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UBC Theses and Dissertations

Streamflow generation in a sub-alpine basin in the coast mountains of British Columbia Barrett, Gary Edward 1981

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STREAMFLOW GENERATION IN A SUB-ALPINE BASIN IN THE COAST MOUNTAINS OF BRITISH COLUMBIA by GARY EDWARD BARRETT B. S c , The U n i v e r s i t y of B r i t i s h Columbia, 1979 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Geography) We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1981 © Gary Edward B a r r e t t , 1981 In p r e s e n t i n g t h i s t h e s i s /in p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree t h a t permission f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head of my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood th a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of Geography The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Da DE-6 (2/79) i i A b s t r a c t Stormflow g e n e r a t i o n was s t u d i e d i n a two square k i l o m e t r e , s u b - a l p i n e , f i r s t - o r d e r b a s i n t r i b u t a r y t o Ryan R i v e r , which i s i n the P a c i f i c Ranges of the Coast Mountains of B r i t i s h Columbia. P r e l i m i n a r y f i e l d work suggested t h a t n e i t h e r s a t u r a t i o n o v e r l a n d f l o w nor s u b s u r f a c e s t o r m f l o w were i m p o r t a n t mechanisms of s t o r m f l o w g e n e r a t i o n H o r t o n i a n o v e r l a n d f l o w appeared t o be dominant. The i n f i l t r a b i l i t y of the s o i l s dropped c o n s i d e r a b l y d u r i n g storm e v e n t s . Three p o s s i b l e causes of t h i s d e c l i n e were c o n s i d e r e d i n i t i a l l y : (1 ) a r e d u c t i o n i n c a p i l l a r y g r a d i e n t s as w e t t i n g p r o c e e d s , ( 2 ) a t e x t u r a l c o n t r a s t i n the p r o f i l e , and ( 3 ) a i r entrapment. A l l of the p r e c e e d i n g were r e j e c t e d on the b a s i s of more d e t a i l e d o b s e r v a t i o n s . I n s t e a d , i t was proposed t h a t a w a t e r - r e p e l l e n t l a y e r e x i s t e d near the t o p of the s o i l p r o f i l e . L a b o r a t o r y e x p e r i m e n t s conducted on i n t a c t s o i l samples demonstrated t h a t a r e p e l l e n t l a y e r on the o r d e r of a few c e n t i m e t r e s t h i c k n e s s d i d e x i s t near the s o i l s u r f a c e . The i m p l i c a t i o n s of t h i s f i n d i n g f o r i n f i l t r a t i o n and s t o r m f l o w g e n e r a t i o n a r e d i s c u s s e d . i i i T a b l e of c o n t e n t s A b s t r a c t i i L i s t Of F i g u r e s v Acknowledgements v i Chapter 1 - I n t r o d u c t i o n 1 Terminology 4 I n f i l t r a b i l i t y 4 C o n t a c t Angle 5 H y d r o p h o b i c i t y 7 W a t e r - r e p e l l e n c y 10 O r g a n i z a t i o n 10 Chapter 2 - The Study Area 12 Chapter 3 - Methods 19 P r e c i p i t a t i o n Measurement 19 Mapping 20 S o i l Sampling 21 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 21 Water Drop P e n e t r a t i o n Time T e s t 22 Chapter 4 - Hypotheses 24 R e d u c t i o n Of C a p i l l a r y G r a d i e n t s 25 T e x t u r a l C o n t r a s t s .