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The generation of stormflow on a glaciated hillslope in coastal British Columbia Utting, Mark Gregory 1978

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THE GENERATION OF STORMFLOW ON A GLACIATED HILLSLOPE IN COASTAL BRITISH COLOMBIA by Mark Gregory U t t i n g B . S c , U n i v e r s i t y of Washington, 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of G e o l o g i c a l S c i e n c e s i n c o n j u n c t i o n with Department of S o i l Sciences 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 December 1978 © Mark Gregory U t t i n g , 1978 In present ing th i s thesis in p a r t i a l f u l f i lment of the requirements for an advanced degree at the Un iver s i t y of B r i t i s h Columbia, I agree that the L ib ra ry sha l l make it f ree l y ava i l ab le for reference and study. I f u r ther agree that permission for extensive copying of th i s thes i s for s cho la r l y purposes may be granted by the Head of my Department or by his representat ives . It is understood that copying or pub l i c a t i on of th i s thes i s for f i nanc ia l gain sha l l not be allowed without my wr i t ten permiss ion. Department of Geological Science The Un iver s i t y o f B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date December 19 th 1978 ABSTRACT-An i n v e s t i g a t i o n i n t o the mechanisms of stormflow generation on a g l a c i a t e d h i l l s l o p e i n c o a s t a l B r i t i s h Columbia has been undertaken. The i n v e s t i g a t i o n i n c l u d e d a c o n t r o l l e d i r r i g a t i o n -r u n o f f experiment on a 30 x 30 m h i l l s l o p e p l o t i n the U.B.C. Research F o r e s t near Haney, B.C. Instrumentation i n c l u d e d 12 r a i n gauges, 45 piezometers, and 2 o u t f l o w - t i p p i n g buckets. Piezometer s l u g t e s t s to measure h y d r a u l i c c o n d u c t i v i t i e s and a ge o l o g i c study to e s t a b l i s h the r e p r e s e n t a t i v e n e s s of the experimental r e s u l t s were conducted t o complement the i r r i g a t i o n experiment . The h y d r o g e o l o g i c u n i t s of the r e s e a r c h p l o t c o n s i s t o f : A) 0.1 to 0.3 m of f o r e s t f l o o r m a t e r i a l c o n s i s t i n g of o r g a n i c m a t e r i a l i n v a r i o u s s t a t e s of decay B) 0.3 to 0.8 m of heterogenous* red-brown B h o r i z o n c o n t a i n i n g many organic r i c h channels made up of l i v e and decayed r o o t s C) 0.5 to 2 m grey to grey-green Vashon t i l l D) f r a c t u r e d to u n f r a c t u r e d g r a n o d i o r i t e bedrock The h y d r a u l i c c o n d u c t i v i t y of the t i l l was approximately 10 -f m/s. A s l i g h t l y higher value of 10~ 6 m/s was found f o r the lower B h o r i z o n matrix. A bulk c o n d u c t i v i t y f o r the lower B ho r i z o n was estimated at 10 -* m/s. The 2 to 3 order-of-magnitude d i f f e r e n c e i s probably a t t r i b u t a b l e t o numerous, high c o n d u c t i v i t y r o o t channels present throughout the lower B ho r i z o n . i i i Stormflow was generated when the water t a b l e rose i n t o the high c o n d u c t i v i t y B h o r i z o n . Outflow a t the stream bank e x i t e d from the B h o r i z o n with most water f l o w i n g from high c o n d u c t i v i t y r o o t channels. The r a t e of outflow was c o n t r o l l e d by the p o s i t i o n of the water t a b l e . S ince the water t a b l e remained p a r a l l e l to the o v e r a l l h i l l s l o p e , the h y d r a u l i c g r a d i e n t remained approximately constant. Only the c r o s s -s e c t i o n a l area a v a i l a b l e f o r flow v a r i e d . Once outflow had commenced, the r a t e of outflow was s e n s i t i v e t o v a r i a t i o n i n the r a i n f a l l r a t e . Input-outflow l a g - t i m e s were as l i t t l e as one hour. The time l a g to i n i t i a t i o n of outflow was 19 hours. Most of t h i s l a g was a t t r i b u t a b l e to the f i l l i n g of storage requirements a f t e r a two month p e r i o d of no r a i n . The d i s t r i b u t i o n of the hydrogeologic u n i t s i n the r e s e a r c h p l o t was found to be r e p r e s e n t a t i v e of the rese a r c h area. Lag times were found to be i n the range found i n another s i m i l a r B.C. mountain b a s i n . I t i s concluded t h a t the mechanism of stormflow generation o p e r a t i n g i n the r e s e a r c h p l o t can be g e n e r a l i z e d to other s i m i l a r b a s i n s . ACKNOWLEDGMENTS -Many people have gi v e n me much needed a d v i c e and a s s i s t a n c e along the way. I would l i k e to thank them here. Jan d e V r i e s , Department of S o i l S c i e n c e , gave much of h i s time f o r d i s c u s s i o n of i d e a s , h a n d - s o i l i n g f i e l d w o r k , and s t a n d i n g under s p r i n k l e r s , piezometer measuring s t i c k i n hand. He a l s o provided me with an on-going res e a r c h p r o j e c t which i n c l u d e d a r e s e a r c h s i t e f a r s u p e r i o r to any I c o u l d have s e t up myself. E. A l l a n Freeze, Department of G e o l o g i c a l Science, gave me an understanding of how to do s c i e n t i f i c r e s e a r c h ; He a l s o amazed me at the f a s t turnaround on my t h e s i s d r a f t s . Dr,i J . L e s l i e Smith, Department of G e o l o g i c S c i e n c e s , spent much of h i s a l l - t o o - p r e c i o u s time as a new f a c u l t y member readi n g and c r i t i c i z i n g my w r i t t e n work. W. H. Mathews, Department of G e o l o g i c a l Sciences, provided i n v a l u a b l e d i r e c t i o n with regard to the g l a c i a l geology. E. Pamela E e i d and Herman Fe l d e r h o f a s s i s t e d me with my f i e l d w o r k w e l l beyond the c a l l of f r i e n d s h i p . C.J. Larsen gave me encouragement during and long before my graduate s t u d i e s began. She a l s o opened doors i n t o the w r i t t e n world which allowed the w r i t i n g of t h i s t h e s i s t o be much l e s s p a i n f u l than i t might have been. At times i t was even f u n l E.H. P e r k i n s helped to de-mystify the wonders of MTS. Gordon Hodge produced b e a u t i f u l l y drawn f i g u r e s from my hasty sketches and C a t h i Lowe f e d my manuscript to the computer at UBC to produce t h i s p r i n t e d copy. Research funds were provided by the N a t i o n a l Eesearch C o u n c i l of Canada. V Thank you a l l , very much. v i TABLE - OF CONTENTS -ABSTRACT w. i i ACKNOWLEDGMENTS i v LIST OF TABLES i x LIST OF FIGURES - ,. X 1.0 INTRODUCTION AND RESEARCH OBJECTIVES 1 2.0 LITERATURE REVIEW 4 2.1 Hortonian Overland Flow 4 2.2 Subsurface Stormflow 8 2.3 Dunne And Black Overland Flow 12 2.4 Work In The Southwest Coast Mountains Of B r i t i s h Columbia - 16 3.0 The Study Area And S i t e 25 3,-1 The Study Area 25 3.1.1 L o c a t i o n And Physiography • «•» 25 3.1.2 Previous D e s c r i p t i o n s : Geology And Pedology .... 26 3.2 The Research P l o t 28 3.2.1 L o c a t i o n And Physiography 28 3.2.2 P l o t Pedology 30 3.2.3 P l o t Geology: T i l l 32 3.2.4 P l o t Geology: Bedrock .... 34 3.3 Near P l o t Geology 35 3.3.1 S u r f i c i a l Mapping 36 3.3.2 V e r t i c a l P r o f i l e s 36 4.0 EXPERIMENTAL PROCEDURE . 43 v i i 4.1 M o d i f i c a t i o n s To The P l o t . ..... 43 4.2 The I r r i g a t i o n System - 44 4.3 E a i n Gauges 45 4.3.1 C o l l e c t i o n Type Rain Gauges 45 4.3.2 T i p p i n g Bucket Rain Gauges 47 4.4 Piezometers .., 48 4.4.1 Standpipes 48 4.4.2 New Piezometers 48 4.4.3 Reading The Piezometers 51 4.5 C o l l e c t i o n Trough And T i p p i n g Buckets ............... 53 4.6 Chemical T r a c e r s . .... . 54 4.7 H y d r a u l i c C o n d u c t i v i t y Determinations 55 4.8 Data Reduction 56 5.0 Experimental R e s u l t s 58 5.1 R a i n f a l l 58 5.2 Piezometers - , 64 5.3 Outflow 66 5.4 Chemistry ...... . 68 5.5 Slug T e s t s . - 70 6.0 ANALYSES, INTERPRETATION, AND DISCUSSION 73 6.1 Non-Stormflow Groundwater Input * 73 6.2 S o i l Moisture Storage 77 6.3 The Water Table 78 6.3.1 C o n f i g u r a t i o n Of The Water Table 78 6.3.2 Role Of The Water Table .. 80 6.4 H y d r a u l i c C o n d u c t i v i t i e s 86 6.4.1 C a l c u l a t e d C o n d u c t i v i t i e s 86 6.4.2 Comparison Of C o n d u c t i v i t i e s -- 87 v i i i 6 . 5 M e c h a n i s m Of S t o r m f l o w G e n e r a t i o n 88 6 .6 G e n e r a l i z a t i o n Of The R e s u l t s . . 92 7 . 0 SUMMARY AND CONCLUSIONS - 97 REFERENCES 100 APPENDIX , 105 i x LIST-OF- FIGURES-FIGOEE 2-1 Hortonian Overlandflow 6 FIGURE 2-2 Subsurface Stormflow C o n t r i b u t i n g Areas 6 FIGUBE 2-3 The Experimental S i t e Of Dunne And Black ........ 13 FIGUEE 2-4 Study S i t e Of Nagpal And DeVries 21 FIGUEE 3-1 Depth To Bedrock In The Eesearch Area .......... 29 FIGUBE 3-2 Diagrammatic C r o s s - s e c t i o n Of The Research P l o t 31 FIGUEE 3-3 Twenty F i v e V e r t i c a l P r o f i l e s 37 FIGUEE 3-4 Bedrock F r a c t u r e s 41 FIGUBE.4-1 Layout Of The Eesearch Plot,....,. ,46 FIGUBE 4-2 The New Piezometer 50 FIGUBE 4-3 Piezometers Below The Eesearch P l o t 52 FIGUBE 5-1 B a i n f a l l From EG1 ,- 61 FIGUEE 5-2 R a i n f a l l From BG2 62 FIGUBE 5-3 Superimposed D a i l y R a i n f a l l 63 FIGURE 5-4 Outflow Hydrograph For The T i p p i n g Buckets ..... 67 FIGURE 5-5 C h l o r i d e C o n c e n t r a r i o n Vs. Time 69 FIGUEE 6-1 Input-outflow Hydrograph 75 FIGUEE 6-2 Schematic Flownet Of The H i l l s l o p e 82 FIGURE 6-3 Water Table P o s i t i o n At The I n i t i a t i o n Of Outflow....... 85 FIGURE 6-4 Schematic Flownet Of The Stream Bank 89 FIGURES A5-1 To A5-23 Piezometer Hydrographs Appendix X LIST - OF - TABLES -TABLE 5-1 C o l l e c t i o n Gauge E a i n f a l l 59 TABLE 5-2 T o t a l P l o t R a i n f a l l .... 60 TABLE 5-3 Slug T e s t R e s u l t s , 71 TABLE 6-1 Timing Of Four Piezometers 811 1 I i O INTRODUCTION - ANJS - RESEARCH • OBJECTIVES -Throughout h i s t o r y many people have s p e c u l a t e d on the o r i g i n of r i v e r s and streams. However, i t has only been i n the l a s t 50 years t h a t attempts have been made to e x p l a i n the mechanisms of stormflow g e n e r a t i o n . B a s i c a l l y , t hree mechanisms have been proposed: overland flow caused by r a i n f a l l i n excess o f i n f i l t r a b i l i t y , s u bsurface stormflow and overland flow caused by a r i s i n g water t a b l e . A l l three mechanisms are known to operate under c e r t a i n circumstances but i t i s not c l e a r which one predominates i n the mountains i n c o a s t a l B r i t i s h Columbia. Recent work i n the south-west Coast Mountains of B r i t i s h Columbia, has shown t h a t extreme s o i l h e t e r o g e n e i t y may f a v o r a subsurface mechanism of c h a n n e l i z e d flow through o r g a n i c zones. DeVries and Chow (1973, 1978) have shown t h a t s o i l h e t e r o g e n e i t y p l a y s a major r o l e i n i n f i l t r a t i o n i n the Seymour Watershed. Continuing t h i s work, Nagpal and d e V r i e s (1976) e s t a b l i s h e d an experimental h i l l s l o p e - s t r e a m system i n the UBC Research F o r e s t i n order to b e t t e r understand stream flow i n f o r e s t e d mountain environments. T h e i r f i n d i n g s can be summarized as f o l l o w s : 1) stormflow t r a v e l e d through the h i l l s l o p e v i a root channels, by-passing the s o i l matrix 2) outflow from the experimental watershed decreased as water broke through i m p e r f e c t i o n s i n the t i l l which underlay the s o i l 3) leakage through t h i s t i l l was up to 75% of the t o t a l r a i n f a l l input The c u r r e n t study r e p o r t s on work c a r r i e d out by the author and 2 d e V r i e s to c r i t i c a l l y examine these conc lus ions ; , One o b j e c t i v e of t h i s s tudy was to examine the u n d e r l y i n g g l a c i a l t i l l with emphasis p l a c e d on the h y d r o l o g i c b e h a v i o r and the s p a t i a l d i s t r i b u t i o n both w i t h i n and near the r e s e a r c h p l o t . Another o b j e c t i v e o f t h i s s tudy was to i n v e s t i g a t e the mechanisms of s tormflow g e n e r a t i o n w i t h i n the e x p e r i m e n t a l h i l l s l o p e - s t r e a m system and to examine the r o l e of r o o t c h a n n e l s i n the B h o r i z o n of the s o i l i P r e v i o u s work by Nagpal and d e V r i e s (1976) had shown t h a t these c h a n n e l s were major c o n d u c t o r s of s tormflow to the stream bank. However, because t h i s mechanism of s tormflow g e n e r a t i o n aroused some c o n t r o v e r s y , i t was dec ided t o re-examine these c o n c l u s i o n s u s i n g more complete i n s t r u m e n t a t i o n . F i n a l l y , i t was i m p o r t a n t t o e s t a b l i s h the g e n e r a l i t y o f the flow mechanisms by examining and comparing the geology i n the r e s e a r c h p l o t to tha t of the area around i t . The p l o t h y d r o l o g y was examined u s i n g a c o n t r o l l e d i r r i g a t i o n exper iment on the same s i t e used by Nagpal and d e V r i e s (1976). In o r d e r to p o s i t i v e l y e s t a b l i s h s teady - s t a t e c o n d i t i o n s and to b e t t e r unders tand f low p a t h s , more complete i n s t r u m e n t a t i o n , c h e m i c a l t r a c e r s and a l o n g e r p e r i o d of i r r i g a t i o n were used . T h i s t h e s i s rev iews the l i t e r a t u r e and p r e s e n t s the t h e o r y of v a r i o u s mechanisms of stormflow g e n e r a t i o n . The l o c a l geology i s then e x p l o r e d u s i n g p r e v i o u s work and t h a t done by the a u t h o r to show the h y d r o g e o l o g i c r e p r e s e n t a t i v e n e s s o f the e x p e r i m e n t a l s i t e i The i r r i g a t i o n experiment and the r e l a t e d i n s t r u m e n t a t i o n are then d e s c r i b e d f o l l o w e d by the r e s u l t s and a n a l y s i s . 3 F i n a l l y , c o n c l u s i o n s are presented along with d i s c u s s i o n about g e n e r a l i z a t i o n of the r e s u l t s to other areas. This study does not answer a l l the q u e s t i o n s concerning stormflow generation i n the c o a s t a l mountains of B r i t i s h Columbia. However, i t i s hoped t h a t through t h i s study the reader w i l l g a i n a b e t t e r understanding of mountain f o r e s t hydrology. 4 2^0 LITERATURE - REVIEW -In t h i s chapter t h r e e mechanisms f o r stormflow are reviewed: Hortonian o v e r l a n d flow, subsurface stormflow, and Dunne and Black overland flow. These are discussed c h r o n o l o g i c a l l y as done by Freeze (1974) and Engman (1974). Recent work i n the southwest c o a s t a l r e g i o n of B r i t i s h Columbia on stormflow g e n e r a t i o n i s then summarized. 2.1 Hortonian Overland Flow The mechanism of stormflow g e n e r a t i o n by overland flow was e x p l a i n e d by Horton (1933). In h i s now c l a s s i c a l i n t e r p r e t a t i o n of the r o l e of i n f i l t r a t i o n , he s t a t e d t h a t p r e c i p i t a t i o n i n excess of t h a t which could i n f i l t r a t e i n t o the ground, f i l l s s u r f a c e d e p r e s s i o n s and then flows along the s u r f a c e to the stream channel i n the form of overland flow. T h i s " p r e c i p i t a t i o n excess" occurs whenever the r a i n f a l l r a t e exceeds the s o i l ' s maximum p o s s i b l e i n f i l t r a t i o n r a t e , or i n f i l t r a t i o n c a p a c i t y as Horton c a l l e d i t . Horton s t a t e d t h a t the i n f i l t r a t i o n c a p a c i t y , or i n f i l t r a b i l i t y as i t i s now o f t e n termed, i s not c o n s t a n t but decreases with time d u r i n g a r a i n f a l l event to an approximately constant r a t e . I t then r e t u r n s to i t s i n i t i a l value w i t h i n a few days a f t e r p r e c i p i t a t i o n has ceased. According t o Horton, the decrease i n i n f i l t r a b i l i t y i s due mainly to three f a c t o r s : packing of the s o i l s u r f a c e by r a i n drop impact, s w e l l i n g of the s o i l and c l o s i n g of s u r f a c e openings, and inwashing and f i l l i n g of s o i l s u r f a c e openings by f i n e m a t e r i a l s . I t was a l s o s t a t e d 5 that the subsequent r e t u r n t o the pre-storm i n f i l t r a b i l i t y i s caused by a r e v e r s a l of the above by sun* wind, and b i o l o g i c a c t i o n on the surface of the s o i l . Horton noted t h a t i n f i l t r a b i l i t y v a r i e s with s o i l type and the time of year during which r a i n occurs. F i n e t e x t u r e d s o i l s have i n f i l t r a b i l i t i e s t h a t decrease more r a p i d l y and l e v e l o f f to lower r a t e s than c o a r s e r s o i l S i He a l s o noted t h a t maximum i n f i l t r a b i l i t i e s occur during summer months when high e r temperatures and more a c t i v e b i o t a produce a g r e a t e r degree of s u r f a c e r e s t o r a t i o n . As Horton viewed i t , overland flow occurs whenever r a i n f a l l r a t e exceeds i n f i l t r a b i l i t i e s , as shown i n F i g u r e 2-1. T h i s type of mechanism i s dominant where r a i n f a l l r a t e s are f r e g u e n t l y higher than n a t u r a l s o i l i n f i l t r a b i l i t i e s ( a r i d t o s e m i - a r i d r e g i o n s with thunderstorm a c t i v i t i e s ) or i n places where the s o i l s u r f a c e i s d i s t u r b e d ( a g r i c u l t u r a l l a n d or urbanized a r e a s ) . These r e g i o n s commonly have exposed s o i l s u r f a c e s and l i t t l e v e g e t a t i o n f o r p r o t e c t i o n from r a i n d r o p impact. The l a c k of v e g e t a t i o n a l s o reduces the o r g a n i c content of the s o i l and t h e r e f o r e reduces the h y d r a u l i c c o n d u c t i v i t y , too. Horton claimed t h a t the decrease i n i n f i l t r a b i l i t y with time i s due to e f f e c t s on the s u r f a c e of the s o i l and not due t o the e f f e c t s of s a t u r a t i o n . Although experimental evidence by Green and Ampt (1911) showed a decrease i n i n f i l t r a t i o n independent of s u r f i c i a l e f f e c t s , Horton d i d not take t h i s i n t o account. His work was e m p i r i c a l , a r e s u l t of many o b s e r v a t i o n s but with no p h y s i c a l b a s i s f o r h i s theory. A t h e o r e t i c a l understanding came l a t e r . 