@prefix vivo: . @prefix edm: . @prefix ns0: . @prefix dcterms: . @prefix skos: . vivo:departmentOrSchool "Land and Food Systems, Faculty of"@en ; edm:dataProvider "DSpace"@en ; ns0:degreeCampus "UBCV"@en ; dcterms:creator "Curtis, John Reeves"@en ; dcterms:issued "2010-01-28T20:49:39Z"@en, "1975"@en ; vivo:relatedDegree "Master of Science - MSc"@en ; ns0:degreeGrantor "University of British Columbia"@en ; dcterms:description "Evapotranspiration measurements using the Bowen ratio/energy balance technique were, made over a dry Douglas fir forest. Soil water matric potential over the period of measurement varied from 0 to -10.5 bars. With the premise that the equilibrium evaporation rate associated with adequate water supply essentially expresses evapotranspiration as a fraction of daily net radiation, the ratio of latent heat flux to net radiation for 24-hour periods is examined as a function of soil water matric potential. This ratio, is reduced to approximately half its maximum value as the matric potential approaches -10 bars for the experimental site. The reduction is not as great-as that implied by changes in soil water storage, indicating that evapotranspiration of water not originating in the root zone is carried on during periods of considerable water stress. While some water may be flowing upward from below the root zone, it is felt that over half of the daily evapotranspired water is released from storage in tree stems during the most severe period."@en ; edm:aggregatedCHO "https://circle.library.ubc.ca/rest/handle/2429/19248?expand=metadata"@en ; skos:note "EVAPOTRANSPIRATION FROM A DRY DOUGLAS FIR FOREST by JOHN REEVES CURTIS B . S c , M c G i l l U n i v e r s i t y , 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department o f S o i l S c i e n c e We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1975 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 D a t e Aon/ 1^7^ - i i -ABSTRACT E v a p o t r a n s p i r a t i o n measurements u s i n g the Bowen r a t i o / e n e r g y b a l a n c e t e c h n i q u e were, made o v e r a dry Douglas f i r f o r e s t . S o i l w ater m a t r i c p o t e n t i a l over the p e r i o d o f measurement v a r i e d from 0 t o -10.5 b a r s . With t h e premise t h a t t h e e q u i l i b r i u m e v a p o r a t i o n r a t e a s s o c i a t e d w i t h adequate water s u p p l y e s s e n t i a l l y e x p r e s s e s e v a p o t r a n s p i r a t i o n as a f r a c t i o n o f d a i l y net r a d i a t i o n , the r a t i o o f l a t e n t h eat f l u x t o net r a d i a t i o n f o r 24-hour p e r i o d s i s examined as a f u n c t i o n o f s o i l w a ter m a t r i c p o t e n t i a l . T h i s r a t i o , i s reduced t o a p p r o x i m a t e l y h a l f i t s maximum v a l u e as the m a t r i c p o t e n t i a l approaches -10 b a r s f o r t h e e x p e r i m e n t a l s i t e . The r e d u c t i o n i s not as g r e a t - a s t h a t i m p l i e d by changes i n s o i l w a ter s t o r a g e , i n d i c a t i n g t h a t evapo-t r a n s p i r a t i o n o f wa t e r not o r i g i n a t i n g i n t h e r o o t zone i s c a r r i e d on d u r i n g p e r i o d s o f c o n s i d e r a b l e w a t e r s t r e s s . W h i l e some water may be f l o w i n g upward from below the r o o t zone, i t i s f e l t t h a t o v er h a l f o f the d a i l y e v a p o t r a n s p i r e d water i s r e l e a s e d from s t o r a g e i n t r e e stems d u r i n g t h e most severe p e r i o d . - i i i -TABLE OF CONTENTS Page ABSTRACT i i LIST OF FIGURES i v ' ACKNOWLEDGEMENTS v i LIST OF SYMBOLS v i i INTRODUCTION 1 EXPERIMENTAL PROCEDURE 4 A. Experimental S i t e 4 B. Experimental Methods 6 RESULTS AND DISCUSSION 10 A. Soil Water Content and Matric Potential 10 B. Daytime Course of Net Radiation and Latent Heat Flux 13 C. The Relationship between LE/R and i> 2 4 n rm D. Evapotranspiration Estimates Using R and i> data 32 ° n rm E. Water Balance of the. Forest 34 CONCLUSIONS 40 REFERENCES ' 43 APPENDIX I 45 APPENDIX I I 4 7 - i v -LIST OF FIGURES F i g u r e 1. Water r e t e n t i o n c u r v e f o r Dashwood s e r i e s . g r a v e l l y sandy loam. Each p o i n t i s t h e average water c o n t e n t 0 o f the 15, 30., 45 and 60 cm depth samples a t a s i n g l e . v a l u e o f s o i l w a ter m a t r i c p o t e n t i a l Bars• i n d i c a t e range o f water .contents. F i g u r e 2. S o i l w ater m a t r i c p o t e n t i a l ^ f o r June;.17-August 19, 19 74. Each p o i n t i s t h e average v a l u e o f t h e m a t r i c p o t e n t i a l s a t t h e 15, 30 and 4 5 cm d e p t h s . F i g u r e 3. Average v o l u m e t r i c water c o n t e n t f o r June 20-August 20 o f t h e 0-60 cm depth o f s o i l from g r a v i m e t r i c s a m p l i n g . F i g u r e 4. D i u r n a l t r e n d i n wet- and d r y - b u l b t e mperature d i f f e r e n c e s AT W and AT r e s p e c t i v e l y o v er 1. m h e i g h t above the canopy f o r June-18 , 1974. Note t h e r e v e r s i n g o f t h e g r a d i e n t s around 0800 PST. F i g u r e 5. Daytime c o u r s e o f s u r f a c e r e s i s t a n c e r (from energy b a l a n c e measurements)and C s t o m a t a l r e s i s t a n c e r s ( p r o j e c t e d l e a f a r e a b a s i s ) f o r June 18 and J u l y 25, 1974. The 'disagreement o f t h e two v a l u e s i s l a r g e on a p e r c e n t a g e b a s i s i n t h e morning. F i g u r e 6a. D i u r n a l t r e n d i n R n and LE f o r June 18, 1974. S o l i d d o t s are energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measurements. The s o l i d l i n e f o r LE was drawn by eye. F i g u r e 6b. D i u r n a l t r e n d i n R n and LE f o r J u l y 25, 1974. S o l i d d o t s are energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measurements. The s o l i d l i n e f o r LE was drawn by eye. F i g u r e 7. F i g u r e s 6a and 6b t o g e t h e r t o i l l u s t r a t e t h e e f f e c t s o f d e c r e a s e d s o i l water m a t r i c p o t e n t i a l i j j m on LE. Note t h a t R n i s h i g h e r on June 18. F i g u r e 8. R n and LE on J u l y 19, t h e . ' f i r s t c l e a r day a f t e r the r a i n y p e r i o d i n m i d - J u l y . The m i d - a f t e r n o o n peak i n LE i s p r e s e n t , but not pronounced. - V -F i g u r e 9. R and LE on J u l y 8, t h e l a s t day o f the f i r s t d r y i n g p e r i o d , and t h e most se v e r e o b s e r v e d day from t h e a s p e c t o f wa t e r s t r e s s . LE/R n was o n l y 0.26. F i g u r e 10. LE/R n as a f u n c t i o n o f ' s o i l w a t e r m a t r i c p o t e n t i a l ^ m f o r t e n s e l e c t e d f i n e days. The number a d j a c e n t t o each p o i n t i s t h e dat e . F i g u r e 11. LE/LE eq- as a f u n c t i o n o f i j j , m where L E e q r e p r e s e n t s t h e d a i l y e q u i l i b r i u m e v a p o r a t i o n r a t e . D i f f e r e n c e s i n t h i s c u r v e and F i g u r e 10 are due o n l y t o d i f f e r e n c e s i n d a i l y mean t e m p e r a t u r e s . F i g u r e 12. L E / L E m a as a f u n c t i o n o f > m ( l o g a r i t h m i c s c a l e ) \"for v a r i o u s a u t h o r s ( t a k e n from van B a v e l , 1967). Note t h a t t h e maximum e v a p o . t r a n s p i r a t i o n r a t e L E m a x i s det e r m i n e d d i f f e r e n t l y f o r d i f f e r e n t a u t h o r s , and i n t h i s s t u d y i s t a k e n as t h e e q u i l i b r i u m r a t e . F i g u r e 13. LE/R n as a f u n c t i o n o f v o l u m e t r i c water c o n t e n t . T h i s and F i g u r e 10 a r e r e l a t e d t h r o u g h t h e water r e t e n t i o n c u r v e f o r t h e s o i l , and a l l o w e v a p o t r a n s p i r a t i o n e s t i m a t e s from water c o n t e n t measurements.to be made. F i g u r e 14. D a i l y t o t a l s o f R R and LE f o r June 17-August 14, 1974 . P o i n t s used t o . - o b t a i n F i g u r e 10 are shown as t r i a n g l e s . F i g u r e 15. Water b a l a n c e o f t h e 0-60 cm depth o f s o i l f o r June 17-August 14, 1974. V a l u e s a re ' d a i l y averages i