^ 33 Coarse Over F i n e 35 F i n e Over Coarse 41 A i r Entrapment 45 Summary 47 Chapter 5 - E v a l u a t i o n Of Hypotheses 50 i v R e d u c t i o n Of C a p i l l a r y G r a d i e n t s 50 T e x t u r a l C o n t r a s t s 51 A i r Entrapment 53 Chapter 6 - H y d r o p h o b i c i t y H y p o t h e s i s 54 Case 1 - Co n t a c t Angle Less Than 90 Degrees 54 Case 2 - Co n t a c t Angle G r e a t e r Than 90 Degrees 58 E v a l u a t i o n .62 Chapter 7 - D i s c u s s i o n 66 Chapter 8 - Summary 76 B i b l i o g r a p h y 79 Appendix A - Major V e g e t a t i o n A s s o c i a t i o n s Of The Goat Meadows Watershed 83 Appendix B - The S u r f a c e T e n s i o n Of E t h y l A l c o h o l Water M i x t u r e s 86 Appendix C- P r e c i p i t a t i o n Data 87 Appendix D - L i s t Of Symbols 88 V L i s t of f i g u r e s 1 C o n t a c t a n g l e 6 2 The e f f e c t of c o n t a c t a n g l e on c a p i l l a r y r i s e ...9 3 L o c a t i o n of the study a r e a 13 4 Pemberton V a l l e y 14 5 Study area 15 6 S o i l p r o f i l e s 18 7 H y d r a u l i c f u n c t i o n s f o r Green and Ampt a n a l y s i s 28 8 Water c o n t e n t - t e n s i o n r e l a t i o n s h i p 31 9 Comparison of the Green and Ampt s o l u t i o n w i t h a n u m e r i c a l s o l u t i o n 32 10 T e x t u r a l c o n t r a s t s 36 11 I n f i l t r a t i o n r a t e , c o a r s e over f i n e c o n t r a s t ...42 12 I n f i l t r a t i o n r a t e v a r i a t i o n s due t o a i r entrapment 48 13 P r o f i l e of an ' a l t e r e d ' s o i l 55 14 I n f i l t r a t i o n r a t e f o r an a l t e r e d s o i l under j u s t - p o n d i n g c o n d i t i o n s .57 15 I n f i l t r a t i o n r a t e f o r an a l t e r e d s o i l , r a i n f a l l r a t e s p e c i f i e d 59 16 I n f i l t r a t i o n r a t e f o r w a t e r - r e p e l l e n t s o i l , ponding depth s p e c i f i e d 61 17 I n f i l t r a t i o n r a t e f o r w a t e r - r e p e l l e n t s o i l , r a i n f a l l r a t e s p e c i f i e d 63 18 Water drop p e n e t r a t i o n t i m e s 65 19 I n f i l t r a t i o n r a t e - time r e l a t i o n s h i p s 69 v i Acknowledgements I extend thanks t o my s u p e r v i s o r - Dr. H. Olav Slaymaker, my second reader - D r . R. A l l a n F r e e z e , and the t h i r d member of my committee - Dr. Jan D e V r i e s . I thank the N a t u r a l S c i e n c e s and E n g i n e e r i n g R esearch C o u n c i l f o r a p o s t g r a d u a t e s c h o l a r s h i p awarded i n 1979, and renewed i n 1980, and f o r support f o r my f i e l d work t h r o u g h O p e r a t i n g Grant A-7073, which was awarded t o Dr. Slaymaker. I thank Rodney Haynes f o r h i s a s s i s t a n c e i n the f i e l d . A l and M a r t i S t a e h l i a re thanked f o r t h e i r h o s p i t a l i t y . R i c h a r d L e s l i e ( t e c h n i c i a n a t the U n i v e r s i t y of B r i t i s h Columbia) i s thanked f o r d e s i g n i n g and c o n s t r u c t i n g a number of i n s t r u m e n t s used i n the f i e l d . Thomas G a l l i e d e s e r v e s s p e c i a l c r e d i t , not o n l y f o r the a s s i s t a n c e o f f e r e d i n the f i e l d , but a l s o f o r many h e l p f u l s u g g e s t i o n s , and f o r many hours of d i s c u s s i o n and debate. 1 Chapter 1 - I n t r o d u c t i o n Much of the d i s c h a r g e of r i v e r s i n the Coast Mountains of B r i t i s h Columbia i s d e r i v e d from a l p i n e and s u b - a l p i n e a r e a s . The t o t a l annual p r e c i p i t a t i o n i n the s e b i o t i c zones i s t y p i c a l l y g r e a t e r than i n lower e l e v a t i o n zones due t o o r o g r a p h i c e f f e c t s . As w e l l , the r a t i o of p r e c i p i t a t i o n t o r u n o f f approaches u n i t y on an annual b a s i s because the p o t e n t i a l f o r e v a p o t r a n s p i r a t i o n i s v e r y low. Hence, c o n s i d e r a b l y more r u n o f f i s g e n e r a t e d i n such environments than i s e x p e c t e d on the b a s i s of t h e i r a r e a l e x t e n t . In s p i t e of t h i s , t h e r e has been r e l a t i v e l y l i t t l e s t udy of the mechanisms of st r e a m f l o w g e n e r a t i o n i n the a l p i n e and sub-a l p i n e of the Coast M o u n t a i n s . In r e c e n t y e a r s , a c o