6 T N E A F T E R O N S E T O F R A I N ( H R S ) FIGURE 2-1 R a i n f a l l , I n f i l t r a t i o n and Hortonian Overland Flow (after Freeze, 1974) FIGURE 2-2 The Subsurface Stormflow Contributing Areas of Hewlett and Nutter (after Hewlett and Nutter, 1970) 7 While Green and Ampt (1911) provided a s e m i - t h e o r e t i c a l e x p l a n a t i o n f o r i n f i l t r a t i o n , i t was not u n t i l the work of Eubin and h i s co-workers i n the 1960s t h a t a complete t h e o r e t i c a l understanding was produced. Richards (1931) developed the equation of unsaturated flow based upon Darcy's law and the equation of c o n t i n u i t y and P h i l i p (1957) provided the f i r s t a n a l y t i c a l s o l u t i o n . Numerical s o l u t i o n s by K l u t e (1952) , Day and L u t h i n (1956), and Hanks and Bowers (1962) helped l e a d to the l u c i d understanding provided by Rubin and S t e i n h a r d t (1963) and Rubin e t a l . (1964) . The work of Rubin and h i s co-workers r e v e a l e d t h a t s u r f a c e ponding occurs only i f r a i n f a l l s at a r a t e g r e a t e r than the s a t u r a t e d c o n d u c t i v i t y of the s o i l and i f i t does so long enough f o r s u r f a c e s a t u r a t i o n to take p l a c e . Thus, overland flow i s produced whenever s a t u r a t i o n occurs at the s u r f a c e . I t i s not j u s t a f u n c t i o n of s u r f a c e e f f e c t s but a l s o of i n i t i a l s o i l moisture and the s o i l p r o p e r t i e s t h a t a f f e c t the s o i l ' s unsaturated response to wetting. H o r t o n 1 s t h e o r i e s i m p l i e d t h a t r a i n f a l l u s u a l l y exceeds i n f i l t r a b i l i t y and t h a t stormflow i s t y p i c a l l y produced by overland flow. He i n f e r r e d t h a t such o v e r l a n d flow g e n e r a t i o n i s a r e a l l y widespread. However, w i t h i n a watershed, s o i l s u s u a l l y have c o n s i d e r a b l e h e t e r o g e n e i t y and r a i n f a l l can vary both temporally and s p a t i a l l y ; Recognizing t h i s , Betson (1964) and TVA (1965) developed a p a r t i a l area concept whereby only c e r t a i n area£, u s u a l l y with l e s s v e g e t a t i o n and higher s o i l moisture c o n t e n t s , c o n s i s t a n t l y produce r u n o f f to streams i n the form of overland flow. T h i s p a r t i a l area concept helped to e x p l a i n why 8 most watersheds i n humid areas t y p i c a l l y generate storm r u n o f f of l e s s than 10% of the t o t a l r a i n f a l l i n p u t . Subseguent work by Whipkey (1965), Eagan (1968), Dunne and Black (1970a, b) , Weyman (1970), and others has shown t h a t r a i n f a l l r a t e s i n temperate, middle l a t i t u d e areas r a r e l y exceed i n f i l t r a b i l i t i e s . Thus, Hortonian overland flow i s r a r e l y seen i n these environments. T h i s i s p a r t i c u l a r l y t r u e i n f o r e s t e d r e g i o n s where dense v e g e t a t i o n p r o t e c t s the s o i l s u r f a c e and where c o n d u c t i v i t i e s are high because of coarse t e x t u r e and high o r g a n i c content. I t i s not s u r p r i s i n g t h a t workers i n these areas proposed a new mechanism f o r the g e n e r a t i o n of stormflow. 2.2 Subsurface Stormflow In vegetated r e g i o n s with l e s s i n t e n s e r a i n f a l l and permeable s o i l s , Hortonian overland flow i s r a r e l y seen. The lack of Hortonian f l o w , coupled with o b s e r v a t i o n s t h a t streams i n these areas do respond to p r e c i p i t a t i o n , l e d to the development of the second major concept of stormflow g e n e r a t i o n : subsurface stormflow. T h i s i s a mechanism whereby storm water flows to the stream channel v i a shallow subsurface paths. I t r e g u i r e s both steep h i l l s l o p e s and l a r g e h y d r a u l i c c o n d u c t i v i t i e s i n a shallow s o i l h o r i z o n (Freeze, 1972) , a s i t u a t i o n commonly found i n upland f o r e s t s Although o r i g i n a l l y p o s t u l a t e d by Hursh (1936, 1944) i t was not u n t i l Hewlett and h i s co-workers (Hewlett, 1961; Hewlett and H i b b e r t , 1963, 1967; and Hewlett and Nutter, 1970) t h a t t h i s mechanism became fa v o r e d by f o r e s t h y d r o l o g i s t s . Hewlett (1961) and Hewlett and H i b b e r t (1963) e s t a b l i s h e d that s o i l c o u l d 9 provide baseflow between r a i n f a l l events via coupled saturated-unsaturated flow. After thoroughly watering a 1 x 1 x 1 4 r a s o i l block and covering i t to exclude atmospheric int e r a c t i o n , they measured an outflow of 1.42 l i t e r s per day a f t e r 60 days. This was admittedly small but i t did demonstrate the f e a s i b i l i t y of downslope movement of water through unsaturated s o i l . Continuing t h i s work, Hewlett and Hibbert (1967) proposed a mechanism where areas nearer the stream bank play a more s i g n i f i c a n t role in stormflow generation than areas farther away. Based on the p a r t i a l area concept of Betson (1964) and TVA (1965) and the "translatory flow" ideas of Horton and Hawkins (1965), they proposed a variable source mechanism of stormflow generation. In t h i s mechanism, runoff i s produced by subsurface stormflow instead of Hortonian overland flow and contributing areas are not s t a t i c but expand with increasing amounts of r a i n f a l l during a storm. According to Hewlett and Hibbert, the expansion of these source areas to meet the subsurface flow paths in the h i l l s l o p e allows the r e l a t i v e l y slow moving s o i l water to reach the stream quickly enough to account for rapid stream r i s e s (Figure 2-2). These low subsurface flow v e l o c i t i e s and the short observed ra i n f a l l - s t r e a m r i s e lag-times are a basic problem with the subsurface stormflow concept. F i e l d measurements by Whipkey (1965) on a layered 1 x 2 meter plot on a 28% h i l l s l o p e i n Ohio, using 24 simulated r a i n f a l l events with both wet and dry antecedent s o i l moisture conditions indicated minimum inflow-outflow lag-times of 1 1/2 hours. Other "storms" of lower and more t y p i c a l r a i n f a l l i n t e n s i t i e s yielded lags that were longer. 10 I n i t i a t i o n of outflow came as l a t e as 2 1/2 hours a f t e r the beginning of r a i n f a l l . E x t r a p o l a t i o n o f t h i s s m a l l s o i l block to watershed dimensions would not c o r r e l a t e w e l l with a c t u a l stream response. Experimental work by Weyman (1970) i n Somerset, England y i e l d e d s i m i l a r c o n c l u s i o n s . On a one meter wide h i l l s l o p e p l o t (length not g i v e n ) , he noted h i l l s l o p e l a g s of 36 hours f o r subsurface stormflow as opposed to l a g s of 3 to 4 hours f o r the a c t u a l stream peak. Weyman's e x p l a n a t i o n f o r t h i s was an unmeasured and more f a v o r a b l e p o r t i o n of the b a s i n upstream from h i s s i t e . Both Whipkey and Weyman had long l a g - t i m e s and h i l l s l o p e flow volumes t h a t were too sm a l l t o account f o r stream r i s e s . More i n f o r m a t i o n on subsurface stormflow was needed to e x p l a i n what was a c t u a l l y happening. This e x p l a n a t i o n was provided by a c a r e f u l f i e l d study by Dunne and Black (1970a, b ) . In a w e l l instrumented b a s i n i n Vermont (discussed i n more d e t a i l i n the next s e c t i o n ) , they concluded t h a t subsurface stormflow was only seen on steep, l a t e r a l l y concave s l o p e s with wet antecednt c o n d i t i o n s and in t e n s e r a i n f a l l . Even with these most f a v o r a b l e c o n d i t i o n s , they measured t o t a l volumes t h a t were too s m a l l and la g - t i m e s that were too long to produce the r i s e - s e e n i n t h e i r stream. These r e s u l t s were v e r i f i e d by Freeze (1972a,b) with computer s i m u l a t i o n of a small watershed. By using f i n i t e d i f f e r e n c e approximations f o r the d i f f e r e n t i a l e q u a t i o n s o f flow, he was able t o simulate a s m a l l watershed by c o u p l i n g boundary c o n d i t i o n s f o r the h i l l s l o p e s u r f a c e , s u b s u r f a c e and stream channel. By v a r y i n g i n i t i a l c o n d i t i o n s , h i l l s l o p e 11 parameters and boundary c o n d i t i o n s , he was able to s i m u l a t e a wide v a r i e t y of watersheds, i n c l u d i n g those thought most f a v o r a b l e f o r subsurface stormflow. He found t h a t subsurface stormflow i s a major peak c o n t r i b u t o r only with l o n g i t u d i n a l l y convex h i l l s l o p e s with h i g h l y permeable s o i l s f e e d i n g s t e e p l y i n c i s e d channels. These s t u d i e s i n d i c a t e d t h a t subsurface stormflow i s r e l a t i v e l y unimportant i n r e g i o n s with homogeneous s o i l s . However, evidence e x i s t s t h a t i n heterogenous systems, subsurface flow i s important. Whipkey (1967), using many ( t o t a l not given) h i l l s l o p e p l o t s of up t o 1,100 m2 on s l o p e s of 19 to 42% with r a i n f a l l s of 12 to 76 mm per hour, s t a t e d t h a t i n f i n e r s o i l s , subsurface flow t r a v e l s v i a b i o l o g i c a l and s t r u c t u r a l channels. His tensiometers i n d i c a t e d a by-passing of the s o i l matrix. Outflow l a g - t i m e s were.as low as 15 t o 25 minutes even with flow paths t h a t passed through 1.22 m of unwatered b u f f e r s t r i p s . V i s u a l o b s e r v a t i o n s at the o u t l e t of the experimental s i t e s i n d i c a t e d t h a t water flowed v i a r o o t h o l e s . In one case, he observed water f l o w i n g i n channels 9 m o b l i g u e l y downslope from the i r r i g a t e d area. Outflow occurred w i t h i n 45 minutes of s u r f a c e ponding on the wetted s l o p e . A s h o r t l a g time such as t h i s i n d i c a t e s t h a t subsurface stormflow c o u l d be important i n heterogenous s o i l s . Other examples of the importance of h e t e r o g e n e i t y i n s o i l s i n c l u d e work by Pond (1971) who s t a t e d t h a t n a t u r a l subsurface flow " p i p e s " can be detected d u r i n g dry weather by s l i g h t s u r f a c e depressions and s p e c i f i c v e g e t a t i o n or l o c a t e d i u s t a f t e r a storm by " l i s t e n i n g to the water g u r g l i n g beneath the 12 s u r f a c e ! " Chamberlin (1972), d e V r i e s and Chow (1973,1978), and Nagpal and d e V r i e s (1976) a l s o demonstrated the importance of subsurface stormflow i n heterogenious s o i l s . T h i s work w i l l be d i s c u s s e d i n d e t a i l i n a l a t e r s e c t i o n . With the e x c e p t i o n of areas with s u i t a b l e s o i l h e t e r o g e n e i t i e s , i t can be concluded t h a t where r a i n f a l l i s moderate and s l o p e s are vegetated, n e i t h e r subsurface stormflow nor Hortonian overland flow i s the dominant r u n o f f - g e n e r a t i n g mechanism. To e x p l a i n r u n o f f i n these r e g i o n s , a t h i r d type of flow mechanism was needed. T h i s e x p l a n a t i o n was provided by Dunne and Black (1970a, b) . 2.3 Dunne and Black Overland Flow Dunne and Black (1970a,b) chose a s m a l l sub~basin of the Sleepers River Experimental Watershed i n Vermont t o look a t a l l the h y d r o l o g i c components of a small watershed. T h e i r o r i g i n a l o b j e c t i v e was t o examine subsurface stormflow. To do t h i s , they instrumented three segments of a h i l l s l o p e (concave, convex, and planar) t h a t c o n s i s t e d of an o r g a n i c r i c h sandy s o i l o v e r l y i n g a low c o n d u c t i v i t y l a c u s t r i n e c l a y (Figure 2-3a). The slope was steep, 30 to 100%, and the h y d r a u l i c c o n d u c t i v i t y of the s o i l was high. These c o n d i t i o n s were thought t o be i d e a l to o b t a i n g u a n t i t a t i v e measurements of volumes and la g - t i m e s of h i l l s l o p e output. They c o n s t r u c t e d a 75 m long c o l l e c t i o n t r e n c h to observe s u r f a c e , shallow subsurface and ground water flow (Figure 2-3b). On the h i l l s l o p e they had 30 r a i n gauges, 12 piezometers and 9 neutron probe access tubes as w e l l as 2 stream gauging weirs (Figure 2-3c). h s p r i n k l e r system allowed them to 13 FIGURE 2-3 The Experimental Site of Dunne and Black (after Dunne and Black,1970a,b) 14 produce a r t i f i c i a l r a i n . T h e i r r e s u l t s were s u r p r i s i n g . In 35 n a t u r a l events with maximum re c u r r e n c e i n t e r v a l s of 2 years, d u r i n g l a t e summer (dry antecedent moisture c o n d i t i o n s ), no s i g n i f i c a n t output from the h i l l s l o p e was produced. They concluded t h a t p r e c i p i t a t i o n went to r e p l e n i s h s o i l moisture l o s t through summer evapo-t r a n s p i r a t i v e demands and through ground water i n p u t . Hortonian flow was not seen as the most i n t e n s e r a i n f a l l of 3.12 i n c h e s per hour was l e s s than the measured i n f i l t r a b i l i t i y of 3.15 inches per hour. L a t e r i n October, c o n d i t i o n s were wetter and, although not s u f f i c i e n t to c r e a t e Hortonian o v e r l a n d flow, they were thought more l i k e l y t o produce subsurface stormflow. In s p i t e of t h i s , r u n o f f was n e g l i g i b l e . Only with a l a r g e a r t i f i c i a l storm on wet s o i l d i d stormflow appear. I t was n e i t h e r subsurface stormflow nor Hortonian overland flow, however. Dunne and Black observed a type of o v e r l a n d flow generated on areas s a t u r a t e d by a r i s i n g water t a b l e . They proposed a flow mechanism t h a t had some s i m i l a r i t i e s to previous stormflow concepts. L i k e Hortonian overland flow under the p a r t i a l area concept of Betson (1964) and TVA (1965), i t too, was generated on s m a l l p o r t i o n s of the watershed only. These areas, though, were more l i k e the v a r i a b l e source areas of Hewlett and Hibbert (1967) and Hewlett and Nutter (1970). They were g e n e r a l l y t o p o g r a p h i c a l l y low with near s u r f a c e water t a b l e s and higher antecedent moisture content. Also c o n s i s t e n t with t h i s concept, these s a t u r a t e d wetlands expanded and c o n t r a c t e d with v a r y i n g amounts of p r e c i p i t a t i o n . Thus F i g u r e 2-2, used to i l l u s t r a t e 15 v a r i a b l e source areas f o r su b s u r f a c e stormflow, can a l s o be used to d e p i c t r u n o f f producing areas l i k e the ones observed by Dunne and Black. Here, i n s t e a d o f expanding channel areas f e d by subsurface stormflow, the near stream c o n t r i b u t i n g areas generate overland flow fed by r a i n f a l l on s a t u r a t e d s u r f a c e s created by r i s i n g water t a b l e s . Like Hortonian o v e r l a n d flow, the Dunne and Black mechanism produces s h o r t l a g - t i m e s and r e l a t i v e l y high r u n o f f volumes. There i s # however, a major d i f f e r e n c e between Hortonian and Dunne and Black o v e r l a n d flow. Hortonian flow i s produced when s a t u r a t i o n occurs at the s u r f a c e because r a i n f a l l r a t e exceeds i n f i l t r a b i l i t y , while Dunne and Black overland flow i s generated when s a t u r a t i o n occurs from below as the water t a b l e r i s e s to the s u r f a c e i n response to i n f i l t r a t i o n . Subsurface c o n d i t i o n s , t h e r e f o r e , do not produce much r u n o f f d i r e c t l y , but are important i n that antecedent moisture and depth t o s a t u r a t i o n i n f l u e n c e the extent of the water t a b l e r i s e . Dunne and Black r u n o f f comes from both d i r e c t p r e c i p i t a t i o n on a s a t u r a t e d zone and from the s a t u r a t e d subsurface d i s c h a r g e to t h i s zone, termed r e t u r n flow. Although Dunne and Black eguated the g u a n t i t y of ov e r l a n d flow d i r e c t l y with the amount of r a i n f a l l on the expanding wetlands and not to the amount of r e t u r n flow, an i s o t o p i c study on fo u r b a s i n s i n Canada by F r i t z et a l . (1976) i n d i c a t e d t h a t ground water discharge may play a more important r o l e i n stormflow g e n e r a t i o n . T h e i r study d i d not i n v o l v e any h i l l s l o p e sampling, however. Freeze (1972a,b;1974) demonstrated t h a t r e t u r n flow i s dominant only when s o i l c o n d u c t i v i t i e s are h i g h , the h i l l s l o p e i s steep, convex, and 16 feeds an i n c i s e d stream. The f i n d i n g s of Dunne and Black were confirmed by Freeze (1972a, b) using the computer s i m u l a t i o n technique d i s c u s s e d i n the previous s e c t i o n . He concluded t h a t i n watersheds of the type s t u d i e d by Dunne and Black, the flow mechanism which they d e s c r i b e d was the norm. He s t a t e d t h a t only i n extreme cases would e i t h e r Hortonian overland flow or subsurface stormflow play a dominant r o l e on humid, vegetated s l o p e s . Freeze*s s i m u l a t i o n s were of s i m p l i f i e d , homogeneous h i l l s l o p e s . Snyder (1973), K n i s e l (1973) and Hewlett (1974) f e l t t hat these were inadequate to d e s c r i b e stormflow i n some re g i o n s . In p l a c e s where l a y e r i n q or extreme h e t e r o q e n e i t y e x i s t , wetting f r o n t s and water t a b l e r i s e s , may not be v e r t i c a l l y c ontinuous. Flowpaths w i l l not be as simple as those shown by Freeze and may i n s t e a d be very complex. One such area i s the mountainous r e g i o n of southwest c o a s t a l B r i t i s h Columbia. Here e x i s t s a s i t u a t i o n t h a t cannot be represented by a simple two l a y e r , continuous model. This i s one p l a c e where subsurface mechanisms other than those d e s c r i b e d by Dunne and Black may apply. 2.4 Work i n the Southwest Coast Mountains of B r i t i s h Columbia Recent work i n the southwest Coast Mountains o f B r i t i s h Columbia by Chamberlin (1972), d e V r i e s and Chow (1973) and Nagpal and deVries (1976) has shown t h a t s o i l - w a t e r flow paths are much more complex than those seen i n r e g i o n s with more homogeneous s o i l s . Instrumented p l o t s t u d i e s i n d i c a t e d t hat i n f i l t r a t i o n does not progress as a continuous wetting f r o n t . 