n mm o f water. The r e s i d u a l t e r m i s d i s c u s s e d i n the t e x t . F i g u r e S o i l water m a t r i c p o t e n t i a l a t \"four depths f o r June 17-August 19, 1974, i n d i c a t i n g d i r e c t i o n o f g r a d i e n t s i n t h e v e r t i c a l a x i s . - v i -ACKNOWLEDGEMENTS I cannot hope t o f u l l y e x p r e s s my a p p r e c i a t i o n o f the guidance,:. i n s i g h t , and e n t h u s i a s m o f my s u p e r v i s o r , Dr. T.A. B l a c k , i n a l l phases o f my M a s t e r ' s s t u d i e s . I s i n c e r e l y f e e l t h a t t h e a c c o m p l i s h -ments r e p r e s e n t e d i n t h i s work are every b i t as much h i s as mine. A p a r t i c u l a r debt o f g r a t i t u d e i s owed t o Mr. Joe Nnyamah, whose m e t i c u l o u s and r e l i a b l e work i n the ' f i e l d o f s o i l m o i s t u r e measurements formed an i n d i s p e n s a b l e c o r n e r s t o n e o f t h i s s t u d y . I w i s h t o thank Mr. P a u l Tang and Mr...Ron Toth f o r t h e i r key r o l e s i n t h e s u c c e s s o f t h e i n s t r u m e n t a t i o n and d a t a l o g g i n g . I would e s p e c i a l l y l i k e t o e x p r e s s thanks t o Mr. C.S. Tan and M i s s Ruth Hardy f o r t h e i r s u p p o r t i n t h e f i e l d and f o r t h e i r f r i e n d s h i p , d i s c u s s i o n , and c r i t i c i s m i n t h e o f f i c e . I am a l s o i n d e b t e d t o t h o s e who s e r v e d on my committee; Drs. C.A. Rowles, T.M. B a l l a r d , T.R. Oke and R.A. F r e e z e . Thanks a r e due t o Mrs. Retha Gerstmar, who took t h e t i m e t o t y p e t h i s t h e s i s . Funds f o r t h i s r e s e a r c h were made a v a i l a b l e t h r o u g h g r a n t s from Canada Department o f t h e Environment (CFS and NCWRR), and from t h e N a t i o n a l Research C o u n c i l o f Canada. - v i i -LIST OF SYMBOLS D Drain a g e E E v a p o t r a n s p i r a t i o n G S o i l h eat f l u x d e n s i t y L L a t e n t heat o f v a p o u r i z a t i o n LE L a t e n t heat f l u x d e n s i t y LE E q u i l i b r i u m l a t e n t heat f l u x d e n s i t y eq ^ J M S t o r a g e o f s e n s i b l e and l a t e n t heat w i t h i n the canopy R n Net r a d i a n t energy f l u x d e n s i t y v.p.d. Vapour p r e s s u r e d e f i c i t AT D r y - b u l b temperature d i f f e r e n c e o v e r h e i g h t AT w Wet-bulb te m p e r a t u r e d i f f e r e n c e o v e r h e i g h t A9 Change i n s o i l w a ter s t o r a g e g Bowen r a t i o 9 V o l u m e t r i c s o i l w a ter c o n t e n t S o i l w ater m a t r i c p o t e n t i a l m r INTRODUCTION E v a p o t r a n s p i r a t i o n i s an i m p o r t a n t t e r m i n b o t h the water b a l a n c e and t h e energy b a l a n c e o f a f o r e s t . However, t h e c o m p l e x i t y o f t h e s u r f a c e f e a t u r e s and p h y s i c a l .geometry, as w e l l as the sheer s i z e o f a f o r e s t canopy, makes t h e a p p l i c a t i o n , o f t h e o r e t i c a l e x p r e s s i o n s f o r e v a p o t r a n s p i r a t i o n a d i f f i c u l t t a s k f o r t h e f o r e s t h y d r o m e t e o r o l o g i s t . N o n e t h e l e s s , r e s e a r c h e r s have been a c t i v e l y s e e k i n g a means o f making u s e f u l e s t i m a t e s o f e v a p o t r a n s p i r a t i o n t a k i n g i n t o a c c o u n t t h e b a s i c m e t e o r o l o g i c a l and h y d r o l o g i c a l parameters o f a v a i l a b l e energy and a v a i l a b l e w a t e r i n a p r a c t i c a l way. I f p o t e n t i a l e v a p o t r a n s p i r a t i o n were t a k e n t o be the e v a p o t r a n s p i r a t i o n when f e t c h and s o i l w a ter a r e adequate, t h e n t h i s p a rameter i s o f n e c e s s i t y a m e t e o r o l o g i c a l one. I t has been shown t h a t t h i s i s a p p r o x i m a t e l y e q u a l t o t h e \" e q u i l i b r i u m \" r a t e (McNaughton, 1974) f o r a w e s t - c o a s t f o r e s t environment. When used as an e s t i m a t e o f e v a p o t r a n s p i r a t i o n under s u i t a b l e c o n d i t i o n s , the e q u i l i b r i u m model i s p h y s i c a l l y j u s t i f i e d and e x t r e m e l y s i m p l e t o a p p l y , s i n c e i t r e p r e s e n t s e s s e n t i a l l y a s l i g h t l y temperature-dependent f r a c t i o n o f the d a i l y net r a d i a t i o n . - 2 -The s i m p l i c i t y o f t h e e q u i l i b r i u m model i s due i n p a r t t o t h e f a c t t h a t i t a p p l i e s o n l y t o a w e l l - w a t e r e d system, and i t a v o i d s t h e q u e s t i o n o f an e v a p o t r a n s p i r a t i o n r a t e w i t h s o i l m o i s t u r e and p h y s i o l o g i c a l parameters t a k e n i n t o a c c o u n t . A t h e o r e t i c a l m o d i f i c a t i o n o f t h e e q u i l i b r i u m model a p p l y i n g t o a l l c o n d i t i o n s o f s o i l m o i s t u r e and p l a n t water s t r e s s would r e n d e r i t u n w i e l d y from a p r a c t i c a l p o i n t o f view. However, t h e f r a c t i o n o f d a i l y net r a d i a t i o n r e p r e s e n t e d i n t h e e q u i l i b r i u m r a t e p r o v i d e s a u s e f u l v e h i c l e f o r a s t r a i g h t f o r w a r d study o f t h e r e d u c t i o n i n e v a p o t r a n s p i r a t i o n under c o n d i t i o n s o f water s t r e s s . Some e a r l i e r s t u d i e s o f t h e r e d u c t i o n o f e v a p o r a t i o n a s s o c i a t e d w i t h water s t r e s s i n a g r i c u l t u r a l p l a n t s were r e p o r t e d by Denmead and Shaw (19 62) and Gardner and E h l i g (1963). F u r t h e r s t u d i e s on c r o p s c a r r i e d - o u t : . i n - . \" f i e l d p l o t s were made by.a number o f o t h e r workers (van B a v e l , 19 67; Denmead and M c l l r o y , 1970; Davie s and A l l e n , 197 3; R i t c h i e , 1973). Only a few have l o o k e d a t t h e problem r e l a t e d t o f o r e s t v e g e t a t i o n , and t h e s e have been p r i m a r i l y on s e e d l i n g s i n t h e l a b o r a t o r y ( Z a t i k o v s k y and F e r r e l , 1968; L o p u s h i n s k y and K l o c k , 1974), owing t o t h e d i f f i c u l t y i n making.in situ measurements o f evapo-t r a n s p i r a t i o n o f a f o r e s t s t a n d . - 3 -The f a c t t h a t such a study o f a f o r e s t s t a n d must u l t i m a t e l y be made i n t h e f i e l d , t o g e t h e r w i t h t h e c o n f i d e n c e g a i n e d i n t h e t e c h n i q u e and a p p a r a t u s used i n t he Bowen r a t i o approach ( B l a c k and McNaughton, 1971), p r o v i d e s t h e b a s i s f o r t h e o b j e c t i v e s o f t h i s s t u d y . These o b j e c t i v e s can be summarized as f o l l o w s : 1. To determine whether t h e e v a p o t r a n s p i r a t i o n r a t e a t t h e e x p e r i m e n t a l s i t e when water i s not l i m i t i n g i s e q u a l t o t h e e q u i l i b r i u m r a t e , t h e r e b y p r o v i d i n g a f u r t h e r check on the r e s u l t s p r e v i o u s l y o b t a i n e d i n a west-c o a s t Douglas f i r s t a n d (McNaughton and B l a c k , 1973). 2. To measure e v a p o t r a n s p i r a t i o n from a dry Douglas f i r s t a n d , and observe t h e e f f e c t o f d e c r e a s i n g s o i l m o i s t u r e on t h e f r a c t i o n o f d a i l y net r a d i a t i o n used i n e v a p o r a t i n g w a t e r . D i u r n a l t r e n d s i n t h e energy b a l a n c e p r o v i d e some i n s i g h t i n t o the p r o c e s s e s c o n t r o l l i n g e v a p o t r a n s p i r a t i o n , and t h e r e f o r e w i l l be touched upon. N e v e r t h e l e s s , t h e emphasis o f t h i s s t u d y w i l l be upon d a i l y changes i n t h e e v a p o t r a n s p i r a t i o n component o f the water b a l a n c e , and i t s r e l a t i o n t o d a i l y changes i n s o i l w a ter s t a t u s and net r a d i a t i o n . - 4 -EXPERIMENTAL PROCEDURE A. Experimental Site S e l e c t i o n o f a s i t e f o r t h i s s t u d y was c o n t i n g e n t upon the n e c e s s i t y f o r an a r e a which c o u l d be c o n s i d e r e d s i m p l e from a m i c r o m e t e o r o l o g i c a l p o i n t o f vi e w , and y e t one which r e p r e s e n t s a n a t u r a l environment from which p r a c t i c a l c o n c l u s i o n s c o u l d be drawn. C o n s i d e r a b l e e f f o r t was t h e n put i n t o f i n d i n g an e x i s t i n g f o r e s t s t a n d which met the f o l l o w i n g c r i t e r i a : 1. The s i t e must e x p e r i e n c e water s t r e s s d u r i n g the summer. 