n t r o v e r s y has a r i s e n over the r e l a t i v e importance of v a r i o u s proposed mechanisms of sto r m f l o w g e n e r a t i o n . These mechanisms i n c l u d e H o r t o n i a n o v e r l a n d f l o w (Horton,1933), s a t u r a t i o n o v e r l a n d f l o w (Dunne,1969), and s u b s u r f a c e s t o r m f l o w (Whipkey,1965). At i s s u e i s the r e l a t i v e s i g n i f i c a n c e of each of these mechanisms, r a t h e r than whether or not they a c t u a l l y o c c u r . F r e e z e ( l 9 7 2 a,b) c o n s i d e r e d - t h e problem t h e o r e t i c a l l y , u s i n g a n u m e r i c a l method of s o l u t i o n f o r u n s a t u r a t e d - s a t u r a t e d f l o w . Only a r a t h e r l i m i t e d range of r a i n f a l l r a t e s , h y d r a u l i c p r o p e r t i e s , and h i l l s l o p e geometry l e d t o s i g n i f i c a n t s u b s u r f a c e s t o r m f l o w ; s a t u r a t i o n o v e r l a n d f l o w was u s u a l l y the dominant mechanism i n these s i m u l a t i o n s . The q u e s t i o n , however, can be r e s o l v e d o n l y by f u r t h e r 2 e x p e r i m e n t a t i o n i n the f i e l d . At p r e s e n t , a l l t h r e e mechanisms must be c o n s i d e r e d i n any study of stormfow generat i o n . H o r t o n ( l 9 3 3 ) found, e m p i r i c a l l y , t h a t the i n f i l t r a b i l i t y of s o i l s i s not a c o n s t a n t ; t y p i c a l l y , i t d e c l i n e s r a d i d l y i m m e d i a t e l y a f t e r the onset of a r a i n f a l l e v e n t , w i t h the r a t e of d e c l i n e d e c r e a s i n g w i t h t i m e . Horton h y p o t h e s i z e d t h a t the d e c l i n e i n i n f i l t r a b i 1 i t y d u r i n g r a i n f a l l e v e n t s was due t o one or more of the f o l l o w i n g p r o c e s s e s : f o r m a t i o n of a low c o n d u c t i v i t y s u r f a c e l a y e r through p a c k i n g by r a i n d r o p s ; s w e l l i n g of c o l l o i d s , r e s u l t i n g i n the c l o s i n g of c r a c k s and o t h e r o p e n i n g s ; and c l o g g i n g of s u r f a c e pores w i t h f i n e s r e d i s t r i b u t e d by r a i n f a l l impact. Between r a i n f a l l e v e n t s , the i n f i l t r a b i l i t y i n c r e a s e s towards a l e v e l s i m i l a r t o t h a t p r e c e e d i n g the event. The r e c o v e r y of the i n f i l t r a b i 1 i t y between e v e n t s was thought t o be e f f e c t e d by p r o c e s s e s such as the u n c l o g g i n g of pores by wind a c t i o n , s h r i n k a g e and c r a c k i n g d u r i n g d r y i n g , and the a c t i o n of earthworms and o t h e r s o i l o rganisms. In the l a t e 1960's, Dunne ( l 9 6 9 ) advanced an a l t e r n a t i v e t o H o r t on's model of s t o r m f l o w p r o d u c t i o n . C a r e f u l comparison of the magnitude of the p r e c i p i t a t i o n i n p u t and the volume of r u n o f f produced d u r i n g storm e v e n t s had demonstrated t h a t , i n many c a s e s , o v e r l a n d f l o w need occur over o n l y a s m a l l p a r t of a b a s i n i n o r d e r t o s u p p o r t the observed volume of r a p i d r u n o f f ( B e t s o n , 1 9 6 4 ) . Dunne 3 e x p l o r e d t h i s o b s e r v a t i o n f u r t h e r i n S l e e p e r s R i v e r Watershed i n Vermont. I t was found t h a t storm p e r i o d r u n o f f was g e n e r a t e d i n and near the c h a n n e l , and i n a dynamic zone a d j a c e n t t o i t . However, the mechanism of o v e r l a n d f l o w p r o d u c t i o n was not H o r t o n i a n . O v e r l a n d f l o w was g e n e r a t e d where the s o i l p r o f i l e was c o m p l e t e l y s a t u r a t e d , u s u a l l y near c h a n n e l margins and i n h o l l o w s . The a r e a of s a t u r a t i o n