 3 . 4 4 2 7 6 9 . 9 8 2 1 . 6 3 6 5 8 4 . 9 3 9 2 9 . 0 2 1 3 0 . 0 0 0 - . 4 3 2 1 0 5 . 0 7 7 1 - . 4 3 2 1 0 4 . 9 8 1 0 4 6 . 3 5 0 4 . 1 7 5 2 5 8 . 1 7 0 3 0 . 0 2 1 3 1 . O O O - 2 . S 2 9 1 1 . 8 6 2 8 1 . 8 6 2 8 1 . 1 0 7 1 9 6 . 9 6 5 1 . 5 2 2 9 8 8 . 6 6 7 3 1 . 0 1 3 3 1 . 8 7 5 - . 9 8 2 4 1 2 . 6 4 2 2 2 . 4 5 6 2 2 . 9 3 1 2 8 4 . 6 5 2 2 . 5 0 1 2 8 5 . 5 7 4 . . . . W E E K L Y T O T A L S : - 1 4 . 8 2 0 2 2 . 5 8 7 1 6 . 0 9 8 . * * * . . . * • A V E R A G E S 1 03 two other methods, the temporal d i s t r i b u t i o n of the calculated melt does not correspond to the lysimeter measured melt. On January 26 the lysimeter measurements indicate a melt of 25.4 mm (BOMMRO minus MMRAIN) while the calculated t o t a l energy (MTOT'') for that day indicates only 11.4 mm of melt. The explanation of th i s event is only speculative as I cannot f i n d any d e f i n i t e cause for such a difference. It i s thought, as i s described in Male and Gray (1981), that the l i q u i d water that i s a transient state in the snowpack may accumulate, and at a cer t a i n undefined period w i l l suddenly " l e t go". However the resu l t s from the energy balance computations for the entire week do match well with the snow survey measurements suggesting that the value of t o t a l melt for the seven day in t e r v a l be close to 40 mm. The summary of the results in the open i s as follows : Energy balance: Total melt (MTOT*') : 37.94 + 3.6 (M ) =41.5 9 Total loss (LTOT'') : 40.61 + 3.6 (M ) =44.2 g Snow survey : 39 mm of snowmelt Lysimeter melt : 38 mm Lysimeter runoff : 110.92 mm FOREST SITE The results obtained in the forest are r e l a t i v e l y easy to interpret and present a clear picture of the snow melting process during the week of January 25 to 31. Because i t is judged that the p r e c i p i t a t i o n of January 30 f e l l as rain in the 1 04 forest the estimation of snowmelt with the snow survey i s simply the difference between the two consecutive snow surveys, being 17 mm. To quantify snowmelt by t h i s method some speculation must be made about the lysimeter data. If a 25% interception loss was assumed, i t would result in a t o t a l melt of 2.0 mm (BFMMRO - (THROUGHFALL*MMRAIN)), t h i s value i s quite low when compared to the snow survey r e s u l t s . Although i t is not in accordance to what the l i t e r a t u r e suggests, i t would seem that evaporative loss would be greater than 25%. The energy balance c a l c u l a t i o n (MTOTF') agrees reasonably well with the melt values obtained with the weekly snow survey. Summary of results : Energy balance: 16.1 mm + 3.3(M ) = 19.4 mm of snowmelt 9 Snow survey : 17 mm of snowmelt Lysimeter melt: 2.0 mm with 25% interception Lysimeter runoff: 56.7 mm Figure 29 depicts well the differences in runoff rates between the forest and the opening during t h i s rain-on-snow event, as measured by the lysimeter. The runoff peaks in the open are quite evident, while the runoff i s more evenly dis t r i b u t e d in the forest where the snowpack i s shallower and patchy and where much of the p r e c i p i t a t i o n i s intercepted by the mature forest canopy. In t h i s f i r s t case, melt calculated by subtracting assumed throughfall from the forest lysimeter data, provides a very low snowmelt value, compared to the two other methods. This would 105 strongly suggest that the canopy above the lysimeter intercepts and routes water away from the lysimeter at a rate greater than 25% of the rain measured in the open. Although i t i s unlikely that much more than 25% of the rain reaching the top of the canopy i s lost to evaporation, the lysimeter i s possibly located in a very 'protected' area. Had there been several lysimeters in the forest, a more representative average value may have been obtained. 5.2.7 Event 2: January 19 - 25, 1983  OPEN SITE On January 19 snowpack water equivalent at the open s i t e was 243 mm, no p r e c i p i t a t i o n occurred u n t i l 23 January when 9 mm w.e. of snow was added to the pack y i e l d i n g a t o t a l w.e. of 252 mm (Figure 30). Although the energy balance c a l c u l a t i o n (MTOT'', Table 14) suggests the presence of s i g n i f i c a n t amounts of energy for melt during 20 and 21 January, the open lysimeter only registered a small amount of runoff on January 20. Although s i g n i f i c a n t amounts of l i q u i d water (up to 20%, see chapter 2 ) can be stored in the snowpack in the transient stage, t h i s only p a r t i a l l y explains the lack of lysimeter runoff. The drop in a i r temperature below 0°C on the night of January 20 most l i k e l y caused the formation of ice lenses near the surface of the pack, impeding the free flow of water to the lysimeter opening. The si t u a t i o n again occurred on January 23, followed by snowfall, creating a layered snowpack. It i s hypothesized that these observed ice lenses caused the rain and 106 Table 14 - Data Table, January 19-25,1983 M I N . D A T E M A X D A T E C A S E S H O U R S M M P R E C M M R A I N M M S N O W M R M C M C O N D 1 9 . 6 4 6 1 9 . 9 7 9 1 7 8 . 5 0 O O 0 . 0 . 0 . 0 . . 3 7 6 8 4 - . 8 6 1 3 6 - 1 2 0 . 0 0 0 2 0 . 9 7 9 4 8 2 4 . 0 0 0 0 . 0 . 0 . 0 . . 7 3 0 9 3 - . 1 7 2 7 6 2 1 . 0 0 0 2 1 . 9 7 9 4 8 2 4 . 0 0 0 0 . 0 . 0 . 0 . . 9 8 6 1 4 - 1 . 1 1 7 2 3 2 2 . 0 0 0 2 2 . 9 7 9 4 8 2 4 . O O O 0 . 0 . 0 . 0 . . 8 5 9 7 7 0 . 2 3 . O O O 2 3 . 9 7 9 4 8 2 4 . 0 0 0 9 . 3 9 8 0 . 5 0 8 0 0 8 . B 9 0 0 . 8 9 9 6 4 - 1 . 5 5 5 0 9 - 1 . 1 5 8 8 0 2 4 . 0 0 0 2 5 . O O O 4 9 2 4 . 5 0 0 3 8 . 1 O 0 3 8 . 1 0 0 0 . 1 . 3 1 1 8 . 5 0 5 2 5 1 . 1 7 1 4 4 7 . 4 9 B 3 8 . 6 0 8 8 . 8 9 0 0 1 . 4 0 1 7 1 . 6 2 9 9 1 . 7 0 6 3 M I N . D A T E M A X D A T E E V A P M R N M T O T M T O T ' L T O T L T O T ' M T O T ' ' L T O T ' ' 1 9 . 6 4 6 1 9 . 9 7 9 0 . . 3 4 3 5 3 . 4 6 7 3 5 1 . 6 9 7 3 . 4 6 7 3 5 1 . 6 9 7 3 1 . 6 9 7 3 1 . 6 9 7 3 2 0 . 0 0 0 2 0 . 9 7 9 . 8 7 9 5 3 - 2 . 9 5 5 5 6 1 . 1 9 2 6 3 . 6 4 6 6 1 . 2 0 1 4 3 . 6 5 5 4 3 . 6 4 6 6 3 . 6 5 5 4 2 1 O O O 2 1 . 9 7 9 . 4 3 7 0 1 - 1 - . 1 6 9 1 0 . 3 0 4 1 8 - 2 4 . 1 9 8 4 . 4 6 7 4 2 - 1 4 . 2 4 2 1 5 . 2 7 2 8 5 . 3 1 6 5 2 2 . O O O 2 2 . 9 7 9 2 . 7 9 6 9 - 1 . 8 9 3 6 - 3 . 8 3 0 8 3 . 3 2 7 2 - 1 . 0 3 3 9 6 . 1 2 4 1 - . 7 5 9 6 2 2 . 0 3 7 3 2 3 . 0 0 0 2 3 . 9 7 9 . 1 0 1 8 1 - 1 . 2 5 6 0 - 1 . 0 5 3 5 2 . 2 8 2 6 - . 9 5 1 6 9 2 . 3 8 4 4 2 . 2 8 2 6 2 . 3 8 4 4 2 4 . 0 0 0 2 5 . 0 0 0 0 . 1 . 1 4 8 8 4 . 1 3 7 2 7 . 8 9 6 9 4 . 1 3 7 2 7 . 8 9 6 9 7 . 8 9 6 9 7 . 8 9 6 9 W E E K L Y T O T A L S : 2 . 9 5 1 2 - . 8 7 0 8 3 . 9 1 5 9 1 2 3 . 0 4 9 3 . 8 6 7 1 2 6 . 0 0 0 2 0 . 0 3 7 2 2 . 9 8 8 M I N . D A T E M A X . O A T E B O M M R O ' M M R L N M S W R M M R A I N F M R F M R N F M M R L N F M T O T F 1 9 . 6 4 6 1 9 . 9 7 9 2 . 1 4 2 3 . 9 9 5 5 0 . 5 7 8 0 O 0 . 0 . - . 8 2 4 8 3 . 8 4 3 2 0 - . 6 2 4 8 3 2 0 . 0 0 0 2 0 . 9 7 9 . 2 9 5 4 9 1 . 7 7 7 6 1 . 6 3 2 0 0 . 0 . - 1 . 6 4 9 5 1 . 3 7 6 2 - 1 . 6 4 9 5 2 1 . O O O 2 1 . 9 7 9 0 . 2 . 3 9 4 3 1 . 6 3 2 0 0 . 0 . - . 9 6 5 3 5 . 8 5 4 2 8 - . 9 6 5 3 5 2 2 . O O O 2 2 . 9 7 9 0 . 3 . 6 3 2 4 1 . 6 3 2 0 0 . 0 . - . 4 1 3 7 5 1 . 8 8 6 1 - . 4 1 3 7 5 2 3 . 0 0 0 2 3 . 9 7 9 0 . . 4 4 8 1 2 1 . 6 3 2 0 5 . 1 6 8 9 . 4 5 6 6 6 - 1 - 1 . 8 8 8 9 . 5 5 5 6 0 - 1 . 8 4 3 2 2 4 . 0 0 0 2 5 . 0 O 0 3 2 . 1 3 4 3 . 2 4 2 5 1 . 6 6 6 0 2 0 . 9 5 5 1 . 0 0 2 6 - 1 . 5 5 9 1 3 . 0 9 1 5 - . 5 5 6 4 7 * * • * W E E K L Y T O T A L S : 3 4 . 5 7 2 1 2 . 4 9 0 8 . 7 7 2 0 2 6 . 1 2 4 1 . 0 4 8 3 - 7 . 3 0 1 4 8 . 6 0 6 9 - 6 . 2 5 3 1 M I N . D A T E M A X . D A T E B F M M R O ' M T O T F ' M T O T F ' ' H T E M P O H R . H . 0 H T E M P F H R H . F 1 9 . 6 4 6 1 9 . 9 7 9 1 . 8 7 2 5 . 8 4 3 2 0 . 8 4 3 2 0 2 . 3 3 7 1 9 8 . 6 4 7 1 . 9 8 3 5 9 0 . 6 0 0 2 0 . O O O 2 0 . 9 7 9 2 . 9 1 9 9 1 . 3 7 6 2 1 . 3 7 6 2 1 . 4 7 9 4 9 8 . 9 9 2 1 . 1 4 9 8 9 0 . 6 5 8 2 1 . 0 0 0 2 1 . 9 7 9 1 . 2 0 6 0 . 8 5 4 2 8 . 8 5 4 2 8 1 . 9 2 5 8 9 2 . 1 0 1 . 7 1 2 5 0 9 0 . 5 6 5 2 2 . 0 0 0 2 2 . 9 7 9 1 . 1 1 0 8 1 . 8 8 6 1 . 5 2 9 0 7 2 . 9 5 0 4 6 5 . 4 7 7 1 . 5 6 8 5 7 4 . 8 1 0 2 3 . 0 0 0 2 3 . 9 7 9 2 . 2 5 3 4 . 6 0 1 2 7 . 5 4 3 4 5 . 3 7 2 7 1 9 7 . 9 6 5 . 4 6 4 3 7 8 7 . 9 2 6 2 4 . 0 0 0 2 5 . 0 0 0 1 7 . 8 0 5 4 . 0 9 4 1 4 . 0 9 4 1 2 . 6 3 5 9 9 B . 8 0 6 2 . 5 1 5 3 8 9 . 2 3 9 | • » * • W E E L K Y T O T A L S : 2 7 . 1 6 7 9 . 6 5 5 2 8 . 2 4 0 3 • * * * • « * • A V E R A G E S • * * * » • * « * » * * * *••**•*»• 107 A I R T E M P E R A T U R E Legend OPEN 150 cm 23 JANUARY 1983 Figure 30 - Measured & computed variables,Jan.22-24,1983 108 melt water to be routed away from the o r i f i c e of the l y s i m e t e r r e s u l t i n g i n erroneous measurements. As the i c e lenses melted, melt water was routed through the l y s i m e t e r g i v i n g a more ac c u r a t e measure of melt, as shown i n the l a t e r h a l f of January 24 ( F i g u r e 30). The d i f f e r e n c e i n the two c o n s e c u t i v e snow survey measurements added to the 23 January s n o w f a l l i n d i c a t e s that a t o t a l of 32.0 mm water e q u i v a l e n t of snow should have e i t h e r been melted or evaporated from the snowpack. The energy balance c a l c u l a t i o n s y i e l d a t o t a l energy input of 20 mm of energy melt (MTOT'', Table 14). When 3.6 mm w.e. of ground heat and 3 mm w.e. of e v a p o r a t i o n are added to the 20 mm a t o t a l l o s s of 26.6 mm w.e. from the snowpack i s o b t a i n e d . The open l y s i m e t e r data o f f e r no comparative r e s u l t s as the snowmelt r o u t i n g was probably a f f e c t e d by observed i c e l e n s e s . On January 24, 38 mm of r a i n f e l l on a primed snowpack with a f u l l y s a t i s f i e d l i q u i d water d e f i c i e n c y and only 32 mm were measured at the outflow. The snowpack i s assumed to have been primed p r e v i o u s to the r a i n because f o r s e v e r a l days the energy balance c a l c u l a t i o n s showed p o s i t i v e v a l u e s of energy f o r melt. The computed runoff f o r t h i s event i s the average melt (29.5 mm) p l u s t o t a l r a i n f a l l (38.6 mm), which equals 68.1 mm. 109 The summary of results for the week of 19 to 25 January is as follows: Energy balance =26.6 mm of snowmelt Snow survey = 32.4 mm of snowmelt Difference = 5.8 mm Average = 29.5 mm of snowmelt Lysimeter melt = affected by ice lenses Computed runoff = 68.1 mm FOREST SITE The two methods for estimating snowmelt in the forest agree quite well over the seven day i n t e r v a l , however again they are subject to speculation on the amount of p r e c i p i t a t i o n reaching the forest f l o o r . The 9.0 mm w.e. snowfall that was recorded in the open on 23 January f e l l on the canopy where a portion was trapped by branches. Snowmelt in the branches begins sooner because the sources of energy for melt and evaporation are greater at the top of the canopy than at the forest f l o o r . This i s due to greater amounts of shortwave radiation, and greater wind speeds which accelerate both the transfer of sensible and latent heat. A portion of the l i q u i d water released from the canopy may be routed along branches and down the trunk of the tree. Such stemflow does not reach the snowpack on the lysimeter because there are no trees on the lysimeter borders, although the canopy does overhang the lysimeter. A l l these factors make i t d i f f i c u l t to estimate how much of the snow recorded in the open 1 10 f e l l to the forest floor either as water or as snow. Because a i r temperatures increased rapidly after the snowfall (Figure 30), canopy dr i p was probably an important process. Thus of the 9 mm w.e. of snow that f e l l on the canopy the portion that reached the forest floor as snow could be anywhere between 30% (2.7 cm) and 70% (6.3 cm), the rest being either lost to evaporation or canopy d r i p . Since the forest snow survey indicated a net loss for the week of 2 mm w.e.; the t o t a l weekly snowmelt would be between 4.7 mm (2.7 mm + 2 mm) and 8.3 mm (6.3 mm +2 mm). The lysimeter data presents some interesting re s u l t s , however calc u l a t i o n s of melt rates cannot be done because of the many speculative assumptions, r e l a t i v e to interception that must be made. It i s evident from data analysis of the forest lysimeter results that snowmelt was continuous throughout the week as runoff was registered in the absence of rain (BFMMRO, Table 14). However during the three day period of January 21 to 23 no runoff occurred in the open. This d e f i n i t e l y shows that snowpack conditions and/or energy d i s t r i b u t i o n are quite d i f f e r e n t between the two s i t e s . The direct comparison of the lysimeter runoff in Figure 30 shows greater runoff from the open s i t e during the rain events. Also interesting i s the presence of lysimeter runoff in the forest immediately following the snowfall (Figure 30), which c l e a r l y represents canopy drip. Although snow did not completely cover the ground (Table 11) the energy balance computations give results that relate to the amount of snow that would be melted i f there was 100% ground 111 coverage with a w.e. at least equal to that amount to be melted. This concept i s something analogous to that of potential evapotranspiration. Thus i t i s to be expected that i f .the snow cover becomes patchy the values obtained from the energy balance computations w i l l be greater than the f i e l d measured values. The energy balance calculations y i e l d a result of 8.24 mm (MTOTF1'), plus 3.6 mm (M ) for a t o t a l of 11.8 mm. 9 Again no considerations were made for turbulent fluxes because of the absence of measurable wind. The summary of the results for the forest s i t e i s as follows. Energy balance: 8.24 mm +3.6 (ground heat) =11.8 mm Snow survey : 9.8 mm with 70% snow throughfall : 4.7 mm with 30% snow throughfall Lysimeter runoff : 27.2 mm The results obtained from the analysis of t h i s event show that snowmelt rates can be greater at the open s i t e and that runoff may be more evenly d i s t r i b u t e d under the forest canopy thus avoiding potential dangerous peak flows. The t o t a l weekly runoff i s s i g n i f i c a n t l y less from the forest s i t e (27.2 mm) than the open s i t e (68.1mm). 