17 Rather, lower l a y e r s of the s o i l sometimes appear to wet-up before l a y e r s above them. The mechanism put forward by the above authors t o e x p l a i n t h i s behavior i s one of concentrated s a t u r a t e d flow. Chamberlin (1972) was the f i r s t t o r e p o r t t h i s type of behavior which he observed i n the Seymour Watershed near Vancouver, B.C. He conducted s e v e r a l i r r i g a t i o n experiments on an instrumented 4 m* p l o t on a h i l l s l o p e of 30%. The p l o t c ontained 1 m of s o i l over g u a r t z d i o r i t e bedrock* The s o i l c o ntained f o r e s t f l o o r m a t e r i a l 0.1 to 0.3 m t h i c k with c a v i t i e s up t o 1 m across under r o o t s and stumps. The s o i l had a w e l l -developed Ae h o r i z o n and a B h o r i z o n up to 1 m t h i c k . Roots and woody m a t e r i a l made up as much as 50% of the upper 0.2 to 0.5 m of the s o i l with l i v e r o o t s extending throughout the B h o r i z o n and along weathered bedrock s u r f a c e s . H i s i n s t r u m e n t a t i o n c o n s i s t e d of 5 ne s t s of 4 tensiometers;. His r e s u l t s were unexpected. Tensiometers a t lower l e v e l s o f t e n responded to i r r i g a t i o n b efore those l o c a t e d at a higher l e v e l . Some of these a l s o i n d i c a t e d s a t u r a t e d c o n d i t i o n s . Chamberlin's e x p l a n a t i o n f o r t h i s behavior was t h a t f r e e water d r a i n s t o lower l e v e l s i n the s o i l v i a i n t e r c o n n e c t e d pathways that bypass the s o i l matrix. He d i d not i d e n t i f y the nature of these pathways but the i m p l i c a t i o n was t h a t they c o n s i s t e d of l i v e and decayed t r e e r o o t s . Chamberlin suggested t h a t these pathways are connected to the s u r f a c e and, when f r e e water i s a v a i l a b l e , e i t h e r from d i r e c t r a i n f a l l at the s u r f a c e or because of c o n c e n t r a t i o n by b u r i e d l o g s or rocks, c h a n n e l i z e d , s a t u r a t e d flow o c c u r s . T h i s 18 happens even though flow r a t e s and s o i l parameters i n d i c a t e that unsaturated c o n d i t i o n s should p r e v a i l s Chamberlin c a l l e d a s o i l with many pathways connected to each other and the atmosphere, an open s o i l . I n r e g i o n s where such s o i l s predominate, the i m p l i c a t i o n s f o r stormflow gener a t i o n are c o n s i d e r a b l e . Chamberlin suggested t h a t s a t u r a t e d subsurface stormflow through "anomalous zones" i n open s o i l s e x p l a i n s the very f l a s h y response of c o a s t a l streams to p r e c i p i t a t i o n . With t h i s mechanism, a subsurface stormflow concept d i f f e r e n t from Hewlett and H i b b e r t (1967) can be e n v i s i o n e d . The problem of the low v e l o c i t i e s of unsaturated flow i s e l i m i n a t e d because s u b s u r f a c e flow would be c h a n n e l i z e d and s a t u r a t e d , and t h e r e f o r e , much f a s t e r . Since Chamberlin d i d not go i n t o d e t a i l on the complete mechanisms of stormflow ge n e r a t i o n , f u r t h e r r e s e a r c h was needed to examine t h i s h ypothesis. T h i s r e s e a r c h was begun by d e V r i e s and Chow (1973, 1978) using an experimental set-up s i m i l a r to Chamberlin's. T h e i r r e s u l t s were comparable. On a 2.5 by 3.5 m h i l l s l o p e p l o t with 25° to 30° s l o p e s at the 300 m e l e v a t i o n i n the Seymour Watershed, they i n s t a l l e d 13 tensiometers i n 3 n e s t s . The s o i l system was s i m i l a r to t h a t s t u d i e d by Chamberlin except t h a t the parent u n i t u n d e r l y i n g the s o i l was a l o w - c o n d u c t i v i t y g l a c i a l t i l l . High i n t e n s i t y i r r i g a t i o n (2.6 cm per hour) was a p p l i e d with the s o i l i n three s t a t e s of a l t e r a t i o n : u n d i s t u r b e d , p a r t i a l l y d i s t u r b e d f o r e s t f l o o r and f o r e s t f l o o r removed. They concluded t h a t water moved downward p r i m a r i l y v i a r o o t channels during i n f i l t r a t i o n but, a f t e r r a i n f a l l stopped, d r a i n e d through 19 the s o i l matrix. Because of extreme s o i l h e t e r o g e n e i t y , i n f i l t r a t e d water d i d not t r a v e l through the s o i l matrix as unsaturated flow; r a t h e r , i t t r a v e l e d as c h a n n e l i z e d , s a t u r a t e d flow. As i n Chamberlin's study, t h i s behavior was i n d i c a t e d by lower tensiometers that responded b e f o r e , and sometime with g r e a t e r magnitude than those nearer t h e i r s u r f a c e . Some of these piezometers a l s o i n d i c a t e d s a t u r a t i o n , even though the r a i n f a l l r a t e and the s o i l parameters would suggest t h a t s a t u r a t i o n would be u n l i k e l y . They found t h i s behavior p u z z l i n g . They s t a t e d t h a t t h i s response was probably not due to a i r entrapment nor due to pressure b u i l d up from output impediment. The i m p l i c a t i o n was t h a t l o c a l i z e d s a t u r a t i o n occurred. By g e n e r a t i n g two-dimensional h y d r a u l i c p o t e n t i a l maps from the 3 tensiometer n e s t s with a d i s t a n c e weighted computer e x t r a p o l a t i o n programme, d e V r i e s and Chow analyzed the complexity of flow paths. They concluded t h a t there was a much gr e a t e r v a r i a t i o n i n h y d r a u l i c p o t e n t i a l during i n f i l t r a t i o n than drainage. T h i s v a r i a t i o n i n d i c a t e d t h a t r o o t channels conducted water during i n f i l t r a t i o n but, because of reverse p o t e n t i a l g r a d i e n t s , not during drainage. Free water d i d not enter these channels from the matrix, however. I t entered at or near the s u r f a c e during i n f i l t r a t i o n and onl y because wood, rocks, and other o b j e c t s concentrated flow such t h a t f r e e water was a v a i l a b l e . The f r e e water then drained i n t o the openings of these high c o n d u c t i v i t y zones which were at atmospheric pressure. 20 By d i s t u r b i n g the f o r e s t f l o o r , the v a r i a b i l i t y i n the d i s t r i b u t i o n of h y d r a u l i c p o t e n t i a l decreased* T h i s was because channel openings near the s u r f a c e were c l o s e d , e l i m i n a t i n g the access of f r e e water. In these " c l o s e d s o i l s " , flow was p r i m a r i l y through the matrix. Supportive evidence was found by Cheng (1976) who used p a i r e d watersheds to observe the e f f e c t s before and a f t e r c l e a r c u t l o g g i n g . He found t h a t storm peaks were delayed and reduced on d i s t u r b e d h i l l s l o p e s , thus tending to c o n f i r m the importance of root channels as flow paths. T h i s i s i n disagreement with Plamondon e t a l . (1972) who claimed that the f o r e s t f l o o r does not e f f e c t stormflow peaks. Continuing mountain watershed r e s e a r c h , Nagpal and d e V r i e s (1976) s t u d i e d a 30 x 30 m h i l l s l o p e p l o t i n the U.B.C. Research F o r e s t , near Haney, B.C. to examine flow paths on a l a r g e r s c a l e . T h e i r i n s t r u m e n t a t i o n (Figure 2-4) i n c l u d e d 7 piezometers, 4 neutron access tubes, 2 tensiometer n e s t s and 2 c a l i b r a t e d t i p p i n g buckets to measure i n p u t and outflow. A r t i f i c i a l storms were c r e a t e d with 8 metered s p r i n k l e r s . The s o i l at the r e s e a r c h s i t e was s i m i l a r to that i n the Seymour Watershed with 0.05 to 0.2 m of f o r e s t f l o o r m a t e r i a l , a t h i n l a y e r of Ae h o r i z o n , approximately 1 m of B h o r i z o n on top of about 1 m or more of g l a c i a l t i l l , a l l u n d e r l a i n by i n t r u s i v e bedrock. This experimental p l o t i s the same one s t u d i e d by the author and r e p o r t e d on i n t h i s t h e s i s . A more d e t a i l e d d e s c r i p t i o n appears i n the f o l l o w i n g s e c t i o n . 21 C SPRINKLERS @ PIEZOMETERS NEUTRON ACCESS TUBES NEUTRON ACCESS TUBES COMBINED WITH TENSIOMETER -MANOMETER SYSTEM M2 SEDIMENT TRAP AND TIPPING BUCKETS CONCRETE COLLECTION TROUGH FIGURE 2-4 The Study Site of Nagpal and deVries (after Nagpal and deVries, 1976) 22 Nagpal and deVries concluded that waterflow from the s o i l to the stream bank was through root channels. This was indicated i n several ways: tensiometer-neutron probe data , stream bank observations and concentration time c a l c u l a t i o n s . Tensiometers again indicated that lower parts of the s o i l responded before some upper l a y e r s . also noted again were pos i t ive tensiometers readings. However, neutron probe data , as well as water retension c h a r a c t e r i s t i c s of the s o i l , ind ica ted that the s o i l was unsaturated. They found th i s puzz l ing . Their explanation for t h i s response was that laboratory-measured c a l i b r a t i o n curves may not have indicated true f i e l d condi t ions . Another explanation seen by th i s author i s that saturated flow occurred in the channels while the surrounding matrix was unsaturated. The neutron probe integrated over both and indicated an average unsaturated condi t ion . Observations at the stream bank gave v i s u a l confirmation to the importance of root channels. During f u l l f l o o d , a major proportion of the outflow was seen to eminate from root channels discharging at the bank. One such channel was measured at 2% of the t o t a l outflow. Also , during the i n i t i a l phases of f l ood ing , output from a d i s t i n c t group of roots produced most of the t o t a l outflow for the whole stream bank. Nagpal and deVries made a "time of concentration ca l cu la t ion" to deduce i f i n i t i a l response times were consistent with root channel flow. This c a l c u l a t i o n was based on the assumption that subsurface flow i s analogous to surface flow i n that outflow timing and volume are d i r e c t functions of path length and that outflow represents the water ac tua l ly applied as 23 r a i n r a t h e r than water from storage. They examined the time r e g u i r e d f o r water to flow along the l o n g e s t path l e n g t h i n t h e i r watershed. Using the t i m i n g of the r i s e t o peak outflow and the l e n g t h of the h i l l s l o p e , they c a l c u l a t e d a s e r i e s of "times of c o n c e n t r a t i o n " as a f u n c t i o n of p o s s i b l e matrix c o n d u c t i v i t i e s . T h i s was done using D a r c y 6 s law and i g n o r i n g storage and v e r t i c a l flow times. They concluded t h a t the c o n d u c t i v i t y r e g u i r e d f o r the observed l a g - t i m e s was too l a r g e to r e p r e s e n t the matrix alone and t h e r e f o r e i n d i c a t e d t h a t a l a r g e p r o p o r t i o n of the t o t a l flow must be conducted by r o o t channels. Nagpal and d e V r i e s drew s e v e r a l other c o n c l u s i o n s . One was that up to 75% of the input water was l o s t through leakage out of the p l o t . Based on o b s e r v a t i o n s of boulders p r o t r u d i n g through the t i l l , surrounded by s m a l l rock fragments, they p o s t u l a t e d t h a t t h i s leakage was due to water "breaking through i m p e r f e c t i o n s i n the compacted t i l l . " In a d d i t i o n , they concluded that the p i e z o m e t r i c data i n d i c a t e d a d i s c o n t i n u o u s water t a b l e . T h i s was a l s o demonstrated i n another i r r i g a t i o n run with 20 piezometers from which they concluded that a water t a b l e on top of the t i l l e x i s t s but t h a t i t i s only l o c a l l y permanent. Other areas are s a t u r a t e d only under wet c o n d i t i o n s (deVries, pers. comm.). They a l s o noted t h a t , u n l i k e c l a s s i c a l homogeneous h i l l s l o p e s with higher moisture c o n d i t i o n s nearer the stream bank, the observed water t a b l e r i s e was not a f u n c t i o n of d i s t a n c e from the stream* On the c o n t r a r y , the f i r s t and highest piezometer r i s e occurred near the the extreme upslope p o s i t i o n while one piezometer near the bank d i d not 24 respond at a l i i This led Nagpal and deVries to postulate a stream flow mechanism where topographic highs on the upper surface of the t i l l act as contributing areas to lower, permanently-saturated areas. During r a i n f a l l , these saturated subsurface depressions f i l l up and overflow to the stream bank via a permeable root mat and network of root channels. These high conductivity zones allow for rapid flow even though the s o i l matrix conductivity i s low* Thus, they postulated that the time lag betwen the i n i t i a t i o n of r a i n f a l l and the beginning of outflow i s due to the f i l l i n g of subsurface basins as well as s o i l moisture recharge. This i s a new concept of subsurface stormflow that i s not yet completely understood. Several problems remain to be examined. The investigation of these problems i s the objective of t h i s thesis. Emphasis w i l l be placed on the g l a c i a l t i l l , i t s d i s t r i b u t i o n and hydroogic properties, including leakage from the h i l l s l o p e system; the examination of the mechanisms of stormflow generation with emphasis on the role of the organic zones; and the geology of the h i l l s l o p e plot and environs to determine the generality of any stormflow generating mechanism operating i n the research p l o t i To accomplish these ends, an i r r i g a t i o n experiment similar to those of Nagpal and deVries was undertaken during the summer of 1977. In addition to t h i s , geologic mapping and a u x i l i a r y hydrologic work were conducted. 25 3-_G The Study Ace a - and - Site -In t h i s chapter the physical c h a r a c t e r i s t i c s of the research area and experimental plot are discussed. Previous pedological and geological work i n the U.B.C. Research Forest i s reviewed. Then, i n order to establish the representativeness of the plot, an investigation of the hydrogeological c h a r a c t e r i s t i c s of the study plot and the surrounding area i s presented. 3.1 The Study Area 3.1.1 Location and Physiography The experimental work was carried out i n the University of B r i t i s h Columbia Research Forest, located on the southern edge of the southwest Coast Mountains* 45 km east of Vancouver , near Haney, B.C. (Figure 3-1). The climate i s P a c i f i c Marine Humid with average da i l y temperatures of somewhat over 15°C for the summer months to just below 0°C during the winter. P r e c i p i t a t i o n ranges from 2.0 to 3.0 m per year with the bulk produced by P a c i f i c f r o n t a l systems during the f a l l - w i n t e r - s p r i n g months. (Rainfall i s not unknown, however, during July and August.) Less than 15% of the t o t a l p r e c i p i t a t i o n occurs as snow because of the moderatng e f f e c t s of the P a c i f i c and the r e l a t i v e l y low elevation. The study area i s around 350 m above sea l e v e l . 26 K r a j i n a (1969) p l a c e s the study area i n the C o a s t a l Western Hemlock B i o g e o c l i m a t i c Zone. Climax growth i s Western Hemlock and Western Red Cedar. Alder predominates i n r e c e n t l y logged or burned areas with D o u g l a s - f i r being the t r a n s i t i o n a l s p e c i e s . Underbrush i n c l u d e s vine maple, brachen f e r n , sword f e r n , b l u e b e r r y , h u c k l e b e r r y , and s a l a l and i s very dense, e s p e c i a l l y i n r e c e n t l y c l e a r e d areas. 3.1.2 Previous D e s c r i p t i o n s : Geology and Pedology The study area i s t y p i c a l l y u n d e r l a i n by i n t r u s i v e bedrock covered by t h i n and d i s c o n t i n u o u s u n c o n s o l i d a t e d d e p o s i t s of g l a c i a l o r i g i n . The bedrock geology was mapped by Roddick (1955,1965). The s u r f i c i a l geology was o r i g i n a l l y d i s c u s s e d by Armstrong (1957) with updates and r e v i s i o n s i n the r e g i o n a l guarternary s t r a t i g r a p h y by Armstrong (19 75), Armstrong and Hickock (1975) and Hicock(1976). S o i l s i n the Research F o r e s t have been mapped e x t e n s i v e l y by K l i n k a (1976) and analyzed at the study p l o t by Nagpal and d e V r i e s (1976) and Bryck (1977). Bedrock c o n s i s t s mainly of Cretaceous g u a r t z d i o r i t e to g r a n o d i o r i t e which belong to the Coast C r y s t a l i n e complex. These" are i n t r u d e d l o c a l l y by minor a n d e s i t e - b a s a l t dykes. Because of r a p i d u p l i f t and r e c e n t g l a c i a t i o n , exposed rock s u r f a c e s are g e n e r a l l y f r e s h with l i t t l e weathering. According to Armstrong (1975), Armstrong and Hicock(1975) and Hicock (1976), the south west c o a s t a l r e g i o n has undergone three major g l a c i a t i o n s : the l a t e Wisconsin (11,000 to 20,000 Y.B. P.), the middle Wisconsin (42,500 tO 52,000 Y.B.P.) and the e a r l y Wisconsin? or pre-Wisconsin? (> 62,000 Y.B.P.). D r i f t from 27 the o l d e r stades are the Semiahmoo and West Lynn. D e p o s i t s from the l a t e Wisconsin F r a s e r stade can be d i v i d e d i n t o three groups: the Sumas d r i f t (10,000 to 11,000 Y.B.P.), the C a p i l a n o sediments (11,000 13,000 Y.B.P.) and the Vashon d r i f t (13,000 to 20,000 Y.B.P.). Evidence e x i s t s f o r a t l e a s t t h r e e l o c a l advances dur i n g the Vashon represented by three t i l l s . Also i n c l u d e d i n the Vashon are g l a c i o - f l u v i a l and i c e - c o n t a c t d e p o s i t s . T i l l i n the study area i s probably Vashon and not the l a t e r Sumas (J.Clague, pers. Comm., 1978). Mathewes (1973) dated p o s t - g l a c i a l marine sediment i n the study area at 12,690 Y.B.P. T h i s date supports the c o n c l u s i o n that the l a t e r Sumas d r i f t was not deposited as high nor as f a r to the west, as the study area. Armstrong (1957) mapped most of t h i s area as " p r e - T e r t i a r y bedrock at or w i t h i n 10 f t (3 m) of the s u r f a c e , commonly o v e r l a i n by t i l l or outwash,." P a r t i c l e s i z e a n a l y s i s of the Vashon t i l l by Armstrong (1957) showed f r a c t i o n s : 57% sand, 41% s i l t , and 2% c l a y (USDA standards: c l a y , l e s s than 0.002 mm; s i l t , 0.002 to 0.05 mm; and sand 0.05 to 2 mm). These analyses were done on lowland samples which Armstrong noted as being l e s s sandy than those of s i m i l a r age i n mountain v a l l e y s . D i s t r i b u t i o n s of t h i s order were supported by d e V r i e s and Chow (1973, 1978) who found a p a r t i c l e d i s t r i b u t i o n of 67.6% sand, 26.5% s i l t , and 5.8% c l a y f o r t i l l i n the Seymour Watershed. Thus, previous work i m p l i e s t h a t t i l l s i n the study area are g e n e r a l l y coarse g r a i n e d and probably over 60% sand. 28 Armstrong a l s o noted t h a t most of the c l a y s i z e d p a r t i c l e s c o n s i s t e d of fragments of quartz, f e l d s p a r , and other rock forming minerals. The high sand content and the l a c k of p l a t e y c l a y minerals probably c r e a t e h y d r a u l i c c o n d u c t i v i t i e s i n the mountain v a l l e y Vashon t i l l t h a t a r e greater than those expected f o r t i l l i n g e n e r a l . K l i n k a (1976) mapped the s o i l s of the UBC Research F o r e s t . He found t h a t the predominant s o i l c l a s s was h u m i c - f e r r i c podzol. T e x t u r a l l y , most s o i l s were g u i t e coarse with Sandy Loam being t y p i c a l . 3.2 The Research P l o t 3,-2.1 L o c a t i o n and Physiography The r e s e a r c h p l o t i s a 30 x 30 m h i l l s l o p e stream-system l o c a t e d near Loon Lake (Figure 3-1) a t an e l e v a t i o n of 354 m. I t has a west-southwest aspect and an average s l o p e of 22° which v a r i e s w i t h i n the p l o t from almost f l a t t o over 40°. Vegetation i s very dense c o n s i s t i n g of D o u g l a s - f i r and Western Hemlock 5 to 7 m high with dense underbrush of f e r n , s a l a l , b l u e b e r r y , e t c . The area was c l e a r c u t i n 1958 and has remained unthinned s i n c e then. FIGURE 3-1 Depth to Bedrock and Location of the Research Area 30 3.2.2 P l o t Pedology The s o i l i n the r e s e a r c h p l o t has been analyzed by Nagpal and d e V r i e s (1976) and Bryck (1977). They d e s c r i b e d i t as a h u m i c - f e r r i c podzol, sandy loam i n t e x t u r e . T h i s i s c o n s i s t e n t with mapping done by K l i n k a (1976). A diagrammatic cross-^section i s presented i n F i g u r e 3-2. The top l a y e r of the s o i l i s the o r g a n i c f o r e s t f l o o r . I t i s 0.05 to 0.25 m t h i c k and i s comprised of l e a v e s , branches, r o o t s , e t c . i n v a r i o u s s t a t e s of decay. Some of t h i s i s s l a s h l e f t over from l o g g i n g with branches and stumps up to 0.5 m i n diameter. T h i s u n i t i s extremely permeable. Underneath the f o r e s t f l o o r i s a d i s c o n t i n u o u s Ae h o r i z o n , 5 to 10 mm t h i c k . T h i s i s u n d e r l a i n by 0.3 to 0.8 m of red-brown B h o r i z o n . This u n i t i s very heterogenous with p a r t i c l e s ranging i n s i z e from c l a y s t o boulders 1 m a c r o s s . The matrix ( p a r t i c l e s under 2 mm) i s t e x t u r a l l y a sandy loam. Also c o n t r i b u t i n g t o h e t e r o g e n e i t y are many r o o t s and r o o t channels. These are found i n high c o n c e n t r a t i o n throughout the u n i t with a very dense mat of r o o t s sometimes found along the s u r f a c e of the u n d e r l y i n g t i l l . T h i s occurs where the c o n t a c t i s w e l l d e f i n e d . In these p l a c e s the t i l l and the B h o r i z o n are e a s i l y d i s t i n g u i s h e d by d i f f e r e n c e s i n c o l o u r and hardness. Where these d i f f e r e n c e s are w e l l d e f i n e d , a l a r g e c o n t r a s t i n p e r m e a b i l i t i e s has caused downward growing r o o t s to fan out along the c o n t a c t , c r e a t i n g a r o o t mat 2 to 3 cm t h i c k . T h i s phenomenon i s s e l f p e r p e t u a t i n g as r o o t s growing i n t o t h i s r e g i o n r a i s e the h y d r a u l i c c o n d u c t i v i t y , a l l o w i n g f o r e a s i e r water e x t r a c t i o n and t h e r e f o r e a b e t t e r environment f o r more r o o t s to c o n c e n t r a t e . 