expands and c o n t r a c t s i n response t o the p r e c i p i t a t i o n i n p u t . The l o c a t i o n of a s a t u r a t e d p r o f i l e depends upon the topography, the h y d r a u l i c p r o p e r t i e s of the s o i l s , and the a n t e c e d e n t c o n d i t i o n s . The exceedence of 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 by the r a i n f a l l r a t e i s not an e s s e n t i a l requirement f o r o v e r l a n d f l o w p r o d u c t i o n , as i t i s i n Horton's model. The s u b s u r f a c e s t o r m f l o w model was i n t r o d u c e d as e a r l y as 1941, by Hursh and Hoover. I t was proposed t h a t r o o t c h a n n e l s may conduct water r a p i d l y t o s u b u r f a c e s a t u r a t e d zones, and thence t o the c h a n n e l , e f f e c t i v e l y b y p a s s i n g the u n s a t u r a t e d s o i l m a t r i x . W h i l e the mechanism of s u b s u r f a c e s t o r m f l o w i s not f u l l y u n d e r s t o o d , i t i s f r e q u e n t l y advanced t o ' e x p l a i n ' the r a p i d response of streams t o storm e v e n t s i n the absence of o b s e r v e d o v e r l a n d f l o w . The . r e s u l t s of s e v e r a l r e c e n t s t u d i e s suggest t h a t t h i s s u b s u r f a c e s t o r m f l o w i s i m p o r t a n t i n a t l e a s t some f o r e s t e d a r e a s of the P a c i f i c Northwest (Chamberlain,1972; D e V r i e s and Chow,1978). I t i s not c l e a r what d e t e r m i n e s whether or not f l o w t h r o u g h r o o t c h a n n e l s i s f a v o u r e d , but D e V r i e s and 4 Chow(l978) found t h a t d i s t u r b a n c e of the o r g a n i c h o r i z o n s a t the t o p of the s o i l p r o f i l e l e d t o a s h i f t away from c o n d u c t i o n through r o o t c h a n n e l s t o c o n d u c t i o n through the m a t r i x . The d i s r u p t i o n of the upper s o i l h o r i z o n s may d e s t r o y the c o n t i n u i t y of r o o t c h a n n e l s , but t h i s p r o p o s i t i o n has not been t e s t e d d i r e c t l y . The p r i m a r y o b j e c t i v e of t h i s study was t o determine which of the t h r e e mechanisms of s t o r m f l o w g e n e r a t i o n o u t l i n e d above a r e i m p o r t a n t i n the study a r e a . T e r m i n o l o g y There a r e s e v e r a l terms used i n t h i s t h e s i s which r e q u i r e some e l a b o r a t i o n ; i n p a r t i c u l a r : i n f i l t r a b i l i t y , c o n t a c t a n g l e , h y d r o p h o b i c i t y , and w a t e r - r e p e l l e n c y . D e f i n i t i o n s of each of t h e s e w i l l be g i v e n i n the pages f o l l o w i n g . I n f i l t r a b i l i t y The ' i n f i l t r a b i l i t y ' of a s o i l i s synonymous w i t h the ' i n f i l t r a t i o n - c a p a c i t y ' d e f i n e d by H o r t o n ( 1 9 3 3 ) . I t i s the i n f i l t r a t i o n r a t e t h a t would be observed i f a j u s t - p o n d i n g c o n d i t i o n were imposed; t h a t i s , i f the t e n s i o n of water at the s u r f a c e i s m a i n t a i n e d a t z e r o . In e s s e n c e , i t i s the maximum r a i n f a l l r a t e t h a t c o u l d be a c c e p t e d w i t h o u t o v e r l a n d f l o w o c c u r r i n g . The i n f i l t r a b i l i t y of a g i v e n s o i l i s not a c o n s t a n t , but v a r i e s w i t h changes i n the m o i s t u r e 5 c o n t e n t of the s o i l , and i n response t o p h y s i c a l d i s t u r b a n c e of the system. The term ' i n f i l t r a b i l i t y ' i s p r e f e r r e d t o ' i n f i l t r a t i o n - c a p a c i t y ' because the p r o p e r t y i n q u e s t i o n i s not a c a p a c i t y i n the u s u a l sense of the word. Co n t a c t a n g l e A c o n t a c t a n g l e may be d e f i n e d f o r systems c o m p r i s