5.2.8 Event 3: January 4 - 1 1 , 1983 As can be observed in Figures 31 and 32 the a i r temperatures throughout the week fluctuated continuously around the 1°C l e v e l . This s i t u a t i o n as e a r l i e r described makes computation of the water balance d i f f i c u l t because the form in 1 12 which the p r e c i p i t a t i o n f a l l s i s unknown. For this analysis i t was decided to use the arb i t r a r y value of 1°C to distinguish between a rain and a snow event. Thus any precipitaton f a l l i n g during a i r temperatures above 1°C was considered as rain and below 1°C as snow. Unfortunately during t h i s week the instrument shed at the open s i t e was flooded and prevented the proper functioning of the tipping bucket, consequently lysimeter runoff data for the open s i t e was not measured. However the atmospheric conditions that prevailed during t h i s event produced some interesting results at the forest s i t e . The analysis of th i s event s h a l l be more q u a l i t a t i v e than quantitative because of the lack of lysimeter data and the d i f f i c u l t y in estimating throughfall. OPEN SITE Although calculations using the 1°C l i m i t y i e l d a t o t a l snowfall of 76.4 mm (Table 15 ), analysis of the plotted results (Figures 31 and 32) suggest the high pr o b a b i l i t y of a mixture of snow and rain during both snow events that occurred January 5 and January 6 . This i s hypothesized because a i r temperatures in the open only dipped to a low of 0.5°C and the lysimeter in the forest continued to register some runoff. Thus to estimate accumulated snowfall during t h i s period i t was thought reasonable to consider half of the calculated snowfall as snow and half as r a i n f a l l . Thus of the 17.5 mm of calculated snowfall in the early part of the week, 8.8 s h a l l be considered as accumulated snow and 8.7 as r a i n f a l l . Using these 113 3 -£ o cn 2 . 5 -(mm/ 2 -NO: 1 . 5 -TAT! 1-CL PRECI 0 . 5 -OH 2.2 ^ 1.7 Q 1.2 0.7 O 00 E^ o 0.2 2.5 2 -1.5-1-0.5 0 PRECIPITATION INTENSITY Legend RAIN  SNOW WIND AND NET RADIATION OPEN RADIATION FOREST RADIATION OPEN LYSIMETER RUNOFF RUNOFF FOREST SITE 60 4 0 20 0 CM -20 O < 4 0 (_ LLI - 6 0 TURBULENT ENERGY MELT IN OPEN SENSIBLE HEAT_ CONDENSATION C E o cn 0.4 H 0.3 H E 0.2 b o.i H LJ 0.0 — 0.1 -H} 1 """ 1 ' " ' RADIATION, RAIN & TOTAL MELT MELT IN OPEN RAIN RADIATION TOTAL JANUARY 1983 Figure 31 - Measured & computed variables,Jan.4-7,1983 114 I1" "" "" I" 9 10 JANUARY 1983 . Figure 32 - Measured & computed variables,Jan.8-10,1983 1 15 Table 15 - Data Table, January 4-11,1983 M I N . D A T E M A X . D A T E C A S E S H O U R S M M P R E C M M R A I N M M S N Q W M R M C O N D M C 4 . 6 6 6 7 5 . 0 0 0 0 1 7 8 . 5 0 0 0 8 . 7 9 0 0 7 . 7 1 0 0 1 . 0 8 0 0 . 1 7 7 3 4 . 8 8 2 6 5 - 1 . 3 6 7 1 6 - 1 5 . 0 2 0 8 6 . 0 0 0 0 4 8 2 4 . 0 0 0 1 0 . 9 6 0 0 . 1 0 . 9 6 0 . 3 5 0 1 0 - 1 . 1 3 7 3 3 . 6 1 1 6 4 - 1 6 . 0 2 0 8 7 . O O 0 O 4 8 2 4 O O O 1 5 . 8 2 0 1 0 . 3 2 0 5 . 5 0 0 0 . 2 5 4 0 2 . 2 3 3 9 6 . 1 0 0 8 7 7 . 0 5 0 8 8 . 0 0 0 0 4 8 2 4 . 0 0 0 7 8 4 0 0 7 8 . 4 0 0 0 . 4 . 1 6 3 4 2 . 0 2 2 9 . 7 8 0 8 3 8 . 0 2 0 8 9 . 0 0 0 0 4 8 2 4 . 0 0 0 7 . 4 3 0 0 7 . 4 3 0 0 0 . . 2 0 3 5 4 . 7 2 1 5 9 . 3 7 5 5 5 9 . 0 2 0 8 1 0 . 0 0 0 4 8 2 4 . 0 0 0 7 5 . 8 2 0 1 7 . 0 0 0 5 8 . 8 2 0 . 1 3 1 8 2 . 1 2 3 6 7 . 3 6 9 9 6 - 1 1 0 . 0 2 1 1 1 . 0 0 0 4 8 2 4 . 0 0 0 4 9 . 1 7 0 4 9 . 1 7 0 0 . 2 . 3 3 2 5 1 . 6 9 3 3 . 6 5 8 3 6 1 1 . 0 2 1 1 1 . 3 7 5 1 8 9 . 0 0 0 0 5 . 4 4 0 0 5 . 4 4 0 0 0 . . 2 7 2 5 2 . 7 6 1 8 7 . 2 9 3 4 5 2 5 1 . 8 3 1 7 5 . 4 7 7 6 . 3 6 0 7 . 5 7 0 2 5 . 7 8 2 9 2 . 3 4 3 9 M I N . D A T E M A X . D A T E E V A P M R N M T O T M T O T ' L . T O T L . T O T ' M T O T ' ' L T O T " 4 . 6 6 6 7 5 . 0 0 O O 0 . . 1 9 1 8 4 . 4 9 4 1 6 1 . 5 7 4 3 . 4 9 4 1 6 1 . 5 7 4 3 1 . 5 7 4 3 1 . 5 7 4 3 S . 0 2 0 8 6 . 0 0 0 0 0 . . 9 2 3 0 0 1 . 1 5 6 5 3 . 1 7 3 5 1 . 1 5 6 5 3 . 1 7 3 5 3 . 1 7 3 5 3 . 1 7 3 5 6 . 0 2 0 8 7 . 0 0 0 0 0 . - . 7 3 2 4 5 - . 1 4 3 6 0 3 . 4 4 5 4 - . 1 4 3 6 0 3 . 4 4 5 4 3 . 4 4 5 4 3 . 4 4 5 4 7 . 0 2 0 8 8 . 0 0 0 0 0 . 1 . 8 6 9 8 8 . 8 3 6 9 1 3 . 3 5 4 8 . 8 3 6 9 1 3 . 3 5 4 1 3 . 3 5 4 1 3 . 3 5 4 8 . 0 2 0 8 9 . 0 0 0 0 . 2 9 0 1 5 - . 5 9 6 8 3 . 4 1 3 7 0 4 . 2 0 7 6 . 7 0 3 8 5 4 . 4 9 7 8 4 . 1 4 0 5 4 . 4 3 0 7 9 . 0 2 0 8 1 0 . 0 0 0 . 3 7 6 3 9 - 1 - 2 . 1 7 5 2 - 1 . 9 2 0 3 1 . 9 4 5 1 - 1 . 8 8 2 7 1 . 9 8 2 7 1 . 9 4 5 1 1 . 9 8 2 7 1 0 . 0 2 1 1 1 . 0 0 0 0 . 1 . 0 3 1 0 5 . 7 1 5 2 1 0 . 9 9 2 5 . 7 1 5 2 1 0 . 9 9 2 1 0 . 9 9 2 1 0 . 9 9 2 1 1 . 0 2 1 1 t . 3 7 5 0 . - . 3 4 3 6 1 . 9 8 4 2 3 3 . 5 7 9 9 . 9 8 4 2 3 3 . 5 7 9 9 3 . 5 7 9 9 3 . 5 7 9 9 * » • W E E K L Y T O T A L S : . 3 2 7 7 9 . 1 6 7 5 5 1 5 . 5 3 7 4 2 . 2 7 2 1 5 . 8 6 5 4 2 . 5 9 9 4 2 . 2 0 4 4 2 . 5 3 2 M I N . D A T E M A X . D A T E M M R L N M S W R M M P R E C F M R F M R N F M M R L N F M T O T F M T O T F ' 4 . 6 6 6 7 5 . 0 0 0 0 . 6 9 3 9 3 . 5 7 8 0 0 4 . 8 3 4 5 . 6 0 2 0 7 - 1 - . 3 2 8 6 6 . 4 9 6 1 0 - . 2 6 8 4 5 . 5 5 6 3 1 5 . 0 2 0 8 6 . 0 0 0 0 1 . 3 0 8 0 1 . 6 3 2 0 6 . 0 2 8 0 . 8 7 5 7 2 - 2 - 2 . 0 8 2 4 . 6 3 5 6 7 - 2 . 0 7 3 7 . 6 4 4 4 3 6 . 0 2 0 8 7 . 0 0 0 0 1 . 2 2 4 5 1 . 6 3 2 0 8 . 7 0 1 0 . 8 1 5 8 8 - 1 - 2 . 4 4 3 4 . 8 5 2 8 0 - 2 . 3 6 1 8 . 9 3 4 3 9 7 . 0 2 0 8 8 . 0 0 0 0 4 . 7 5 4 8 1 . 6 3 2 0 4 3 . 1 2 0 1 . 3 9 6 7 - 2 . 3 5 3 1 3 . 3 2 4 6 - . 9 5 6 3 6 4 . 7 2 1 4 8 . 0 2 0 8 9 . 0 0 0 0 1 . 5 6 5 1 1 . 6 3 2 0 4 . 0 8 6 5 . 6 4 4 6 1 - 1 - 2 . 7 2 5 6 . 8 0 6 2 0 - 2 . 6 6 1 2 . 8 7 0 6 6 9 . 0 2 0 8 1 0 . 0 0 0 . 5 8 2 1 1 - 1 1 . 6 3 2 0 4 1 . 7 0 1 - . 9 3 6 0 8 - 1 - 1 . 3 4 8 4 - . 2 8 9 9 4 - 1 . 4 4 2 0 - . 3 B 3 5 5 1 0 . 0 2 1 1 1 . 0 0 0 4 . 6 7 5 7 1 . 6 3 2 0 2 7 . 0 4 3 . 6 7 6 8 8 - . 8 0 8 1 5 2 . 8 6 5 1 - . 1 3 1 2 7 3 . 5 4 2 0 1 1 . 0 2 1 1 1 . 3 7 5 1 . 6 4 0 0 . 6 1 2 0 0 2 . 9 9 2 0 . 8 4 1 0 9 - 1 - . 4 1 8 1 7 1 . 0 6 3 1 - . 3 3 4 0 6 1 . 1 4 7 2 * • • * W E E K L Y T O T A L S : 1 5 . 9 2 0 1 0 . 9 8 2 1 3 8 . 5 1 2 . 2 7 9 1 - 1 2 . 5 0 8 9 . 7 5 3 7 - 1 0 . 2 2 9 1 2 . 0 3 3 M I N . D A T E M A X . O A T E M T O T F ' • B F M M R O ' H T E M P O H R . H . 0 H T E M P F H R . H . F 4 . 6 6 6 7 5 . 0 0 0 0 . 5 5 6 3 1 7 . 2 0 4 7 1 . 6 3 3 5 9 9 . 7 4 7 1 . 1 7 1 8 1 0 1 . 7 6 5 . 0 2 0 8 6 . 0 0 0 0 . 6 4 4 4 3 8 . 3 1 9 8 1 . 0 8 7 1 9 9 . 3 S 6 . 5 3 2 9 2 1 0 2 . 1 7 6 . 0 2 0 8 7 O O O O . 9 3 4 3 9 8 . 1 7 6 8 1 . 0 2 4 4 9 9 . 5 7 9 . 7 1 4 7 9 1 0 2 . 1 1 7 . 0 2 0 8 8 . 0 0 0 0 4 . 7 2 1 4 5 6 . 8 0 9 3 . 9 1 6 9 9 9 . 3 9 2 2 . 7 5 6 7 1 0 1 . 5 0 8 . 0 2 0 8 9 . 0 0 0 0 . 8 7 0 6 6 1 1 . 4 3 6 1 . 2 9 3 1 9 4 . 1 1 8 . 6 7 1 2 5 1 0 1 . 9 3 9 . 0 2 0 8 1 0 . 0 0 0 - . 3 8 3 5 5 1 6 . 9 2 5 . 4 4 1 6 7 - 1 9 9 . 7 6 2 - . 2 4 5 6 2 9 5 . 9 7 3 1 0 . 0 2 1 1 1 . 0 0 0 3 . 5 4 2 0 5 1 . 6 6 3 3 . 8 5 2 1 9 9 . 2 0 8 2 . 3 8 0 0 1 0 0 . 0 4 1 1 . 0 2 1 1 1 . 3 7 5 1 . 1 4 7 2 1 0 . 0 9 2 3 . 6 1 0 6 9 9 . 8 7 8 2 . 3 5 6 7 9 9 . 7 2 8 I •••• W E E K L Y T O T A L S : 1 2 . 0 3 3 1 7 0 . 6 3 A V E R A G E S * * * 116 calculations the t o t a l r a i n f a l l for the week becomes 184 mm and the addition of snow to the snowpack in the open i s 67.6 mm w.e. Energy balance calculations suggest that during the one week period enough energy was available to melt 42.2 mm of water equivalent (MTOT'',Table15). If we add to th i s the amount of the reduction of s.w.e. due to evaporation 0.3mm (EVAP,Table 15) we obtain 42.5 mm (LTOT*',Table 15). To th i s we add the t o t a l melt caused by ground heat (M ) of 3.6 mm for a t o t a l 9 weekly loss of 46.1 mm w.e. in the open. Since the t o t a l weekly snowfall was 67 mm w.e. and the snow survey showed a 5 mm s.w.e. increase (Table 11) in the snowpack, this indicates a t o t a l melt of 67mm-5mm=62mm. The calculated runoff for thi s event i s 230.1 mm (184 mm + 46.1 mm). In summary the measured snowmelt rates in the open for the week of January 4 to 11, 1983 are as follows : Energy balance method : 46.1 mm Snow survey method : 60 mm Calculated runoff :230.1 mm FOREST SITE January and February a i r temperatures t y p i c a l l y remain close to 0°C at our study s i t e . This means that p r e c i p i t a t i o n type i s continuously fluctuating between rain and snow. This has some d e f i n i t e implications to the analysis of our results, as s h a l l be demonstrated with th i s event. As has been stated several times, estimates of snow and rain throughfall values are necessary to analyse both snowmelt and runoff data at the forest s i t e . Alternating sequences of 1 17 rain and snow, due to the fluctuating temperatures, make i t especially d i f f i c u l t to estimate throughfall. Estimating snow throughfall i s d i f f i c u l t because, although not much i s lost to evaporation, there may be very important amounts that melt in the canopy and reach the forest f l o o r in the form of r a i n . This canopy melt water may percolate through the snowpack, thus not adding to the water equivalency of the l a t t e r . Thus, under th i s type of conditions three estimates must be made: 1) the quantity of snow throughfall 2) the quantity of rain throughfall 3) the quantity of canopy drip. Because I have no good information on which to base these estimates, I s h a l l abstain from doing so. However, I think i t i s int e r e s t i n g to go through th i s event and make some descriptive comments on the re s u l t s . The data from Table 15 and Figures 31 and 32 w i l l be used to describe how the forest canopy can affect snowmelt rates and consequently runoff at our forest s i t e . Late January 4 and early January 5, 12 mm w.e. of snow f e l l on the forest canopy (Table 15, MMSNOW). During the January 4 f i e l d v i s i t i t was noted that no snow was present in the canopy, thus providing a maximum storage capacity for any new f a l l i n g snow. Because a i r temperatures and most l i k e l y foliage temperatures were above the freezing point some of the warm snow f a l l i n g on the canopy probably melted quickly and f e l l from the fo l i a g e as canopy dri p . What didn't immediately melt or f a l l d i r e c t l y to the ground was temporarily stored in the canopy. This stored snow could melt and drip to the forest floor l a t e r , after actual 118 p r e c i p i t a t i o n has ceased. T h i s can be seen i n F i g u r e 31 on the f o r e s t l y s i m e t e r runoff curve. Here runoff i s continuous . f o r both days of 4 and 5 January and shows a s l i g h t peak mid-day January 5 i n response to an i n c r e a s e i n a i r temperature and net r a d i a t i o n . For both days the l y s i m e t e r i n the f o r e s t r e g i s t e r e d a t o t a l of 15.52 mm of runoff (Table 15, BFMMRO) while the p r e c i p i t a t i o n gauge i n the open r e g i s t e r e d a t o t a l of 19.74mm. Mowing f o r some eva p o r a t i o n from the f o r e s t canopy, these r e s u l t s s t r o n g l y suggest that most of the snow f a l l i n g on the f o r e s t canopy reached the f o r e s t f l o o r as canopy d r i p and only a small amount of snow was added to the f o r e s t pack. A s i m i l a r s c e n a r i o o c c u r r e d on January 6 when again probably much of the 5.5mm of snow r e g i s t e r e d i n the open reached the f o r e s t f l o o r as canopy d r i p . I t i s i n t e r e s t i n g to analyse the curve of the t o t a l c a l c u l a t e d melt (Figure 31). In t h i s f i g u r e , the curve of the t o t a l c a l c u l a t e d melt f o r the open s i t e i s very s i m i l a r i n shape to that of the f o r e s t r u n o f f . T h i s i s i n t e r e s t i n g because i t shows, f o r t h i s case, a r a p i d response of snowmelt and r e l e a s e of water from the f o r e s t snowpack with v a r i a t i o n s i n energy i n p u t s . T h i s can be a t t r i b u t e d to the probable presence of snow in the t r e e s and subsequent canopy d r i p ( s i t u a t i o n 2 ,Table 12). The a v a i l a b l e energy at the top of the canopy i s s i m i l a r to that measured i n the open. However the snow caught at s e v e r a l l a y e r s in the canopy w i l l respond much qu i c k e r ( i . e . , m e l t ) to v a r i a t i o n s i n a v a i l a b l e energy because of i t s l e s s e r s.w.e., 119 consequently i t s smaller i n t e r n a l heat s t o r a g e s . In the second h a l f of t h i s event, no snow f e l l t i l l January 9 when 58.8 mm of snow was assumed to have f a l l e n i n the open based on the 1°C r u l e , f o l l o w e d by 79 mm of r a i n . Most of the snow that f e l l on the canopy on January 9 was e i t h e r caught in the branches or f e l l to the f o r e s t f l o o r , as no l y s i m e t e r runoff was measured t i l l l a t e i n the day (Figure 32). As the r a i n s t a r t e d the l y s i m e t e r r e g i s t e r e d r u n o f f i n t e n s i t i e s s l i g h t l y g r e a t e r than p r e c i p i t a t i o n i n t e n s i t i e s suggesting that the r a i n f a l l was mixed with m e l t i n g snow that had p r e v i o u s l y been caught i n the canopy. Thus i t becomes evident that not a l l of the 59mm of snow that f e l l on the canopy reached the f o r e s t f l o o r as snow, but r a t h e r some as canopy d r i p . For t h i s seven day event, the t o t a l c a l c u l a t e d melt f o r the f o r e s t s i t e i s 12 mm (MTOTF'',Table 15). When t h i s amount i s added to the ground melt of 3.6 mm, the t o t a l melt equals 15.6 mm. However, I do not t h i n k that t h i s value i s r e p r e s e n t a t i v e of the amount of melt that r e a l l y o c c u r r e d at the f o r e s t s i t e . T h i s i s because the i n s t r u m e n t a t i o n used to c a l c u l a t e t h i s value was l o c a t e d near the f o r e s t f l o o r , when a c t u a l l y much of the runoff o r i g i n a t e d from melt i n the canopy. Thus f o r t h i s event, f o r the above s t a t e d reasons, none of our three methods can be used to estimate snowmelt at the f o r e s t s i t e . The only value d i r e c t l y measured was the t o t a l l y s i m e t e r runoff (170.6 mm). 