0 1 I I metres 'FIGURE 3-2 Diagrammatic Cross-section of the Research 32 T h i s s i t u a t i o n r e s u l t s i n two d i s t i n c t h y d r o l o g i c u n i t s : the B ho r i z o n with high c o n d u c t i v i t y because of the l a r g e c o n c e n t r a t i o n of organic m a t e r i a l and the l o w - c o n d u c t i v i t y t i l l . Where t h i s c o n t r a s t o c c u r s , a perched water t a b l e o f t e n e x i s t s as was seen i n s e v e r a l l o c a t i o n s i n and near the r e s e a r c h p l o t . In other p l a c e s , a sharp c o n t a c t does not e x i s t . Rather, th e r e i s a gr a d u a l change from s o f t to hard and from red-brown to grey over a d i s t a n c e of one meter or more. Where t h i s occurs, the t i l l and the B h o r i z o n form a s i n g l e h y d r o l o g i c u n i t which has c o n d u c t i v i t i e s t h a t i n c r e a s e toward the s u r f a c e due to i n c r e a s i n g c o n c e n t r a t i o n of o r g a n i c pathways and decreasing degree of c o n s o l i d a t i o n . I t was observed t h a t t h i s s i t u a t i o n predominates i n the study p l o t and i s t h e r e f o r e of major h y d r o l o g i c s i g n i f i c a n c e . 3.2.3 P l o t Geology: T i l l As the nature of the c o n t a c t s would i n d i c a t e , the t i l l i t s e l f i s v a r i a b l e . I t grades from grey to green-grey i n an unweathered s t a t e to red-brown where extremely weathered. Where a r o o t has penetrated i t s s u r f a c e , a red-brown weathered zone a few c e n t i m e t e r s wide and up to 0.5 m i n l e n g t h i s sometimes surrounded by f r e s h , unweathered t i l l . Some p a r t s of the t i l l are reasonably s o f t while other p a r t s are so hard they can ba r e l y be broken with a pi c k . T h i s v a r i a b i l i t y i s d i s c u s s e d more f u l l y i n a f o l l o w i n g s e c t i o n . 33 The t i l l i s u s u a l l y found at a depth of 1 m. T h i s depth i s v a r i a b l e , i n p a r t from the o r i g i n a l p o s t - g l a c i a l and c u r r e n t topography and i n p a r t from the u n c e r t a i n t y of d e f i n i n g the c o n t a c t . Exposures along the seepage face and i s o l a t i o n troughs around the p l o t show t h i s v a r i a b i l i t y i n depth. I n order to expand these two-dimensional s e c t i o n s to a t h r e e - d i m e n s i o n a l s u r f a c e , an attempt was made to probe the s o i l with a s t e e l rod. T h i s rod was hammered i n t o the ground with the hope that d i f f e r e n c e s i n hardness would i n d i c a t e a c o n t a c t . U n f o r t u n a t e l y , t h i s technique was not s u c c e s s f u l as a marked c o n t r a s t between the t i l l and the B h o r i z o n d i d n o t always e x i s t . The l a c k of a well d e f i n e d contact a l s o precluded the use of q e o p h y s i c a l methods to l o c a t e the c o n t a c t . The t i l l , l i k e i t s dauqhter the B h o r i z o n ^ i s p o o r l y s o r t e d . I t c o n t a i n s p a r t i c l e s from c l a y s i z e up to angular b o u l d e r s 1 m a c r o s s . To g u a n t i f y the p a r t i c l e s i z e d i s t r i b u t i o n , seven samples were analysed. Three of these came from the r e s e a r c h p l o t while the other f o u r came from l o c a t i o n s up to 5 km away. A l l samples were a i r d r i e d and gently crushed to break up aggregates. Non-matrix m a t e r i a l was removed f o l l o w i n g which samples were Rotap si e v e d i n t o 7 s i z e f r a c t i o n s : 4.76, 2.00, 0.421, 0.210, 0.106, 0.074, and s m a l l e r than 0.074 mm. The under 0.074 mm f r a c t i o n s were then separated u s i n g the hydrometer method (Day 1965). A t o t a l of 18 s i z e f r a c t i o n s were separated from each sample. The average f o r these samples: 83.9% sand, 8.4% s i l t , and 7.7% c l a y (USDA s t a n d a r d s ) , i s c o n s i d e r a b l y s a n d i e r than the 57% sand noted by Armstrong (1955, 1957) f o r lowland Vashon t i l l . 34 However, i t i s c o n s i s t a n t with h i s statement t h a t mountain v a l l e y t i l l s c o n t a i n more sand than t h e i r lowland c o u n t e r p a r t s . I t i s p o s s i b l e t h a t uncrushed s i l t - c l a y aggregates make up p a r t of the sand s i z e f r a c t i o n , even though every attempt was made to minimize t h i s p o s s i b i l i t y . C o n s i s t e n t r e s u l t s between d u p l i c a t e d samples tends to i n d i c a t e the v a l i d i t y of the technigue. A l a c k of v a r i a t i o n between the seven l o c a t i o n s i n d i c a t e s t h a t the t i l l i s l a t e r a l l y homogeneous i n p a r t i c l e s i z e d i s t r i b u t i o n . 3.2.4 P l o t Geology: Bedrock D i r e c t l y u n d e r l y i n g the Vashon t i l l at v a r i a b l e depth i s the bedrock g r a n o d i o r i t e of the Coast C r y s t a l i n e complex. This rock i s hard, f r e s h and r e l a t i v e l y unweathered. In the r e s e a r c h p l o t , i t i s found at a depth of more than 1 m but u s u a l l y not more than 3 m below the undisturbed s u r f a c e . Along the man-made seepage face of the re s e a r c h p l o t , a c r o s s - s e c t i o n of the t i l l -bedrock c o n t a c t can be seen. I t v a r i e s from a depth of 1 meter i n the northern h a l f t o below road l e v e l i n the southern h a l f . T h i s r e f l e c t s a depth of more than 3 m from the undisturbed s u r f a c e . I s o l a t i o n troughs and s o i l sample h o l e s w i t h i n the p l o t a l s o i n d i c a t e t h a t depth to bedrock i s u s u a l l y around 2 to 3 m. The rod-hammer probe d i s c u s s e d above was a l s o i n c o n c l u s i v e f o r l o c a t i n g the three dimensional t i l l - b e d r o c k c o n t a c t as l a r g e boulders i n both the B h o r i z o n and t i l l were i n d i s t i n g u i s h a b l e from bedrock. Thus, the t i l l - b e d r o c k i n t e r f a c e i s not a c c u r a t e l y known. 35 Where bedrock outcrops along the seepage f a c e , f r a c t u r e s are widely spaced* T h i s i s the only p l a c e where bedrock i s exposed i n the p l o t and would, i f r e p r e s e n t a t i v e , imply t h a t bedrock h y d r a u l i c c o n d u c t i v i t i e s are low. However, the bedrock i n the p a i r e d r e s e a r c h p l o t to the south i s w e l l f r a c t u r e d along i t s s u r f a c e and must have r e l a t i v e l y high c o n d u c t i v i t y as p r e v i o u s work by d e V r i e s (pers. comm. 1977) i n d i c a t e d t h a t very l i t t l e a p p l i e d i r r i g a t i o n water appears as output. Thus, the h y d r o l o g i c r o l e of bedrock i n the c u r r e n t r e s e a r c h p l o t c o u l d not be surmized from g e o l o g i c i n v e s t i g a t i o n as t h e r e was no way to t e l l f r a c t u r e d e n s i t y beneath the surface,. As p r e v i o u s experimental work (Nagpal and d e V r i e s , 1976) i n d i c a t e d a high leakage r a t e , i t was a t f i r s t assumed t h a t f r a c t u r i n g was s i g n i f i c a n t . F u r t h e r work reported i n a l a t e r s e c t i o n does not support t h i s conclusion,. 3.3 Near P l o t Geology Because t h i s r e p o r t i s process o r i e n t e d , i t was f e l t t h a t the g e n e r a l i t y of the r e s e a r c h p l o t should be e s t a b l i s h e d . Two technigues were used to i n v e s t i g a t e the h y d r o g e o l o g i c a l c o n d i t i o n s of the r e s e a r c h p l o t v i s - a - v i s those of the area: s u r f i c i a l mapping and an examination of a s e r i e s of v e r t i c a l road cut s e c t i o n s . 36 3.3.1 S u r f i c i a l Mapping An attempt at s u r f i c i a l mapping was made i n the area around the research plot. This was done using a rod-hammer probe, an Oakfeld auger, and a shovel. Because the s o i l contained many large rocks and was often covered by 1-2 m of slash, these technigues were seldom useful. I t was possible, however, to map the regions with bedrock at or near surface (within^ 1/2 m) and those regions with bedrock at greater depth. I t was postulated that i n areas with deeper bedrock, hydrogeologic conditions would be similar to those of the study plot. In shallow bedrock areas, i t i s possible that flow mechanisms are d i f f e r e n t . The results are shown in Figure 3.1. It can be seen that i n 60 to 70% of the area around the study plot, depths to bedrock are similar to those within the plot; This technique i s l i m i t e d i n that i t does not indicate the actual depth or nature of bedrock-t i l l or t i l l - B horizon contacts. To augment t h i s , a v e r t i c a l section technique was used. 3*3.2 V e r t i c a l P r o f i l e s Since the mapping technigue used did not show actual thickness-contact relationships, 23 v e r t i c a l road cut sections within 2 km of the study plot were examined in d e t a i l . In addition, two other sections from the Seymour Watershed were reconstructured from the l i t e r a t u r e ( Chamberlin, 1972; deVries and Chow, 1973, 1978). The depths and thicknesses of the forest f l o o r , B horizon, t i l l , and bedrock were noted. These are presented i n Figure 3-3. Sections one through 23 represent O-i CO £ 0.2-1 2 3 4 5 6 7 8 > " 2 v 1 Z \ , I *"/ ' A -. - / „ . 1 , L u \ 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 TT fc&J 0.4-I 0.6-Q 1.0^ h1, i /•V ' o 9 c 7 o o.; c I 0 0 O C r r a 129 "0 T -"~ 1 \ "/ i -\ t i / -' 1 -~~^\ O-J Lrv Legend P.. 3. V A i O FOREST FLOOR B HORIZON TILL BEDROCK i f WELL-DEFINED CONTACT GRADATIONAL CONTACT FIGURE 3-3 Twenty Five V e r t i c a l P r o f i l e s from near the Research Plot 38 p r o f i l e s near the r e s e a r c h p l o t , while s e c t i o n 24 i s based on Chamberlain (1972) and s e c t i o n 25 i s based on d e V r i e s and Chow (1973). I t can be seen t h a t 12 out of 25 s e c t i o n s had d e f i n i t e to s e m i - d e f i n i t e (± 0. 15 m) c o n t a c t s , s i x had broader g r a d a t i o n a l c o n t a c t s ( u s u a l l y over 0.50 m wide) and seven s e c t i o n s had f o r e s t f l o o r r e s t i n g on bedrock, or a t most with a very t h i n B h o r i z o n i n t e r s p a c e d between f o r e s t f l o o r and bedrock. Thus, i t can be seen t h a t r e l a t i v e t h i c k n e s s of the fo u r u n i t s i n the r e s e a r c h p l o t are f a i r l y t y p i c a l of those i n the area. The f o r e s t f l o o r m a t e r i a l was s i m i l a r t o t h a t seen i n the re s e a r c h p l o t . I t contained decayed and semi-decayed twigs, branches, l e a v e s , e t c . , 0.1 to 0.45 m t h i c k . In a l l c a s e s , i t appeared to be very open and permeable. The B h o r i z o n was a l s o s i m i l a r to t h a t seen i n the r e s e a r c h p l o t . I t was brown-grey to red-brown i n c o l o u r and v a r i e d from 0.05 to g r e a t e r than 1 m t h i c k . I t had many l i v e r o o t s as we l l as decayed ro o t zones. As these were areas of higher water flow, g r e a t e r weathering was o f t e n seen as i n d i c a t e d by red^brown l i m o n i t e - h e m a t i t e s t a i n s . The presence of these weathered zones near the bottom of the s o i l p r o f i l e made i d e n t i f i c a t i o n o f the co n t a c t with the t i l l d i f f i c u l t , as these zones o f t e n f i n g e r e d i n t o what was unguestionably t i l l . The B h o r i z o n was u s u a l l y not s a t u r a t e d . T h i s was not s u r p r i s i n g as the study was conducted during June f o l l o w i n g a record dry winter. However, i n 4 of the 25 l o c a t i o n s the B h o r i z o n was s a t u r a t e d and had observable d i s c h a r g e . I n two of these s a t u r a t e d areas, a perched water t a b l e e x i s t e d above 0.3 39 to 0.5 m of unsaturated t i l l . T h i s s i t u a t i o n was a l s o observed at one l o c a t i o n i n the r e s e a r c h p l o t . The t i l l i n the surrounding area was s i m i l a r t o t h a t i n the r e s e a r c h p l o t . I t ranged i n t h i c k n e s s from as l i t t l e as 0.1 m to more than 0.60 m. I t may have been t h i c k e r but there were only two l o c a t i o n s where i t was p o s s i b l e to see both top and bottom. I t s c o l o u r ranged from an unweathered grey to a weathered red-brown. I t was a l s o seen to be very hard i n some p l a c e s and s o f t i n o t h e r s . T h i s d i f f e r e n c e i n hardness was a l s o observed i n the study p l o t . T e x t u r a l l y , the nearby t i l l was s i m i l a r t o t h a t i n the research p l o t but with one e x c e p t i o n . In s e v e r a l l o c a t i o n s a t h i n (1-3 cm) l a y e r of c l a y was present at the t i l l - b e d r o c k c o n t a c t . This l a y e r , which undoubtably r e f l e c t s some i c e - r o c k i n t e r a c t i o n , may be s i g n i f i c a n t h y d r o l o g i c a l l y i n t h a t the c l a y would form a l o c a l l o w - c o n d u c t i v i t y zone and tend to minimize flow i n t o bedrock j o i n t s . T h i s c l a y l a y e r was not observed on any bedrock with j o i n t s , however. In s h o r t , the t i l l i n the r e s e a r c h p l o t i s s i m i l a r t o that in the surrounding area, e s p e c i a l l y i n the v a r i a t i o n s e x h i b i t e d . T h i s v a r i a b i l i t y , caused by d i f f e r e n c e s i n degree of weathering, l e d p r e v i o u s workers to the c o n c l u s i o n t h a t both an a b l a t i o n and a compacted t i l l e x i s t ( Nagpal and d e V r i e s , 1976; Bryck, 1977). However, t h i s i s probably not the case. Nowhere could two d i s t i n c t t i l l s i n c o n t a c t with each other be seen. Nor d i d there seem to be any d i f f e r e n c e s i n pebble f a b r i c between the s o f t e r and harder t i l l s . D e t a i l e d examination of the 23 v e r t i c a l s e c t i o n s showed t h a t zones where the term a b l a t i o n t i l l c o u l d be 40 a p p l i e d graded i n t o compacted zones; P a r t i c l e s i z e a n a l y s i s of both hard and s o f t t i l l s y i e l d e d the same r e s u l t s . I t may be p o s s i b l e that two d i f f e r e n t t i l l s d i d e x i s t but, as c o l l u v i a l processes have been very a c t i v e , any d i f f e r e n c e s which were once present are no l o n g e r seen. Thus, i t i s concluded t h a t only one t i l l i s present i n the study area. H y d r o l o g i c a l l y , the question of whether there are one or two t i l l s i s unimportant. Hydr o l o g i c behavior i s not based on g e o l o g i c a l h i s t o r y , but on a c t u a l p h y s i c a l c h a r a c t e r i s t i c s . S o f t , weathered t i l l with o r g a n i c channels behaves l i k e B h o r i z o n and can be i n c l u d e d with i t i n one h y d r o l o g i c u n i t . Thus, a s e p a r a t i o n of h y d r o l o g i c u n i t s i s based on o r g a n i c content and degree of compaction, and not on genesis. The bedrock i n the v e r t i c a l s e c t i o n s was a l l g r a n o d i o r i t e . Depths ranged from 0 to more than 2 m below the u n d i s t u r b e d ground l e v e l . In some l o c a t i o n s , i t was not p o s s i b l e t o determine the t o t a l depth as the bedrock c o n t a c t was below the s u r f a c e of the road cut. In depth t o bedrock. F i g u r e 3-3 shows that the study p l o t i s r e p r e s e n t a t i v e f o r 18 out of 25 s e c t i o n s . Most bedrock exposures were more f r a c t u r e d than those seen i n the research p l o t . In s e v e r a l p l a c e s l a r g e s u r f a c e f r a c t u r e s 2 to 3 cm a c r o s s c o u l d be seen (Figure 3-4). Large f r a c t u r e a pertures combined with f r a c t u r e spacings of 10/m c o u l d g i v e a r e l a t i v e l y high h y d r a u l i c c o n d u c t i v i t y . The o r i g i n of these f r a c t u r e s i s not known. T h e i r o r i e n t a t i o n appeared to be random and as the only p l a c e s where well-exposed rock f a c e s c o u l d be viewed were road c u t s , i t i s p o s s i b l e t h a t these f r a c t u r e s were caused by road b u i l d i n g . O r i g i n a s i d e , i t i s probable t h a t FIGURE 3-4 Bedrock Fractures Tape length: 0.37 m 42 f r a c t u r e a p e r t u r e s become much s m a l l e r with depth. T h i s decrease i n s i z e coupled with the c l a y r i c h t i l l along the c o n t a c t , suggests that flow i n t o and through the bedrock should be minimal and most l i k e l y c o n f i n e d t o shallow depths. Thus, the d i f f e r e n c e s i n f r a c t u r i n g seen i n the p l o t and surrounding areas do not l i m i t g e n e r a l i z a t i o n of the experiment to the study area. 0 43 4.0- EXPERIMENTAL PROCEDURE -In order to examine the mechanism of stormflow g e n e r a t i o n , a c o n t r o l l e d i r r i g a t i o n experiment was conducted. T h i s experiment c o n s i s t e d of monitoring water i n f l o w , water outflow, and pi e z o m e t r i c pressures i n a study watershed during r i s i n g , s t e a d y - s t a t e and f a l l i n g outflow c o n d i t i o n s . The experiment was conducted on the 8th through 23rd o f August 1977. The watershed c o n s i s t e d of a 30 m sguare s e c t i o n of a h i l l s l o p e , an a r t i f i c i a l stream, r a i n gauges, piezometers, and outflow t i p p i n g buckets. In a d d i t i o n , chemical t r a c e r s were i n t r o d u c e d during the f i r s t p a rt of i r r i g a t i o n with samples taken at four stages o f the system: at the source, on the h i l l s l o p e , at the " b a s i n " o u t l e t , and below the h i l l s l o p e . These samples were examined i n order to deduce flow paths taken by r a i n f a l l on i t s journey to the stream. In t h i s chapter, the i n s t r u m e n t a t i o n and procedure of the experiment are d i s c u s s e d . In a d d i t i o n , the a u x i l i a r y measurements t h a t were made t o c a l c u l a t e h y d r a u l i c c o n d u c t i v i t i e s are d e s c r i b e d . A diagrammatic r e p r e s e n t a t i o n o f the h i l l s l o p e , stream, and i n s t r u m e n t a t i o n used i n the experiment, i s i n c l u d e d i n F i g u r e 3-2. 4.1 M o d i f i c a t i o n s to the P l o t In order to i n c r e a s e the s i g n i f i g a n c e of the experimental r e s u l t s , s e v e r a l m o d i f i c a t i o n s to the n a t u r a l s i t e have been made. A roof over the a r t i f i c i a l stream was made to e l i m i n a t e d i r e c t p r e c i p i t a t i o n i n t o the "stream" and the subseguent measurements of water not fl o w i n g through the h i l l s l o p e . 