e d of a s o l i d and two l i q u i d phases, or f o r a s o l i d , a l i q u i d , and a gas - i n t h i s , c a s e , f o r a s o l i d , w ater, and a i r . In heterogeneous porous media, the s o l i d i s a c t u a l l y a m u l t i p h a s e system; hence, f o r each s o l i d phase, a c o n t a c t a n g l e may be s p e c i f i e d . At any chosen p o i n t of c o n t a c t between the t h r e e phases, a l i n e i s d e f i n e d by the i n t e r s e c t i o n of the p l a n e tangent t o the s o l i d s u r f a c e w i t h the p l a n e tangent t o the water - a i r i n t e r f a c e . The c o n t a c t a n g l e i s the a n g l e between these two p l a n e s , p r o c e e d i n g from the s o l i d - w a t e r i n t e r f a c e t o the w a t e r - a i r i n t e r f a c e , normal t o the l i n e of i n t e r s e c t i o n (see F i g u r e 1). The s i z e of the c o n t a c t a n g l e i s d e t e r m i n e d by the i n t e r a c t i o n s between the t h r e e phases. At e q u i l i b r i u m , t h e r e must be a b a l a n c e of f o r c e s a t the c o n t a c t p o i n t . Movement normal t o the s o l i d s u r f a c e i s i m p o s s i b l e ; hence, o n l y the e q u a t i o n f o r a b a l a n c e of f o r c e s p a r a l l e l t o the s o l i d s u r f a c e i s g i v e n ( H i l l e l , 1 9 7 1 ) : <ysa = tfsw + tfwa«cos(o) Where: tysa = s o l i d - a i r i n t e r f a c i a l t e n s i o n Figure 1 - Contact angle 7 <rsw = s o l i d - w a t e r i n t e r f a c i a l t e n s i o n <ywa = w a t e r - a i r i n t e r f a c i a l t e n s i o n T h e r e f o r e : a = A r c c o s ( ( t f S a - *sw)/tfwa) There a r e c e r t a i n n a t u r a l l i m i t s t o the a p p l i c a t i o n of t h i s e q u a t i o n ; one of t h e s e i s : crsa > <rwa + tfsw When t h i s i s the c a s e , t h e r e w i l l be no s o l i d - w a t e r i n t e r f a c e - t h e r e w i l l o n l y be w a t e r - a i r and s o i l d - w a t e r i n t e r f a c e s . In o t h e r words, the s o l i d w i l l be c o m p l e t e l y w e t t e d , and t h e r e w i l l not be a c o n t a c t p o i n t . The o t h e r l i m i t i s : <rsw > <rsa + «ywa I f t h i s i s the c a s e , a s o l i d - w a t e r i n t e r f a c e w i l l n o t . f o r m -t h e r e w i l l o n l y be s o l i d - a i r and w a t e r - a i r i n t e r f a c e s . In o t h e r words, the s u r f a c e w i l l not wet. Between the two l i m i t s j u s t o u t l i n e d , the c o n t a c t a n g l e i s governed by the e q u a t i o n d e v e l o p e d e a r l i e r . For a s o l i d - w a t e r - a i r system, the s u r f a c e t e n s i o n of water - «wa - i s e s s e n t i a l l y c o n s t a n t (72.7 dynes/cm at 20C). Hence, the c o n t a c t a n g l e i s d e t e r m i n e d almost s o l e l y by the r e l a t i v e v a l u e s of the s o l i d - a i r i n t e r f a c i a l t e n s i o n and the s o l i d - w a t e r i n t e r f a c i a l t e n s i o n . Hydrophobic i t y A s o l i d s u r f a c e i s h y d r o p h i l l i c i f i t wets s p o n t a n e o u s l y , and h y d r o p h o b i c i f i t does no t . The c o n t a c t 8 a n g l e may be used as a c r i t e r i o n f o r s p o n t a n e i t y . I f the c o n t a c t a n g l e i s l e s s than 90 degree s , the s u r f a c e w i l l wet s p o n t a n e o u s l y , but i f the a n g l e i s g r e a t e r than 90 degree s , the s u r f a c e w i l l not wet s p o n t a n e o u s l y . Thus, h y d r o p h i l l i c and h y d r o p h o b i c s u r f a c e s may be d i f f e r e n t i a t e d by d e t e r m i n i n g whether the c o n t a c t a n g l e i s l e s s t h a n , or g r e a t e r than 90 degrees. I t i s i n s t r u c t i v e t o c o n s i d e r the c a p i l l a r y r i s e f o r m u l a (Osipow, 1977): h = 2 • <ywa «cos ( o )/( pgr ) Where: h = h e i g h t of c a p i l l a r y r i s e p - d e n s i t y of water g = g r a v i t a t i o n a l a c c e l l e r a t i o n r = r a d i u s of c a p i l l a r y tube I f the c o n t a c t a n g l e i s 90 de g r e e s , then the h e i g h t of c a p i l l a r y r i s e w i l l be z e r o . S i m i l a r l y , a s o i l pore w i l l not wet s p o n t a n e o u s l y i f the c o n t a c t a n g l e i s 90 d e g r e e s . I f the c o n t a c t a n g l e i s l e s s than 90 degree s , water w i l l r i s e i n the tube up t o some c h a r a c t e r i s t i c h e i g h t , but i f the a n g l e i s g r e a t e r than 90 d e g r e e s , water w i l l not e n t e r the tube u n l e s s the l e v e l of the water about the tube i s r a i s e d t o the c h a r a c t e r i s t i c h e i g h t , as i l l u s t r a t e d i n F i g u r e 2. In an analogous manner, s o i l p ores which have h y d r o p h o b i c s u r f a c e s r e q u i r e the a p p l i c a t i o n of a p o s i t i v e p r e s s u r e head b e f o r e they w i l l f i l l . Figure 2 - The effect of contact angle on capillary rise 10 W a t e r - r e p e l l e n c y I t i s i m p o s s i b l e t o d e f i n e a s i n g l e c o n t a c t a n g l e f o r heterogeneous porous media, because the s o l i d i s a c t u a l l y c o m p r i s ed of many d i s t i n c t phases. U n l e s s a l l the c o n t a c t a n g l e s of a l l the s o l i d phases a r e e i t h e r a l l l e s s than 90 d egrees, or a l l g r e a t e r than 90 d e g r e e s , the medium can not be c l a s s e d as e i t h e r h y d r o p h i l l i c or h y d r o p h o b i c . In t h i s s t u d y , the term ' w a t e r - r e p e l l e n t ' i s used t o d e s c r i b e media which have at l e a s t some pores w i t h h y d r o p h o b i c s u r f a c e s . U n f o r t u n a t e l y , some i n d i c e s of w a t e r - r e p e l l e n c y do not d i s t i n g u i s h between media which have pore s u r f a c e s w i t h c o n t a c t a n g l e s g r e a t e r than z e r o d e g r e e s , but s t i l l l e s s than 90 d e g r e e s , and media which have pore s u r f a c e s w i t h c o n t a c t a n g l e s g r e a t e r than 90 degrees (Debano,1981). The d i s t i n c t i o n between the s e two p o s s i b i l i t i e s s h o u l d be m a i n t a i n e d , however, because the u n s a t u r a t e d f l o w p r o c e s s e s are f u n d a m e n t a l l y d i f f e r e n t f o r each of t h e s e c a s e s . O r g a n i z a t i o n The c h a r a c t e r i s t i c s of the s t u d y a r e a a r e o u t l i n e d i n the next c h a p t e r . The methods used i n the f i e l d and the l a b o r a t o r y are then o u t l i n e d . F o l l o w i n g t h a t , the p o s s i b l e mechanisms of s t o r m f l o w g e n e r a t i o n a r e r e v i e w e d i n l i g h t of the f i e l d o b s e r v a t i o n s . More s p e c i f i c hypotheses are then advanced, and e v a l u a t e d by comparing the t h e o r e t i c a l i m p l i c a t i o n s of each w i t h the f i e l d o b s e r v a t i o n s , and w i t h 11 the r e s u l t s of l a b o r a t o r y e x p e r i m e n t s . A l l of the i n i t i a l h ypotheses a r e r e j e c t e d . In t h e i r p l a c e , the h y p o t h e s i s t h a t a w a t e r - r e p e l l e n t l a y e r e x i s t s a t , or near, the s u r f a c e i s advanced. The t h e o r e t i c a l i m p l i c a t i o n s of t h a t p r o p o s i t i o n are examined, and the h y p o t h e s i s i s r e c a s t i n more ex a c t terms, a l l o w i n g e v a l u a t i o n by a s i m p l e l a b o r a t o r y e x p e r i m e n t . The i m p l i c a t i o n s of the r e s u l t s a r e d i s c u s s e d i n the p e n u l t i m a t e c h a p t e r . The f i n a l c h a p t e r i s a summary. 