1 20 5.2.9 Event4 : February 8 - 12,1983 From 8 to 10 February 1983, 72 mm w.e. of wet snow f e l l on the study s i t e f ollowed by i n c r e a s e s i n a i r temperature of 6°C and a two day t o t a l r a i n f a l l of 173 mm ( F i g u r e 33). C o n c u r r e n t l y a major d e b r i s t o r r e n t o c c u r r e d on A l b e r t a Creek, in the g e n e r a l v i c i n i t y of the study s i t e , causing two deaths, the d e s t r u c t i o n of a highway bri d g e and many thousands of d o l l a r s of p r o p e r t y damage. These events put i n t o p e r s p e c t i v e the importance of problems a s s o c i a t e d with r a i n on snow events. U n f o r t u n a t e l y the l y s i m e t e r data f o r t h i s time p e r i o d are not r e l i a b l e ; both s i t e s showed l e s s r u n o f f than o c c u r r e d . The open l y s i m e t e r developed a leak around the o r i f i c e of the outflow p i p e , while twigs and needles that had c o l l e c t e d on the snow duri n g January were p a r t i a l l y o b s t r u c t i n g the f r e e flow of water from the f o r e s t l y s i m e t e r . However, some very u s e f u l o b s e r v a t i o n s were made du r i n g the three s i t e v i s i t s of t h i s p e r i o d , and the energy balance computations o f f e r some i n t e r e s t i n g r e s u l t s . A f t e r a prolonged wet s n o w f a l l at the study s i t e , we have f r e q u e n t l y observed runoff to be g r e a t e r under the f o r e s t canopy than i n the open s i t e . The warm snow that gets trapped i n the branches has no l i q u i d requirement or c o l d content to be s a t i s f i e d and any p o s i t i v e energy input w i l l r e s u l t i n canopy d r i p . An e x c e l l e n t example of such an event i s from the night of February 9, 1983 to the a f t e r n o o n of February 10 (Figure 33). Although the f o r e s t s i t e computations show l i t t l e a v a i l a b l e 121 - 0 . 0 5 -9 10 11 FEBRUARY 1983 Figure 33 - Measured & computed variables,Feb.8-12,1983 1 22 energy for melt, i t can be reasonably assumed that there was greater turbulent energy at the top of the canopy where the snow was caught. The fact that the freshly f a l l e n snow melted quickly in the forest, whereas i t accumulated in the clearcut has some important consequences. This i s evident in Figure 33, which shows that on February 9 and 10 the canopy dr i p (melted snow) is causing greater lysimeter runoff in the forest than in the open. When the intense rains began in the late evening of February 10 1983 (Figure 33), a s i g n i f i c a n t amount of the recent snowfall in the forest had already melted, thus giving a more even runoff d i s t r i b u t i o n (Figure 33). However in the clearcut s i t e , due to small amounts of available energy (Figure 33), very l i t t l e melt occurred before the onset of the rains. This i s the type of situation where i t can be e a s i l y argued that extensive cle a r c u t t i n g may have negative e f f e c t s by increasing peak flows. The analysis of events 1 and 2 suggest that our method for computing the energy balance i s reasonable. On t h i s basis, results from a similar analysis of this event should provide useful information. A s i g n i f i c a n t increase in t o t a l available energy occurred very early on February 11 in the open, with over 55% of the t o t a l energy supplied by transferred heat from rain and condensation. The period of maximum availa b l e energy, between midnight February 10 and midnight February 11, t h e o r e t i c a l l y melted 19.4 mm w.e. during a t o t a l r a i n f a l l of 118 mm. The cumulative runoff (snowmelt and rain) for the 24 hour period would thus be 137.7 mm (MTOT'' + MMRAIN , Table 16), 1 23 an increase of 16% over the runoff that the p r e c i p i t a t i o n alone would have produced. The forest s i t e experienced less runoff because of the absence of turbulent energy, lack of snow on the lysimeter (50% covered, Table 11) and most important the influence of the canopy on interception losses. For February 8 to 12 the energy balance computations suggest a potential melt of 11 mm. The rain reaching the forest floor i s again d i f f i c u l t to estimate during t h i s event because of the problems in estimating throughfall. As in a l l events where there i s a mixture of snow and rain two estimates must be made. 1) During what int e r v a l was the p r e c i p i t a t i o n in the form of snow in the open and rain in the forest? 2) What percentage of the melted snow and/or rain was intercepted by the canopy and did not reach the forest floor? The f i e l d v i s i t of February 10 showed a d e f i n i t e accumulation of snow during the two previous days at the forest s i t e as the lysimeter was once again almost completly covered (Table 11). However, th i s v i s i t also permitted actual observation of the "snow in the open, rain in the forest" s i t u a t i o n . Because of the numerous assumptions that must be made about canopy interception losses and transformations of canopy snow to drip, any estimations of l i q u i d throughfall would be much too speculative. However due to evaporative losses that seem to be important, coupled with smaller calculated snowmelt rates for the forest, peak runoff rates should be less from the forest lysimeter. 124 Table 16 - Data Table, February 8-12, 1983 M I N D A T E M A X . D A T E C A S E S H O U R S M M P R E C M M R A I N M M S N O W B O M M R O ' M C O N D E V A P 3 9 5 4 2 4 0 . 0 0 0 2 3 1 1 . 5 0 0 9 . 1 4 4 0 0 . 9 . 1 4 4 0 0 . . 4 0 1 4 9 - 2 . 2 4 3 4 8 - 1 4 0 0 2 1 4 1 . 0 0 0 4 8 2 4 . 0 0 0 3 0 . 4 8 0 0 . 3 0 . 4 8 0 . 5 4 5 2 4 . 2 2 0 8 0 - 1 . 2 3 4 9 3 - 2 4 1 0 2 1 4 2 . 0 0 0 4 8 2 4 . 0 0 0 4 6 . 7 8 7 1 4 . 2 7 5 3 2 . 5 1 2 6 . 5 1 0 8 . 2 3 7 4 8 0 . 4 3 0 2 1 4 3 . 0 0 0 4 8 2 4 . O O O 1 1 8 . 2 6 1 1 8 . 2 6 0 . 9 9 . 2 6 6 3 . 7 7 7 6 0 . 4 3 0 2 1 4 3 . 4 7 9 2 3 1 1 . 5 0 0 4 0 . 3 1 0 4 0 . 3 1 0 0 . 4 8 . 4 3 0 1 . 1 6 7 7 0 . W E E K L Y T O T A L S : 2 4 4 . 9 8 1 7 2 . 8 5 7 2 . 1 3 6 1 5 4 . 7 5 5 . 2 0 8 9 . 2 6 6 9 7 - 1 M I N . D A T E M A X . D A T E M C M R M T O T M T O T ' L . T O T L . T O T ' M T O T ' ' L T O T ' ' 3 9 . 5 4 2 4 0 . 0 0 0 . 1 7 0 1 7 - 1 - . 6 6 2 4 1 - 1 . 5 5 9 1 5 1 . 1 7 3 0 . 5 8 3 5 0 1 . 1 9 7 4 1 . 1 8 6 3 1 . 2 1 0 7 4 0 0 2 1 4 1 . 0 0 0 . 1 1 3 5 8 - 1 . 3 3 9 9 5 - 1 - 1 . 2 1 2 8 1 . 9 8 4 8 - 1 . 2 1 0 4 1 . 9 8 7 1 1 . 9 8 4 8 1 . 9 8 7 1 4 1 . 0 2 1 4 2 . 0 0 0 . 1 0 8 3 3 . 3 9 8 2 2 . 3 2 1 0 8 3 . 2 4 5 0 . 3 2 1 0 8 3 . 2 4 5 0 3 . 2 4 5 0 3 . 2 4 5 0 4 2 . 0 2 1 4 3 . 0 0 0 1 . 4 4 1 3 6 . 9 3 9 4 1 3 . 8 4 1 1 9 . 4 3 1 1 3 . 8 4 1 1 9 . 4 3 1 1 9 . 4 3 1 1 9 . 4 3 1 4 3 . 0 2 1 4 3 . 4 7 9 . 4 4 2 5 4 2 . 2 1 B 6 4 . 4 2 8 5 7 . 1 7 2 9 4 . 4 2 8 5 7 . 1 7 2 9 7 . 1 7 2 9 7 . 1 7 2 9 * • * * W E E K L Y T O T A L S : 2 . 0 2 0 6 9 . 5 2 4 0 1 7 . 9 3 7 3 3 . 0 0 7 1 7 . 9 6 4 3 3 . 0 3 4 3 3 . 0 2 0 3 3 . 0 4 7 M I N D A T E M A X D A T E M R N M M R L N M S W R M M R A I N F B F M M R O ' M R F M M R L N F M R N F 3 9 . 5 4 2 4 0 . 0 0 0 . 6 0 4 3 6 . 4 3 6 2 4 . 7 8 2 0 0 4 . 2 9 7 7 . 2 2 0 8 9 - . 6 6 2 1 2 - 2 . 2 0 1 9 1 - . 7 2 4 8 3 4 0 . 0 2 t 4 1 . 0 0 0 - 1 . 2 8 0 2 . 2 8 5 3 6 1 . 6 3 2 0 1 4 . 3 2 6 7 . 8 8 9 0 . 4 7 9 7 8 - 2 . 5 5 9 2 1 - 1 - 2 . 1 1 6 1 4 1 . 0 2 1 4 2 . 0 0 0 - . 4 2 2 9 5 . 8 6 8 9 2 1 . 6 3 2 0 2 1 . 9 9 0 1 8 . 6 8 1 . 1 1 3 5 3 . 4 3 4 5 4 - 1 . 4 5 0 4 4 2 . 0 2 1 4 3 . O 0 O 1 . 6 8 2 6 5 . 6 4 0 9 1 . 6 3 2 0 5 5 . 5 8 3 3 8 . 9 7 2 2 . 6 0 0 8 4 . 4 6 7 6 - . 6 8 2 7 3 4 3 . 0 2 1 4 3 . 4 7 9 . 5 9 9 6 2 2 . 5 6 2 1 . 7 8 2 0 0 1 8 . 9 4 6 1 5 8 1 0 . 8 9 6 5 1 2 . 1 9 5 4 - . 9 0 7 1 5 W E E K L Y T O T A L S : 1 . 1 8 3 4 9 . 7 9 3 5 6 . 4 6 0 0 1 1 5 . 1 4 8 1 . 5 7 2 3 . 6 0 9 0 7 . 3 5 5 4 - 5 . 0 6 4 8 M I N D A T E M A X . D A T E M T O T F M T O T F ' H T E M P O H R . H . 0 H T E M P F H R . H . F 3 9 5 4 2 4 0 . 0 0 0 - . 7 3 1 4 5 . 1 9 5 2 9 . 7 3 7 3 9 9 2 . 6 1 7 . 3 5 1 3 0 8 8 . 7 8 3 4 0 . 0 2 1 4 1 . 0 0 0 - 2 . 1 1 1 3 . 6 0 7 1 8 - 1 . 2 3 9 7 9 9 9 . 5 2 7 . 4 7 0 8 3 - 1 8 8 . 9 6 7 4 1 . 0 2 1 4 2 . 0 0 0 - 1 . 3 3 6 9 . 5 4 8 0 7 . 7 2 7 5 0 9 9 . 5 1 0 . 3 6 4 5 8 8 9 . 1 3 5 4 2 0 2 1 4 3 . 0 0 0 1 . 9 1 8 1 7 . 0 6 8 4 4 . 6 2 4 2 9 8 . 8 8 3 3 . 6 8 1 7 9 0 . 0 7 1 4 3 0 2 1 4 3 . 4 7 9 . 8 0 5 7 9 3 . 0 9 1 9 4 . 3 9 6 1 9 9 . 5 9 6 3 . 7 7 9 6 8 9 . 5 9 1 • • -W E E K L Y T O T A L S : - 1 . 4 5 5 8 1 0 . 9 6 4 •*•••* • • • • » • A V E R A G E S »**«**•»* 1 25 It i s important to r e c a l l that the open s i t e results are for a small clearcut (16 hectares) where wind speeds above the snow surface were generally less than 2 meters/second. Unfortunately because of the lysimeter problems i t i s d i f f i c u l t to compare the two actual runoff rates and determine qua n t i t a t i v e l y the increase in t o t a l runoff that occurred due to the forest harvesting. It i s clear however that a larger clearcut, where wind speeds and consequently turbulent energy would be greater, would cause higher melt rates. 5.2.10 Event 5: February 12 - 16,1983 This event cannot be analysed t o t a l l y separately from the previous one, as a rigourous snow survey was not performed between the two events. A f i e l d v i s i t was done on February 12 because of the need to empty the rain gauges and the general importance of t h i s event, at which time instruments were checked. This included a cleaning of the o r i f i c e of the forest lysimeter, permitting once again the free flow of water to the tipping bucket. Unfortunately, the leak in the open lysimeter was not detected t i l l a l a t e r s i t e v i s i t , r e s u l t i n g in erroneous measurements from the open lysimeter. OPEN SITE Between February 8 and 16 the snow survey indicated a net gain of 32 mm w.e. of snow at the open s i t e . The t o t a l snowfall during t h i s period, as estimated by the p r e c i p i t a t i o n gauge and a i r temperature, was 101 mm w.e. The difference between these 1 26 Table 17 - Data Table, February 12-16,1983 M A X . O A T E C A S E S H O U R S B O M M R O ' 4 3 . 7 7 1 4 4 . 0 0 0 1 2 6 . 0 0 0 0 1 5 . 2 4 0 1 5 . 2 4 0 O . 1 5 . 0 4 2 . 3 6 9 2 2 0 . 4 4 . 0 2 1 4 5 . 0 0 0 4 8 2 4 . 0 0 0 3 2 . 5 1 2 2 8 . 7 0 2 3 . 8 1 0 0 1 6 . 6 7 B . 5 8 9 3 4 O . 4 5 . 0 2 1 4 6 . 0 0 0 4 8 2 4 . 0 0 0 3 1 . 1 9 1 9 . 4 7 4 2 2 1 . 7 1 7 3 . 0 1 4 9 . 4 0 7 0 0 O . 4 6 . 0 2 1 4 7 . 0 0 0 4 8 2 4 . 0 0 0 1 5 . 4 9 4 1 2 . 1 9 2 3 . 3 0 2 0 4 . 1 0 5 3 . 7 2 7 2 4 0 . 4 7 . 0 2 1 4 7 . 2 9 2 1 4 7 . 0 0 0 0 1 1 . 4 3 0 1 1 . 4 3 0 O . 4 . 1 0 5 3 . 2 3 3 5 6 0 . . . . . . . . . . . . . . . . . . . . W E E K L Y T O T A L S : 1 0 5 . 8 7 7 7 . 0 3 8 2 B . 8 2 9 4 2 . 9 4 6 2 . 3 2 6 4 0 . M I N . O A T E M A X . D A T E M C M T O T ' L . T O T L . T O T ' L T O T ' 4 3 . 7 7 1 4 4 . 0 0 0 . 1 3 8 0 3 . 6 3 3 7 9 1 . 2 9 0 2 2 . 5 5 3 3 1 . 2 9 0 2 2 . 5 5 3 3 2 . 5 5 3 3 2 . 5 5 3 3 4 4 . 0 2 1 4 5 . O O O . 2 1 9 2 3 . 9 0 1 5 2 3 . 3 6 9 9 6 . 2 4 1 7 3 . 3 6 9 9 6 . 2 4 1 7 6 . 2 4 1 7 6 . 2 4 1 7 4 5 . 0 2 1 4 6 . 0 0 0 . 1 4 4 1 1 . 3 4 9 8 7 1 . 0 5 5 7 3 . 9 7 5 5 1 . 0 5 5 7 3 . 9 7 5 5 3 . 9 7 5 5 3 . 9 7 5 5 4 6 . 0 2 1 4 7 . O O O . 3 4 4 5 6 . 3 0 9 1 0 3 . 0 8 3 0 6 . 7 9 4 9 3 . 0 8 3 0 6 . 7 9 4 9 6 . 7 9 4 9 6 . 7 9 4 9 4 7 . 0 2 1 4 7 . 2 9 2 . 1 0 3 6 7 . 3 4 5 3 6 . 9 3 6 3 8 2 . 0 4 6 5 . 9 3 6 3 8 2 . 0 4 6 5 2 . 0 4 6 5 2 . 0 4 6 5 • * * • W E E K L Y T O T A L S : . 9 4 9 6 0 2 . 5 3 9 6 9 . 7 3 5 2 2 1 . 6 1 2 9 . 7 3 5 2 2 1 . 6 1 2 2 1 . 6 1 2 2 1 . 6 1 2 M A X D A T E M M R L N M M R A I N F B F M M R O ' M M R L N F M R N F 4 3 . 7 7 1 4 4 . 0 2 1 4 5 . 0 2 1 4 6 . 0 2 1 4 7 . 0 2 1 4 4 . 0 0 0 4 5 . 0 0 0 4 6 . 0 0 0 4 7 . 0 0 0 4 7 . 2 9 2 . 1 4 9 1 4 1 . 6 5 9 8 . 1 5 4 7 3 1 . 7 0 2 1 . 2 5 3 8 0 W E E K L Y T O T A L S : 3 . 9 1 9 6 1 . 0 0 4 3 2 . 8 9 9 7 1 . 4 4 2 5 3 . 7 8 2 0 . 8 8 7 8 9 1 0 . 0 1 6 . 4 0 8 0 0 1 . 6 3 2 0 1 . 6 3 2 0 1 . 6 3 2 0 . 4 7 6 0 0 5 . 7 8 0 0 7 . 1 6 2 8 1 5 . 2 8 1 1 4 . 6 6 0 7 . 2 8 2 2 5 . 