2. The s i t e must be f r e e o f a d v e c t i v e e f f e c t s , r e q u i r i n g an e x t e n s i v e f o r e s t e d a r e a o f r e a s o n a b l y u n i f o r m v e g e t a t i o n , f l a t t o p o g raphy, and good f e t c h . 3. The st a n d s h o u l d be p r i m a r i l y Douglas f i r . 4. The s i t e s h o u l d be a c c e s s i b l e by r o a d . The s i t e chosen was on Crown Z e l l e r b a c h Company p r o p e r t y a p p r o x i m a t e l y 17 m i l e s n o r t h w e s t o f Co u r t e n a y , B. C., on t h e e a s t e r n c o a s t o f Vancouver I s l a n d . The s i t e met most o f t h e r e q u i r e m e n t s f o r t h e study from both a m i c r o m e t e o r o l o g i c a l and an o p e r a t i o n a l p o i n t o f view. The measurements were made i n an u n t h i n n e d - 5 -sta n d o f Douglas f i r p l a n t e d i n 1953, l o c a t e d on the r e l a t i v e l y e x t e n s i v e c o a s t a l p l a i n between Courtenay and Campbell R i v e r . The wet, m i l d w i n t e r s and d r y , warm summers e x p e r i e n c e d by the a r e a p r o v i d e d t h e range o f s o i l m o i s t u r e needed f o r the s t u d y . The topography i s g e n e r a l l y f l a t , a l t h o u g h t h e r e a r e s e v e r a l r i d g e s o f a p p r o x i m a t e l y 20-30 m r e l i e f , and some d e p r e s s i o n a l a r e a s o f a swampy n a t u r e which d r y out d u r i n g the summer. The s i t e was l o c a t e d a t an e l e v a t i o n o f 150 m. At t h e time o f t h e r e s e a r c h , t h e t r e e s ranged i n h e i g h t from 7 t o 10 m, and averaged a p p r o x i m a t e l y 15 cm i n d i a m e t e r . There were a p p r o x i m a t e l y 17 3 0 stems -1 2 -1 ha , and the b a s a l a r e a was 2 7.5 m ha , w i t h a c o m p e t i t i v e s t r e s s i n d e x o f 34 0. The s o i l , b e l o n g i n g t o t he Dashwood s e r i e s , was a w e l l - d r a i n e d g r a v e l l y sandy loam o f 45-6 0 cm depth o v e r l y i n g a deep l a y e r o f compacted b a s a l t i l l . Measurements o f r o o t d e n s i t y i n d i c a t e d t h a t t h e b u l k o f the r o o t s were i n t h e upper 45 cm o f the s o i l . O p e r a t i o n a l l y , the s i t e p r o v i d e d good a c c e s s w i t h main l o g g i n g r o a d s l e a d i n g t o w i t h i n 2 m i l e s o f the study a r e a from the I s l a n d Highway. S m a l l e r r o a d s l e d r i g h t t o t h e s i t e , a l o n g s i d e which were p l a c e d t h e two t r a i l e r s and the g e n e r a t o r . The 12.2 m h i g h i n s t r u m e n t a t i o n tower was p l a c e d 30 m from the t r a i l e r s . - 6 -I t i s f e l t t h a t t h e a r e a i s an e x c e l l e n t one f o r a m i c r o m e t e o r o l o g i c a l s t u d y . A d v e c t i v e e f f e c t s were f e l t not t o be p r e s e n t , w i t h f i v e m i l e s o f f o r e s t e d l a n d s e p a r a t i n g the s i t e from the S t r a i g h t o f G e o r g i a , and f o r e s t e x t e n d i n g upwards o f 20 m i l e s i n . e i t h e r d i r e c t i o n a l o n g the c o a s t . A l t h o u g h t h e w e s t - t o - s o u t h e a s t s e c t o r i n c l u d e d i r r e g u l a r t e r r a i n and d e f o r e s t e d a r e a s on the s l o p e s o f mountains r i s i n g i n t o t h e F o r b i d d e n P l a t e a u a r e a w i t h i n 2 m i l e s o f t h e s i t e , winds were v i r t u a l l y never from t h a t d i r e c t i o n . P r e v a i l i n g winds were from t h e n o r t h t o n o r t h e a s t . B. Experimental Methods Continuous h a l f - h o u r l y measurements o f e v a p o t r a n s -p i r a t i o n were made from June 14 u n t i l August 15, 1974 u s i n g t h e Bowen r a t i o / e n e r g y b a l a n c e t e c h n i q u e . The Bowen r a t i o g was measured a t the 8.5 m l e v e l u s i n g t h e p s y c h r o m e t r i c a p p a r a t u s d e s c r i b e d i n B l a c k and McNaughton (1971). Optimum placement o f t h e s e n s o r over the canopy was sought a f t e r some e x p e r i m e n t a t i o n w i t h v a r i o u s h e i g h t s and p o s i t i o n s r e l a t i v e t o t h e t o p o f the tower. The s e n s o r remained i n t h e same l o c a t i o n from June 17 u n t i l August 1. On August 1, a second Bowen r a t i o a p p a r a t u s i d e n t i c a l t o t h e f i r s t was p l a c e d above t h e f i r s t one, and the p o s i t i o n s o f the two machines were p e r i o d i c a l l y s w i t c h e d i n o r d e r t o check on the o p e r a t i o n o f each, and t o t e s t f o r any s e n s o r placement problems. Placement o f t h e s e n s o r s i s d i s c u s s e d b r i e f l y i n t h e next s e c t i o n . Net r a d i a t i o n R was measured a t t h e top o f the n ^ canopy w i t h a S w i s s t e c o S - l net r a d i o m e t e r , c o n t i n u o u s l y v e n t i l a t e d w i t h d r i e d a i r from an aquarium-type pump. S o i l heat f l u x G was measured a t t h e 5 cm l e v e l w i t h two heat f l u x p l a t e s , and c o r r e c t e d f o r s t o r a g e i n t h e upper 5 cm u s i n g an i n t e g r a t e d t e m p e r a t u r e measured w i t h a diod e i n t e g r a t i n g thermometer (Tang et a l . , 1974). S t o r a g e o f s e n s i b l e and l a t e n t heat w i t h i n the canopy M was e s t i m a t e d from wet and d r y b u l b t e m p e r a t u r e s t a k e n e v e r y 15 minutes a t t h e 3, 5 and 7 m l e v e l s , and from e s t i m a t e s o f the heat c a p a c i t y o f the canopy based on the work o f St e w a r t and Thorn (1973). E v a p o t r a n s p i r a t i o n E was t h e n c a l c u l a t e d u s i n g the e q u a t i o n E = (R - G - M ) / [ L ( 1 + 6)] where L i s the l a t e n t heat o f v a p o u r i z a t i o n . S u p p o r t i n g c l i m a t o l o g i c a l measurements were made of i n c i d e n t s o l a r r a d i a t i o n R g a t t h e top o f the tower, and a t v a r i o u s l e v e l s t h r o u g h t h e canopy a t s e l e c t e d t i m e s . Wind speed and d i r e c t i o n were m o n i t o r e d u s i n g a C a s s e l a s e n s i t i v e anemometer and C l i m e t wind vane - 8 -mounted a t the top o f the tower. P r e c i p i t a t i o n was r e c o r d e d each day w i t h a 10.2 cm (4\") d i a m e t e r r a i n gauge a t the t r a i l e r . Data s i g n a l s were c a r r i e d back t o the d a t a l o g g i n g t r a i l e r by 7 5 m s h i e l d e d c a b l e s where they were r e c o r d e d w i t h a H e w l e t t - P a c k a r d 2707 A d a t a l o g g e r , and R n, R , and Bowen r a t i o d a t a s i g n a l s were i n t e g r a t e d u s i n g v o l t a g e i n t e g r a t o r s ( B l a c k et al., 1974). Bowen r a t i o d a t a was f u r t h e r m o n i t o r e d on a Honeywell s t r i p -c h a r t r e c o r d e r . The system was powered by a 6.5 kW K o h l e r d i e s e l g e n e r a t o r . S o i l w ater c o n t e n t was measured b o t h g r a v i m e t r i c a l l y and by use o f the n e u t r o n m o i s t u r e meter. Neutron m o i s t u r e measurements were made eve r y 2 t o 3 days i n s i x a c c e s s t u b e s . The b a s a l t i l l l i m i t e d the depth of the tubes t o 45-60 cm. G r a v i m e t r i c s a m p l i n g o f the r o o t zone-was t a k e n - e v e r y 5 t o 16 da y s , depending on the s t a t e o f the d r y i n g p e r i o d . S o i l water p o t e n t i a l between 0 and -1 bar was measured by a t e n s i o m e t e r - t r a n s d u c e r system. Four t e n s i o m e t e r s were used and were l o c a t e d a t depths o f 15, 30, 45 and 60 cm. S o i l water