1 2 Chapter 2 - The study a r e a The f i e l d - w o r k was done w i t h i n a two square k i l o m e t r e , unnamed, s u b - a l p i n e , f i r s t - o r d e r b a s i n t r i b u t a r y t o Ryan R i v e r (see F i g u r e s 3 and 4 ) . The study a r e a i s w i t h i n the P a c i f i c Ranges of the Coast M o u n t a i n s , and has a c o l d perhumid c l i m a t e . The s o i l s a r e m a i n l y B r u n i s o l s and R e g o s o l s , which are deve l o p e d i n s u r f i c i a l d e p o s i t s comprised p r i m a r i l y of l o e s s over t i l l . W i t h i n the l o e s s a re two d i s c o n t i n u o u s p y r o c l a s t i c ash l a y e r s . The bedrock c o n s i s t s m a i n l y of g n e i s s e s and s c h i s t s of the Gambier Group, which o v e r l i e s the g r a n o d i o r i t e of the Coast Mountains P l u t o n as a r o o f pendant. The d r a i n a g e system i s shown i n some d e t a i l i n F i g u r e 5. Many of the streams above M i d d l e Pond a r e ephemeral; some f l o w i n response t o v i r t u a l l y any r a i n s t o r m , w h i l e o t h e r s f l o w o n l y as a r e s u l t of p r o l o n g e d e v e n t s . Most of the streams b e g i n w i t h i n , or a t , the base of t a l u s cones, or a t s p r i n g s i s s u i n g from the base of the e a s t s l o p e . One stream f l o w s from G a l l i e Pond, but i t t o o i s ephemeral. There i s some o r g a n i z a t i o n of d r a i n a g e w i t h i n the t a l u s cones on the south s i d e ; water f l o w s i n 'ch a n n e l s ' a t some depth w i t h i n the t a l u s , f e d m a i n l y by m e l t w a t e r from the s c a t t e r e d snow patc h e s t h a t remained throughout the f i e l d season. S e v e r a l major v e g e t a t i o n a s s o c i a t i o n s have been i d e n t i f i e d i n t h i s a r e a ( G a l l i e , p e r s o n a l communication, 1980). The s p e c i e s c o m p o s i t i o n of the a s s o c i a t i o n s a r e g i v e n Figure 3 - L o c a t i o n of study area (adapted from Farley,1979) 14 Figure 4 - Pemberton V a l l e y (adapted from Government of Canada,1970) 16 i n Appendix A. A d j a c e n t t o stream c h a n n e l s and the ponds, and i n s u r f a c e s t o r a g e d e t e n t i o n s i t e s , the Sedge, and the Luetkea-Moss-Lichen A s s o c i a t i o n s dominate. The Heather-Dwarf T r e e s , Heather-Sedge-Forb, and Cassiope-Moss A s s o c i a t i o n s occupy the s i d e s l o p e s . The Tree I s l a n d a s s o c i a t i o n - which i s c omprised p r i n c i p a l l y of A l p i n e F i r , Mountain Hemlock, B l a c k Mountain H u c k l e b e r r y , White Rhododendron, Red Heat h e r , White Moss Heather, and Luetkea - i s f r e q u e n t l y found on convex breaks i n s l o p e , and on t o p o g r a p h i c h i g h s . Other a s s o c i a t i o n s a re found w i t h i n the study a r e a , but a r e not a r e a l l y e x t e n s i v e and a r e not p a r t i c u l a r l y i m p o r t a n t f o r the purposes of t h i s s t u d y . C l i m a t i c r e c o r d s f o r the a r e a a r e r a t h e r s p a r s e , but r u n o f f from L i l l o o e t R i v e r i s known t o average about 1840mm a n n u a l l y . P r e c i p i t a t i o n r e c o r d s f o r the n e a r e s t c l i m a t e s t a t i o n - Pemberton Meadows - show t h a t t h e r e i s a s t r o n g p r e c i p i t a t i o n maximum i n the w i n t e r months. At h i g h e r e l e v a t i o n s , most of the w i n t e r p r e c i p i t a t i o n o c c u r s as snow. Not s u r p r i s i n g l y , the maximum d i s c h a r g e of L i l l o o e t R i v e r u s u a l l y c o i n c i d e s w i t h the temperature maximum i n m i d - J u l y , as p o i n t e d out by T e t i ( l 9 7 9 ) . Nearby snow c o u r s e