3 7 2 1 4 9 . 7 5 8 8 . 3 6 2 4 1 8 . 4 9 2 1 7 . 8 6 1 8 . 4 8 8 6 4 . 6 7 0 3 5 7 . 8 7 4 . 2 7 1 7 3 . 4 0 1 7 9 . 1 3 7 5 1 . 1 4 2 1 7 . 1 4 8 9 9 1 . 1 0 2 2 . 9 1 4 5 1 2 . 5 7 3 6 1 . 2 6 9 6 2 . 3 7 2 9 . 7 9 6 1 7 7 . 9 2 6 8 - . 1 1 2 7 3 - . 9 2 0 4 3 - 1 . 6 4 3 4 - . 1 5 1 7 6 - . 5 7 5 3 9 - 3 . 4 0 3 7 M I N . D A T E M A X . D A T E M T O T F ' H T E M P F H R . H . F 4 3 . 7 7 1 4 4 . 0 0 0 4 4 . 0 2 1 4 5 . O O O 4 5 . 0 2 1 4 6 . 0 0 0 4 6 . 0 2 1 4 7 . 0 0 0 4 7 . 0 2 1 4 7 . 2 9 2 • • • » W E E K L Y T O T A L S . 1 5 9 0 0 1 . 1 8 6 2 3 . 3 2 1 7 - . 5 1 8 6 4 2 . 9 7 5 4 2 . 4 0 8 5 - 1 . 5 0 5 9 1 . 4 0 7 1 1 . 2 0 4 4 - . 9 5 8 6 3 - 2 2 . 5 1 5 1 3 . 0 9 3 5 - . 4 2 6 4 0 . 9 4 5 1 6 2 . 5 2 7 9 - 2 . 3 0 1 5 9 . 0 2 9 0 1 0 0 . 6 8 3 . 0 3 0 0 8 9 . 1 2 5 1 0 1 . 1 0 2 . 1 4 1 7 8 9 . 2 7 3 1 0 1 . 4 9 1 . 0 6 0 4 8 9 . 2 8 1 9 7 . 0 6 9 1 . 9 7 2 5 8 8 . 7 4 2 9 8 . 2 7 9 2 . 2 7 0 0 8 8 . 7 2 1 . . . . . . . A V E R A G E S » • • • » « • • » * • 1 27 two values (69 mm) indicates the amount of snowmelt that occurred during this seven day period. It must be remembered that snowfall can only be roughly estimated when the a i r temperature range is in between 0.5 and 2°C and there are no .visual observations. The r e l a t i v e humidity remained above 95% during the entire period between 12 and 16 February and no evaporation from the snowpack occurred (Table 17). The sum of the energy balance computations for both half week events equals 55 mm (LTOT', Table 16 + LTOT' ,Table 17), which agrees reasonably well with the snow survey r e s u l t s . For the entire 7 day period r a i n f a l l was 249 mm, the snowmelt added to the r a i n f a l l increased the runoff by 22% over a similar r a i n f a l l only event. FOREST SITE No snow survey was caried out on February 16 at the forest s i t e as the percentage snow cover was considered too low and too unevenly d i s t r i b u t e d to obtain any sort of representative areal average. Thus no estimates of snowmelt can be made with the snow survey method. Again the mixture of snow and rain made the analysis of the forest lysimeter data unfeasible. An interception value in the area of 50% must be used i f the lysimeter data are to make any sense. As has been discussed e a r l i e r t h i s value i s too high to represent an areal average. The t o t a l snowmelt in the forest as calculated by the energy balance computations for the entire 7 day period was 20 mm, which i s the only method available for this i n t e r v a l , compared to 55 mm for the open s i t e . 128 5.2.11 Summary Of 1982-83 Results Comparative analysis of snowmelt rates and subsequent runoff, with emphasis on rain-on-snow events was performed between a forested and a clearcut plot during the 1982-83 winter. Two methods were used to measure snowmelt d i r e c t l y and were compared with results from the semi-empirical USACE (1956) energy balance equations. Direct measurement of snowmelt was .obtained using a weekly snow survey and a snowmelt lysimeter. Unfortunately frequent instrument problems associated with the lysimeters resulted in only one d i r e c t measurement during certain periods. The results of the energy balance computations generally compared favorably with the snow survey and lysimeter data for the open s i t e . Problems in determining canopy interception losses for both rain and snowfall resulted in d i f f i c u l t i e s with the interpretation of the lysimeter data. Most results showed greater melt rates during rain-on-snow events in the open than in the forest. Of greater importance than the melt rates alone is the subsequent runoff (snowmelt + r a i n ) . A l l calculations and measurements of runoff during rain-on-snow events showed greater weekly runoff occurring at the open s i t e . A summary of the results obtained from the analysis of fiv e rain-on-snow events that occurred in early 1983 is presented in Table 18. Along with the measured and calculated values of both weekly snowmelt and runoff, the percentage differences between the forest and clearcut plots of runoff are also presented. 129 Table 18 - Summary of 1983 r e s u l t s EVENT METHOD OPEN SITE FOREST SITE % INCREASE MELT (mm) RUNOFF (mm) MELT (mm) RUNOFF (mm) RUNOFF 4 + 5 EBAL 2 S.S. 3 LYSI 4 EBAL S.S. LYSI EBAL S.S. LYSI EBAL S.S LYSI 41.5 39 38 26.6 32 46. 1 60 55 69 1 1 1 68 18.4 1 7 2.8 11.8 7.0 57 27.2 230 1 70 309 20 35 1 50 35 1 measured as (open - f o r e s t ) / f o r e s t 2 EBAL=ENERGY BALANCE 3 S.S.=SNOW SURVEY * LYSI=LYSIMETER 1 30 The. results for the open s i t e were obtained from a small cleacut of 16 hectares bordered by an old growth forest where wind speeds above the snow surface were generally less than 2 meters/second. Under these conditions the turbulent energy for melting snow was low and the snowmelt process was dominated by the net radiation balance. Because wind speeds remained very low in the forest s i t e , turbulent energy fluxes were considered to be n e g l i g i b l e thus only the net radiation balance and the rain and ground heat were used to compute snowmelt in the forest. 5.3 Winter 1983-1984 Results Three main rain-on-snow events, comprised of several smaller events, were chosen to show the differences in runoff patterns between the forest and open s i t e s during rain-on-snow. These events seem to be t y p i c a l of what we have observed and measured during the winters of 1982-83 and 1983-84. The analysis of these events w i l l be divided into sub-events to f a c i l i t a t e comprehension of the d i f f e r e n t processes involved during rain-on-snow in the transient snow zone. 5.3.1 Event 1: February 9 - 14, 1984 (5 Subevents) General Weather patterns at our study s i t e , which seem to be c h a r a c t e r i s t i c of the transient snow zone during January and February, generally bring in p r e c i p i t a t i o n in the form of snow followed by increases in a i r temperatures and transformation of t h i s p r e c i p i t a t i o n to rain. This is an important observation 131 and, as s h a l l be demonstrated, explains very well the differences in d i s t r i b u t i o n and i n t e n s i t i e s of the measured runoff at both s i t e s . This seven day period saw temperatures constantly fluctuating around the 1°C mark resulting in a succession of rain and snow events (Figures 34 to 37). Figures 35-2 and 37-2 show the runoff i n t e n s i t i e s measured at both s i t e s . Although the highest peak was recorded at the open s i t e , the difference between this and the highest recorded peak in the forest i s small. As Figures 35-2 and 37-2 show, the peak runoff rates alternate between the forest and open s i t e s , with changes in p r e c i p i t a t i o n type (rain changing to snow or snow to r a i n ) . Peak runoff rates are greater from the forest when the r a i n f a l l has been immediatly preceded by snowfall. This i s explained by the melting of the snow in the canopy and i t s subsequent canopy drip, a process that s h a l l be discussed in further d e t a i l later in t h i s chapter. Figures 35 and 37 compare the measured runoff with the computed runoff, for both forest and open s i t e s . The computed runoff was obtained by adding the sum of the calculated melt rates, using the USACE (1956) equations. For the open s i t e the summation i s as follows: Calculated runoff = rainfall+M +M +M +M +M +M c e r g sw lw Where M , M , and M are melt from the ground, melt from g sw lw shortwave radiation and melt from longwave radiation respectively. M and M are constant values of 0.5 mm/day and g sw 1 32 AIR TEMPERATURE(I) Legend OPEN 60 cm FOREST 60 cm £ 100 Q ij| 8 0 X 8 0 : LU > 7 0 : <c eo-UJ r r 6 0 R E L A T I V E H U M I D I T Y & W I N D S P E E L X 2 ) r 4 S 3.6 — 7 3 C E r2.6 3 r O .5 g 0 h-I - -0 .B f C -1 < -2 RELATIVE HUMIDITY OPEN WIND 1 1 1 1 n 1 1 1 1 ii 1 1 1 I I 1 1 I I 1 1 1 1 ii 1 1 1 1 1 1 1 i i 1 1 1 1 1 M I I i ii 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ii i i i i 1 1 1 1 1 1 1 M 1 1 1 1 1 1 1 1 1 1 1 1 1 u T U R B U L E N T ENERGY(3) MELT IN OPEN SENSIBLE HEAT CONOEIWATTON EVAPORATION E o CO 1= E 0.20 0.15 0.10 0.05 0.00 -0.05 n M l h i i l i H i l M i i l n i i l i l l l M l i ill i i i i l i l i l l i | l h i l h i i m i l l i i H l M i l i i l l l l i M i i l l n l l i l l l i l R A D I A T I O N , R A I N &. T O T A L MELT(A) -1.6 £ -1.4 Q 1.2 2 -, 5 - 0 . 8 •0.6 _ 0.10 ^ E o 0.06 CO 0.00 ~ -0.06 5 MELT IN OPEN M W ftAOIATION TOTAL i i i i i i i i m i i u i 9 10 FEBRUARY 1984 Figure 34 - Measured & computed variables,Feb. 9-10,1984 1 33 P R E C I P I T A T I O N I N T E N S I T Y f l ) 2 . B -E E o 2 « E B-1.6 — z o ,10.6 Sz u 11 I 11 I I I 11 I L Y S I M E T E R RUN0FF{2) i i i i i i i i i i n i ' i i i RUNOFF OPEN SITE FOREST SITE 111111111111111111111111 TTTT?fiii Vi t if? ift m iftrfilYh r iWifl i 111 RUNOFF MEASURED CALCULATED O P E N S I T E RUNOFF(3) c E o CO E O z ^ 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 n 11111111 il ii [I Ti FumYrnWrT'iT H I T i <; 7i PiT'n'i"?rt T?I YiT"! Ti h c E o CO 1-1.5 E j E u. O 0 . 6 2 3 h o = 2 1.6 1 0 . 6 0 - f F O R E S T SITE RUNOFF(A) j d RUNOFF MEASURED CALCULATED g 1 1 1 1 II n 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 nfi i n i m 1 1 1 1 1 iTni 1 1 1 1 1 1 ii I'I I'I i 1 1 ffi fn IT H Ti Ti TI'i I'I \ \ 9 10 FEBRUARY 1984 Figure 35 - Pr e c i p i t a t i o n and runoff, -Feb. 9-10,1984 1 34 I Legend OPEN 60 cm FOREST 60 cm (- 100-2 90 D X 80-1 Ld > 7 0 ^ b < 60 _J LD AIR TEWPERATURE(I) mrnmmm • • • • • • • 11 • I • • • • • • • • • • • • • 11 • • • • i • • • • • • • • • • • ifnmnfvtfifjTf^pff^p^mvnftftfmfnnn^pnfmTHfvtvfifnnnmv RELATIVE HUMIDITY & WIND5PEED(2) RELATIVE HUMIDITY OPEN WIND tmrnmr A A. Illllllllllllllllllllllllllllllllllllll1 5 4.5 4 3.5 3 2.5 2 1.5 1 U LU cr: D t* Cr: LU CL LU MELT IN OPEN SENSIBLE HEAT CONDENSATION EVAPORATION TURBULENT ENERGY(3) E o cn \ E LU 0.25-0.20-0.15-i n i n i n m 1111 n MI n i T m i m 111 m i H iiiiiiiiiiiiiiiiiiiiTnwpitiwHwnw RADIATION, RAIN & TOTAL MELT(A) 0.5 r-0 cr: -0.5 < •1.8 ^ cn •1.6 £ •1.4 Q •1.2 ^ •1 •0.8 0.6 _ 0.10 c E o n 0 . 0 5 \ E E 0 . 0 0 w - 0 . 0 5 UJ MELT IN OPEN RAIN  RADIATION TOTAL - 0 . 0 5 -11 12 13 FEBRUARY 1984 Figure 36 - Measured & computed variables,Feb.11 -14,1984 C •-t fD 0 0 - J RUNOFF (mm/30min) RUN0FF(mm/30min) RUNOFF (mm/30min) PRECIPITATION ( m m / 3 0 m i n ) 136 1.63 mm/day respectively, as discussed in sections 5.2.5 and 5.2.2 of t h i s thesis. Because of the almost continuous p r e c i p i t a t i o n and very high r e l a t i v e humidity values throughout th i s event, the constant presence of a low cloud cover was assumed. Thus longwave radiation exchange could be estimated with the use of equation 5.17. The value of 1.25°C was chosen as the cut point to determine whether p r e c i p i t a t i o n would be considered as snow or rain . Of course in a c t u a l i t y t h i s may vary quite a b i t , and in some cases could explain the differences observed between calculated and measured runoff. At the forest s i t e , M and M were considered n e g l i g i b l e . c e M was taken as 20% of the value used for the open, as sw suggested by Federer (1971) and Deardoff (1978). R a i n f a l l rates for the forest were assumed to be 85% of those measured in the open. This 85% value i s quite a r b i t r a r y but seems to correspond well with the measured r e s u l t s . During the week of February 9 to February 14, 1984, fi v e individual rain-on-snow events occurred. Each of these w i l l be analysed i n d i v i d u a l l y . Emphasis w i l l be directed towards comparing peak and t o t a l runoff rates between the two si t e s and explaining the differences and s i m i l a r i t i e s observed between calculated and measured runoff. 