44 Another m o d i f i c a t i o n was an i s o l a t i o n t r e n c h around the s i d e s and back of the p l o t , dug down to the l e v e l of the t i l l . The trench was excavated i n order to reduce the unmeasurable a d d i t i o n of subsurface flow from o u t s i d e the p l o t . U n f o r t u n a t e l y , because the s o i l was so stony, the t r e n c h could not be dug deep enough. A f i n a l m o d i f i c a t i o n was the c o n s t r u c t i o n of a c o n c r e t e r u n o f f trough and placement of a 4 mil p l a s t i c sheet below the p l o t to d r a i n o f f s p r i n k l e r overspray. These m o d i f i c a t i o n s ensured t h a t below-plot piezometer r i s e s were due to subsurface flow o r i g i n a t i n g from i r r i g a t i o n water a p p l i e d t o the study p l o t . 4.2 The I r r i g a t i o n System Water was a p p l i e d to the r e s e a r c h p l o t v i a an e i g h t s p r i n k l e r i r r i g a t i o n system. Placement and s p r i n k l e r design was such t h a t " r a i n f a l l " was as uniform as p o s s i b l e . The s p r i n k l e r s rose approximately 6 m above the ground s u r f a c e and* f o r a l l but a few of the t a l l e s t t r e e s , were 1/2 meter or more above crown he i g h t . Water was pumped from an aerated sewage treatment lagoon 500 m west of the s i t e . Using a metered pump, i t was planned to i r r i g a t e at a constant r a t e throughout the experiment . However, plugging of the i n t a k e screen by algae sometime before the morning of the t h i r d day of i r r i g a t i o n (August 10) caused a " r a i n f a l l " r a t e which decreased c o n t i n u a l l y u n t i l 2:30 h rs on the f i f t h day (August 12), when the s i t u a t i o n was d i s c o v e r e d and c o r r e c t e d . L u c k i l y , t h i s problem turned out t o be of b e n e f i t t o the experiment because i t produced a h y d r a u l i c wave which was 45 measurable throughout the system. The l o c a t i o n of the s p r i n k l e r s i s i n d i c a t e d i n F i g u r e 4-1 by the l e t t e r s. 4.3 Rain Gauges Ten c o l l e c t i o n gauges and two t i p p i n g bucket continuous-r e c o r d i n g r a i n gauges were used t o monitor input to the study p l o t . In order to measure n a t u r a l r a i n f a l l which c o u l d have occurred i n a d d i t i o n t o i r r i g a t i o n , one continuous r e c o r d i n g r a i n gauge was monitored o u t s i d e of the i r r i g a t e d area. 4.3.1 C o l l e c t i o n Type Rain Gauges C o l l e c t i o n type r a i n gauges were placed 1/2 m above the ground surface at 10 l o c a t i o n s w i t h i n the study p l o t . These c o n s i s t e d of beveled edge p l a s t i c f u n n e l s with c o l l e c t i o n areas of 82.52 cm 2, f e e d i n g s e a l e d storage b o t t l e s . Water i n these b o t t l e s was measured once or twice d a i l y with a one l i t e r graduated c y l i n d e r . R a i n f a l l r a t e s a t the ground s u r f a c e v a r i e d a c c o r d i n g to p o s i t i o n i n r e l a t i o n to the s p r i n k l e r s and amount of v e g e t a t i o n cover. In order to c a l c u l a t e average r a i n f a l l as a c c u r a t e l y as p o s s i b l e f o r the e n t i r e p l o t , Thiessen weighted polygons were used (Dunne, 1974). Rain gauges were placed such t h a t areas d e f i n e d by Theissen polygons were approximately the same as areas d e f i n e d by v e g e t a t i o n cover. Thus, each r a i n gauge was r e p r e s e n t a t i v e of p r e c i p i t a t i o n t h a t a c t u a l l y h i t the s u r f a c e i n i t s r e s p e c t i v e area. From the c o l l e c t i o n gauges, average 46 0 - j — i — i — i — i — | — i — i — i — i — j — i — i — i — i — i — i — i — i — i — j — i — i — i — i — | — i — i — i — i — 0 5 10 15 20 25 30 DISTANCE, metres • 5 Standpipe *R8 Collection rainguage Outflow tipping bucket OD New piezometer A RG2 Continous event rainguage C~\ Thiessen • L14 Lower piezometer Polygon ° S Sprinkler FIGURE 4-1 Layout of the Research P l o t 47 r a i n f a l l r a t e s f o r the e n t i r e p l o t were c a l c u l a t e d . These r a i n gauges, l a b e l e d R1 to R10, and Thiessen polygons are shown i n F i g u r e 4-1. 4.3.2 T i p p i n g Bucket Rain Gauges In order t o c o r r o b o r a t e c o l l e c t i o n r a i n gauges and t o monitor d i u r n a l v a r i a t i o n , two continuous t i p p i n g bucket r a i n gauges were used. These had a diameter of 25.4 cm and an area of 506.7 cm 2. Monitoring was done with an E s t e r l i n e - A n g u s , paper-r o l l , continuous-event r e c o r d e r with each event equal to 16.0 cm 3 (0.0316 cm r a i n depth e q u i v a l e n t ) . Mean r a i n f a l l r a t e s were c a l c u l a t e d by countinq the number of events and e s t i m a t i n q to 10% the f r a c t i o n of the uncompleted event i n each h o u r l y p e r i o d . R e s o l u t i o n was ± 4% f o r low r a i n f a l l r a t e s and a s f i n e as ± 1/2% f o r the hiqhest r a i n f a l l r a t e s . T h i s deqree of r e s o l u t i o n made i t p o s s i b l e to d e t e c t and measure d i u r n a l r a i n f a l l v a r i a t i o n . Data from these r a i n qauqes were not used d i r e c t l y t o c a l c u l a t e i n p u t r a t e s . Rather, the measured percentaqe d i u r n a l v a r i a t i o n was superimposed on the values obtained from c o l l e c t i o n type qauqes. Thus, i t was assumed t h a t d i u r n a l v a r i a t i o n was uniform throuqhout the p l o t . F i q u r e 4-1 shows the l o c a t i o n of the continuous event qauqes, l a b e l e d RG1 and RG2. 48 4.4 Piezometers In order to understand h i l l s l o p e flow paths, 50 piezometers were used. Twenty of these had been i n s t a l l e d and used by Nagpal and d e V r i e s (1976) f o r e a r l i e r i n f i l t r a t i o n s t u d i e s . For convenience these are c a l l e d standpipes. An a d d i t i o n a l 30 piezometers were designed, b u i l t and i n s t a l l e d by the author i n order to help examine flow i n the t i l l and flow out of the research p l o t . These are c a l l e d the new piezometers . 4.4.1 Standpipes Twenty piezometers were i n s t a l l e d i n the B h o r i z o n by Nagpal and d e V r i e s to study the water t a b l e c o n f i g u r a t i o n . They were made of 1.91 cm O.D. g a l v a n i z e d s t e e l pipe approximately 1.45 m long and had ten 2 mm h o l e s i n the lower 1/4 m f o r an i n t a k e screen. These were not t r u e piezometers as they were not sealed at the t i p and t h e r e f o r e d i d not measure head at a p o i n t . However, they were s e a l e d at the s u r f a c e with c l a y to prevent "stem" flow down the tube* These piezometers , or s t a n d p i p e s as they are more a c c u r a t e l y termed, measured the water t a b l e l e v e l i n the permeable B h o r i z o n . T h e i r l o c a t i o n s are shown i n F i g u r e 4-1 numbered 1-20. 4.4.2 New Piezometers The 30 piezometers i n s t a l l e d by the author c o n s i s t e d of 4.45 cm O.D. g a l v a n i z e d s t e e l c o n d uit, 1.45 t o 4.01 m long. They were tapered at the t i p to prevent c l o g g i n g d u r i n g i n s t a l l a t i o n and had 34, 1 mm by 30 mm s l i t s i n the lower 13 cm f o r the i n t a k e 49 screen (Figure 4-2). A two-person Groundhog power auger was used to d r i l l the 6 cm hole r e q u i r e d . Because the t e r r a i n was both steep and rugged and e s p e c i a l l y because there were many cobbles and b o u l d e r s i n the B h o r i z o n and t i l l , i t was not p o s s i b l e t o d r i l l deeply enough to l o c a t e these piezometers where they c o u l d u n e q u i v o c a l l y e s t a b l i s h flow r a t e s i n the t i l l . (For t h i s reason I do not recommend t h i s method of i n s t a l l a t i o n f o r those c o n s i d e r i n q research i n s i m i l a r l o c a t i o n s . ) A f t e r d r i l l n g , sand was placed i n the bottom of the h o l e to ensure h y d r o l o g i c c o u p l i n q . A volume of sand c a l c u l a t e d t o f i l l to a depth j u s t above the i n t a k e screen was a p p l i e d with a tube i n s e r t e d t o the bottom of the hole. The piezometer was then d r i v e n i n t o the sand and s e a l e d i n t o p l a c e . The s e a l i n q of these piezometers was attempted i n two ways. The f i r s t nine were s e a l e d with b e n t o n i t e expandinq c l a y ( Q u i c k g e l ) . T h i s was attempted with both dry and s l u r r y forms. I t was not c e r t a i n with e i t h e r of these methods that s e a l i n g took p l a c e j u s t above the i n t a k e screen. For t h i s reason, some of these piezometers may, i n e f f e c t , be standpipes* F o r t u n a t e l y , only one of the g u e s t i o n a b l e piezometers (A) was i n the r e s e a r c h p l o t . F i v e of the remaining e i q h t were so f a r below the p l o t , they d i d not respond to i r r i q a t i o n and so s e a l i n g was unimportant. The remaining piezometers were w e l l - s e a l e d with a concrete s l u r r y . Fourteen piezometers were i n t s t a l l e d i n the r e s e a r c h p l o t to b e t t e r understand the r o l e of the Vashon t i l l . These were placed i n n e s t s of two or t h r e e which u s u a l l y i n c l u d e d one of the FIGURE 4-2 The New Piezometer 51 standpipes;. From these n e s t s , v e r t i c a l g r a d i e n t s and response times as f u n c t i o n s of depth were examined. Most of these piezometers were placed within the t i l l , but u n f o r t u n a t e l y none g r e a t e r than 0.5 m below the B h o r i z o n - t i l l content. The l o c a t i o n s of the new i n - p l o t piezometers are i n d i c a t e d i n F i g u r e 4-1, l a b e l e d A through N. To study leakage from the i r r i g a t e d a r e a , 16 piezometers were i n s t a l l e d below the study p l o t : t hree j u s t below the c o l l e c t i o n trough, e i g h t below the road near the n a t u r a l stream, downslope from the p l o t and the l a s t 5 a t some d i s t a n c e (up to 125 m) away. Because the lower f i v e showed no response, they are not d i s c u s s e d i n t h i s r e p o r t . The other 11 piezometers are l a b e l e d L6 through L16 i n F i g u r e 4-3. 4.4.3 Reading the Piezometers Piezometer water l e v e l s were measured using an a c r y l i c p l a t i c tube c o n t a i n i n g an expanded p o l y s t y r e n e (Styrofoam) f l o a t which adhered to the tube wall v i a s u r f a c e t e n s i o n at the he i g h t i t was f l o a t i n g . The measuring tube was i n s e r t e d i n t o a piezometer and then withdrawn with the d i s t a n c e from the t i p to the f l o a t measured t o the nearest m i l l i m e t e r . Because s o i l i n the water caused the p o s i t i o n o f the f l o a t to vary, readings were taken two t o f i v e times f o r each piezometer to e s t a b l i s h a r e p r e s e n t a t i v e water l e v e l . Accuracy of the average obtained i s estimated to be ± 0.5 cm. 52 FIGURE 4-3 Location of the Piezometers Below the Research Plot 53 Readings were taken at 90 minute to h a l f day i n t e r v a l s , depending on whether the experiment was i n a t r a n s i e n t or s t e a d y - s t a t e c o n d i t i o n . 4.5 C o l l e c t i o n Trough and T i p p i n g Buckets Outflow from the seepage face (Figure 3-2) was c o l l e c t e d by an a r t i f i c i a l stream (concrete trough) on the s u r f a c e of the t i l l , 2 m below the o r i g i n a l ground s u r f a c e . Although t h i s trough had been c o n s t r u c t e d f o r previous experiments by Nagpal and deVries (1976), i t was overhauled to reduce the p o s s i b i l i t y of l o s s e i t h e r a t the t r o u g h - t i l l i n t e r f a c e or through c r a c k s i n the o r i g i n a l c o n crete. T h i s trough was c o n s t r u c t e d with two o u t l e t s such t h a t outflow from the southern 2/3 of the study p l o t was measured s e p a r a t e l y from the n o r t h e r n 1/3. Outflow was measured with c a l i b r a t e d t i p p i n g buckets. These were c o n s t r u c t e d with a d j u s t a b l e volumes of up to 2.3 l i t r e per t i p . The north bucket was c a l i b r a t e d at 1.96 l i t r e per t i p while the south bucket was o r i g i n a l l y s e t at 2.25 l i t r e per t i p . T h i s volume d i d not remain constant however, as the adjustment screw s h i f t e d twice d u r i n g the experiment. To compensate, a c o r r e c t i o n f a c t o r was used f o r the p e r i o d s d u r i n g which no a c t u a l c a l i b r a t i o n e x i s t e d . T h i s f a c t o r was based on the o b s e r v a t i o n that during both c a l i b r a t e d p e r i o d s , the average outflow measured by the south bucket was approximately 1.5 (± 0.3) times the outflow measured by the north bucket. M a i n t a i n i n g t h i s r a t i o , f i v e c a l i b r a t i o n volumes f o r the south bucket were c a l c u l a t e d . With these, the a c t u a l t i m i n g s and r e l a t i v e volumes (within each c a l i b r a t i o n period) were preserved while a f a i r l y 54 accurate e s t i m a t i o n o f outflow -volume was made p o s s i b l e . The c a l i b r a t i o n of the north bucket remained c o n s t a n t throughout the experiment. Outflow was recorded on two channels of the same event recorder used f o r r a i n f a l l . Hourly outflow r a t e s were c a l c u l a t e d by counting the number of t i p s d u r i n g the 15 minute p e r i o d past each hour and m u l t i p l i e d by f o u r . Because the number of events during t h i s p e r i o d was l a r g e ( u s u a l l y 80 t o 140), i t was n e i t h e r necessary nor p r a c t i c a l to estimate the remaining volume o f water f o r the uncompleted t i p at the end of each p e r i o d . T h i s l e d to a r e s o l u t i o n of 1 % to 4%, depending on outflow volume. Such p r e c i s i o n i s comparable to other elements of the system. 4.6 Chemical T r a c e r s In order to deduce flow paths through the h i l l s l o p e , chemical t r a c e r s of known c o n c e n t r a t i o n were a p p l i e d to the h i l l s l o p e . These t r a c e r s c o n s i s t e d of C l ~ , K +,N0~ , NH*, and P03-. Tagged water mixed i n a sewage treatment lagoon 500 m to the west was a p p l i e d f o r the f i r s t seven days of the experiment, at which time a water-flux steady s t a t e was we l l e s t a b l i s h e d . Clean l a k e water was then a p p l i e d f o r the remaining f i v e days with hopes t h a t the c l e a n water " f r o n t " would be observable. Samples were taken f o u r times d a i l y a t f o u r p a r t s o f the system: at the i n t a k e to the e i g h t s p r i n k l e r s , i n the p l o t a t piezometer C and B, a t both outflow t i p p i n g buckets, and below the p l o t at piezometers L15 and L9. E l e c t r i c a l c o n d u c t i v i t y measurements were a l s o made on the outflow water to observe s o l u t e breakthrough. 55 Samples wera analyzed by the P o l l u t i o n C o n t r o l E n g i n e e r i n g Laboratory, U.B.C, using an a a u t o a n a l y z e r . Analyses were done wi t h i n 48 hours, a f t e r being t r e a t e d with s u l f u r i c a c i d and st o r e d near 0°C to reduce i o n i c s p e c i e s t r a n s f o r m a t i o n . E l e c t r i c a l c o n d u c t i v i t y measurements were made i n the f i e l d using a Y e l l o w f i e l d model 33 c o n d u c t i v i t y meter. 4.7 H y d r a u l i c C o n d u c t i v i t y Determinations The h y d r a u l i c c o n d u c t i v i t y o f the t i l l and the lower B h o r i z o n was determined by s e v e r a l methods. D i r e c t measurements were made by s l u g t e s t s and i n f i l t r o m e t e r t e s t s . Values were a l s o c a l c u l a t e d i n d i r e c t l y from the r e s u l t s of the i r r i g a t i o n experiment . I n t e r p r e t a t i o n o f s l u g t e s t data i s based on H v o r s l e v ' s (1951) method. T h i s approach combines an e m p i r i c a l shape f a c t o r and the d i f f e r e n t i a l form of Darcy's law t o produce formulae f o r c o n d u c t i v i t y as a f u n c t i o n of head change over time from an i n j e c t e d s l u g of water. T h i s t e s t was performed on s i x of the new piezometers during a p e r i o d of s t e a d y - s t a t e flow i n the h i l l s l o p e , well a f t e r the i r r i g a t i o n experiment was run. The homogeneous i s o t r o p i c form of the Hvorslev equation was used. T h i s approach should y i e l d at l e a s t an order of maqnitude accuracy. In a d d i t i o n to the piezometers t e s t e d , L15 was pumped f o r chemical a n a l y s i s d u r i n q the experiment . T h i s allowed the use of the subsequent water l e v e l r i s e to be used i n the Hvorslev equation as a b a i l t e s t . 56 An i n f i l t r o m e t e r , designed and b u i l t by J . DeVries and C. Paul with some m o d i f i c a t i o n s by the author, was used to measure i n f i l t r a t i o n r a t e s on c l e a r e d - o f f s e c t i o n s of t i l l and lower B h o r i z o n T h i s device c o n s i s t e d of a 0-5 x 0.5 m g r i d of 400 hypodermic needles which produced a r t i f i c i a l r a i n d r o p s , supported approximately 0.8 m above a t e s t s e c t i o n of t i l l or s o i l . Water was s u p p l i e d by a c o n t r o l l e d b u r e t t e system. The theory behind t h i s instrument i s based on P h i l i p (1957) who s t a t e d t h a t the s t e a d y - s t a t e i n f i l t r a t i o n r a t e i s e g u a l to the 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 y . U n f o r t u n a t e l y , the i n f i l t r a b i l i t y o f the t i l l was too low f o r the i n f i l t r o m e t e r to maintain and measure the uniform j u s t - p o n d i n g c o n d i t i o n s r e g u i r e d f o r the use of P h i l i p ' s eguation. To overcome t h i s problem, higher i n p u t r a t e s were used with an attempt to measure ru n o f f and c a l c u l a t e i n f i l t r a t i o n r a t e from the d i f f e r e n c e between i n p u t and output. This technique d i d not y i e l d s a t i s f a c t o r y r e s u l t s as i t was not l o g i s t i c a l l y p o s s i b l e to run the apparatus long enough t o ensure s t e a d y - s t a t e c o n d i t i o n s For these reasons, the r e s u l t s of t h i s work are not presented i n t h i s r e p o r t . 4.8 Data Reduction Data a n a l y s i s was s i m p l i f i e d by the use of the computing f a c i l i t i e s a t the U n i v e r s i t y of B r i t i s h Columbia . Raingauge data, read as a volume of water at an a r b i t r a r y time, were converted d i r e c t l y to average r a i n f a l l r a t e s and i n f i l t r a t i o n volumes. Piezometers , which were read i n s e v e r a l ways ( i . e . r e l a t i v e t o the top or to the bottom depending on depth to 57 water) were converted to a common datum. Hydrographs f o r r a i n f a l l , piezometers , and outflow were drawn using Calcorop s u b r o u t i n e s and p r i n t e d by a T e k t r o n i x 4012 CRT d i s p l a y t e r m i n a l with a hard copy attachment. Thus, many t e d i o u s c a l c u l a t i o n s were avoided with the bonus of i n c r e a s e d accuracy. 58 5.0- Experimental R e s u l t s -In t h i s chapter the r e s u l t s of the f i e l d r a i n f a l l - r u n o f f experiment are presented. Hydrographs and t a b l e s f o r a r t i f i c i a l r a i n f a l l ( h e r e a f t e r c a l l e d r a i n f a l l ) , p i e z o m e t r i c l e v e l s , and outflow are l i s t e d . These are f o l l o w e d by r e s u l t s from the s l u g t e s t s and chemical analyses. Included with these data are minor d i s c u s s i o n p e r t a i n i n g t o b a s i c i n t e r p r e t a t i o n . The major d i s c u s s i o n of analyses of the f i e l d experiment are presented i n the next chapter a f t e r a l l b a s i c data have been presented. 