p o t e n t i a l l e s s t h a n -1 b a r was measured u s i n g a Campbell S c i e n t i f i c HR-33T dew p o i n t m i c r o v o l t m e t e r and PT-10 hygrometers. S i x - 9 -hygrometers, two each a t the 15 and 3 0 cm d e p t h , and one each a t t h e 45 and 6 0 cm d e p t h , were used. S o i l w a ter p o t e n t i a l by the t e n s i o m e t e r - t r a n s d u c e r system was r e c o r d e d a t 15 minute i n t e r v a l s , w h i l e s o i l w a ter p o t e n t i a l s by the hygrometers was measured t h r e e t i m e s each day. I n t h i s s t u d y , o n l y d a i l y v a l u e s o f s o i l w ater p o t e n t i a l a r e used i n the a n a l y s i s . S o i l w ater r e t e n t i o n c u r v e s were de t e r m i n e d i n the l a b o r a t o r y by ( i ) hanging column method from s a t u r a t i o n t o a water p o t e n t i a l o f -0.12 bar and ( i i ) p r e s s u r e p l a t e e x t r a c t i o n from -0.33 bar t o -15 b a r s . In a d d i t i o n , t e x t u r e , b u l k d e n s i t y , and r o o t d i s t r i b u t i o n measurements were made. .Stomatal r e s i s t a n c e r a t t h e 10 m h e i g h t was s & measured on a r o u t i n e b a s i s t h r e e t i m e s a day on the m a j o r i t y o f days d u r i n g the p e r i o d June 14 t o August 18 and measurements were made e v e r y two hours d u r i n g the daytime on 16 s e l e c t e d days. The measurement was made u s i n g t h e v e n t i l a t e d porometer d e s c r i b e d i n B l a c k et al. (1974). A t l e a s t two samples o f f o u r n e e d l e s each were used t o o b t a i n a v a l u e o f r . - 10 -RESULTS AND DISCUSSION A. Soil water content and matric potential The average water r e t e n t i o n c u r v e f o r the 0-60 cm depths i s shown i n F i g u r e 1. The c u r v e r e p r e s e n t s the average o f l a b o r a t o r y a n a l y s e s f o r f o u r d e p t h s , w i t h the b a r s i n d i c a t i n g t h e range o f d e v i a t i o n s . F i e l d measure-ments o f s o i l water c o n t e n t and m a t r i c p o t e n t i a l c o n f i r m t h i s c u r v e . The a v a i l a b l e water c a p a c i t y i s a p p r o x i m a t e l y 3 - 3 0.14 cm cm f o r t h e m a t r i c p o t e n t i a l range -1/3 t o -15 b a r . The r e s u l t s o f the m e c h a n i c a l a n a l y s i s o f s o i l p a r t i c l e s w i t h a d i a m e t e r l e s s than 2 mm was 63.5% sand, 21.9% s i l t , and 14.6% c l a y by w e i g h t . The volume f r a c t i o n o f s t o n e s ( p a r t i c l e s g r e a t e r than 2 mm) was 20%. F i g u r e 2 shows the average s o i l water m a t r i c p o t e n t i a l it. f o r the p e r i o d June 17 t o August 19 , 1974 f o r t h e r o o t m c zone (0-45 cm). D a i l y v a l u e s o f p r e c i p i t a t i o n i n mm a r e i n c l u d e d f o r r e f e r e n c e . I t can be seen t h a t t h e summer can be d i v i d e d i n t o t h r e e p e r i o d s : ( i ) a d r y i n g p e r i o d i n the l a t e s p r i n g , which was a l r e a d y i n p r o g r e s s when the equipment was s e t up i n June, ( i i ) a w e t t i n g p e r i o d i n m i d - J u l y , when the b u l k o f the summer's p r e c i p i t a t i o n o c c u r r e d ( p r e c i p i t a t i o n f o r J u l y , 1974 was a b n o r m a l l y h i g h ) , - 11 -i 1 1 1 1 1 1 r 1 I I I 1 r F i g u r e 1. Water r e t e n t i o n c urve f o r Dashwood s e r i e s g r a v e l l y sandy loam. Each p o i n t i s the average water c o n t e n t 6- o f t h e 15, 30, 45 and 60 cm depth samples a t a s i n g l e v a l u e ,of s o i l w ater m a t r i c p o t e n t i a l tym. Bars i n d i c a t e range o f water c o n t e n t s . - .12 -1 i I i i i i i r o - 5 ( 0 \" 4 5 c m depth, bar) I -10 J I ft-fl i n 20 30 June JL J i t_ Precip. 20 (mm) 0 20 30 July 1974 10 20 August F i g u r e 2... S o i l w ater m a t r i c p o t e n t i a l f o r June ' 17-August 19, 1974. Each p o i n t i s . t h e average v a l u e o f the m a t r i c p o t e n t i a l s a t t h e 15, 30 and 4-5 cm depths. - 13 -r e s t o r i n g the s o i l t o z e r o m a t r i c p o t e n t i a l , and ( i i i ) a second d r y i n g p e r i o d w i t h no p r e c i p i t a t i o n o c c u r r i n g t h r o u g h o u t u n t i l the end o f the p e r i o d . T h i s weather a f f o r d e d two s e p a r a t e p e r i o d s a t the d r y end o f t h e range o f water c o n t e n t s i n t h e s o i l . Average s o i l w a ter c o n t e n t , shown i n F i g u r e 3, ranged from 9-23 p e r c e n t on a v o l u m e t r i c b a s i s which i n c l u d e s s t o n e s . The 0 t o 60 cm l a y e r was used i n t h i s average because i t i s f e l t t h a t water c o n t e n t changes down t o 60 cm may be s i g n i f i c a n t i n the f o r e s t water b a l a n c e . T h i s m a t t e r w i l l be d i s c u s s e d i n more d e t a i l l a t e r . P r e c i p i t a t i o n i s a g a i n i n c l u d e d i n t h e f i g u r e f o r r e f e r e n c e . B. Daytime course of net radiation and latent heat flux D e t a i l e d c o m p u t a t i o n o f h a l f - h o u r l y energy b a l a n c e s was r e s t r i c t e d p r i m a r i l y t o f i n e days when r e s o l u t i o n was good i n d i u r n a l f l u c t u a t i o n s o f energy f l u x e s . I t was found t h a t t h e s m a l l g r a d i e n t s o f wet and d r y b u l b t e m p e r a t u r e s over t h e d r y f o r e s t produced more s c a t t e r i n the evapo-t r a n s p i r a t i o n measurement than, had been e x p e c t e d . The s i z e o f t h e s e g r a d i e n t s i s e v i d e n t when F i g u r e 4, showing AT and AT t h r o u g h the c o u r s e o f the day on June 18, i s compared w i t h t h e g r a d i e n t s shown i n F i g u r e 6 i n B l a c k and McNaughton (1971). - 14 -T 1 1 i i i i i I r F i g u r e 3. Average v o l u m e t r i c water c o n t e n t f o r June 2 0-August 20 o f the- 0-60 cm depth o f s o i l from g r a v i m e t r i c s a m p l i n g . - 15 -T ~~ i i : — i 1 1 — i 1 r J -i 1 1 I J I i i L L 0 4 8 12 16 20 Hour PST F i g u r e 4. - 16 -N e g a t i v e v a l u e s o f AT, c a u s i n g o v e r e s t i m a t e s o f LE f r e q u e n t l y e x c e e d i n g R , were c o n s i s t e n t l y seen i n the p e r i o d from s u n r i s e u n t i l a p p r o x i m a t e l y 1000 PST. I t was f e l t t h a t t h e problem was caused by the s e n s o r b e i n g p l a c e d too low and c o n s e q u e n t l y measuring g r a d i e n t s w i t h i n the canopy r a t h e r t h a n over i t a t c e r t a i n t i m e s . A s e t t l i n g o f the maximum temperature o f the p r o f i l e i n t o the canopy a f t e r 1000 PST c o u l d e x p l a i n morning i n v e r s i o n s and s u c c e s s f u l a f t e r n o o n Bowen r a t i o measurements. A l s o , s i n c e t h e t e r r a i n s l o p e d g e n t l y t o t h e n o r t h e a s t , the apparent h e i g h t o f t h e s e n s o r was g r e a t e r when winds were from the n o r t h e a s t , which was the predominant wind d i r e c t i o n i n the a f t e r n o o n . S i n c e a d v e c t i v e e f f e c t s were c o n s i d e r e d a b s e n t , i t was f e l t t h a t t h e t r u e v a l u e o f LE s h o u l d be somewhat l o w e r t h a n R d u r i n g t h i s t ime p e r i o d . R e s u l t s from the f o r e s t n 6 ^ a t Haney (McNaughton and B l a c k , 1973) would suggest t h a t a v a l u e c l o s e r t o o n e - h a l f o f R would be a b e t t e r e s t i m a t e . n Independent e s t i m a t e s o f LE u s i n g t h e M o n t e i t h e q u a t i o n and observed v a l u e s o f s t o m a t a l r e s i s t a n c e t o c a l c u l a t e s u r f a c e r e s i s t a n c e shew good agreement w i t h energy b a l a n c e measure-ments i n the a f t e r n o o n , but tend t o s u p p o r t t h e o n e - h a l f R R r e l a t i o n s h i p i n the morning. F i g u r e 5 compares s t o m a t a l r e s i s t a n c e r _ w i t h s u r f a c e r e s i s t a n c e r _ c a l c u l a t e d from - 17 -6 9 12 15 18 Hour PST F i g u r e 5. Daytime c o u r s e ' o f s u r f a c e r e s i s t a n c e