1 37 5.3.2 Event 1a: February 9, 1984 This event began with r a i n f a l l , was followed by a drop in a i r temperature and a snowfall of 14 mm w.e. Following t h i s , temperature rose and r a i n f a l l resumed for several hours before the onset of colder weather and snow again on February 10 (Figure 35-1 ) . OPEN SITE Figure 35-3 compares measured runoff rates with calculated rates. As would be expected, the measured runoff in the open shows a time lag in respect to the calculated runoff. It takes a certain amount of time before the snowpack responds to instantaneous changes in energy and water inputs at the surface. This i s well i l l u s t r a t e d in Figure 35-3 ("a" on graph). Here calculated runoff drops to almost zero in response to a change from rain to snow, while measured runoff i s at a peak. During t h i s snowfall the pack continues to drain. Following this measured runoff reaches a low while calculated runoff i s on the r i s e . This corresponds to a renewal in r a i n f a l l . The double peak at "a'" (calculated runoff) r e f l e c t s an increase in p r e c i p i t a t i o n . The need to s a t i s f y the water holding capacity of the newly f a l l e n snow could explain the low i n t e n s i t i e s in measured runoff at t h i s time. The highest instantaneous measured peak for the open s i t e , for t h i s event, i s 1.62 mm/30min. The t o t a l measured runoff i s 21.2 mm, while the t o t a l calculated runoff i s 19.2 mm. The fluctuations of a i r temperature around the 1.25°C mark could e a s i l y explain the small difference observed between the 1 38 c a l c u l a t e d and measured r u n o f f . FOREST SITE Fig u r e 35-4 shows the r e l a t i v e l y good correspondence between measured and c a l c u l a t e d f o r e s t r u n o f f peaks, i f an allowance i s made fo r a time l a g . F i g u r e 35-4 at "b" i l l u s t r a t e s t h a t , although i t i s snowing i n the open, runoff i n the f o r e s t c o n t i n u e s to i n c r e a s e towards a peak, i n d i c a t i n g the p r o b a b i l i t y of canopy d r i p . The v a l l e y f o l l o w i n g the f i r s t peak, at " c ", corresponds to a drop i n a i r temperature and p r e c i p i t a t i o n i n t e n s i t y , thus a corresponding change to snow and a slowdown i n canopy d r i p . The second peak at "d" i s of s i m i l a r i n t e n s i t y as the high peak recorded at the open s i t e ( F i g u r e s 35-4, 35-2). At "e", " f " and "g" i n F i g u r e 35-4 i t i s i n t e r e s t i n g to n o t i c e the runoff i n the open stopping while the f o r e s t r u n o f f peaks s e v e r a l more times. T h i s corresponds t o peaks i n p r e c i p i t a t i o n , measured as s n o w f a l l i n the open but f a l l i n g as canopy d r i p or r a i n i n the f o r e s t . The t o t a l measured runoff from the f o r e s t s i t e f o r February 9 and 10 i s 33.5 mm while the computed runoff i s only 15.43 mm. For t h i s same p e r i o d the t o t a l p r e c i p i t a t i o n i s 55.2 mm. Th i s example, the f i r s t of s e v e r a l , demonstrates that the use of the USACE (1956) energy balance r o u t i n e i s not very e f f i c i e n t to estimate runoff from a f o r e s t e d s i t e d u r i n g r a i n -on-snow s i t u a t i o n s . The l a s t peak recorded at the f o r e s t s i t e "g" i s d e f i n i t e l y canopy d r i p . T h i s can be assumed because, durin g t h i s same p e r i o d , no r a i n f a l l i s recorded at the open s i t e . 1 39 5.3.3 Event 1b; Evening February 11, 1984 OPEN SITE In this event the time lags are again quite evident between calculated runoff and measured runoff (Figure 37-3). About 1 to 2 hours pass before changes in energy and rain inputs at the surface of the snowpack are recorded as runoff. This time lag can change substantially and depends on the qu a l i t y of the snowpack and i t s depth. Prior to t h i s event the snowpack in the open contained approximately 140 mm w.e. and was 450 mm deep. During t h i s period the a i r temperature measured at 60 cm was very close to 1.25°C and fluctuated across t h i s l i n e (Figure 36-1). This results in the jagged peaks observed in the calculated runoff (Figure 37-3) where snowfall i s alternating with r a i n f a l l . In r e a l i t y however, because temperatures were so close to 1.25°C i t i s quite possible that the p r e c i p i t a t i o n f e l l exclusively as rai n . The t o t a l measured runoff for thi s event is 7.5 mm with the peak being 1 mm/30 min. FOREST SITE Just prior to this event 61.3 mm of snowfall was registered in the open s i t e . Much of t h i s snow had most l i k e l y been caught in the forest canopy. In the afternoon of February 11 temperatures began to r i s e . At t h i s time an important runoff peak, both in intensity and duration, was registered by the forest lysimeter (Figures 37-2 and 37-4). This was caused by the combination of the rapid melting of the snow in the trees, the r a i n f a l l and the melt from the snow on the ground. As can 1 40 be observed in Figure 37-4, the calculated melt is not even close to the measured melt. This i s because the calculations do not account for the energy available at the crown l e v e l where much snowmelt may occur. The peak runoff rate for thi s event i s 2.65 mm/30min with a t o t a l measured runoff of 24.5 mm. During thi s six hour period the r a i n f a l l t o t a l l e d 3.5 mm, suggesting that 22 mm (25.4 mm - 3.5 mm) of snowmelt occurred from both the canopy and ground l e v e l s . 5.3.4 Event 1c; Morning Of February 12, 1984 OPEN SITE Considering the lag between variations in energy inputs at the snow surface and runoff measurements at the tipping bucket, the measured runoff corresponds reasonably well with the calculated runoff (Figure 37-3). The measured peak runoff i s 2.7 mm/30min, while the t o t a l measured runoff i s 31.0 mm. For the same period the t o t a l computed runoff i s 32.2 mm. This suggests once again that the USACE (1956) snowmelt equations do quite well at the open s i t e . FOREST SITE At the onset of thi s event there was probably very l i t t l e snow l e f t in the canopy, as most of i t was melted during the preceding event. In thi s s i t u a t i o n (no snow in the canopy) the computed runoff agrees well with the measured runoff, and runoff patterns at both si t e s are s i m i l a r . The open runoff tends to peak a l i t t l e higher (Figure 37-2), however the t o t a l runoffs 141 are very similar for both s i t e s . Figure 36-3 i l l u s t r a t e s that convective and condensation melt in the open i s very small. Thus, i t can be expected that runoff patterns at the forest s i t e be s i m i l a r , where these two energy inputs are also assumed to be very small. Of course t h i s only holds in conditions when there i s no snow in the canopy. The s l i g h t l y smaller peaks at the forest s i t e l i k e l y result from the e f f e c t s of canopy interception. Although the throughfall rate of 85% chosen to simulate the runoff at the forest s i t e was an a r b i t r a r y choice, i t seems to do well under conditions such as the one presented in t h i s event (Figure 37-4). 5.3.5 Event Id; Noon February 12 To Noon February 13, 1984 During this event, the lysimeter at the open s i t e responds twice to sudden r a i n f a l l (Figure 37-2 and 37-3). This indicates that the pack i s well primed with i t s l i q u i d water requirements s a t i s f i e d . The measured runoff was 10 mm while the calculated runoff was 6.5 mm. These two p r e c i p i t a t i o n events only caused a small response from the forest lysimeter. This can be explained by the concept of interception losses during small rain events. It is feasible to hypothesize that the increase in wind and temperature, and the drop in r e l a t i v e humidity at mid-day February 12 (Figures 36-1,36-2), would have somewhat dried out the canopy. This would create room for interception loss, explaining the lack of throughfall for such a small p r e c i p i t a t i o n event (3.5 mm) 142 5.3.6 Event 1e: Afternoon Of February 13, 1984 Immediately prior to this event 11 mm w.e. of snow f e l l on the old snowpack at the open s i t e . At 1500 h r a i n f a l l began. The calculated runoff at the open s i t e suggests about 2 mm of runoff. However the open lysimeter did not register any runoff during t h i s period (Figure 37-3). These results imply that the small amount of rain and snowmelt that occurred, simply served to f i l l the l i q u i d water requirements of the freshly f a l l e n snow and/or was stored in a transient state. At the forest s i t e , the lysimeter results portray a much dif f e r e n t s i t u a t i o n (Figure 37-4) . The 11 mm snow w.e. that f e l l in the canopy, during the f i r s t half of the day, melted and dripped to the forest floor in response to increased temperature and r a i n f a l l . The t o t a l p r e c i p i t a t i o n on February 13 was 14.7 mm while the measured runoff at the forest s i t e was 11 mm. This 11 mm of runoff occurred during the l a s t 11 hours of February 13 during which time there was only 3.4 mm of p r e c i p i t a t i o n . This would suggest that most of the runoff causing t h i s peak resulted from melting snow in the canopy. 5.3.7 Event 2: February 18 To 21, 1984 On the night of February 14 to 15, 27 mm of snow f e l l at the study s i t e . During the following four days temperatures were warm, there was an absence of p r e c i p i t a t i o n and daytime re l a t i v e humidities were low. Small peak runoffs were recorded from both s i t e s during mid-day on February 16. This type of weather pattern usually causes any snow caught in the canopy to 1 43 f a l l to the forest floor in clumps of dense wet snow. Thus i t i s reasonable to assume that at the onset of the r a i n f a l l of February 19 very l i t t l e snow was l e f t in the canopy. The analysis of th i s event provides interesting information as the results d i f f e r substantially from the previously analysed s i t u a t i o n . This i s due to the d i f f e r e n t antecedant conditions; a dry canopy devoid of snow prior to r a i n f a l l and a canopy storage capacity at or near maximum. OPEN SITE During t h i s event the a i r temperature rose to an average daytime high of 5°C on February 19 and 20, with mean wind speeds remaining at about 1m/s (Figures 38-1, 38-2). P r e c i p i t a t i o n i n t e n s i t i e s peaked at 2.8 mm/30 min with the t o t a l r a i n f a l l for these two days amounting to 102 mm. These atmospheric conditions provided substantial amounts of energy for snowmelt (Figure 38-3 and 38-4) with calculated t o t a l melt peaking at 0.51 mm/30min. During the peak open s i t e runoff (3.38 mm/30 min) the USACE snowmelt calculations suggest that snowmelt accounts for 15% of the t o t a l runoff (0.51/3.38). Figure 39-3 shows remarkable agreement between measured and calculated runoff for the open s i t e . As i s to be expected, there i s a time lag between the start of the r a i n f a l l and the measured runoff. In thi s case the lag time i s about four hours. As the snowpack, which includes some fresh snow, s a t i s f i e s i t s l i q u i d water deficiency, this lag time decreases to about one and one half hours. This can be observed during peak events and is most noticeable in the evening of February 20 after the main 144 Legend OPEN 60 cm FOREST 60 cm >• g => X LU > llililiiiiluimiiiiiliimilililiiiiiiimmnhil 100 80-80-70 80 60 • Mini i i i l l in i lMhi i i i i i iu i 111 III! H i l l fflffififfiiffiiinpm iliiiiiiiillliilililliiiinillllll RELATIVE HUMIDITY & WINDSPEED(2) L Tl I •»• 111 in O P E N W I N D iiuiiiiiiiii|iilmffrfinmi™iiiM 6 5 6 6 jjj 4 5 oc 4 3 3.6 3 2 5 2 1.6 1 0 6 ?O.BE -1 < < oc UJ QL UJ -1.4 -1.2 T U R B U L E N T E N E R G Y O ) MELT IN O P E N SENSIBLE MEAT CONOEJWATiON EVAPORATION 0 . 8 0 - ffiiiniiiliinw^^ c E o CO E E 1 0 8 0 6 u.20 0.16 -0 10 - 0 06 0 00 o z R A D I A T I O N , R A I N & T O T A L MELT(A) E o CO E E MELT IN OPEN BAIN FEBRUARY 1984 Figure 38 - Measured & computed variables,Feb.18-22,1984 145 F O R E S T SITE RUNOFF(4) RUNOFF MEASURED FEBRUARY 1984 F i g u r e 39 - P r e c i p i t a t i o n and r u n o f f , Feb.18-22,1984 1 46 rain event has ceased, when second and t h i r d much smaller events occur (Figure 39-3). For t h i s four day period the t o t a l calculated runoff in the open i s 138.2 mm compared to 136.8 mm recorded by the lysimeter. Again these re s u l t s support the use of the USACE snowmelt equations for use in an open area. The snow survey data provides further authentication of the r e s u l t s . The snow surveys of 14 and 23 February show a net increase in the open s i t e snowpack of 24 mm w.e. During t h i s same period the t o t a l snowfall was estimated at 62 mm w.e. This suggests a t o t a l snowmelt of 38 mm (62 mm - 24 mm). This corresponds very well with the t o t a l calculated melt of 37 mm and the lysimeter measured melt of 41 mm, for t h i s 9 day period. FOREST SITE There i s no quick r i s e , high intensity peak runoff from the forest s i t e lysimeter with the start of the r a i n f a l l (Figures 39-2 and 39-4). This suggests the absence of snow in the canopy. The 4-6 h lag time between the onset of the rain and a s i g n i f i c a n t increase in lysimeter runoff suggests that t h i s period was used to s a t i s f y the storage capacity of the forest canopy. Starting at around 2100 h on January 19, the curve of calculated runoff follows quite well the curve of the measured runoff. However, the calculated runoff i s constantly higher than the measured (Figure 39-4). For February 18 to 21 the t o t a l calculated runoff is 105 mm while the measured runoff i s 94 mm. This could mean that the use of a throughfall value of 85% i s too high. However, thi s difference of only 10% between 1 47 the measured and calculated runoff values i s quite acceptable. These results indicate that when there i s no snow in the canopy, runoff from the open s i t e , during rain-on-snow, i s substantially greater in peak flow and t o t a l runoff than at the forest s i t e . The factors that account for thi s greater runoff are as follows: 1) Greater snowmelt due to more available energy. In the open there i s more convection, condensation, radiation and rain melt. 2) Greater p r e c i p i t a t i o n reaching the snowpack. This results from the absence of interception losses caused by the forest canopy. However the conditions described in the previous event, where snow i s present in the canopy, do not represent an exceptional case. This s i t u a t i o n , which causes greater runoff from the forest, was often observed at the study s i t e and may be just as frequent as the situ a t i o n described in event 2. 5.3.8 Event 3: January 22 - 25, 1984 This four day event combines several d i f f e r e n t situations in a succession of six sub-events of 8 to 22 hours duration. Among these sub-events two show forest runoff far exceeding open runoff. These events occurred immediately after snowfall and were accompanied by a rapid r i s e in a i r temperatures, which caused canopy drip. Succeeding the second forest peak there was a sequence of three short duration events. These three events a l l had greater peak runoffs at the open s i t e . During t h i s four day period the highest peaks in the open are not s i g n i f i c a n t l y 1 48 greater than those in the forest. 