5.1 R a i n f a l l Mean r a i n f a l l r a t e s f o r each of the c o l l e c t i o n type r a i n gauges are l i s t e d i n Table 5-1. T o t a l p l o t i n p u t expressed both v o l u m e t r i c a l l y and averaged over the i r r i g a t e d area i s shown i n Table 5-2. R a i n f a l l from continuous r e c o r d i n g type gauges i s graphed i n F i g u r e s 5-1 and 5-2. A d i u r n a l r a i n f a l l v a r i a t i o n was i n d i c a t e d by both r e c o r d i n g gauges. In order t o b e t t e r demonstrate t h i s v a r i a t i o n , superimposed d a i l y r a i n f a l l from RG1, f o r the p e r i o d of August 13th through 20th, i s shown i n F i g u r e 5-3. Minimum i n p u t occurred between 13:00 and 19:00 h r s . There was no s i n g l e , well d e f i n e d i n p u t maximum except on the 15th, 16th, and 17th when peak r a i n f a l l o c c u r r e d at 10:00 h r s . The minimum c o i n c i d e d with the h o t t e s t p a r t of the day when much mist and vapour was observed over the p l o t . I t i s l i k e l y t h a t i n t e n s e e v a p o r a t i o n and low humidity around the p l o t , caused by d a i l y temperatures near 30°C, produced the d i u r n a l v a r i a t i o n i n r a i n f a l l r eaching Table 5-1. Average rainfall rates for the ten collection type rain gauges. Rain gauge 1 2 3 4 5 6 7 8 9 10 August Cm/Hr Cm/Hr Cm/Hr Cm/Hr Cm/Hr Cm/Hr Cm/Hr Cm/Hr Cm/Hr Cm/Hr 9.63 0.33 0.40 0.50 0.31 0.55 0.35 0.48 0.41 0.32 0.33 10.52 0.33 0.07 0.50 0.26 0.54 0.32 0.44 0.37 0.27 0.23 11.48 0.23 0.08 0.36 0.15 0.41 0.27 0.29 0.24 0.20 0.21 12.08 0.15 0.03 0.22 0.06 0.22 0.13 0.08 0.02 0.08 0.18 12.85 0.34 0.10 0.48 0.25 0.48 0.28 0.42 0.38 0.22 0.24 13.32 0.36 0.24 0.55 0.41 0.59 0.33 0.64 0.67 0.27 0.27 13.81 0.42 0.14 0.55 0.35 0.57 0.28 0.49 0.48 0.14 0.24 14.49 0.39 0.16 0.57 0.30 0.58 0.34 0.72 0.62 0.29 0.28 15.37 0.35 0.17 0.27 0.33 0.57 0.36 0.48 0.59 0.18 0.27 16.37 0.33 0.12 0.51 0.32 0.54 0.36 0.42 0.13 0.26 0.24 17.37 0.33 0.10 0.51 0.32 0.53 0.35 0.48 0.10 0.27 0.23 18.39 0.35 0.10 0.52 0.30 0.54 0.33 0.51 0.09 0.32 0.23 19.36 0.34 0.20 0.52 0.25 0.55 0.36 0.62 0.17 0.26 0.25 20.36 0.35 0.10 0.52 0.31 0.54 0.35 0.55 0.23 0.25 0.25 21.40 0.36 0.09 0.51 0.35 0.59 0.33 0.48 0.28 0.24 0.23 21.76 0.38 0.16 0.56 0.34 0.58 0.37 0.52 0.42 0.26 0.23 Table 5-2. Total plot input and average rainfall rate for the whole plot. Total Input Average Rate August m3/hr cm/hr m/s(x 10" ) 9.633 2.386 0.33 8.2 10.524 2.053 0.28 7.8 11.480 1.462 0.20 5.6 12.078 0.617 0.08 0.2 12.851 1.956 0.27 7.5 13.323 2.757 0.38 10.6 13.813 2.253 0.31 8.6 14.493 2.691 0.37 10.3 15.372 2.292 0.31 10.6 16.369 1.906 0.26 7.2 17.368 1.902 0.26 7.2 18.394 1.967 0.27 7.5 19.360 2.065 0.28 7.8 20.365 2.076 0.28 7.8 21.396 2.128 0.29 8.1 21.764 2.340 0.32 8.9 6r AUGUST 19-20 9* 71 \T» 9* S i t itf FIGURE 5 - 3 Superimposed daily r a i n f a l l from rain gauge R G 1 64 the s u r f a c e . The c o l l e c t i o n and continuous r a i n gauges gave s i m i l a r r e s u l t s . Both types of gauges i n d i c a t e d the decrease i n p r e c i p i t a t i o n because of the p a r t i a l c l o g g i n g of the i r r i g a t i o n pump input screen. A l s o , c o l l e c t i o n gauge R3 and r e c o r d i n g gauge EG 1, l o c a t e d near each other (Figure 4-1), both i n d i c a t e d a mean r a i n f a l l r a t e of 0.52 cm/hr. T h i s aggreement i n d i c a t e s t h a t c a l i b r a t i o n was c o n s i s t e n t f o r both types o f gauge. Seemingly c o n t r a r y to t h i s c o n c l u s i o n , EG2 showed a marked decrease i n r a i n f a l l d uring the p e r i o d of the 16th through the 20th t h a t was not seen i n surrounding c o l l e c t i o n type gauges. Such behavior was due to m a l f u n c t i o n i n g of the mercury switch on the t i p p i n g bucket. T h e r e f o r e , t h i s p r e c i p i t a t i o n decrease was not r e a l . 5.2 Piezometers Piezometer hydrographs are shown i n two ways: i n d i v i d u a l l y and i n groups of two to f i v e . These groups are not t r u e v e r t i c a l n e s t s because the h o r i z o n t a l s p a c i n g between some of these piezometers i s g r e a t e r than t h e i r v e r t i c a l s e p a r a t i o n . A l l piezometers were r e f e r e n c e d t o a common datum at the road (0.0 m). The piezometer hydrographs are l i s t e d i n the Appendix, F i g u r e s A5-1 to A5-23. Most piezometers w i t h i n the p l o t (1 through 20, B through M ex c l u d i n g F and K) were i n the B h o r i z o n and responded g u i c k l y to the i n i t i a t i o n o f r a i n f a l l . Piezometer 19 rose w i t h i n 1 1/2 hours while two others (3 and 20) began to r i s e i n l e s s than 3 hours. The mean i n i t i a l response time was 10.3 hours f o r the standpipes (shallower) and 16.9 hours f o r the new piezometers 65 (deeper). The l a t e r response of deeper piezometers i s c o n t r a r y to unsaturated i n f i l t r a t i o n theory i n homogeneous media (Rubin and S t e i n h a r d t , 1963). S a t u r a t i o n occurred at shallower l e v e l s f i r s t because v e r t i c a l l y c h a n n e l i z e d flow from the s u r f a c e produced l o c a l l y s a t u r a t e d zones. V e r t i c a l l y c h a n n e l i z e d s a t u r a t e d flow was d i s c u s s e d by deVries and Chow (1973, 1978). The lower B h o r i z o n piezometers rose and f e l l g u i c k l y as r a i n f a l l v a r i e d , showing a d i u r n a l v a r i a t i o n of i 2 to 5 cm. Th i s v a r i a t i o n i s only apparent on the piezometer hydrographs d u r i n g the f i r s t h a l f of the experiment when water l e v e l s were recorded more f r e g u e n t l y . Piezometers 1, 6, 7, 13, and 14 i n d i c a t e d d i s c o n t i n u o u s s a t u r a t i o n . Because these piezometers were shallow, they probably were i n the s a t u r a t e d zone only duri n g the higher phase of the d i u r n a l c y c l e . The piezometers i n the lower B h o r i z o n a l s o responded t o the decrease i n r a i n f a l l on the t h i r d and f o u r t h day of i r r i g a t i o n (August 10th and 11th) caused by of the p a r t i a l c l o g g i n g of the i n p u t screen on the pump. This response was shown by f a l l i n g heads s t a r t i n g l a t e on the 10th and c o n t i n u i n g through the 11th. A subseguent r i s e of up to 0.20 m occurred on the morning of the 12th when i r r i g a t i o n r eturned t o normal. F i v e piezometer t i p s were l o c a t e d at depths g r e a t e r than 0.30 m i n t o the t i l l . Four o f these (A, F, K, and N) demonstrated delayed and damped response. Damping was shown by reduced d i u r n a l v a r i a t i o n and s m a l l e r response t o the l e s s e r r a i n f a l l of the 10th and 11th. (The l a r g e head drop and recovery i n piezometer N on the e a r l y 11th remains a mystery. The r i s e occurred too soon to be i n d i c a t i v e of the r e t u r n to h i g h e r 66 r a i n f a l l , Figure A5-9). The delayed response in the t i l l i s shown by: l a t e r i n i t i a l r i s e , greater time required to reach steady-state, and lonqer laq between the temination of r a i n f a l l and the i n i t i a t i o n of f a l l i n g head. Piezometers below the i r r i g a t e d area responded to r a i n f a l l applied to the plot. Piezometer L16 ( d i r e c t l y below the outflow trough) i n i t i a l l y rose 0.30 m and then indicated a head drop caused by the decrease i n r a i n f a l l on the 10th and 11th. Piezometer L15 was not read freguently enough to note th i s reduction* Downslope response was also seen i n the piezometers farther below the plot (L6 through L13). The r i s e i n these piezometers was approximately 0.10 m. However, because the r i s e occurred gradually, time lags could only be resolved to between 70 and 130 hours. Piezometer L8 indicated flowing artesian conditions and therefore a natural discharge area. As an extension to contain the water at i t s equilibrium l e v e l was not properly sealed u n t i l the 16th, no head r i s e s were observed in t h i s piezometer. 5.3 Outflow Outflow from the north, south and combined, tipping buckets i s shown i n Figure 5-4. The flood wave began 19 1/2 hours aft e r r a i n f a l l began with the f i r s t peak occurring after 40 1/2 hours. Outflow rose and f e l l c y c l i c a l l y u n t i l steady state was reached after 6 days. This c y c l i c v a r i a t i o n was diurnal and amounted to + 25% of the mean dai l y outflow. Based on the steady state period August 13th through 20th, the mean dai l y maximum occurred at 9:15 hrs (± 15 minutes) with the mean minimum at 19:50 hrs (± 68 15 m i n u t e s ) . A f t e r p r e c i p i t a t i o n c e a s e d , out f low began to dercease w i t h i n 1 1/2 to 2 h o u r s . F o r t y hours l a t e r , ou t f l ow was 1/12 the r a t e p r e v a i l i n g be fore i r r i g a t i o n was shut o f f -5.4 Chemis try The r e s u l t s o f the c h e m i c a l ana lyse s were s u r p r i s i n g . I t had been h y p o t h e s i z e d t h a t h i g h i n f l o w c o n c e n t r a t i o n s of P0 3 ~ and NH£ (10 m g / l and 17 m g / l r e s p e c t i v e l y ) and s h o r t c o n t a c t t ime with the m i n e r a l s o i l would produce h igh out f low c o n c e n t r a t i o n s o f these s p e c i e s . I n s t e a d , a n a l y s e s showed no d e t e c t a b l e amounts at any sample l o c a t i o n (except i n p u t ) . These r e s u l t s i n c l u d e samples taken at i n - p l o t p i ezometers B and C , which were l e s s than 1.4 m below the s u r f a c e . I t appears t h a t exchange c a p a c i t i e s , s e s g u i o x i d e - p h o s p h a t e r e a c t i o n s and n i t r o g e n f i x i n g r e a c t i o n s w i t h i n the f o r e s t f l o o r a n d / o r B h o r i z o n were more s i g n i f i c a n t than p r e v i o u s l y e x p e c t e d . Al though these s p e c i e s were a p p l i e d a t r a t e s c o n s i d e r a b l y h i g h e r than those o f Bryck (1977), these f i n d i n g s are s t i l l s i m i l a r to h i s ; a l l P0 3~ and NH* were t i e d up. Because the s o i l was so a d s o r p t i v e , t h i s c o n c e n t r a t i o n data c o u l d not be used to e l u c i d a t e f low p a t h s . C h l o r i d e r e s u l t s were a l s o s u r p r i s i n g . The c h l o r i d e d a t a f o r i n p u t , i n - p l o t (p iezometers B and C ) , o u t f l o w , and below p l o t (piezometers L15 and L9) a r e graphed i n F i g u r e 5 -5 . C o n c e n t r a t i o n a t the i n i t i a t i o n o f ou t f l ow was n e g l i g i b l e . E i s i n g s t e a d i l y , c o n c e n t r a t i o n peaked at a p p r o x i m a t e l y 1/2 the i n p u t l e v e l , more than 2 days a f t e r s w i t c h i n g t o c l e a n water. A n a l y s e s of N0~ and K + r e v e a l e d s i m i l a r t i m i n g s , but wi th i n p u t - o u t f l o w c o n c e n t r a t i o n r a t i o s t h a t were l ower . The two-day-120 - i 100 -O) £ 80 h" 60 cr LU o z o o 40 -2 0 -INPUT CHLORIDE CONCENTRATION AT SIX LOCATIONS SOUTH BUCKET 10 TCURIi ~T~ 12 I 14 NORTH BUCKET PIEZOMETER B PIEZOMETER C .• / ^ PIEZOMETER L15 16 ~r 18 20 22 n 24 DATE : AUGUST 1977 ON VO Chloride Concentration vs. Time at Four Sample Locations: Input, In-plot (Piezometers B and C),Outflow and Below-plot 70 plus concentration-peak l a g and i n p u t - o u t f l o w c o n c e n t r a t i o n r a t i o s of l e s s than one, i n d i c a t e one or more of the f o l l o w i n g : the occurrence of flow path r e a c t i o n s t r a n s f o r m i n g i n p u t s p e c i e s i n t o unanalysed ones, c a t i o n and anion exchange with the s o i l , d i s p e r s i o n , and / or the mean flow path t r a v e l time (average l i n e a r v e l o c i t y ) . I t had been hoped t h a t c h l o r i d e would a c t as a non - r e a c t i n g base l e v e l t r a c e r with which to compare phosphate and ammonium. T h i s could not be done and t h e r e f o r e l i t t l e flow path i n f o r m a t i o n was gained. 5.5 Slug T e s t s R e s u l t s of the s l u g t e s t s are l i s t e d i n Table 5-3. H y d r a u l i c c o n d u c t i v i t i e s ranged from T O - 7 to 10 ~ 6 m/s with a mean value around 5x10~ 6 m/s. The lowest c o n d u c t i v i t y of 10~ 7 m/s was measured i n piezometer F l o c a t e d i n an area of w e l l d e f i n e d , more compacted t i l l with a sharp B h o r i z o n c o n t a c t . Upper t i l l c o n d u c t i v i t i e s (piezometers B, D, L16, and L15) as w e l l as lower B h o r i z o n c o n d u c t i v i t y (piezometer G) were a l l approximately 8 x 10~ 7 m/s. These areas r e f l e c t a B h o r i z o n t o t i l l t r a n s i t i o n zone i n s t e a d o f a w e l l d e f i n e d c o n t a c t . Thus, i n t r a n s i t i o n areas, the h y d r a u l i c c o n d u c t i v i t i e s i n the upper t i l l and the lower B h o r i z o n are probably s i m i l a r but with a s l i g h t i n c r e a s e toward the s u r f a c e . Piezometer A i n d i c a t e d a c o n d u c t i v i t y of 9 x 10 -* m/s. This was not accurate as the water l e v e l from the i n j e c t e d s l u g f e l l a t two d i s t i n c t r a t e s , c o n f i r m i n g a s u s p i c i o n that t h i s piezometer was not s e a l e d j u s t above the i n t a k e screen. The c a l c u l a t e d value was too high as improper s e a l i n g produced an 71 Table 5-3. Hydraulic conduct iv i t ies calcula ted from Hvorslev (1951). R2m(^) (Where K = ? [ J R = 2.5 cm, L = 15 cm, r = 1.75 cm), o Piezometer TQ(min) K(m/s) Unit Slug Test A 13 9 x 10" 6 * T i l l & B B 330 3 x 10" 7 T i l l D 180 6 x 10" 7 T i l l F 840 1 x 10" 7 T i l l G 140 9 x 10" 7 B L16 72 1 x IO" 6 T i l l Ba i l Test L15 < 120 > 9 x IO" 7 T i l l See tex t . 72 i n f l o w area l a r g e r than t h a t used i n the Hvorslev (1951) eguation. Therefore, t h i s c o n d u c t i v i t y i s not i n d i c a t i v e of the t i l l or the B h o r i z o n . 73 6..0 - ANALYSES, - INTERPBETATIOJU AND -DISCUSSION-In t h i s chapter, the r e s u l t s from the p r e v i o u s chapter are i n t e r p r e t e d and d i s c u s s e d . To begin with, the eguation of c o n t i n u i t y i s used t o c a l c u l a t e the non-stormflow ground water i n p u t and storage volume of the p l o t * The water t a b l e i s then d i s c u s s e d : i t s nature, p o s i t i o n , and r o l e i n determining flow paths and g r a d i e n t s . F o l l o w i n g t h i s , the bulk h y d r a u l i c c o n d u c t i v i t y of the h i l l s l o p e p l o t i s c a l c u l a t e d and compared with measured values. From a l l of these r e s u l t s and analyses the mechanism of stormflow g e n e r a t i o n i s then summarized. F i n a l l y , two p o s s i b l e o b j e c t i o n s to g e n e r a l i z i n g t h i s mechanism to other watersheds are d i s c u s s e d . 6.1 Non-Stormflow Groundwater Input In previous work by Nagpal and d e V r i e s (1976) the term leakage was used to d e s c r i b e the water l o s t from the s o i l i n the form of flow i n t o the t i l l . Because the approach i s being taken t h a t the s a t u r a t e d and unsaturated zones make up one system, the term non-stormflow ground water input w i l l be used i n s t e a d of leakage. Non-stormflow ground water i n p u t i s the component o f s a t u r a t e d flow which flows i n t o the t i l l and does not e x i t s h o r t l y t h e r e a f t e r . To c a l c u l a t e the non-stormflow ground water i n p u t the eguation of c o n t i n u i t y can be used: I - (0S + q ) = AS Where: I = E a i n f a l l i n p u t 0 S = Outflow at the c o l l e c t i o n trough 74 Of = Outflow i n t o the t i l l A S = The change i n storage At steady s t a t e , &S = 0 and the flow i n t o the t i l l i s egual to the d i f f e r e n c e between r a i n f a l l input and outflow a t the c o l l e c t i o n trough. Steady-state i n p u t d u r i n g the l a t t e r p a r t of the r a i n f a l l - r u n o f f experiment averaged 2.11 m 3/hr or 7.97 x 10~ 7 m/s i n t e g r a t e d over the t o t a l i r r i g a t e d area of 735 m2. Outflow d u r i n g steady s t a t e had a mean r a t e o f 1.70 m 3/hr or 6.43 x 10 " 7 m/s. The d i f f e r e n c e between these values i n d i c a t e s a l o s s of 1.54 x 10 " 7 m/s or l e s s than 20% of i n p u t . (This d i f f e r e n c e can be seen as the area between i n p u t and outflow i n Fi g u r e 6-1). The a c t u a l l o s s was undoubtably l e s s than t h i s f i g u r e as piezometers A, F, and N were s l o w l y r i s i n g d u r i n g t h i s p e r i o d , i n d i c a t i n g t h a t some water was s t i l l going i n t o storage. Non-storm ground water flow out of the p l o t was confirmed by pi e z o m e t r i c response below the i r r i g a t e d area. There are two reasons why t h i s c a l c u l a t e d 20% l o s s i s s i g n i f i c a n t l y l e s s than the 75% r e p o r t e d by Nagpal and d e V r i e s . The f i r s t i s t h a t r a i n f a l l r a t e s were c a l c u l a t e d d i f f e r e n t l y . Nagpal and d e V r i e s , c a l c u l a t e d r a i n f a l l a t 0.6 cm/hr as opposed to the mean value of 0.287 cm/hr f o r the l a t e s t experiment. T h e i r f i g u r e i s s i m i l a r to the h i g h e s t r a t e s measured i n exposed ( i . e . no v e g e t a t i o n cover) r a i n gauges i n t h i s l a t e s t experiment where r a i n f a l l , r a t e s v a r i e d from 0.07 cm/hr to 0.72 cm/hr. Thus, the use of Theissen weighted polygons and r e p r e s e n t a t i v e placement of r a i n gauges gave a r a i n f a l l r a t e t h a t was l e s s than h a l f of t h a t estimated p r e v i o u s l y . T h i s helped produce a non-stormflow ground water component that was l e s s than f i v e times 76 s m a l l e r than t h a t c a l c u l a t e d by Nagpal and d e V r i e s . The lower r a i n f a l l r a t e was supported by a comparison of r a i n f a l l and pumping r a t e s . The r a i n f a l l r a t e of 0.287 cm/hr i s e g u i v a l e n t to a n o - l o s s pumping r a t e of 2.11 m 3/hr. The a c t u a l pumping r a t e averaged 3.11 m 3/hr. The 32% d i f f e r e n c e was supported by o b s e r v a t i o n s of s p r i n k l e r overspray and clouds of mist advected out of the p l o t during i r r i g a t i o n . The higher r a i n f a l l r a t e of 0.60 cm/hr would have r e g u i r e d a pumping r a t e of 4-41 m 3/hr without t a k i n g i n t o account a d v e c t i v e and overspray l o s s e s . Thus, i f the higher r a i n f a l l r a t e was a c c u r a t e , the pumping r a t e would have to have been at l e a s t 42% higher than that measured i n the l a t e s t experiment. Since the same c o n s t a n t - r a t e i r r i g a t i o n system was used f o r both experiments i t i s u n l i k e l y t h a t the higher r a t e i s v a l i d * A second reason f o r the discrepancy i n the ground water i n p u t values i s t h a t Nagpal and d e V r i e s ' c a l c u l a t i o n s were not based on t r u e s t e a d y - s t a t e . I t can now be seen that the l e v e l i n g o f f of outflow which they a t t r i b u t e d to the onset of steady-s t a t e c o n d i t i o n s was r e a l l y j u s t an outflow d i u r n a l peak. Had they continued i r r i g a t i o n , outflow would have i n c r e a s e d to a h i g h e r r a t e through s e v e r a l more d i u r n a l c y c l e s . Thus, some o f the d i f f e r e n c e between input and outflow which they a t t r i b u t e d to leakage from the s o i l was a c t u a l l y due to water going i n t o storage. For t h i s reason as w e l l as t h e i r overestimated r a i n f a l l r a t e , Nagpal and d e V r i e s ' e s t i m a t i o n of the non-stormflow ground water i n p u t was probably much too high- The 20% l o s s r a t e suggested here i s more l i k e l y to be c o r r e c t . 