r c (from energy b a l a n c e measurements) and s t o m a t a l r e s i s t a n c e r s ( p r o j e c t e d l e a f a r e a b a s i s ) f o r June 18 and J u l y 25, 1974. The disagreement o f the two v a l u e s i s l a r g e on a p e r c e n t a g e b a s i s i n t h e morning.. the energy b a l a n c e e s t i m a t e s o f LE and t h e M o n t e i t h e q u a t i o n . Note t h a t i n t h e morning r g and r c o f t e n d i f f e r by o v er a f a c t o r o f two, whereas i n the a f t e r n o o n t h e y show good agreement on a p e r c e n t a g e b a s i s . At t i m e s when the second Bowen r a t i o a p p a r a t u s was p l a c e d above the f i r s t one, i t s measurements a l s o suggest t h a t t h e o n e - h a l f R n r e l a t i o n s h i p i s a good. e s t i m a t e . The s u c c e s s o f the second machine i n t h i s p o s i t i o n tends t o s u p p o r t the c o n c l u s i o n t h a t w i t h such s m a l l g r a d i e n t s , placement o f t h e s e n s o r s can be c r i t i c a l : t oo low a s e n s o r may be s u b j e c t t o i n v e r s i o n e f f e c t s i n the morning, and too h i g h a s e n s o r may not be a b l e t o s u f f i c i e n t l y r e s o l v e the s m a l l e r g r a d i e n t s . I t may w e l l be t h a t the most v i a b l e a l t e r n a t i v e t o v a r i a b l e placement of the s e n s o r w i t h h e i g h t o v e r th e day i s an i n c r e a s e d s e p a r a t i o n o f the s e n s o r s f o r d r y f o r e s t measurements. F i g u r e 6 shows the d i u r n a l c o u r s e o f R and LE f o r ° n (a) June 18, 1974, and (b) J u l y 25, 1974. The s o l i d d o t s show the energy b a l a n c e v a l u e s o f LE, w h i l e the c i r c l e s r e p r e s e n t LE e s t i m a t e s from s t o m a t a l r e s i s t a n c e measurements. D a i l y t o t a l s o f e v a p o t r a n s p i r a t i o n were o b t a i n e d from th e s o l i d l i n e , drawn by eye. Note the a p parent o v e r e s t i m a t i o n o f measured LE over what i s f e l t t o be the b e t t e r approxim-a t i o n i n the d a y l i g h t p e r i o d b e f o r e 1000 h r s . T h i s - 19 -T I I r r 1 1 1 1 r _i _ L j t i i I 1 1 i i i I 0 4 8 12 16 20 24 Hour PST F i g u r e 6a. D i u r n a l t r e n d i n R n and LE f o r June 18, 1974. S o l i d dots a re energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measurements.. The s o l i d l i n e f o r LE was drawn by eye. - 20 -6 0 0 Wm 4 0 0 r -2 0 0 Hour PST F i g u r e 6b. D i u r n a l t r e n d i n R n and LE f o r J u l y 25, 1974. S o l i d d o t s are energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measure- • ments.. The s o l i d l i n e f o r LE was drawn by eye. - 21 -o v e r e s t i m a t i o n u s u a l l y amounted t o a p p r o x i m a t e l y t e n p e r c e n t o f the d a i l y t o t a l e v a p o t r a n s p i r a t i o n . F i g u r e s 6 (a) and (b) a r e combined i n F i g u r e 7 t o c o n t r a s t t h e i r c u r v e s o f LE and R . J u l y 25 was a f i n e n J day s u f f i c i e n t l y l o n g a f t e r the r a i n y p e r i o d t h a t t h e r e was no f r e e water on t h e t r e e s , y e t a day when s o i l water c o u l d be c o n s i d e r e d adequate a t a m a t r i c p o t e n t i a l o f -0.7 b a r . The l a t e n t heat f l u x on J u l y 25 does not d i s p l a y the m i d - a f t e r n o o n peak obse r v e d a t Haney by McNaughton and B l a c k (1973). In f a c t , a l t h o u g h i t was p r e s e n t , i t was not pronounced i n the l a t e n t heat f l u x p l o t f o r J u l y 19 (shown i n F i g u r e 8 ) , the f i r s t c l e a r day a f t e r t h e w e t t i n g p e r i o d . T h i s s u g g e s t s more r e s t r i c t i o n t o t r a n s p i r a t i o n t h a n was p r e s e n t a t Haney i n the p e r i o d o f peak a f t e r n o o n vapour p r e s s u r e d e f i c i t . The 24-hour v a l u e s o f LE/R o f 0.54 and 0.58 f o r J u l y 19 n J and 2 5 r e s p e c t i v e l y a r e s l i g h t l y l o wer t h a n th e r e s p e c t i v e v a l u e s of 0.61 and 0.63 p r e d i c t e d by the e q u i l i b r i u m e v a p o r a t i o n r a t e . The r e s t r i c t i o n may w e l l be a f u n c t i o n o f the low h y d r a u l i c c o n d u c t i v i t y a s s o c i a t e d w i t h a v e r y c o a r s e s o i l . June 18, a day when the m a t r i c p o t e n t i a l was -6.5 b a r s f o l l o w i n g s e v e r a l weeks o f g e n e r a l l y c l e a r weather, p r e s e n t s a much d i f f e r e n t p i c t u r e . Note, f i r s t , t h a t R - 22 -6 0 0 Wm'2 4 0 0 2 0 0 25 July, 1974 Y m = -0 .7 bar — 18 June, 1974 Y m = - 6.5 bars 0 0 8 12 Hour 16 PST 20 24 F i g u r e 7. F i g u r e s 6a and 6b t o g e t h e r t o i l l u s t r a t e t h e e f f e c t s o f • d e c r e a s e d s o i l water m a t r i c p o t e n t i a l i | ^ m on L E . Note t h a t R i s h i g h e r on June 18. n to - 23 -l_ I 1 I : i I I 1 L I I I I I 0 4 8 12 16 20 24 Hour PST F i g u r e 8. •' R n and LE on J u l y 19, the f i r s t c l e a r day a f t e r t h e r a i n y p e r i o d i n m i d - J u l y . The m i d - a f t e r n o o n peak i n LE i s p r e s e n t , b u t . n o t pronounced. - 24 -i s h i g h e r on June 18, b e i n g o n l y 3 days from the summer s o l s t i c e . The c u t b a c k i n LE was apparent as e a r l y as 10 00 PST, a t whi c h time the e v a p o t r a n s p i r a t i o n r a t e reached i t s d a i l y maximum. The l a t e n t heat f l u x d e c r e a s e d d u r i n g t h e l a t e morning and e a r l y a f t e r n o o n t o a minimum o f 80 Wm~2 a t 1430 PST when t h e v.p.d. and s t o m a t a l r e s i s t a n c e b o t h r e a c h e d maximum d a i l y v a l u e s ( B l a c k et al., 1975). R e s i s t a n c e t o t r a n s p i r a t i o n i s o b v i o u s i n t h i s case w i t h the 24-hour v a l u e o f LE/R n b e i n g o n l y 0.38. J u l y 8, whose d i u r n a l c o u r s e s o f LE and R n a r e shown i n F i g u r e 9, can be c o n s i d e r e d t h e most severe o b s e r v e d day o f the summer, w i t h an average ^ m o f -10.5 b a r s , R n . • o f 6.75 mm \" e q u i v a l e n t , and LE/R n o f o n l y 0.26. G r a d i e n t s i n the wet and d r y b u l b t e m p e r a t u r e s were so s m a l l t h a t the observ e d vapour p r e s s u r e g r a d i e n t was n e g l i g i b l e a t 1600 PST. C. The relationship between LE/Rn and i>m As mentioned i n the i n t r o d u c t i o n , the e q u i l i b r i u m e v a p o r a t i o n r a t e p r e d i c t s an a p p r o x i m a t e l y c o n s t a n t v a l u e of t h e r a t i o LE/R n f o r adequate s o i l w a t e r . S i n c e the e v a p o t r a n s p i r a t i o n r a t e depends on e v a p o r a t i v e demand as w e l l as water s u p p l y , we w i l l examine the e f f e c t o f d e c r e a s i n g s o i l w a ter m a t r i c p o t e n t i a l on LE/R n, F i g u r e 10 shows t h e 24-hour v a l u e o f t h e r a t i o LE/R n as a f u n c t i o n o f f o r t e n s e l e c t e d f i n e days i n c l u d i n g - 2 5 -T i : i 1 r r- 1 1 — — \\ 1 T J i i i i i i i i i i i I 0 4 8 12 16 20 24 Hour PST F i g u r e 9. R n and LE on J u l y 8, the l a s t day o f the f i r s t d r y i n g p e r i o d , and the most severe o b s e r v e d day from t h e a s p e c t o f water s t r e s s . LE/R was o n l y 0.26. - 2 6 -0-8, 0-6 LE 0-4 0-2 0 1 - 2 5 / 7 i i i i i i i x e .19/7 \\ ^ . 