5.3.9 Event 3a: Afternoon Of January 22, 1984 Prior to t h i s event 57 mm w.e.' of snow f e l l at the open s i t e . This was immediately followed by an increase in a i r temperature to a high of 6°C and a drop in r e l a t i v e humidity from 99% to 40% (Figure 40-1 and 40-2). This would suggest a probable break-up in the clouds and an afternoon of sunny weather (afternoon, January 22). These conditions caused massive canopy drip at the forest s i t e , where the peak runoff reached 2.6 mm/30min (Figure 41-2). At the open s i t e , however, there was very l i t t l e lysimeter runoff. This demonstrates that fresh snow caught in a forest canopy may be e a s i l y melted with the r e s u l t i n g d r i p quickly percolating through the shallow f o r e s t - f l o o r snowpack. However freshly f a l l e n snow on an old snowpack, as in the open s i t e , seems to need more l i q u i d water (from melt or rain) before i t s a t i s f i e s i t s l i q u i d water holding capacities and begins to drain. Although the t o t a l amount of runoff generated at the forest s i t e during t h i s event i s not extreme (20 mm), the peak runoff is quite important. However i t i s • d i f f i c u l t to determine i f such a high intensity, low duration peak has the potential to cause detrimental effects to the watershed. Another interesting, although speculative, observation may be made with the use of these re s u l t s . This concerns the snow interception capacity of the forest canopy. Of the 20 mm of runoff registered during th i s event, the f i r s t 16 mm occurred in 149 >- 100-1-Q 90-ID 80-X IVE 70-60 J < _J LU CC 50-IIIIIIIIIIIIIIIIIIMIIIIIIIMIIIIIIIIIIIIIIMIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIHIIIIIIIIIIIIIIIIIIII RELATIVE HUMIDITY & WINDSPEED T U R B U L E N T ENERGY MELT IN O P E N SENSIBLE HEAT -1 < 2 1 8 ^ 1.6 1 -0.05 - iiiiiiiiiiiiiiihiiiiiiiiiiiiiiiiiiiiiiiiiiiiii|iiiiiiiiiiiiiiiiiiiiiiniiiiii 22 23 JANUARY 1984 F i g u r e 40 - Measured & computed variables,Jan.22-25,1984 150 JANUARY 1984 o o c o o a Figure'41 - P r e c i p i t a t i o n and r u n o f f , Jan.22-25,1984 151 the absence of any p r e c i p i t a t i o n . Thus, th i s 16mm must be interpreted as snowmelt. In th i s case there are two possible sources of snowmelt; snow from the canopy and snow from the ground. Because no melt was registered from the snowpack in the open, i t seems unlikely that there would be much melt from the snowpack on the forest f l o o r , where available energy i s less than the open. Thus i t would be reasonably safe to assume that most of the registered runoff had i t s source as canopy drip. Using t h i s assumption we can estimate the canopy interception capacity of the snow as a value around 16 mm w.e. 5.3.10 Event 3b: Evening And Night Of January 22-23, 1984 OPEN SITE At 1600 h on January 22 temperatures rose and r a i n f a l l began. Seven hours l a t e r the temperature dropped and p r e c i p i t a t i o n changed to snow (Figure 40-1). The lysimeter at the open s i t e responded 4 h after the start of the r a i n f a l l , with a t o t a l runoff of 11.7 mm. The calculated runoff was 12.7 mm which i s reasonably close. Calculated peak runoff corresponded well with the measured peaks (Figure 41-3). FOREST SITE At the forest s i t e runoff had not t o t a l l y ceased before the onset of the rain, but canopy drip had dropped to a low intensity of 0.35 mm/30 min. With the presence of a certain lag the calculated runoff seems to simulate adequately the measured runoff, after the cessation of canopy drip (Figure 41-4, morning of January 23). In this sub-event calculated runoff for both 152 the forest and open s i t e corresponds well to the measured runoff. Also the peaks and t o t a l runoff recorded from the open s i t e are greater than those recorded at the forest s i t e . These two c h a r a c t e r i s t i c s seem to repeat themselves whenever i t i s raining on a canopy devoid of snow, with snow on the ground. 5.3.11 Event 3c: Night Of January 23 - 24, 1984 This event was preceded by 21 mm of snow, which ended at 1700 h on January 23. This was followed by low intensity r a i n f a l l and a r i s e in temperature to 2°C (Figure 40-1). This sequence of events resulted in high intensity runoff (2.4 mm/30min) from the forest s i t e , with very l i t t l e runoff at the open s i t e (Figure 41-2). The calculated runoff was neither able to simulate the runoff measured at the open s i t e nor at the forest s i t e (Figure 41-3 and 41-4). This i s explained at the open s i t e by two main factors: 1) Much of the p r e c i p i t a t i o n f a l l i n g during t h i s event occurred at temperatures near 1°C. While the a r b i t r a r i l y chosen cut off point suggests p r e c i p i t a t i o n was rain, i t might have been snow. 2) As has been noted previously, for the open s i t e , i t takes some time before the rain f a l l i n g on fresh snow s a t i s f i e s the l i q u i d water deficiency and percolates through the pack. During t h i s event the peak runoff at the forest s i t e was 2.4 mm/30 min, while the t o t a l runoff was 20mm and the r a i n f a l l was only 4.2 mm. At the open s i t e only 1.3 mm was measured from the lysimeter. This again demonstrates well the importance of 1 53 canopy drip, and, that peak runoffs can be generated from a forested s i t e , while a clearcut s i t e generates very l i t t l e runoff. This event also provided information to v e r i f y the hypothesis of a canopy interception capacity of 16mm of snow w.e. On January 23, 21 mm w.e. of snow f e l l at the study s i t e , some or a l l of i t was caught by the forest canopy while the rest f e l l to the ground. This was followed by a r a i n f a l l of 4.2 mm and a recorded forest runoff of 20 mm. This suggests that 15.8 (20 mm - 4.2 mm) of runoff was provided by snowmelt. As has been previously discussed the snowmelt from the ground was probably small, suggesting that most of the 15.8 mm of snowmelt originated from the forest canopy. This simple calculation supports the previously suggested canopy interception capacity of about 16 mm. 5.3.12 Events 3d, 3e And 3f; January 24-25, 1984 This series of three events ( i d e n t i f i e d by d, e and f on Figure 41-2) was generated by almost 48 hours of continuous p r e c i p i t a t i o n , a l l in the form of rai n . The p r e c i p i t a t i o n i n t e n s i t i e s during these events were highly i r r e g u l a r , with sharp peaks followed by deep valleys (Figure 41-1). It i s assumed that l i t t l e snow was l e f t in the canopy at the onset of these events, as most of i t had probably melted and dropped to the ground during event 3c. Runoff rates during these three events were continuously higher at the open s i t e compared to the forest s i t e (Figure 41-2). Peaks were s i g n i f i c a n t l y higher at the open s i t e , as were 1 54 t o t a l amounts. Event 3d In event 3d the open peak reached a le v e l of 1.85 mm/30 min, while the corresponding peak in the forest was only 0.8 mm/30 min . This can be explained by the increasing available energy at the open s i t e as the wind increases {Figure 40-2) and the absence of interception losses. The calculated runoff at the open s i t e seems to simulate well the measured peaks, however the t o t a l runoffs do not match so well. For event 3d the t o t a l calculated runoff was 21 mm while the measured runoff was 26 mm. The only reasonable explanation for t h i s difference i s that the presence of wind might have caused a difference between the catch of the pr e c i p i t a t i o n gauge and that of the lysimeter l y i n g on a 26°C slope. The calculated runoff uses the p r e c i p i t a t i o n measured at the gauge for computation. If the wind was blowing in a north-easterly d i r e c t i o n , right into the slope, the lysimeter would catch more rain per hori z o n t a l l y projected square meter of surface area than would the rain gauge. This could explain the greater values obtained from the measured runoff compared to those obtained from the calculated runoff. For event 3d the calculated forest runoff does not simulate well the measured runoff, except for one peak. However this i s reasonable and quite e a s i l y explained. The rapid and frequent changes in p r e c i p i t a t i o n i n t e n s i t i e s at the top of the forest canopy are, most l i k e l y , not registered as such at the forest f l o o r . This i s because the forest canopy, by intercepting the 1 55 r a i n f a l l , would tend to even out the p r e c i p i t a t i o n i n t e n s i t i e s . This would cause a more even d i s t r i b u t i o n of the throughfall as i t reaches the ground. The t o t a l measured forest runoff for thi s event i s 16.2 mm, while the t o t a l calculated runoff i s 12.7 mm, with a measured peak of 1.26 mm/30 min. Event 3e In event 3e the peak runoff recorded from the open s i t e was 2.7 mm/30 min compared to 1.3 mm/30 min for the forest s i t e (Figure 46-2). A l l three calculated peaks for the open s i t e are quite close to measured peaks, however t o t a l calculated runoff (11.8 mm) i s once again quite d i f f e r e n t from measured runoff (26.7 mm). The explanation for t h i s difference i s the same as previously given. The measured forest runoff follows nicely the pattern of the calculated runoff with the peaks being subdued by the presence of the canopy. It i s interesting to note that in t h i s case the measured runoff (12.5 mm) is very similar to the t o t a l calculated runoff (12.8 mm). Event 3f In the f i n a l event, 3f, the open runoff peaks at an intensity of 2.25 mm/30 min and yield s a t o t a l measured runoff of 33.4 mm (Figure 41-3). Although the wind has died down (Figure 40-2) the calculated runoff for the open s i t e i s only 20.2 mm. Most of th i s can be accounted for by the fact that although rain has ceased, runoff continues and i s draining the water kept in temporary storage. In the forest a similar sit u a t i o n exists where the measured runoff i s 18.5 mm and the 156 calculated i s only 14.2 mm. The peak runoff rate at the forest s i t e for this event is 1.4 mm/30 min . 5.3.13 Summary Of 1983-84 Results During the f a l l of 1983 improved lysimeters and flow measuring devices were i n s t a l l e d at both the open and forest s i t e s . Based on the experience of the two preceding years in monitoring rain-on-snow, several other improvements were brought to the experimental set-up. As a result of these improvements only very few instrumentation problems were encountered and most of the major rain-on-snow events were monitored successfully. It was thus possible to analyse a variety of situations with greater confidence in the r e s u l t s . A t o t a l of 12 events were analysed which varied in duration from 8 h to greater than two days. These events were analysed in greater d e t a i l than those of the previous winter. Energy balance calculations for runoff, based on the USACE equations, were compared to measured runoff on a half hourly basis, rather than on a d a i l y basis as with the 1983 data. During conditions of rain-on-snow, when no snow was caught in the forest canopy, the calculated runoff compared very favorably with the measured runoff at both s i t e s . However, when there was presence of snow in the canopy (a common event), the measured forest runoff far exceeded the calculated runoff. This was explained by the greater energy available at the top and throughout the canopy to melt the snow caught therein, than at the forest floor where the data-collecting instruments were located. In other words the instrumentation was measuring 1 57 energy sources at the ground l e v e l , when actually the greatest part of the melt was occuring above; in the forest canopy. The presence or absence of large amounts of snow in the forest canopy, prior to rain-on-snow, was i d e n t i f i e d as a major determinant of runoff i n t e n s i t i e s . Greater runoff i n t e n s i t i e s were monitored from the forest s i t e during rain events when snow was caught in the canopy, as was observed in 1983. However the 1984 analysis resulted in a much clearer demonstration of the importance of these "snow in the canopy" events. It was shown that these peaks were often no smaller than many of the peak i n t e n s i t i e s registered from the open s i t e . It was consistently noted that after a l l the snow had melted from the canopy, and r a i n f a l l continued, the peak runoffs would then be generated from the open s i t e . This was attributed to the greater energy available at the snow surface in the open s i t e and to the absence of interception losses that occur at the forest s i t e . The following table presents a summary of the 1984 re s u l t s . 1 58 Table 19 - Summary of the 1984 r e s u l t s EVENT DURATION PEAK RUNOFF INTENSITY TOTAL RUNOFF (mm) CON. 1 (hours) (mm/30 min OPEN FOREST OPEN FOREST SITE SITE SITE SITE 1a 21 .0 1 .70 1 .60 21.2 25.0 (1) 1b 8.5 1 .00 2.70 7.5 24.5 (2) 1c 11.5 2.90 2.60 31 .0 29.0 (3) Id 14.0 0.90 0.40 11.4 5.0 (3) 1e 12.0 0.10 1 .20 2.4 11.1 (2) 2 60.0 3.38 2.80 1 36.8 94.0 (4) 3a 8.5 0.00 2.61 0.0 20.0 (2) 3b 18.0 1.15 0.85 11.6 16.3 (3) 3c 12.0 0. 