77 The d i u r n a l v a r i a t i o n i n both i n p u t and outflow coupled with a much lower non-stormflow ground water i n p u t r a t e i n d i c a t e d that the previous i n t e r p r e t a t i o n of water breaking through i m p e r f e c t i o n s i n the compacted t i l l i s not necessary to e x p l a i n the l o s s from the system. I t i s u n l i k e l y t h a t a temporally and s p a t i a l l y d i s c o n t i n u o u s process such as h y d r o l o g i c breakthrough would have occ u r r e d at the same time each day. I t i s more l i k e l y t h a t the v a r i a t i o n i n outflow was d i r e c t l y caused by the v a r i a t i o n i n i n p u t as both of these volumes and t i m i n g s were s i m i l a r (Figure 6-1). Synchonous t i m i n g s were a l s o seen i n h i l l s l o p e piezometers. Thus, the v a r i a t i o n i n i n p u t caused a v a r i a t i o n i n p i e z o m e t r i c l e v e l s which i n t u r n caused a v a r i a t i o n i n outflow. 6.2 S o i l Moisture Storage The s o i l moisture storage volume was c a l c u l a t e d u s i n g the equation of c o n t i n u i t y and the assumption t h a t the d i f f e r e n c e between t r a n s i e n t and steady s t a t e flow r a t e s i n t o the t i l l was not s i g n i f i c a n t . T h i s assumption seems reasonable because the i n c r e a s e d flow i n t o the low c o n d u c t i v i t y t i l l (induced by t r a n s i e n t g r adients) should be p r o p o r t i o n a l l y much s m a l l e r than e i t h e r outflow or r a i n f a l l i n put. Thus, f o r example, i f the e s t i m a t i o n of flow i n t o the t i l l were o f f by 50%, the e r r o r i n storage would be l e s s than 10%. The t o t a l r a i n f a l l d uring the 6 day t r a n s i e n t p e r i o d was 302.1 m3 while outflow measured 145.6 m3. Flow i n t o the t i l l , at 20% of r a i n f a l l i n p u t , e q u a l s 60.4m3. Then: 78 I - (Os + 0 ) = AS Symbols d e f i n e d above AS = 91.2 m3 or = 0.131 m water depth e q u i v a l e n t Based on an estimated p o r o s i t y of 0.4 f o r the lower B h o r i z o n and the assumption t h a t the coarse t e x t u r e of the B h o r i z o n allowed f o r l i t t l e unsaturated s t o r a g e . This S value produced a water t a b l e r i s e of 0.38 m. T h i s r i s e was c o n s i s t a n t with that i n d i c a t e d by the i n - p l o t piezometers. 6.3 The Water Table 6.3.1 C o n f i g u r a t i o n of the Water Table Water going i n t o storage caused e i t h e r a continuous r i s e i n the s a t u r a t e d zone or the formation of a perched water t a b l e . The p i e z o m e t r i c data d i d not always i n d i c a t e where each of these s i t u a t i o n s predominated. In p l a c e s t h a t were s a t u r a t e d both above and below the B h o r i z o n - t i l l c o n t a c t before i r r i g a t i o n began (such as near piezometers A, B, C, D, and N), the s a t u r a t e d zone remained v e r t i c a l l y c ontinuous throughout the experiment. I n i t i a l piezometer response at these l o c a t i o n s was g e n e r a l l y f a s t e r than i n p l a c e s where piezometers were i n i t i a l l y dry. Such behavior i s c o n s i s t e n t with c l a s s i c a l i n f i l t r a t i o n theory (Rubin and S t e i n h a r d t , 1963) as the water t a b l e r o s e , not because the i n f i l t r a t i o n r a t e was higher than the s a t u r a t e d c o n d u c t i v i t y but because storage was being f i l l e d . 79 In areas where piezometers were i n i t i a l l y dry, the water t a b l e c o n f i g u r a t i o n was not as c l e a r . E i t h e r the f i l l i n g of storage caused a v e r t i c a l l y continuous s a t u r a t e d zone t o r i s e or the i n f i l t r a t i o n r a t e o f 7 x 1 0 ~ 7 m/s l e d to the f o r m a t i o n of a perched water t a b l e above the s u r f a c e of the t i l l (K = 10 " 7 m/s). The i n i t i a l p i e z o m e t r i c response t i m e - l a g c o u l d not be used to d i s t i n g u i s h between a perched or continuous water t a b l e . I t was not p o s s i b l e to t e l l whether t i m e - l a g s were a f u n c t i o n o f : flow path-length, a f u n c t i o n of the f i l l i n g of storage above the t i l l t o form a perched s a t u r a t e d zone, or a f u n c t i o n of the depth to the s a t u r a t e d zone. Where the B h o r i z o n - t i l l c o n t a c t was well d e f i n e d , i t i s most l i k e l y t h a t a perched water t a b l e e x i s t e d (as seen at other l o c a t i o n s , see 3.3.2). A g r a d a t i o n a l c o n t a c t probably hosted a v e r t i c a l l y continuous s a t u r a t e d zone because the lower c o n d u c t i v i t y of t h i s zone would produce slow drainage. Thus, water would be i n storage long enough f o r a s a t u r a t e d f r o n t to move downward, e l i m i n a t i n g any u n d e r l y i n g unsaturated r e g i o n . Since both w e l l - d e f i n e d and g r a d a t i o n a l c o n t a c t s e x i s t e d w i t h i n the p l o t , i t i s l i k e l y t h a t both types of water t a b l e c o n f i g u r a t i o n s occurred. F o r t u n a t e l y , the nature of the s a t u r a t e d zone d i d not c o n t r o l where subsurface stormflow occurred. In both perched and continuous water t a b l e s i t u a t i o n s , most stormflow t r a v e l e d through the high c o n d u c t i v i t y B h o r i z o n because the c o n d u c t i v i t y c o n t r a s t with the t i l l and B h o r i z o n - t i l l t r a n s i t i o n zone was 2 to 3 orders of magnitude. The low c o n d u c t i v i t y of the t i l l and t r a n s i t i o n zones kept stormflow c o n t r i b u t i o n s from t h i s r e g i o n to a minimum. Thus, i t was not important t o the o v e r a l l volume 80 of stormflow whether the u n d e r l y i n g low c o n d u c t i v i t y zones were s a t u r a t e d or not as c o n t r i b u t i o n s would have been s m a l l i n e i t h e r s i t u a t i o n . 6.3.2 Role of the Water Table The r o l e of the water t a b l e was r e v e a l e d by the p i e z o m e t r i c and outflow data. The piezometers i n d i c a t e d t h a t the o v e r a l l h y d r a u l i c g r a d i e n t remained approximately constant while water l e v e l s and outflow v a r i e d d i u r n a l l y . Four piezometers r e p r e s e n t a t i v e of the lower, middle, and upper slope demonstrated t h a t w i t h i n the r e s o l u t i o n of four r e a d i n g s per day, d i u r n a l t iming was independent of h i l l s l o p e p o s i t i o n (Table 6 -1). Piezometer v a r i a t i o n at each of these l o c a t i o n s was a l s o approximately the same (± 5 to 10 cm). Thus, the water t a b l e rose and f e l l p a r a l l e l t o the o v e r a l l h i l l s l o p e . The p a r a l l e l response of the water t a b l e and the r e l a t i v e t h i n n e s s of the zone through which most s a t u r a t e d stormflow occurred produced flow paths t h a t were approximately p a r a l l e l t o the h i l l s l o p e and g r a d i e n t s t h a t were s i m i l a r to t h a t of the o v e r a l l h i l l s l o p e (Figure 6-2). These g r a d i e n t s remained approximately c o n s t a n t (egual t o the tangent of the h i l l s l o p e ) because l o c a l g r a d i e n t d i f f e r e n c e s from v a r i a t i o n s i n water t a b l e p o s i t i o n were unimportant r e l a t i v e t o the o v e r a l l g r a d i e n t of the h i l l s l o p e . Thus, the main component of the h y d r a u l i c g r a d i e n t was g r a v i t a t i o n a l . 81 Table 6-1. Maximums occurred in la te morning (8:00tol0:00) with minumum at ear ly evening (18:00). Values on the 12th are not representative because of r a i n f a l l reduction. Piezometer 2 8 11 L Pos i t ion August 9 10 11 12 13 14 Lower Mi n Max Rising 17:00 11:00 1:00 6:00 19:00 12:00 18:00 6:00 18:00 10:00 Lower Min Max 12: 00 8 00 18: 00 8 00 2: 00 7 00 20: 00 13 00 Lost 18: 00 11 00 Middle Min Max 17:00 7:00 17:00 8:00 19:00 7:00 20:00 8:00 19:00 7:00 19:00 11:00 Upper Mi n Max Ris ing 17:00 3:00 24:00 7:00 19:00 13:00 18:00 7:00 18:00 11:00 R A I N F A L L •io, >8 O U T F L O W 0 met res FIGURE 6-2 Schematic Flownet of part of the H i l l s l o p e Flow i s p a r a l l e l to the h i l l s l o p e because the saturated stormflow zone i s r e l a t i v e l y thin compared to the t o t a l h i l l s l o p e length. Gradients are primarily g r a v i t a t i o n a l . 83 C l a s s i c a l h i l l s l o p e behavior, where the magnitude of water t a b l e f l u c t u a t i o n s decrease with d i s t a n c e from the stream, was not observed. There are s e v e r a l p o s s i b l e reasons f o r t h i s . F i r s t , the h i g h - c o n d u c t i v i t y B h o r i z o n was probably a b l e to accommodate ths g r e a t e r flow nearer the stream bank with only a s l i g h t l y g r e a t e r water t a b l e r i s e than i n upslope r e g i o n s . Second, t h i s r i s e was not seen because the l a r g e h e t e r o g e n e i t y of the B h o r i z o n produced water t a b l e r i s e s t h a t were extremely v a r i a b l e f o r a given d i s t a n c e from the stream bank. Thus, the water l e v e l i n d i c a t e d by each piezometer may not have been i n d i c a t i v e of the mean water t a b l e height f o r i t s d i s t a n c e from the stream. A s l i g h t l y l a r g e r d i u r n a l v a r i a t i o n near the stream bank would probably have been seen with a l a r g e r piezometer a r r a y . Gradients and s a t u r a t e d c o n d u c t i v i t i e s remained approximately c o n s t a n t with time, i n d i c a t i n g t h a t v a r i a t i o n s i n outflow were caused by v a r i a t i o n s i n the s a t u r a t e d c r o s s * s e c t i o n a l area a v a i l a b l e f o r stormflow . Within t h i s area, d e l i n e a t e d by the water t a b l e on top and by the low c o n d u c t i v i t y t i l l and B h o r i z o n - t i l l t r a n s i t i o n zones on the bottom, a stormflow area v a r i a t i o n of ± 10 to ± 20% caused an i n c r e a s e and decrease i n outflow of ± 25%. The s m a l l e r v a r i a t i o n i n s a t u r a t e d flow area c a u s i n g a l a r g e v a r i a t i o n i n outflow i n d i c a t e d t h a t a g r e a t e r p r o p o r t i o n of the stormwater was f l o w i n g i n the upper p o r t i o n of the s a t u r a t e d zone. T h i s c o n c l u s i o n was supported by low c o n d u c t i v i t y measurements i n the t i l l and lower B h o r i z o n . 84 P a r a l l e l water t a b l e response and p r o p o r t i o n a l outflow were a l s o observed during the t e r m i n a l phase of the experiment. A f t e r i r r i g a t i o n was shut o f f most p i e z o m e t r i c l e v e l s began to f a l l i n l e s s than two hours as d i d the outflow r a t e . For the f o l l o w i n g few hours water l e v e l s f e l l , m a i n t a i n i n g an approximately constant g r a d i e n t , but with d e c r e a s i n g c r o s s - s e c t i o n a l flow area and a corresponding decrease i n outflow r a t e . A f t e r t h i s time, r u n o f f decreased while water l e v e l s f e l l only s l i g h t l y s u p p o r t i n g the c o n c l u s i o n t h a t higher c o n d u c t i v i t y and a g r e a t e r p r o p o r t i o n of flow occurred i n the upper p a r t of the s a t u r a t e d zone. The importance of the p o s i t i o n of the water t a b l e i n g e n e r a t i n g outflow was c l e a r l y demonstrated i n i t s behavior j u s t before outflow began. Piezometer L16, j u s t below the c o l l e c t i o n trough, began to r i s e 9 t o 10 hours before any r u n o f f was produced. Outflow commenced only a f t e r the water t a b l e rose and i n t e r s e c t e d the s u r f a c e of the stream bank as evidenced by water l e v e l s i n piezometers L16 and A (Figure 6-3). Before outflow began, water was going i n t o storage causing e i t h e r a water t a b l e r i s e i n the already s a t u r a t e d B h o r i z o n - t i l l t r a n s i t i o n zones or the formation of a perched water t a b l e where the B h o r i z o n t i l l t r a n s i t i o n was i n i t i a l l y unsaturated. A f t e r t h i s time the r i s i n g water t a b l e produced an i n c r e a s i n g amount of stormflow as the zone of s a t u r a t i o n rose i n t o the o r g a n i c r i c h zones of the lower B h o r i z o n . Outflow was at a maximum when the c r o s s -s e c t i o n a l area through which i t flowed was a l s o at a maximum. Thus, the p o s i t i o n of the water t a b l e d i r e c t l y c o n t r o l l e d the r a t e of outflow. ' o & % °°. i (fid « 0 /OV r-Piezometer L16 Outflow Trough ° o 'r>o °A 0 ° * - 2 y 0 0 D ^  .VV, H I - O O n O , 0 - 0 / ) ^ . O ^ T P C 3 • - n <D (/> ^ 4 % ^ ^ CO IDIOO co <o "ro CD" N o 3 CD oc 86 6.4 H y d r a u l i c C o n d u c t i v i t i e s 6.4.1 C a l c u l a t e d C o n d u c t i v i t i e s A f t e r i t was e s t a b l i s h e d that stormflow was p a r a l l e l to the h i l l s l o p e , the bulk c o n d u c t i v i t y of the p l o t was c a l c u l a t e d using Darcy's law. T h i s c a l c u l a t i o n was made f o r a zone a s h o r t d i s t a n c e i n from the seepage f a c e because, at the f a c e , e x i t e f f e c t s and the e f f e c t s of the steeper seepage f a c e angle produced g r a d i e n t s and flow paths t h a t were hard to esti m a t e . Darcy's law can be w r i t t e n : Q/A = K grad h Where: Q = outflow (L 3/T) A = c r o s s - s e c t i o n a l flow area (L 2) K = 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 y (L/T) grad h = h y d r a u l i c g r a d i e n t (L/L) The mean s t e a d y - s t a t e outflow r a t e of 2.11 m 3/hr was c a l c u l a t e d f o r Q. A c r o s s - s e c t i o n a l flow area of 15 m2 was estimated using the p l o t width of 30 m and a s a t u r a t e d depth to the low c o n d u c t i v i t y l a y e r of 1/2 m. Th i s depth of the stormflow area was based on piezometer r i s e s recorded near the stream bank and on v i s u a l o b s e r v a t i o n s at the seepage f a c e d u r i n g steady s t a t e . The h i l l s l o p e g r a d i e n t of 0.4 was used f o r the h y d r a u l i c g r a d i e n t , a value t h a t i s r e a l i s t i c f o r a l l p o i n t s throughout the h i l l s l o p e except f o r those r i g h t at the seepage f a c e . 87 With these v a l u e s , the bulk h y d r a u l i c c o n d u c t i v i t y of the research p l o t was c a l c u l a t e d at 8 x 1 0 _ 5 m/s. T h i s c o n d u c t i v i t y i s probably a minimum as the s a t u r a t e d flow depth of 1/2 m was the upper l i m i t of v a l u e s i n d i c a t e d by the i n - p l o t piezometers. The a c t u a l flow depth was probably l e s s because most stormflow occurred i n the upper p a r t of the s a t u r a t e d zone- Thus, p a r t of t h i s 1/2 m i n c l u d e d area through which only a s m a l l p r o p o r t i o n of stormflow t r a v e l e d . T h e r e f o r e , the bulk c o n d u c t i v i t y of the stormflow t r a n s m i s s i o n zone was probably somewhat g r e a t e r than the c a l c u l a t e d value of 8 x 10~ 5 m/s. 6.4.2 Comparison of C o n d u c t i v i t i e s The c a l c u l a t e d bulk h y d r a u l i c c o n d u c t i v i t y i n d i c a t e d that most storm r u n o f f flowed through a u n i t with c o n d u c t i v i t i e s three orders of magnitude higher than those measured f o r the t i l l and lower B h o r i z o n . Yet, examination o f the seepage face and piezometer r i s e s r e v e a l e d t h a t most flow d i d occur i n the lower p a r t of the B h o r i z o n . T h i s paradox i s most l i k e l y e x p lained by the presence of o r g a n i c zones, mostly l i v e and decayed r o o t m a t e r i a l which produced a much higher o v e r a l l c o n d u c t i v i t y . Water f l o w i n g through the B h o r i z o n passed p r e f e r e n t i a l l y through these zones, which, having a very high c o n d u c t i v i t y , gave a bulk c o n d u c t i v i t y 2 to 3 orders of magnitude l a r g e r than the mineral matrix of the lower B h o r i z o n alone. 88 Most r u n o f f e x i t e d from the stream bank v i a these r o o t channels. The s m a l l p r o p o r t i o n of outflow t h a t came d i r e c t l y from the s o i l matrix d i d not i n d i c a t e t h a t the matrix was uninvolved i n stormflow generation nor d i d i t i n d i c a t e t h a t the orga n i c pathways were i n t e r c o n n e c t e d channels. A schematic flow net (Figure 6-4) demonstrates t h a t the high c o n d u c t i v i t y c o n t r a s t between the roo t channels and the s o i l matrix i s s u f f i c i e n t to cause a major p o r t i o n of flow to e x i t v i a the organic channels. Up the h i l l s l o p e , water t r a v e l e d between adjacent r o o t channels v i a the s a t u r a t e d matrix of the lower B h o r i z o n . Thus, the bulk c o n d u c t i v i t y of the s o i l was determined mostly by the c o n c e n t r a t i o n of high c o n d u c t i v i t y organic channels. 