2 / 8 -19/6 - 3 / 8 ^ ~ ^ 17/6 -- l 8 / 6 ^ ~ ~ — — -2 5 / 6 -i i i • i I I 8 / 7 -0 - 2 -4 - 6 . - 8 ^ m ( 0 - 4 5 c m depth, bar.)' -10 F i g u r e 10. LE/R as a f u n c t i o n o f s o i l water m a t r i c n . p o t e n t i a l ^ m f o r t e n s e l e c t e d f i n e days. The number a d j a c e n t t o each p o i n t i s t h e date . - 27 -the f o u r days p r e v i o u s l y d i s c u s s e d . ^ m r e p r e s e n t s t h e average v a l u e f o r the r o o t zone, as i n F i g u r e 2, A c u r v e o f b e s t f i t was drawn by eye. The c u r v e i n d i c a t e s t h a t LE/R^ d e c r e a s e d c o n t i n u o u s l y t o a p p r o x i m a t e l y h a l f i t s maximum v a l u e as IJJ^ approached -10 b a r s . I t i s p o s s i b l e t h a t l e s s s c a t t e r would be o b t a i n e d by t a k i n g i n t o a ccount such f a c t o r s as s t o m a t a l r e s i s t a n c e , v.p.d,, and t h e magnitude o f R , but the c o m p l e x i t i e s i n v o l v e d i n d o i n g t h i s d e f e a t the purpose o f the s i m p l e approach used h e r e . The h i g h v a l u e o f LE/R n f o r June 19 (0.41) may be due t o the somewhat h i g h e r v.p.d. on t h a t day, but f u r t h e r s p e c u l a t i o n would not be j u s t i f i e d w i t h o u t a g r e a t d e a l more d a t a . Workers i n a g r i c u l t u r e have o f t e n used v a r i a t i o n s on the Penman p o t e n t i a l e v a p o r a t i o n e q u a t i o n r a t h e r t h a n R R t o n o r m a l i z e LE t o e v a p o r a t i v e demand. The d i f f i c u l t i e s i n c a l c u l a t i n g '-and i n t e r p r e t i n g the Penman p o t e n t i a l e v a p o r a t i o n r a t e have been d i s c u s s e d by McNaughton and B l a c k (1973). F i g u r e 11 shows the r a t i o LE/LE as a eq f u n c t i o n o f , where LE i s the e q u i l i b r i u m e v a p o r a t i o n m eq ^ ^ r a t e d e t e r m i n e d from d a i l y a i r t e m p e r a t u r e s and net r a d i a t i o n . The s i m i l a r i t y between t h i s graph and F i g u r e 10 e s s e n t i a l l y u n d e r l i n e s the degree t o which th e e q u i l i b r i u m r a t e can be e x p r e s s e d as a c o n s t a n t f r a c t i o n o f R . Because - 28 -0.4 r-I I • I I I I I 1—. 1 1 1 0 - 2 - 4 - 6 - 8 -10 (bar) F i g u r e 11. L E / L E e q as a f u n c t i o n o f tym where L E e q r e p r e s e n t s t h e d a i l y e q u i l i b r i u m e v a p o r a t i o n r a t e . D i f f e r e n c e s i n t h i s c u rve and F i g u r e 10 are due o n l y t o d i f f e r e n c e s i n d a i l y , mean t e m p e r a t u r e s . - 29 -o f the f r e q u e n t l y s t r o n g dependence o f water c o n t e n t on h i g h v a l u e s o f ^ m f o r most a g r i c u l t u r a l s o i l s , c u r v e s such as t h a t i n F i g u r e 11 a r e o f t e n shown w i t h il> on a m l o g a r i t h m i c s c a l e t o expand t h e h i g h ^ m range. Such a c u r v e , t a k e n from F i g u r e 11, i s shown i n F i g u r e 12 a l o n g w i t h v a r i o u s o t h e r c u r v e s t a k e n from van B a v e l (19 67) f o r comparison. I t s h o u l d be noted t h a t d e t e r m i n a t i o n o f maximum r a t e s o f LE i s not the same f o r a l l a u t h o r s . A l s o i n the c o a r s e s o i l o f t h i s s t u d y , h y d r a u l i c c o n d u c t i v i t y tends t o drop o f f f a s t e r w i t h d e c r e a s i n g m a t r i c p o t e n t i a l than i n most a g r i c u l t u r a l s o i l s . W h i l e the energy-based parameter I|J i s used e x t e n s i v e l y by e n v i r o n m e n t a l p h y s i c i s t s , h y d r o l o g i s t s and a g r i c u l t u r a l i s t s o f t e n f i n d i t more c o n v e n i e n t t o use v o l u m e t r i c water c o n t e n t 6 as a measurement o f s o i l w a t e r . I t s h o u l d be p o s s i b l e , t h e n , t o \"draw a r e l a t i o n s h i p .between LE/R n and 8 by use. o f F i g u r e 10 and a w a t e r ; r e t e n t i o n c u r v e f o r the s o i l under s t u d y . Such a r e l a t i o n s h i p , d e r i v e d from F i g u r e s 1 and 10, i s shown i n F i g u r e 13. The use o f such a c u r v e would a l l o w t h e use o f a d a i l y g r a v i m e t r i c o r n e u t r o n meter measurement o f 8 a l o n g w i t h d a i l y R n d a t a t o e s t i m a t e d a i l y v a l u e s o f e v a p o t r a n s p i r a t i o n . - 30 -r~ i i 1 r a u t h o r s ( t a k e n from van B a v e l , 1967). Note t h a t the maximum e v a p o t r a n s p i r a t i o n r a t e L E m a x i s d e t e r m i n e d d i f f e r e n t l y f o r d i f f e r e n t a u t h o r s , and i n t h i s s t u d y i s t a k e n as the e q u i l i b r i u m r a t e . - 31 -.25 .20 .6 .10 .05 9 (cm3 cm\"3) F i g u r e 13. LE/R n as. a f u n c t i o n o f v o l u m e t r i c water c o n t e n t . T h i s and F i g u r e 10 are r e l a t e d t h r o u g h the water r e t e n t i o n c u r v e f o r the s o i l , and a l l o w e v a p o t r a n s p i r a t i o n e s t i m a t e s from water c o n t e n t measurements t o be made. - 32 -D. Evapotranspiration estimates usinq R and i> data c r u n m A r e l a t i o n s h i p such as t h a t i n F i g u r e 10 can be used t o e s t i m a t e e v a p o t r a n s p i r a t i o n u s i n g measured v a l u e s o f d a i l y and i j ^ . T h i s approach r e q u i r e s o n l y one more measurement, t h a t o f t h a n th e e q u i l i b r i u m a p p r o a c h , and y e t i t a p p l i e s t o a wide range o f s o i l m o i s t u r e . F i g u r e 14 shows the s e a s o n a l t r e n d i n e v a p o t r a n s p i r a t i o n i n mm e s t i m a t e d from from F i g u r e 2 and d a i l y v a l u e s m • • J o f R n ( e x p r e s s e d i n mm), u s i n g t h e . r e l a t i o n s h i p b e t w e e n ,LE/R n and if; drawn i n Figure,. 10. V a l u e s o f - - : e v a p o t r a n s p i r a t i o n f o r days used t o o b t a i n F i g u r e 10 were used i n F i g u r e 14 f o r t h o s e days, and a r e shown as t r i a n g l e s t o d i s t i n g u i s h them from v a l u e s o f LE d e r i v e d from th e r e l a t i o n s h i p between LE/R and i> . W h i l e n m i t may be argued t h a t F i g u r e 10 s h o u l d a p p l y o n l y t o f i n e days of h i g h R n, i t can be seen i n F i g u r e 14 t h a t e v a p o t r a n s p i r a t i o n on days when R n i s low i s o n l y a s m a l l f r a c t i o n o f the summer t o t a l , and s m a l l . e r r o r s i n e s t i m a t i n g e v a p o t r a n s p i r a t i o n on days o f low demand are not c r i t i c a l f o r l o n g term p r e d i c t i v e e s t i m a t e s . V a l u e s o f R n and LE shown i n F i g u r e 14 a r e g i v e n i n t a b u l a t e d form i n Appendix I . - 3 3 -8 r and L E (mm) 01 -T 1 1 1 r IV, Rn 2 0 3 0 June i r j i_ 10 2 0 3 0 July 1974 10 2 0 August F i g u r e 14. D a i l y t o t a l o f R n and LE f o r June 17-August 14, 19 74. P o i n t s used t o o b t a i n F i g u r e 10 are shown as t r i a n g l e s . .R ' and LE are shown as mm wa t e r e q u i v a l e n t , - 34 -E. Water balance of the forest W h i l e t h e changes i n s o i l w a ter c o n t e n t shown i n F i g u r e 3 c o u l d be used t o check on t h e e s t i m a t e d v a l u e s o f e v a p o t r a n s p i r a t i o n , the s i g n i f i c a n c e o f such a comparison i s d o u b t f u l s i n c e no independent measurements o f d r a i n a g e were made. On the o t h e r hand, a water b a l a n c e c o u l d be o b t a i n e d u s i n g e v a p o t r a n s p i r a t i o n and s o i l water c o n t e n t v a l u e s , w i t h d r a i n a g e as the r e s i d u a l u s i n g t h e water b a l a n c e e q u a t i o n P = E + -A9 + D where P i s t h e p r e c i p i t a t i o n , A9 i s t h e change i n water c o n t e n t i n t h e 0-60 cm l a y e r o f s o i l , and D i s t h e ' d r a i n a g e . S u r f a c e r u n o f f was not o b s e r v e d a t t h e s i t e . Average d a i l y v a l u e s o f A 9 were o b t a i n e d from the c u r v e i n F i g u r e 3 f o r the e i g h t p e r i o d between d a t a p o i n t s . D a i l y v a l u e s o f P and E f o r the c o r r e s p o n d i n g p e r i o d s were averaged from F i g u r e s 3 and 14 r e s p e c t i v e l y , and average d a i l y v a l u e s o f d r a i n a g e were th e n c a l c u l a t e d . The water b a l a n c e components thus o b t a i n e d a r e p r e s e n t e d i n F i g u r e 15, and g i v e n i n t a b u l a r form i n Appendix I I . Note t h a t d r a i n a g e appears as an i n p u t as w e l l as an o u t p u t t o t h e r o o t zone. Whil e t h e n e g a t i v e d r a i n a g e - • 35 -~T i i 1 — — i ( 1 1 1 1 r Depth (mm) 16 • | 12 8 4 i 0 - 4 - 8 I2 | 8 [ Outputs Depth (mm) 0 - 4 Storage change - f t I 1 1 : I I U I T Precipitation [\"^] Evapotranspiration-| | Residual J L I l_ 2 0 3 0 10 20 3 0 10 June July 1974. August F i g u r e 15- Water b a l a n c e 1 o f the 0-60 cm depth o f s o i l f o r June 17-August 14, 19 74. V a l u e s are d a i l y averages i n mm o f w a t e r . The r e s i d u a l term i s - d i s c u s s e d i n t h e t e x t . - 36 -c a l c u l a t e d f o r the p e r i o d J u l y 14-17 c o u l d e a s i l y be caused by s m a l l e r r o r s i n measurements o f p r e c i p i t a t i o n and s o i l water s t o r a g e f o r t h i s s h o r t p e r i o d , t h e r e i s an o b v i o u s t r e n d i n t h e r e s i d u a l o f t h e wa t e r b a l a n c e f o r d r i e r p e r i o d s t h a t i n d i c a t e s t h a t e v a p o t r a n s p i r a t i o n c o n t i n u e d a t p r o g r e s s i v e l y h i g h e r r a t e s t h a n would be p r e d i c t e d from s o i l m o i s t u r e d e p l e t i o n . S i n c e t h e r e i s l i t t l e e v i d e n c e t o i n d i c a t e t h a t r o o t s extended below the 4 5 cm l e v e l , t h e r e i s an apparent s o u r c e o f water which i s not t a k e n i n t o account i n a c o n v e n t i o n a l w ater b a l a n c e o f t h e r o o t zone. There i s some e v i d e n c e t h a t t h i s s o u r c e i s , i n p a r t , upward movement o f water i n t o t h e 0-4 5 cm l a y e r . F o r t h i s r e a s o n t h e water c o n t e n t a t t h e 6 0 cm l e v e l was i n c l u d e d i n c a l c u l a t i n g A6. I n s p e c t i o n o f t h e p r o f i l e o f s o i l m a t r i c p o t e n t i a l ( F i g u r e 16) i n d i c a t e s t h a t i n d e e d g r a d i e n t s e x i s t e d d u r i n g t h e d r i e r p e r i o d s t h a t would t e n d t o in d u c e f l o w upwards from below t h e 6 0 cm l e v e l . A p p l i c a t i o n o f . C a r e y ' s / law t o t h e s e g r a d i e n t s and t h e magnitude o f t h e n e g a t i v e d r a i n a g e v a l u e s g i v e s v a l u e s o f u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s t h a t a r e r e a s o n a b l e f o r a c o a r s e s o i l . However, t h i s c o n d u c t i v i t y s h o u l d become p r o g r e s s i v e l y l ower as t h e apparent n e g a t i v e d r a i n a g e i n c r e a s e s . I t i s a l s o d i f f i c u l t t o - 37 -T 1 r~ i 1 1 1 ~T i~ r 20 30 10 2 0 3 0 10 2 0 June July 1974 August F i g u r e 16. S o i l w ater m a t r i c p o t e n t i a l a t f o u r depths f o r June 17-August 19, 1974, i n d i c a t i n g d i r e c t i o n o f g r a d i e n t s i n t h e v e r t i c a l a x i s . \\f - 38 -b e l i e v e t h a t o v er h a l f o f t h e wa t e r t r a n s p i r e d d u r i n g t h e d r y p e r i o d i n . mid-August, as i n d i c a t e d by t h e water b a l a n c e , o r i g i n a t e d from the: n e a r l y impermeable l a y e r o f b a s a l t i l l below 60 cm. I t has been r e c e n t l y suggested t h a t t h e amount o f water c a p a b l e o f b e i n g s t o r e d i n t h e wood o f c o n i f e r s can be i m p o r t a n t i n the t r a n s p o r t o f water t h r o u g h t h e t r e e s (Running et al. , 1975). S i g n i f i c a n t amounts o f water may be r e l e a s e d by t h e wood when s u p p l y a t t h e r o o t s becomes i n a d e q u a t e , i n o r d e r tormeet e v a p o r a t i v e demand. T h i s i s i n a c c o r d w i t h some r e c e n t .experimental f i n d i n g s f o r Douglas- f i r (Lasso'ie, 1973 ) and White Oak ( H i n c k l e y and B r u c k e r h o f f , 1975). The f a c t t h a t water p o t e n t i a l s i n t h e t w i g s and r o o t s o f t h e t r e e s under s t u d y d e c r e a s e t h r o u g h o u t t h e d r y i n g c y c l e ( B l a c k et al. , 1975) i n d i c a t e s t h a t t h e t r e e s a r e l o s i n g water which has not e x p l i c i t l y been t a k e n i n t o account i n t h e water b a l a n c e . A l t h o u g h measurements o f t r e e water c o n t e n t were not made, i t i s r e a s o n a b l e t o s p e c u l a t e t h a t p r o g r e s s i v e l y more o f t h e \" n e g a t i v e d r a i n a g e \" term i n t h e water b a l a n c e i s due t o changes i n t r e e water s t o r a g e t h a n i n s o i l w a ter s t o r a g e . T h i s amounts t o over h a l f t h e d a i l y average e v a p o t r a n s p i r a t i o n i n t h e l a s t p e r i o d . I t i s e x p e c t e d t h a t t h e n e g a t i v e d r a i n a g e term would - 39 -b e g i n t o decrease under more extreme d r y i n g c o n d i t i o n s , c a u s i n g a more marked d e c r e a s e i n t r a n s p i r a t i o n . The water b a l a n c e a n a l y s i s s e r v e s t o i l l u s t r a t e t h e u s e f u l n e s s o f m i c r o m e t e o r o l o g i c a l measurements i n u n d e r s t a n d i n g t h e water regime o f a f o r e s t , and i n d e e d i t has p r o v i d e d some i n s i g h t i n t o t h e p r o c e s s o f water t r a n s p o r t t h r o u g h t r e e s i n a w a t e r - s t r e s s e d f o r e s t . I t i s q u i t e c o n c e i v a b l e t h a t w i t h a b e t t e r u n d e r s t a n d i n g o f t h e r e l a t i o n s h i p between s o i l m o i s t u r e and t h e p r o p o r t i o n o f t r a n s p i r e d water r e l e a s e d from s t o r a g e i n t h e wood, a r e a s o n a b l y a c c u r a t e water b a l a n c e c o u l d be p r e d i c t e d from e s t i m a t e s o f d a i l y , net r a d i a t i o n and p r e c i p i t a t i o n a l o n e . B e g i n n i n g w i t h a known c o n d i t i o n o f s o i l m o i s t u r e i n t h e e a r l y p a r t o f t h e season, e v a p o t r a n s p i r a t i o n f o r each day can be used, a l o n g w i t h an e s t i m a t e o f t h e p r o p o r t i o n o f t h e day's t r a n s p i r e d w ater t a k e n from t r e e stem s t o r a g e , t o c a l c u l a t e a v a l u e o f IJ f o r the next day. T h i s w i l l i n t u r n be used m J t o d e termine LE/R^ f o r t h e next day. I n t h i s way good h y d r o l o g i c a l p r e d i c t i o n s c o u l d be made from the s t r a i g h t -f o r w a r d measurements o f p r e c i p i t a t i o n and net r a d i a t i o n . CONCLUSIONS E v a p o t r a n s p i r a t i o n from a water s t r e s s e d Douglas f i r f o r e s t has been measured th r o u g h o u t t h e range o f s o i l m a t r i c p o t e n t i a l s from 0 t o -10.5 b a r s , and d a i l y v a l u e s o f LE/R have been c a l c u l a t e d f o r s e l e c t e d days. n J These v a l u e s have been used t o e x p r e s s a r e l a t i o n s h i p between e v a p o t r a n s p i r a t i o n , net r a d i a t i o n , and s o i l m a t r i c p o t e n t i a l t o be used t o p r e d i c t e v a p o t r a n s p i r a t i o n from r a d i a t i o n and s o i l w a ter d a t a . The f o l l o w i n g c o n c l u s i o n s can be drawn: 1. E v a p o t r a n s p i r a t i o n a t t h e e x p e r i m e n t a l s i t e i s n e a r l y e q u a l t o t h e e q u i l i b r i u m r a t e when s o i l water . i s f e l t not t o be l i m i t i n g . C a l c u l a t e d d a i l y v a l u e s o f LE/R n were s l i g h t l y l o w e r t h a n t h o s e a s s o c i a t e d w i t h e q u i l i b r i u m r a t e s even f o r f i n e days f o l l o w i n g r a i n , and i t i s seen from the d i u r n a l t r e n d i n LE t h a t t h e r e appears t o be a r e s t r i c t i o n t o t r a n s p i r a t i o n i n t h e a f t e r n o o n p e r i o d . T h i s r e d u c t i o n may be t h e r e s u l t o f low h y d r a u l i c c o n d u c t i v i t y near the r o o t s p r e v e n t i n g t h e m i d - a f t e r n o o n peak, i n LE obser v e d a t Haney. While i t appears t h a t t h e e q u i l i b r i u m r a t e would be a good e s t i m a t e o f e v a p o t r a n s p i r a t i o n when th e s o i l i s c o m p l e t e l y s a t u r a t e d , t h e s e r e s u l t s - 41 -suggest t h a t i t may s l i g h t l y o v e r e s t i m a t e t h e t r u e v a l u e even a t m a t r i c p o t e n t i a l s as h i g h as -0.5 bar i f the s o i l d i s p l a y s poor water r e t e n t i o n c h a r a c t e r i s t i c s . 