10 1 .90 2.3 19.8 (2) 3d 18.0 1 .90 1.15 26.3 16.2 (4) 3e 8.5 2.70 1 .20 26.7 12.5 (4) 3f 22.0 2.20 1 .30 33.4 18.5 (4) : The numbers i n t h i s column correspond to one of the f i v e c o n d i t i o n s d e s c r i b e d i n Table 1 2 - F i e l d o b s e r v a t i o n s . 159 VI. CONCLUSIONS The i n i t i a l objective of t h i s research work was to answer two main questions: 1) How does forest c l e a r c u t t i n g affect snowmelt rates during rain-on-snow events in Southwestern B r i t i s h Columbia ? 2) Can the USACE (1956) snowmelt equations be used to predict snowmelt during the above mentioned type of event? As occurs in most research work additional questions surfaced as work progressed, and the o r i g i n a l problem and i t s solutions had to be focused on from a d i f f e r e n t perspective. For example the i n i t i a l concern for snowmelt rates proved to be much less important than the quantity and timing of the t o t a l runoff, produced by the addition of rain to meltwater. Also when considering the role of the forest canopy, our attention had to be s h i f t e d from i t s effects on reducing wind v e l o c i t i e s at the snow surface, to how i t influenced quantities and timing of throughfall. Thus the problem of forest removal and i t s effects on snowmelt during r a i n f a l l became more complex than o r i g i n a l l y envisioned. The analysis of the results obtained from the open s i t e proved to be generally straightforward. The areal d i s t r i b u t i o n of p r e c i p i t a t i o n and of most energy inputs to the snowpack, such as net radiation, could be assumed to be quite uniform, because of the absence of a forest canopy. As the results have shown, the energy balance calculations in the open s i t e agreed reasonably well with the direct methods of estimating snowmelt. When rain-on-snow became an issue in B r i t i s h Columbia, 160 mostly as a result of the papers by Toews and Wilford (1978), Harr and McCorison (1979) and Harr (1981), the major concern seemed to be focused towards the increased wind v e l o c i t i e s at the snow surface that would result from forest removal. Because of low wind v e l o c i t i e s experienced at our clearcut pl o t , the calculated amounts of convective and condensation melt remained small. Thus wind did not play an important role in explaining the differences in runoff rates that occurred between the forest and the open s i t e s . In the i n i t i a l preparation of t h i s research work, l i t t l e concern was expressed for what ef f e c t s the forest canopy might have on the interception of p r e c i p i t a t i o n , be i t in the form of rain or snow. Most of the e f f o r t had been focused on methods and instrumentation needed to measure the energy balance at the snow surface. With time i t became evident that the measurement of a precise energy balance under the forest canopy was almost impossible. F i e l d observations made i t obvious that other factors were more important in explaining the differences in runoff occurring between the two s i t e s than the d i f f e r e n t energy inputs. The forest canopy played an important role in capturing snow and r a i n f a l l and transforming i t into a d i f f e r e n t state. Where, in the case of snow, i t could be transformed either to l i q u i d and f a l l as canopy dr i p or, as in the case of rain, d i r e c t l y to vapour. In most of the 1983 and 1984 events presented in t h i s thesis, snowfall occurred at a i r temperatures close to 0°C and often between 0 and 1°C. Because of the dir e c t energy supplied 161 by the forest canopy either as longwave radiation or sensible heat, the 'warm' snow f a l l i n g on the canopy was often transformed to 'rain' or caught in the branches where melting was delayed for several hours. Thus when snow was accumulating in the open, runoff was occurring in the forest. This process is thought to be the major cause for the large differences in snow accumulation patterns observed between the two s i t e s . Because of the deeper and consequently the more layered structure of the snowpack in the open, i t tended to hold more l i q u i d water in a transient state. Our data seemed to show (January 24 and 27, 1983) that af t e r a period of r a i n f a l l the snowpack in the open would suddenly " l e t go" of i t s transient water. These observations have also been noted by Gray and Male (1981). This caused greater peak flows from the open than from the forest s i t e . These sudden bursts in runoff could not be explained by the energy balance c a l c u l a t i o n s ; however, when t o t a l l e d over a seven day period the energy balance calculations agreed well with the direct measurements of snowmelt. The forest canopy also played an important role in r a i n f a l l interception. Our data suggested that in many cases over 30% of the r a i n f a l l was intercepted by the mature forest canopy and was lost to evaporation. This value i s high but not t o t a l l y inconceivable when compared to the l i t e r a t u r e . It was found that during r a i n — o n l y events the presence of a forest canopy caused a more even d i s t r i b u t i o n of the lysimeter runoff and smaller t o t a l quantities. These results were primarily attributed to canopy interception loss and canopy-caused delay 162 of throughfall. Thus through t h i s mechanism i t seems that the higher peak flows experienced in the open were avoided in the forest. This work also pointed out that rain-on-snow i s only an issue i f forest harvesting i s located in the transient snow zone. As the 1981-82 results showed, when the snowpack i s deep and the frequency of winter rain events i s low, winter runoff from these s i t e s w i l l remain small. This i s because the deep snowpack i s usually not continuously ripe and primed, and much r a i n f a l l w i l l be needed before s i g n i f i c a n t amounts of runoff w i l l occur. Because of the instrumentation problems in 1983 and the unfavorable weather in 1982, the results from these two years lack d e f i n i t e conclusions. However the improved instrumentation in 1984 and acquired experience have provided clear r e s u l t s . The answer to the f i r s t question, how does forest c l e a r c u t t i n g a f f e c t snowmelt rates (runoff rates) during r a i n -on-snow, may be Ogi'^en. in thi s fashion: Two d i s t i n c t situations may occur during rain-on-snow in the transient snow zone, d i r e c t l y influencing the generated runoff. These are the presence or absence of large quantities of snow in the forest canopy before the onset of r a i n f a l l . In the former s i t u a t i o n runoff amounts are greater under the forest canopy and peak runoff i n t e n s i t i e s are generally not much smaller than the major peaks registered in the open. In the l a t t e r s i t u a t i o n , when there i s no snow in the canopy, generally both peak i n t e n s i t i e s and t o t a l runoff are greater from the open 163 s i t e . Observations of several storms at our study s i t e suggests that p r e c i p i t a t i o n often begins in the form of snow, f i l l i n g the interception capacity of the forest canopy, followed by r i s i n g a i r temperatures with snow changing to rai n . During the f i r s t hours of thi s r a i n f a l l , the l i q u i d water requirements of the freshly f a l l e n snow are being s a t i s f i e d at the open s i t e , while massive canopy dr i p occurs in the forest. This, as has been well demonstrated, results in intense peak runoff rates in the forest while very l i t t l e runoff occurs from the,open. As r a i n f a l l continues, the runoff in the forest decreases because of the absence of snow in the canopy. At the same time the open runoff increases to surpass the forest runoff. As has been noted, the peak runoff in the open, registered after the snow in the canopy has melted, i s not usually much greater than those registered in the forest during massive canopy drip. These results seem to offer some important forest management p o s s i b i l i t i e s . We have shown that runoff rates, during rain-on-snow, can peak at d i f f e r e n t times, depending on whether they are generated from a forest or a clearcut s i t e . It would thus seem possible to set out cutting patterns that would desynchronize runoff peaks resu l t i n g in lower peak streamflows. To c l a r i f y t h i s l e t us take the often observed sequence; snowfall followed by intensive rains. The results of t h i s work suggest that the area l e f t forested would generate peak flows 10 to 12 hours e a r l i e r than the clearcut area. Under these circumstances i t seems l o g i c a l to hypothesize that the re s u l t i n g 164 peak streamflow would be lower than had the entire watershed been l e f t forested or had i t been e n t i r e l y clearcut. The answer to the second question stated in the objectives, seems now to be quite c l e a r . From the results of both the 1983 and 1984 winters, t h i s work has shown that the measured runoff in the open agrees well with the calculated runoff using the USACE (1956) snowmelt equations. However because of the phenomenon of canopy drip, and the d i f f i c u l t y in quantifying interception losses, i t i s often very d i f f i c u l t to predict forest runoff. Thus a simple analysis such as the one used by Toews and Wilford (1978) i s not adequate for the forested s i t u a t i o n . In summary, the results of t h i s work have shown that clearcut harvesting does not necessarily increase runoff during rain-on-snow. Also, the snowmelt and subsequent runoff process under the forest canopy i s very complex-s. Because of the importance that i s often associated with the downstream resources such as human habitation and f i s h survival these results could have important management implications. However many questions s t i l l need to be answered, such as : 1) How long does i t take for the regrowth to assume the same role as the mature forest in r e l a t i o n to interception and windspeed? 2) Was our forest s i t e representative of average conditions of mature forests in the transient snow zone? 3) In situations where wind is more important, does convective and condensation melt play a s i g n i f i c a n t role? 1 65 Further research should include r e p l i c a t i o n of forest plots to account for the v a r i a b i l i t y of the wind, net radiation, and p r e c i p i t a t i o n intensity under the canopy. 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R a i n f a l l interception in forest and moorland. In Forest Hydrology, (W.E.Sopper and H.W.Lull, eds.), p163-178, Pergamon, Oxford. Makkonnen,L., P.Pyder and A.K.Kemp. 1981. The heat balance of wet snow. Meteorological Magazine, 110(1304):82-84. Ma1e,D.H. and R.J.Granger. 1978. Energy mass fluxes at the snow surface in a prarie environment. In Proceedings  Modeling of Snow Cover Runoff, Colbeck and Ray, Editors, U.S. Army CRREL, pplOl-124. Ma1e,D.H. and R.J.Granger. 1981. Snow surface energy exchange. Water Resources Research, 17(3):609-627. Male,D.H. and D.M.Gray. 1975. Problems in developing a phy s i c a l l y based snowmelt model. Can. Jour. Civ. Eng. , 2:474-488. Miller,D.H. 1966. Transport of intercepted snow from trees during snow storms. U.S. Forest Service Research Paper  P.S.W.-33, Berkly, C a l i f o r n i a . 1 69 Miller,D.H. 1977. Water at the Surface of the Earth, N.Y. Academic Press 250 p. Mckay,D.C. 1978. An energy budget of a snow cover. In Proceedings Modeling of Snow Cover Runoff, Colbeck and Ray, Editors, U.S. Army CRREL, pp125-l34. Mckay,D.C. and G.W.Thurtell. 1978. Measurements of the energy fluxes involved in the energy budget of a snow cover. J.  Appl. Meteorol. ,17:339-349. McMinn,R.G. 1960. Water relations and forest d i s t r i b u t i o n in the Douglas-fir region of Vancouver Island. Div. Forest  B i o l . , Dept. Agr. Can. Pub. 1091. Obled,C.H. and H.Harder. 1979. A review of snowmelt in the mountain environment. In Proceedings Modeling of Snow  Cover Runoff, Colbeck and Ray, Editors, U.S. Army CRREL, pp179-204. Oke,T.R. 1978. Boundary Layer Climates, Methuen and co. l t d . New York, 372pp. Orloci,L. 1964. Vegetational and environmental variations in the ecosystems of the coastal western hemlock zone. unpublished Ph.D. thesis, University of B r i t i s h Columbi, Vancouver, B r i t i s h Columbia. Patric,J.H. 1966. R a i n f a l l interception by mature coniferous forests of southeast Alaska. S o i l and Water Conserv. , 21(6):229-231. Priestly,C.H.B. 1959. Turbulent Transfer in the Lower atmosphere. The University of Chicago Press. 130pp. Price,A.G. and T.Dunne. 1976. Energy balance computations of snowmelt in a subartic area. Water Resources Research, 12(4):685-694. Province of B r i t i s h Columbia 1980. Summary of Snow Measurements  in B r i t i s h Columbia 1935-1980, Ministry of the Environment, Inventory and Engineering Branch, 219pp. Pysklywec,D.W. 1966. Correlation of snowmelt with the c o n t r o l l i n g meteorological parameters, Unpublished Master's  thesis, Dept. of C i v i l Engineering, Univ. of New Brunswick, Fredricton, New Brunswick. deQuervain,M.R. 1948. Ueber den abbau der alpinen schneedecker. Int. Union Geophys, Gen. Assem. Oslo.[Vol  2,Snow and Ice], Int. Assoc. S c i . Hydrol. Publ.30, pp.55-68. 