6.5 Mechanism of Stormflow Generation On the b a s i s of t h i s study, and the work of d e V r i e s and Chow (1973, 1978) and Nagpal and d e V r i e s (1976), the f o l l o w i n g mechanism of stormflow g e n e r a t i o n i s e n v i s i o n e d : Rain h i t s the ground and flows along l e a v e s , branches, l o g s , rocks, e t c . i n t o and through the f o r e s t f l o o r . T h i s flow i s not unsaturated as the p o r o s i t y and c o n d u c t i v i t y of the f o r e s t f l o o r might suggest. Rather, flow i s l o c a l l y s a t u r a t e d i n v e r t i c a l channels because of the extremely open nature of the f o r e s t f l o o r and the c o n c e n t r a t i n g e f f e c t s of l o g s , branches, e t c . These c o n c e n t r a t i n g elements allow f o r the formation of l o c a l i z e d f r e e water which can then enter the l a r g e open pores i n the s o i l . The open nature and high c o n d u c t i v i t y of the f o r e s t f l o o r a l s o preclude ponding and Hortonian overland flow except 89 FIGURE 6-4 Schematic Flownet of the Stream Bank showing prefered Path Through High-conductivity Root Zone 90 i n l o c a l l y d i s t u r b e d a r e a s . Water f lows downward through the f o r e s t f l o o r u n t i l i t encounters the B h o r i z o n . I t then e n t e r s and c o n t i n u e s t o f low downward through l o c a l l y s a t u r a t e d pathways, p r i m a r i l y through high c o n d u c t i v i t y o r g a n i c r o o t zones . V e r t i c a l l y c h a n n e l i z e d , s a t u r a t e d flow predominates because the lower s a t u r a t e d c o n d u c t i v i t y of the s o i l matr ix ( r e l a t i v e to the o r g a n i c channels ) a l l o w s o n l y a s m a l l amount o f outward f low i n t o the m a t r i x . Because v e r t i c a l f low through the B h o r i z o n i s c o n c e n t r a t e d i n t o h i g h - c o n d u c t i v i t y , s a t u r a t e d r e g i o n s , f low downward to the water t a b l e o r , i f i s encountered f i r s t , t o the low c o n d u c t i v i t y t i l l , i s rap id , . In the lower p a r t o f the B h o r i z o n , t h i s downward moving water f i l l s s t o r a g e and causes e i t h e r the water t a b l e to r i s e or. the f o r m a t i o n of a perched water t a b l e where the lower c o n d u c t i v i t y o f the t i l l impedes downward f l o w . As water goes i n t o s t o r a g e , the water t a b l e r i s e s . D u r i n g t h i s p e r i o d o n l y a s m a l l amount o f water f lows out a t the a r t i f i c i a l stream bank because the water t a b l e i s below the s u r f a c e of the c o l l e c t i o n t r o u g h . Any out f low which does occur d u r i n g t h i s p e r i o d i s probab ly due to a l o c a l l y perched water t a b l e near the s tream bank. Major out f low beg ins when the water t a b l e i n t e r s e c t s the s u r f a c e o f the stream bank. At t h i s po in t* most s u b s u r f a c e storm water i s f l o w i n g through the h i g h c o n d u c t i v i t y B h o r i z o n . The high c o n d u c t i v i t y i s due to the c o n c e n t r a t i o n o f o r g a n i c zones , p r i m a r i l y l i v e and decayed r o o t s . At the stream bank most out f low o c c u r s thr ough these roo t channe l s because the c o n t r a s t 91 i n c o n d u c t i v i t y between the channels and the s o i l matrix i s s e v e r a l orders of magnitude. Only a s m a l l p r o p o r t i o n of r u n o f f e x i t s through the s a t u r a t e d matrix. V a r i a t i o n i n outflow i s a d i r e c t f u n c t i o n of change i n the c r o s s - ^ s e c t i o n a l area through which storm water f l o w s and not of changes i n h y d r a u l i c g r a d i e n t . The water t a b l e r i s e s and f a l l s p a r a l l e l to the o v e r a l l h i l l s l o p e . Thus, the h y d r a u l i c g r a d i e n t remains approximately constant. Only the v e r t i c a l c r o s s -s e c t i o n a l s a t u r a t e d flow area changes. Once outflow has begun, shor t l a g - t i m e s between i n p u t and outflow v a r i a t i o n s are p o s s i b l e because s h o r t flow paths from the s u r f a c e to the s a t u r a t e d zone produce f a s t water t a b l e response. A v a r i a t i o n i n the water t a b l e p o s i t i o n causes an almost i n s t a n t a n e o u s change i n outflow; During t h i s p e r i o d the h i l l s l o p e - s t r e a m system i s very s e n s i t i v e t o r a i n f a l l v a r i a t i o n s . During outflow, l e s s than 20% of the i n f i l t r a t e d r a i n f a l l e n t e r s a deeper, non-stormflow, ground water system. This water moves through the t i l l and p o s s i b l y the f r a c t u r e d bedrock. Flowing downward and out of the r e s e a r c h p l o t , t h i s water undoubtedly c o n t r i b u t e s to the base flow of the n a t u r a l stream below the p l o t . With the t e r m i n a t i o n of r a i n f a l l , outflow f a l l s o f f r a p i d l y as the water t a b l e drops. T h i s drop does not change the o v e r a l l h y d r a u l i c g r a d i e n t a p p r e c i a b l y as the water t a b l e c o n t i n u e s to remain approximately p a r a l l e l t o the h i l l s l o p e . Only the v e r t i c a l c r o s s - s e c t i o n a l flow area of the s a t u r a t e d high c o n d u c t i v i t y B h o r i z o n v a r i e s . Thus, as the water t a b l e f a l l s , 92 the decrease i n flow a r e a causes a decrease i n the outf low r a t e . A f t e r a few h o u r s , the r a t e o f out f low c o n t i n u e s to f a l l , but with only a s m a l l accompanying drop i n water t a b l e p o s i t i o n . D u r i n g t h i s p e r i o d , the water t a b l e f a l l s i n t o the p a r t of the lower B h o r i z o n where r o o t channe l c o n c e n t r a t i o n i s l o w e r . A f t e r the water t a b l e drops below the s u r f a c e o f the seepage f a c e , out f low i s produced o n l y where the water t a b l e i s l o c a l l y perched near the stream . bank. Baseflow i s not s u s t a i n e d by s a t u r a t e d f low through the t i l l because the water t a b l e i s below the c o l l e c t i o n t r o u g h . Low out f low c o n t i n u e s (from the d e c r e a s i n g l y s m a l l e r perched water t a b l e r e g i o n s near the bank) u n t i l the next r a i n f a l l event o c c u r s . The c y c l e then r e p e a t s . 6*6 G e n e r a l i z a t i o n o f the R e s u l t s There are two o b j e c t i o n s t h a t c o u l d be r a i s e d towards g e n e r a l i z i n g t h i s mechanism of s tormflow g e n e r a t i o n from the r e s e a r c h p l o t to s u r r o u n d i n g watersheds . The f i r s t of these p e r t a i n s to the r e p r e s e n t a t i v e n e s s of the man-made e x p e r i m e n t a l system i n comparison to n a t u r a l systems. The second c o n c e r n s the observed l a g - t i m e s . I w i l l t r y to show t h a t n e i t h e r of these o b j e c t i o n s i s s e r i o u s . I t was demonstrated i n a p r e v i o u s p a r t of t h i s s tudy t h a t the h y d r o g e o l o g i c u n i t s of the r e s e a r c h p l o t were s i m i l a r i n type and t h i c k n e s s to s u r r o u n d i n g a r e a s . What remains open to g u e s t i o n i s whether the h i l l s l o p e - s t r e a m system and c o r r e s p o n d i n g s tormflow g e n e r a t i o n mechanism are t y p i c a l of nearby watersheds . 93 Comparison of the n a t u r a l stream below the r e s e a r c h p l o t with the a r t i f i c i a l stream bank and c o l l e c t i o n trough shows s e v e r a l d i f f e r e n c e s . In c o n t r a s t with the a r t i f i c i a l stream, the p i e z o m e t r i c data near the n a t u r a l stream r e v e a l e d s t r o n g discharge g r a d i e n t s as w e l l as a water t a b l e near the s u r f a c e . In such an area, stormflow generation by the Dunne and Black mechanism (1970a, b) c o u l d occur. In the r e s e a r c h p l o t however, the g r e a t e r depth to the s a t u r a t e d zone and the higher bulk c o n d u c t i v i t y of the B h o r i z o n make t h i s an u n l i k e l y mechanism. Dunne and Black r e p o r t e d a major p r o p o r t i o n of r u n o f f generated near the stream channel with some upslope areas c o n t r i b u t i n g l i t t l e , i f any, to d i r e c t runoff;* T h e r e f o r e , the p o s s i b i l i t y e x i s t s t h a t i n a complete b a s i n , l a r g e r than the experimental p l o t , r u n o f f generated by the Dunne and Black mechanism might predominate. However, t h i s p o s s i b i l i t y i s s m a l l . An examination of nearby h i l l s l o p e s r e v e a l e d t h a t the p r o p o r t i o n of wet, near channel source areas was much too s m a l l t o account f o r r a i n f a l l -r u n o f f r a t i o s r e p o r t e d by Cheng (1975) f o r l o c a l b a s i n s . Thus, i t i s probable t h a t the Dunne and Black mechanism i s only l o c a l l y s i g n i f i c a n t i n B.C. Coast Mountain b a s i n s . Examination of upland watersheds i n the v i c i n i t y of the research p l o t r e v e a l e d aspects of the experimental p l o t t h at were p h y s i c a l l y s i m i l a r t o the n a t u r a l systems. T y p i c a l l y , steep s l o p e s fed i n c i s e d stream channels which were dry during the summer months. The area around these channels was not marshy nor was the water t a b l e at or near the s u r f a c e . The only major d i f f e r e n c e between the experimental s i t e and surrounding upland b a s i n s was t h a t i n the experimental s i t e the t i l l s u r f a c e was 94 exposed a t the stream bank while i n the n a t u r a l system t h i s was r a r e l y the case. The exposure of the t i l l and the p o s i t i o n o f the a r t i f i c i a l stream channel change the i n i t i a l outflow response time l a g . T h i s i s d i s c u s s e d f u r t h e r , below. Input-outflow l a g s c o n s t i t u t e a second p o s s i b l e o b j e c t i o n to g e n e r a l i z i n g the experimental r e s u l t s . The i n i t i a l response l a g of 19 hours appears to be too long to be r e p r e s e n t a t i v e of l a g s i n n a t u r a l systems. For example, Dunne and Black reported stream response w i t h i n s e v e r a l minutes a f t e r the i n i t i a t i o n of r a i n f a l l . However; these lag-times were f o r summer storms with stream response due only t o d i r e c t p r e c i p i t a t i o n i n t o the stream channel. Undoubtedly, had the r o o f over the stream bank and c o l l e c t i o n trough been removed, s h o r t l a g times of a s i m i l a r nature would a l s o have been observed f o r the r e s e a r c h p l o t . Response times r e p o r t e d by Cheng (1975) are more r e p r e s e n t a t i v e of n a t u r a l B.C. South Coast Mountain b a s i n s . In studying 33 n a t u r a l r a i n f a l l events which occurred i n the Jamison Creek basin l o c a t e d i n the Seymour Watershed , Cheng noted i n i t i a l reponse l a g - t i m e s of 5 to 15 hours. T h i s b a s i n had g e o l o g i c (and probably hydrologic) c h a r a c t e r i s t i c s s i m i l a r to the watershed used f o r t h i s study. Shorter l a g s f o r a l a r g e r b a s i n seem to i n d i c a t e that the r e s e a r c h p l o t i s not r e p r e s e n t a t i v e of the l o c a l s i t u a t i o n . For s e v e r a l reasons these s h o r t e r l a g s are not p e r c e i v e d as a problem. One of these reasons i s t h a t i n the mechanism o p e r a t i n g i n the r e s e a r c h p l o t , the i n i t i a l response l a g i s due to water going i n t o storage. T h e r e f o r e , t h i s l a g i s a f u n c t i o n of antecedent moisture and t h i c k n e s s of the unsaturated u n i t s . The 95 exper iment a t the r e s e a r c h p l o t was conducted a f t e r two months of no r a i n . Cheng's da ta were f o r summer r a i n f a l l event s f o l l o w i n g an u n u s u a l l y wet p e r i o d (N. Penny, p e r s . comm. 1978) as we l l as f o r events d u r i n g the wet ter months of f a l l and w i n t e r . F o r a more d i r e c t comparison with Cheng, i t i s p o s s i b l e to o b t a i n q u a l i t a t i v e w a t - a n t e c e d e n t i n p u t - o u t f l o w response l a g s by c o n s i d e r i n g the s e n s i t i v i t y of the e x p e r i m e n t a l h i l l s l o p e once out f low had begun. E e f e r i n g to F i g u r e 6 - 1 , over the p e r i o d August 15-17 (when the e f f e c t i s most c l e a r l y seen) i n p u t -out f low maximum l a g s were l e s s than 1 h o u r . I n p u t - o u t f l o w minimum l a g s were from 3 to 7 h o u r s . Thus , once m o i s t u r e s t o r a g e regu irements had been s a t i s f i e d , response t imes were much s h o r t e r and as a r e s u l t , c o n s i s t e n t w i th Cheng. As the t h i c k n e s s e s o f the g e o l o g i c u n i t s i n the Jamison Creek b a s i n were not g i v e n , i t i s a l s o p o s s i b l e t h a t the l o n g e r l a g observed i n the r e s e a r c h p l o t was due to deeper s o i l p r o f i l e s . The d i f f e r e n c e i n s i z e between Cheng' s b a s i n and the r e s e a r c h p l o t i s not a major f a c t o r i n the d i f f e r e n c e i n response l a g s because out f low i s generated by a water t a b l e r i s i n g p a r a l l e l to the h i l l s l o p e i n a synchronous manner. Lag time i s not a f u n c t i o n o f i n d i v i d u a l p a r t i c l e flow path t r a v e l t imes but of mois ture c o n t e n t , depth to s a t u r a t i o n , and f low paths from the s u r f a c e * These f a c t o r s vary w i t h , but are not d i r e c t l y c o n t r o l l e d by , b a s i n s i z e . Cheng ' s l a r g e r b a s i n does not n e c e s s a r i l y r e q u i r e l o n g e r l a g - t i m e s than the s m a l l e r e x p e r i m e n t a l watershed used i n t h i s s t u d y . 96 A f i n a l reason f o r the longer response l a g i n the r e s e a r c h p l o t was the a r t i f i c i a l nature of the c o l l e c t i o n trough. The trough s a t on the s u r f a c e of the exposed t i l l with the water t a b l e w e l l below i t . Because the s a t u r a t e d zone below the r u n o f f trough s t a r t e d to r i s e 9 to 10 hours before the i n i t i a t i o n o f outflow, i t i s p o s s i b l e t h a t i f the a r t i f i c i a l stream were t o p o g r a p h i c a l l y lower (and more r e p r e s e n t a t i v e of a n a t u r a l stream with the water t a b l e c o i n c i d e n t with the stream) outflow would have commenced sooner. Thus, i n i t i a l response l a g - t i m e s c o u l d have been as s h o r t as 13 1/2 hours ( i n i t i a l response time of piezometer L16), a f i g u r e i n the range found by Cheng. I t i s t h e r e f o r e concluded t h a t even though the experimental h i l l s l o p e - s t r e a m system i s not n a t u r a l , the mechanism of stormflow generation d e s c r i b e d i n t h i s r e p o r t can be g e n e r a l i z e d (cautiously) t o surrounding watersheds. 97 7-.0 SUMMARY - AND - CONCLUSIONS -The o b j e c t i v e s of t h i s study were t h r e e - f o l d . The f i r s t was to examine the u n d e r l y i n g g l a c i a l t i l l and t o determine i t s p h y s i c a l c h a r a c t e r i s t i c s , s p a t i a l d i s t r i b u t i o n and h y d r o l o g i c behavior. Emphasis was placed on the mechanism of h y d r o l o g i c breakthrough proposed by Nagpal and deVries (1976) and the r o l e of the t i l l i n c o n t r o l l i n g leakage out of the s o i l system. The second was to i n v e s t i g a t e the mechanisms of stormflow g e n e r a t i o n o p e r a t i n g w i t h i n the experimental p l o t and to examine the r o l e of o r g a n i c channels. The t h i r d o b j e c t i v e was to e s t a b l i s h whether t h i s mechanism c o u l d be g e n e r a l i z e d to s i m i l a r watersheds. A l l three of these o b j e c t i v e s have been met. I t was determined that the u n d e r l y i n g Vashon t i l l has an average p a r t i c l e s i z e d i s t r i b u t i o n of 83.9% sand, 8.4% s i l t , and 7.7% c l a y . I t can e i t h e r be hard and w e l l compacted or s o f t and much l o o s e r with the d i f f e r e n c e a t t r i b u t a b l e to the degree of weathering and not to ge n e t i c d i f f e r e n c e s . The s p a t i a l d i s t r i b u t i o n of the t i l l i s v a r i a b l e . T i l l i s present i n 60% of the area near the r e s e a r c h p l o t a t depths of 0.15 t o more than 1.5 m. The h y d r a u l i c c o n d u c t i v i t y of the t i l l i s 10 ~ 6 to 10 ~ 7 m/s. Leakage r a t e s through the t i l l are l e s s than 20% of i n p u t . The proposed mechanism of h y d r a u l i c breakthrough i s unnecessary to account f o r the observed outflows. The v a r i a t i o n i n outflow i s a t t r i b u t a b l e t o d i u r n a l v a r i a t i o n . Stormflow i s generated when i n f i l t r a t e d r a i n f a l l causes a water t a b l e r i s e i n t o the high c o n d u c t i v i t y lower B h o r i z o n The bulk c o n d u c t i v i t y of the lower B h o r i z o n i s high due to the 98 presence of numerous h i g h c o n d u c t i v i t y zones c o n s i s t i n g o f l i v e and decayed r o o t s . Most storm water presumably t r a v e l s through these r o o t c h a n n e l s because the c o n d u c t i v i t y of the lower B h o r i z o n m a t r i x as determined by p iezometer t e s t s i s very low, 10 ~ 6 to 10~ 7 m/s. A major p r o p o r t i o n o f the s tormflow e x i t s a t the stream bank from these r o o t c h a n n e l s because o f the c o n d u c t i v i t y c o n t r a s t . Only a s m a l l amount o f water e x i t s from the s a t u r a t e d matr ix o f the lower B h o r i z o n . Outflow i s c o n t r o l l e d by the p o s i t i o n o f the water t a b l e . V e r t i c a l f l u c t u a t i o n s i n response to r a i n f a l l causes changes i n the c r o s s - s e c t i o n a l flow area p e r p e n d i c u l a r t o f l o w . The area a v a i l a b l e f o r s a t u r a t e d f low c o n t r o l s the out f low r a t e as the h y d r a u l i c g r a d i e n t s s t a y a p p r o x i m a t e l y e g u a l to the h i l l s l o p e g r a d i e n t . Outf low ceases when the water t a b l e drops below the c o n t a c t between the B h o r i z o n and the lower c o n d u c t i v i t y t i l l . T h i s mechanism of stormflow g e n e r a t i o n can be g e n e r a l i z e d ( c a u t i o u s l y ) to s i m i l a r watersheds . A g e o l o g i c s tudy demonstrated t h a t the h y d r o g e o l o g i c u n i t s of the r e s e a r c h p l o t are r e p r e s e n t a t i v e of the s u r r o u n d i n g a r e a . The out f low t r o u g h , however, i s d i f f e r e n t from a n a t u r a l s t ream. T h i s d i f f e r e n c e probably causes a d e l a y i n i n i t a l outf low response which h e l p s t o e x p l a i n why i n i t i a l outf low l a g - t i m e s a t the e x p e r i m e n t a l p l o t are l o n g e r than those of a nearby watershed . I t i s conc luded t h a t these l o n g e r l a g s do not l i m i t g e n e r a l i z a t i o n because most of t h i s l a g t ime i s a f u n c t i o n of s o i l mo i s ture s t o r a g e regu irements and p o s i t i o n o f the out f low t r o u g h . Once mois ture requ irements have been met and out f low has commenced, the h i l l s l o p e p l o t i s j u s t as s e n s i t i v e to i n p u t v a r i a t i o n s as 99 are n a t u r a l h i l l s l o p e s . I t i s concluded t h a t the experimental p l o t i n t h i s wetter s t a t e y i e l d s l a g - t i m e s t h a t are c o n s i s t e n t with l o c a l catchments. T h i s r e p o r t has attempted t o e x p l a i n stormflow g e n e r a t i o n i n a southwest Coast Mountain environment. Many guestions have a r i s e n d u r i n g the course of t h i s r e s e a r c h to j o i n those g u e s t i o n s that have yet to be asked. I hope t h a t t h i s r e p o r t w i l l be used as a stepping stone toward answering these guestions and t h a t a more complete understanding of h i l l s l o p e h y d r o l o g i c processes w i l l r e s u l t . 100 REFERENCES -Armstrong, J.E. 1957. S u r f i c i a l geology of New Westminster map area, B r i t i s h Columbia, Canada. Dept. of Mines and T e c h n i c a l Surveys, Paper 57-5. Armstrong, J.E. 1975. Quaternary geology, s t r a t i g r a p h i c s t u d i e s , and r e v a l u a t i o n of t e r r a i n i n v e n t o r y maps, F r a s e r lowland, B r i t i s h Columbia, Geol. Surv. Can., Paper 75-1, pt A, pp. 377-380. Armstrong J.E. And S.R. Hicock, 1975. Quaternary m u l t i p l e v a l l e y development of the lower Coguitlam v a l l e y , Coguitlam, B r i t i s h Columbia. Geol; Surv. Can., Paper 75-1, pt B, pp. 99-103. Betson, R. P. 1964. What i s watershed r u n o f f ? Jour. Geophys. Res. 69(8): 1541-1551. Bryck, J.M. 1977. 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Personal Communication. 1978. J . Clague. G e o l o g i c a l Survey of Canada, Vancouver. Personal communication. 1978. J . DeVries. Dept. of S o i l S c iences, U n i v e r s i t y of B r i t i s h Columbia, Vancouver . 104 Personal communication. 1978. N. Penny. C l i m a t o l o g i s t , P a c i f i c Weather S e r v i c e , Richmond, B.C. 1 0 5 APPENDEXl PIEZOMETEB-HYDBQGRAPHS-901 FIGURE A5-2 Hydrograph for piezometers 11, 12, B, C, and D 6 0 1 3 1 1 e n in FIGURE A5-9 Hydrograph for piezometers 3 and N 116 CM DATE:AUGUST FIGURE A5-12 Hydrograph for piezometers L15 and L16 PIEZOMETER 1 m 0 3 . LU X in 3-n — L2.0 n 1 1— 14.0 16.0 18.0 DATE:AUGUST 1 — 20.0 1 24 8.0 10.0 22.0 FIGURE A5-13 Hydrograph for piezometer 1 1 o>" PIEZOMETER 6 in 9 * C H O I U J oi" n — 10.0 n 1 14.0 16.0 DATEsAUGUST ~ i — 22.0 a.o 12.0 10.0 20.0 24 FIGURE A5-15 Hydrograph for piezometer 6 T 21 ZZl SZI 9ZI LZl 

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