2. The r a t i o LE/R n d e c r e a s e s t o a p p r o x i m a t e l y h a l f i t s maximum v a l u e as \\\\>. d e c r e a s e s t o -10 b a r s m at t h i s s i t e . The r e l a t i o n s h i p between LE/R n p r o v i d e s a s i m p l e approach t o e s t i m a t i n g e v a p o t r a n s p i r a t i o n from a water s t r e s s e d f o r e s t u s i n g o n l y d a i l y v a l u e s o f R and . The n m dependence o f LE/R on R i t s e l f i s f e l t t o be v n n o f l e s s e r i mportance when e s t i m a t i n g e v a p o t r a n s - : p i r a t i o n o v er t h e l o n g term. The r e l a t i o n s h i p of LE/R n t o such f a c t o r s as s p e c i e s , s o i l t y p e , p h y s i o l o g y and m a c r o - c l i m a t e has y e t t o be de t e r m i n e d . 3. The d e c r e a s e i n LE/R f o r t h e e x p e r i m e n t a l s i t e n r appears t o be immediate f o l l o w i n g a drop i n e v a p o t r a n s p i r a t i o n c o n t i n u e s a t a r a t e h i g h e r t h a n t h a t r e f l e c t e d i n the s o i l w a t er s t o r a g e change, i n d i c a t i n g a sou r c e o f water t h a t i s r e l e a s e d o n l y when s o i l w a ter i s not adequate. W h i l e some upward f l o w o f water from below the r o o t zone i s i n e v i d e n c e from m a t r i c p o t e n t i a l g r a d i e n t s , i t i s b e l i e v e d t h a t : r e l e a s e o f water s t o r e d w i t h i n t h e t r e e stems w i l l p r i m a r i l y account f o r t h e c o n t i n u i n g e v a p o t r a n s -p i r a t i o n under f a i r l y extreme d r y i n g c o n d i t i o n s . - 43 -REFERENCES B l a c k , T.A. and K..G. McNaughton. 1971. P s y c h r o m e t r i c apparatus, f o r Bowen r a t i o d e t e r m i n a t i o n over f o r e s t s . Boundary-Layer M e t e o r o l . 2: 246-254. B l a c k , T.A., P.A. Tang, C.S. Tan, J.R. C u r t i s and K.G. McNaughton. 19 74. Measurement t e c h n i q u e s used i n f o r e s t h y d r o m e t e o r o l o g y . F i n a l Report f o r t h e D i r e c t o r , P a c i f i c F o r e s t R e s e a r c h C e n t r e , Department o f t h e Environment, V i c t o r i a , B.C. B l a c k , T.A., C.S. Tan, J.R. C u r t i s and J.U. Nnyamah. 1975. F a c t o r s a f f e c t i n g e v a p o t r a n s p i r a t i o n o f an. u n t h i n n e d D o u g l a s - f i r f o r e s t . F i n a l R e p o r t t o t h e D i r e c t o r , P a c i f i c F o r e s t Research C e n t r e , Department o f t h e Environment, V i c t o r i a , B.C. D a v i e s , J.A. and C D . A l l e n . 1973. E q u i l i b r i u m , p o t e n t i a l , and a c t u a l e v a p o r a t i o n from cropped s u r f a c e s i n Southe r n O n t a r i o . J . A p p l . M e t e o r o l . 12: 649-657. Denmead, O.T. and I.C. M c l l r o y . 1970. Measurements o f n o n - p o t e n t i a l e v a p o r a t i o n from wheat. A g r i c . M e t e o r o l . 7: 285-302. Denmead, O.T. and R.H. Shaw. 1962. A v a i l a b i l i t y o f s o i l w a ter t o p l a n t s as a f f e c t e d by s o i l m o i s t u r e c o n t e n t and m e t e o r o l o g i c a l c o n d i t i o n s . Agronomy J . 54: 385-390. Gardner, W.R. and C F . E h l i g . 1963. The i n f l u e n c e o f s o i l w a ter on t r a n s p i r a t i o n by p l a n t s . J . Geophys. Res. 68: 5719-5724. H i n c k l e y , T.M. and D.N. B r u c k e r h o f f . 1975. The e f f e c t s o f drought on water r e l a t i o n s and stem s h r i n k a g e o f Quercus alba. Can. J . Bot. 53: 62-72. L a s s o i e , J.P. 1973. D i u r n a l d i m e n s i o n a l f l u c t u a t i o n i n a D o u g l a s - f i r stem i n response t o t r e e water s t a t u s . F o r e s t S c i e n c e 1 9 ( 4 ) : 251-255. L o p u s h i n s k y , W. and G.O. K l o c k . 1974. T r a n s p i r a t i o n o f c o n i f e r s e e d l i n g s i n r e l a t i o n t o s o i l w a ter p o t e n t i a l . F o r e s t S c i e n c e 20: 181-186. - 44 -McNaughton, K.G. 1974. A s t u d y o f t h e energy b a l a n c e o f a D o u g l a s - f i r f o r e s t . Ph.D. T h e s i s , Department o f S o i l S c i e n c e , U n i v e r s i t y o f B r i t i s h C olumbia, Vancouver, B.C. McNaughton, K.G. and T.A. B l a c k . 1973. A s t u d y o f e v a p o t r a n s p i r a t i o n from a D o u g l a s - f i r f o r e s t u s i n g the energy b a l a n c e approach. Water Resources Res. 9: 1579-1590. R i t c h i e , J.T. 1973. I n f l u e n c e o f s o i l water s t a t u s and m e t e o r o l o g i c a l c o n d i t i o n s on e v a p o r a t i o n from a c o r n canopy. Agron. J . 65: 893-897. Running, S.W., R.H. Waring and R.A. R y d e l l . 19 7^. P h y s i o l o g i c a l c o n t r o l o f water f l u x i n c o n i f e r s . O e c o l o g i a • 1 8 : 1-16. S t e w a r t , J.B. and A.S. Thorn. 197 3. Energy budgets i n a p i n e f o r e s t . Quart. J . R. Meteorol-. Soc. 99 :! 154-170. Tang, P.A., K.G. McNaughton and T.A. B l a c k . 1974. I n e x p e n s i v e diode thermometry u s i n g i n t e g r a t e d c i r c u i t components. Can. J . F o r e s t Res . 4 :.. 2 5 0-2 54 . van B a v e l , C.H.M. 1967. Changes i n canopy r e s i s t a n c e t o water l o s s from a l f a l f a induced.by s o i l water d e p l e t i o n . Agr. M e t e o r o l . 4: 165-176. Z a v i t k o v s k i , J . and W.K. F e r r e l l . 1968. E f f e c t o f drought upon r a t e s o f p h o t o s y n t h e s i s , r e s p i r a t i o n , and . t r a n s p i r a t i o n o f s e e d l i n g s o f two e c o t y p e s o f D o u g l a s - f i r . Bot. Gaz. '129: 346-350. - 45 -APPENDIX I Date 1974 R (mm) LE (mm) LE/R ^„ ( b a i —n n —m June 17 7.03 2 .69 . 38 -5 . 8 18 7 .27 2 .62 . 36 -6 . 5 19 7.29 2.99 .41 -7 . 3 20 7 . 41 2.45 .33 -7 .4 21 4.69 1.50 . 32 -7 . 6 22 6 .57 2 . 04 . 31 -8.1 23 5 . 44 1.69 . 31 -8 . 3 24 A at* _f_ a-* ft A A A rft 4a 4\\ ,»? j». -8.4 25 6 .49 1.78 .27 -8 . 7 26 0.97 0 .27 .28 -9.4 27 4.66 1.33 . 28 -9 . 3 28 2 .84 0 .80 .28 -9 .-4 29 1.92 0 .55 .28 -9 . 3 30 3 .43 0 .98 .28 -9 . 3 J u l y 1 3.73 1.05 .28 -9 . 5 2 5 .58 1.56 .28 -9.6 3 4 .28 1.16 . 27 -9.9 4 4.73 1.28 .27 -10 .1 5 7 .40 2 .00 .27 -10.1 6 7 . 38 1.96 .26 -10. 3 7 6 .87 1. 79 .26 -10 . 4 8 6 .75 1.74 .26 -10 . 5 9 1.23 0 .32 .26 -10 . 6 10 1.76 ' 0 .45 .25 -10 . 8 11 4.60 1.38 .30 -8 . 8 12 3.40 1.09 .32 -7.6 13 6.61 2 .38 .26 -6 .1 14 5.62 2 .19 . 39 -4.9 15 3.03 1.27 .42 -3.9 16 0 . 84 0 . 36 .43 -3.4 17 3 .97 2 .38 .60 -0.1 18 3.72 2 . 22 . 59 -0.2 19 6 . 54 3 .55 .54 -0 . 2 20 6 .55 3.83 .58 -0 . 3 21 3 .32 1.94 .58 -0 . 3 2 2 3.69 2 .14 . 58 -0.3 23 4.99 2 . 89 .58 -0.4 24 5 .65 3 .14 .55 -0.4 25 5 .66 3 . 31 .58 -0.7 - 46 -Date 1974 J u l y 2 6 27 28 29 30 31 Aug. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 R (mm) LE (mm) 5 . 69 3 . 07 5 .77 3 .12 5 .67 3 .00 5.96 3 .16 5 . 39 2.80 5 . 82 2 .73 6 . 29 2.92 6 .51 2.98 6 .49 2 . 75 6.44 2 .84 5 .99 2 .55 6 .00 2 . 46 6.27 2 . 51 6 . 26 2 . 31 6 .11 2 . 29 5.68 2 . 04 5 .52 1.88 6.03 1.99 6.23 2.03 2.08 0.66 LE/R tjj ( b a r s ) -n ' Tin . 54 -0.9 . 54 -0 . 9 .53 T - l . 0 .53 -1.1 .52 -1.3 .47 -2 . 3 .46 -2.4 .46 -2 . 7 . 42 -3.0 .44 -3.2 .42 -3 . 6 .41 -4 . 2 . 40 -4 . 6 . 37 -5 . 0 . 37 -5.4 . 36 -6 .1 . 34 -6 . 7 .33 -7.1 .32 -7 . 5 . 31 . -7.9 - 47 -APPENDIX I I Water Balance Data D a i l y averages g i v e n i n mm P e r i o d P E A 0 June 20-June 2 6 June 2 6-J u l y 5 J u l y 5-J u l y 14 0.00 1.50 -0.90 1.19 1.19 0.00 5.50 1.48 3.27 JTn } y J\"!} 12.33 1.34 14.80 J u l y 17 T U3\" y i o 0-80 2 . 74 -7 . 32 J u l y 2 2 J T U ^ Y ll~ 0-00 3.09 -2.60 J u l y 2 8 ^Uly I8' 0.00 2.88 -2.14 Aug. 4 Aug. -4-Aug. 14 0 .00 2 .07 -0 .90 "@en ; edm:hasType "Thesis/Dissertation"@en ; edm:isShownAt "10.14288/1.0093176"@en ; dcterms:language "eng"@en ; ns0:degreeDiscipline "Soil Science"@en ; edm:provider "Vancouver : University of British Columbia Library"@en ; dcterms:publisher "University of British Columbia"@en ; dcterms:rights "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en ; ns0:scholarLevel "Graduate"@en ; dcterms:title "Evapotranspiration from a dry Douglas fir forest"@en ; dcterms:type "Text"@en ; ns0:identifierURI "http://hdl.handle.net/2429/19248"@en .