1 70 deQuervain,M.R. 1952. Evaporation from the snowpack. Snow  Investigations Research Note No.8, U.S. Army Corps Eng, North P a c i f i c Div. Portland Oregon. Roddick,J.A. 1965. Vancouver North, Coquitlam and P i t t Lake  map areas, B r i t i s h Columbia with s p e c i f i c emphasis on the  evolution of the plutonic rocks. Canada Dept. Mines and Tech. Surveys. Geol. Survey of Canada, Memoir 335, 276pp. Rothacher,J. 1973. Does harvest in the west slope Douglas-fir increase peak flows in small forest streams? USDA, P a c i f i c  Northwest Forest Range Experimental Station, Forest Service  Research Paper PNW-163, 13pp. Rutter,A.J. 1963. Studies in the water relations of Pinus S y l v e s t r i s in plantation. J . Ecol. ,51:191-203. Rutter,A.J., A.J.Morten and P.C.Robins 1975. A predictive model of r a i n f a l l interception in forests. Appl. Ecol. , 12:367-380. Smith,J.L. 1974. Hydrology of warm snowpacks and their e f f ects upon water delivery some new concepts. In Advanced  Concepts and Techniques in the Study of Snow and Ice  Resources. National Academy of Sciences, Washington D.C. pp 76-85. Sverdrup H.V. 1936. The eddy conductivity of the a i r over a smooth snow f i e l d . Geopys. Pub. , vol 4, no 7. Tetens,0. 1930. Mber einige meteorologisch B e r g r i f f e . Z. Geophys. 6:297-309. Thorn A.S. and H.R.Oliver. 1977. On Penman's equation for estimating regional evaporation. Quart. J. Roy. Met.  Soc. ,103:345-357. Toews,D.A. abd D. Wilford 1978. Watershed management considerations for operational planning on T.F.L. #39 (Blk 64). Graham Island. Fisheries and Marine Service  Manusript report no. 1473, Fisheries and Environment Canada, 32pp. USACE 1956. Snow Hydrology. U.S. Army Corps of Engineers, Portland, Oregon, 437 pp. USACE 1960. Runoff from Snowmelt. U.S. Army Corps of Engineers, Portland, Oregon, 59 pp. Zeeman, L.J. 1973. Chemistry of topospheric f a l l o u t and streamflow in a small mountainous watershed near Vancouver, B r i t i s h Columbia . Unpublished Ph.D. thesis, Faculty of  B r i t i s h Columbia, University of B r i t i s h Columbia, 154pp. 171 Zinke,P.J. 1966. Forest interception studies in the United States. In Forest Hydrology, (W.E. Sopper and H.W.Lull,eds) p 137-161. Pergamon, Oxford. Zuzel,J.F., R.N.Greenwalt and R.R.Almaras. 1983 . Rain-on-snow: transient snowpacks with frozen s o i l s . Proceedings  of the 51st Annual Western Snow Conference, Vancouver Washington, pp. 676-85. 172 APPENDIX A - LIST OF SYMBOLS (by order of appearance in text) W : cold content of snow c F'' : l i q u i d water holding capacity of snow P S : l i q u i d water requirements of the snowpack P Q : energy flux due to radiation, conduction or i convection mh : energy transfer due to p r e c i p i t a t i o n dV/dt or Q : change in internal energy of the snowpack 9 Q : net radiation transfer n Q : latent heat transfer e Q : sensible heat transfer h Q : transfer of heat from rain water r Q : heat transfer across the snow s o i l interface 9 Q : energy used to melt snow m K : shortwave radiation z : v e r t i c a l distance A : extinction c o e f f i c i e n t for shortwave radiation c : s p e c i f i c heat of a i r P p : a i r density 173 q r s p e c i f i c humidity q' : fluctuating component of the s p e c i f i c humidity T' : fluctuating component of the a i r temperature w' : v e r t i c a l component of the wind j3 : Bowen's r a t i o L : Latent heat of sublimation s P : a i r pressure a T : a i r temperature at height a a T : surface a i r temperature o e : vapour pressure at height a a e : surface vapor pressure o K : eddy d i f f u s i o n c o e f f i c i e n t for momentum m K : eddy d i f f u s i o n c o e f f i c i e n t for sensible heat h K : eddy d i f f u s i o n c o e f f i c i e n t for water vapour e T : surface shearing stress du/dz : mean gradient of the momentum flux dT/dz : mean gradient of the sensible heat flux dp /dz : mean gradient of the latent heat flux v u*x? : f r i c t i o n v e l o c i t y k : Von Karman's constant u : wind speed T : temperature 174 D : bulk transfer c o e f f i c i e n t for latent heat e D : bulk transfer c o e f f i c i e n t for sensible heat h e : vapour pressure e' rsaturation vapour pressure e : base of Naperian logarithms s.m.w.e. : snow melt water equivalent s.w.e. : snow water equivalent w.e. : water equivalent R * : net longwave radiation exchange lw a : the Stephan-Boltzman constant E : evaporation from a wet surface wet 7 : pyschometric constant A : slope of the saturation vapor pressure versus temperature curve r : aerodynamic resistance a From the U.S.A.C.E. equations P : a i r pressure at the measurement location P : a i r pressure at sea le v e l Z and Z : heights of measurement above the snow surface a b T : a i r temperature a T : snow temperature s v : wind speed b 175 e : vapour pressure a e : snow surface vapour pressure s P : d a i l y r a i n f a l l r M : snowmelt from convective energy c M : snowmelt from condenstion energy e M : snowmelt from the transfer of rain heat r e* : saturated vapour pressure R.H. : r e l a t i v e humidity M : snowmelt rate 1 76 APPENDIX B - THE POINT SNOWMELT EQUATIONS FROM USACE (1956) The six natural sources of heat in melting snow are : a) absorbed solar radiation, H r s b) net longwave radiation, H r l c) covective heat transfer from the a i r , H c d) latent heat of vaporization by condensation from the a i r , H e e) conduction of heat from the ground, H 9 f) heat content of rain water, H P The summation of net exchange from a l l sources of heat represents the amount of energy available for melting the snowpack, and may be expressed by the general formula: M = I H/203B in which M i s snowmelt in inches of water equivalent, I H i s the algebraic sum of a l l heat components, in c a l o r i e s per square centimeters, and B i s the thermal qualit y , which i s equal to the r a t i o of heat required to melt a unit weight of the snow to that of ice at 0°C. The constant 203 i s the number of c a l o r i e s per square centimeter required to melt 1 inch of w.e. of ice at 0°C. (80cal/g*2.54cm/inch) Shortwave radiation The melt equivalent for shortwave radiation, in inches per day i s : M =(1-a)l /203B rs i where a i s the albedo expressed as a decimal f r a c t i o n and I i s the d a i l y incident solar radiation in langleys. for a melting mountain snowpack where thermal qua l i t y is assumed to be .97: M =0.00508(1 (1-a)) rs i Longwave radiation: 1 77 For clear sky conditions the net exchange by longwave radiation (R ) i s : n R =0.76aT 11-0 . 459 (Ly/min ) n a for low clouds or under the forest: R =CTT *-0.459(Ly/min) n a For clear skies in the open, the s i m p l i f i e d formula for heat exchange to the melting snowpack by longwave radiation can be expressed as : M =0.0212(T -32)-.84 r l a where T i s the a i r temperature (°F) 10 feet above the snow, a Daily snowmelt under he forest canopy or condition with complete cloud cover for a melting snowpack: M =0.29(T -32) r l a Covection melt The melt of a ripe snpowpack by convection : M =.00629(P/P )(Z Z (T -T ) (v ) c o a b a s b Condensation melt: Melt from the energy released when vapour condenses on the snowpack: M =.054(Z Z )" 1A ( e -e )v e a b a s b Rain melt Snowmelt by the transfer of heat from ra i n : M =.007 P (T -32) p r a where P and P are the a i r pressures at the location and at sea 0 l e v e l , respectively, Z and Z are the heights of measurement in a b feet above the snow surface, of a i r temperature and wind speed respectively, T is the a i r temperature in °F, T i s the snow a s surface temperature in °F (32°F when melting), v i s the wind b speed in miles per hour, e is the vapour pressure in mb, e is a s the snow surface vapour pressure in mb (6.11 mb for a melting snow surface) and P i s the da i l y r a i n f a l l in inches. r 1 78 APPENDIX C - USACEQ956) BASIN SNOWMELT EQUATIONS The g e n e r a l equation f o r t o t a l b asin melt d u r i n g r a i n : t o t a l melt, M, i s expressed by the r e l a t i o n s h i p : M=M +M +M +M +M rs r l ce g p a) f o r open or p a r t l y f o r e s t e d basin areas : M=(0.029+0.0084kv+0.007P )(T -32)+0.09 r a b) f o r h e a v i l y f o r e s t e d areas, M=(0.074+0.007P )(T -32)+0.05 r a where T i s the mean temperature of s a t u r a t e d a i r at the 10 foot a l e v e l i n °F, v i s the mean windspeed at the 50 foot l e v e l i n miles per hour, P i s the rate of p r e c i p i t a t i o n i n inches per r day and k i s a bas i n constant, which represents the mean exposure of the bas i n or segment thereof to wind, c o n s i d e r i n g topographic and f o r e s t e f f e c t s . The g e n e r a l equations f o r bas i n snowmelt d u r i n g r a i n - f r e e  p e r i o d s : H e a v i l y f o r e s t e d area : M=0.074(0.53T '+0.47T ') a d F o r e s t e d area: M=k(0.0084v)(0.22T '+0.78T ')+0.029T ' a d a P a r t l y f o r e s t e d a r e a : M=k'(1-F)(0.000401 ) (1-a)+k(0.0084v) i (0.22T '+0.78T ')+F(0.029T ') a d a Open area M=k'(0.005081 )(1-a)+(1-N)(0.0212T '-0.84)+N(0.029)T ')+ 1 79 k(0.0084v)(0.22T '+0.78T ') a d where : T ' : i s the difference beween the a i r temperature a measured at 10 feet and the snow surface temperature, in ° F. T ' : i s the difference between the dew point temperature d measured at 10 feet and the snow surface temperature, in 0 F. k' : i s the bassin shortwave radiation melt factor. It depends upon the average exposure of the open areas to shortwave radiation in comparison with an unshielded horizontal surface. F : i s the estimated average basin forest canopy cover, e f f e c t i v e in shading the area from solar radiation, expressed as a decimal f r a c t i o n . T ' : i s the difference between the cloud base temperature c and snow surface temperature, in 0 F. It is estimeted from upper a i r temperatures or by lapse rate from surface station, preferably on snow free s i t e . N : Is the estimated cloud cover. 180 APPENDIX D - SNOWMELT CALCULATIONS USgJ) $Y TOEWS AND WILFORD ( 1978) (adapted from tfoews and VJilford 1978) ( Equations from Carps of Engineers, 1956 Ibtal snovmelt M = Mrs + Mrl + Mc + Me+Mr + Mg where M - total melt Mrs - melt resulting front short wave radiation Mrl - melt resulting from long wave radiation Mc - malt resulting from exchange of heat between atmosphere and snow (convection) Me - melt resulting from condensation of water vapour into snowpack Mr - melt resulting frcm heat frcm rain Mg - melt resulting from heat from the ground Based on work by the Corps of Engineers (1956) Mrs and Mg are small (average 0.07 in/day and 0.02 in/day respectively) and for the purpose of these c a l -culations w i l l be considered to be negligible. The various components for melt during a storm w i l l be compared far a forested and clearcut plot. The main difference in melt rates results from the d i f -fering wind speeds. The time period i s one day. Assume a winter storm with the following conditions: - 1 inch rain in 24 hours - 40°F. (4.44^C) - Average temperature - Snowpack throughout the watershed with a minimum of 5 inches water equivalent - Snow i s ripe (i.e., consists of large crystals) and transmits water rapidly - Mean wind velocity above forest canopy i s 30 mph (at 30 ft.) - Saturated vapour pressures at 32°F and 40? and 6.11 mb and 8.02 mb respectively - Relative humidity i s 95% and vapour pressure of overlying a i r w i l l then be 7 . 6 2 mb 181 The 30mph wind at 30ft. was calculated to be reduced to 12 mph ( at 30 f t . ) within the forest canopy. Ihis reduction of 60% i s within the range of 40 - 90% suggested by various authors (see l i s t below). A 0.55 correction factor, as suggested by the Corps of Engineers (1956) was used to calculate the wind speed at the 1 f t . level. Snowmelt from a Forested plot Radiation Melt Long Wave Radiation Melt Mrl = 0.029 (Ta - Us) where Mrl - long wave radiation melt i n inches Ta - air temperature i n degrees F Ts - temperature of melting snowpack (assume 32^F) Mrl = 0.029 (40 - 32) = 0.23 in (0.59 cm) Convection Melt Mc = 0.00629 (Ti - TS) Vi where Mc - melt i n inches Ti - air temperature Ts - temperature of melting snowpack (assume 32^F) Vi - wind velocity in mph at one foot level Mc = 0.00629 (40 - 32) (0.55 x 12) = 0.33 i n (0.84 cm) Condensation Melt Me = 0.054 (ei - es) Vi where Me - melt i n inches e i - vapour pressure of overlying air es - vapour pressure at the snow surface Vi - wind velocity i n mph at one foot level Me = 0.054 (7.62 - 6.11) (.55 x 12) = 0.54 inches (1.37 cm) 182 Rain Melt Mr = 0.00695 (Tr - 32) Pr where Mr - rain melt (inches) Tr - temperature of rain (°F) Pr - rainfall (inches) Mr = 0.00695 (40 - 32) 1 = 0.05 inches (0.13 cm) Precipitation = 1 inch (2.54 cm) as rain Snowmelt from Clearcut plot Radiation Melt Mrl = 0.23 in (0.59 cm) (similar to forested plot) (some sources suggest that radiation melt may be slightly greater in a forested plot because of long wave radiation from the forest canopy) Convection Melt Mc = 0.00629 (Ti - Ts) Vi Mc = 0.00629 (40 - 32) (30 x 0.55) = 0.83 inches (2.11 cm) Condensation Melt Me = 0.054(ei - es) Vi = 0.054 (1.51) (.55 x 30) •= 1.34 inches (3.42 cm) Rain Melt Mr = .05 in (0.13 cm)(similar to forested plot) Precipitation = 1 inch (2.54 cm) 183 Total Melt Forested Plotfc, m) Clearcut Plot (c Mrl 0.59 0.59 Mc 0.84 2.11 Me 1.37 3.42 Mr 0.13 0.13 M (Total melt) 2.93 . 6.25 Rain 2.54 2.54 Total Rain and melt 5.47 8.79 

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