s have average snowpacks t h a t range from 766mm a t W h i s t l e r Mountain ( e l e v a t i o n 1450m) t o 1354mm a t Diamond Head ( e l e v a t i o n 1420m). Average h o u r l y r a i n f a l l r a t e s r e c o r d e d i n August and September of 1980 w i t h the t i p p i n g bucket r a i n gauge are r e p o r t e d i n Appendix C. The s o i l s i n the study a r e a a r e m a i n l y D y s t r i c 1 7 B r u n i s o l s and R e g o s o l s . L o c a l l y , t h e r e a r e Or g a n i c s o i l s -m a i n l y a c c u m u l a t i o n s of sphagnum moss. There i s a d i s t i n c t i v e sequence of pa r e n t m a t e r i a l s over much of the f i e l d a r e a ; t h i s sequence i s i l l u s t r a t e d i n F i g u r e 6, a l o n g w i t h a common v a r i a n t . In a d d i t i o n t o the v a r i a n t i l l u s t r a t e d , b u r i e d stream d e p o s i t s c r e a t e l o c a l l y d i s t i n c t i v e sequences. The lower ash i s p r o b a b l y Mazama (6600 BP), w h i l e the upper ash i s p r o b a b l y B r i d g e R i v e r (2600 BP). The upper ash i s more a r e a l l y e x t e n s i v e than the l o w e r , i s l e s s weathered, and c o a r s e r . Both ash l a y e r s a re c o a r s e r than the l o e s s l a y e r s ; hence, t h e r e i s a sequence of changes i n t e x t u r e and a s s o c i a t e d h y d r a u l i c p r o p e r t i e s . The bedrock i s a r o o f pendant mapped as Gambier Group (Woodsworth, 1977). The exposures a t the f i e l d s i t e suggest t h a t h e a v i l y f r a c t u r e d g n e i s s e s predominate, a t l e a s t l o c a l l y . F r a c t u r e s range i n s i z e from ones which a r e o n l y m i l l i m e t r e s wide, c e n t i m e t r e s deep, and metres l o n g t o ones which are more than a metre wide, s e v e r a l metres deep, and perhaps hundreds of metres l o n g . loess loess upper ash upper ash oU cm l oess loess oU cm lower ash loess ablation till ablation till lodgment till lodgment till1  1 Figure 6 - Soil profiles 19 Chapter 3 - Methods The method of ' s t r o n g i n f e r e n c e ' a d v o c a t e d by P l a t t ( l 9 6 4 ) was f o l l o w e d as c l o s e l y as p o s s i b l e . In s h o r t , the method i s the s y s t e m a t i c a p p l i c a t i o n of i n d u c t i v e i n f e r e n c e . E x periments a r e d e s i g n e d t o e x c l u d e one or more of the hypotheses advanced as c l e a r l y and as s i m p l y as p o s s i b l e . Each s t e p of an i n q u i r y f o l l o w i n g t h i s approach depends upon the outcome of the p r e v i o u s one; hence, i t i s not p o s s i b l e t o o u t l i n e the c o u r s e of a study i n advance. The r a t i o n a l e f o r the c h o i c e of methods, and d i s c u s s i o n of the e x p e r i m e n t a l d e s i g n i s g i v e n i n the t e x t as the r e s e a r c h e v o l v e s . In t h i s c h a p t e r , o n l y the p r o c e d u r e s t h a t r e q u i r e p h y s i c a l m a n i p u l a t i o n of s o i l samples, or d i r e c t measurement of some parameter are d e s c r i b e d . P r e c i p i t a t i o n measurement Changes i n r a i n f a l l r a t e s over time were m o n i t o r e d u s i n g a t i p p i n g bucket r a i n gauge, which was c o n n e c t e d t o an event r e c o r d e r ( c o n s t r u c t e d by R i c h a r d L e s l i e , T e c h n i c i a n , Department of Geography, U.B.C.). Each t i p of the bucket r e p r e s e n t s 0.03 cm of r a i n , w i t h the number of t i p s i n each 20 minute p e r i o d b e i n g r e c o r d e d . There i s a b a s i c u n c e r t a i n t y of one t i p per time p e r i o d ; hence, the r a i n f a l l r a t e i s known t o a p p r o x i m a t e l y p l u s or minus 2.5x10" 5 cm/s (or 0.09 cm per hour f o r each 20 minute time p e r i o d . 20 T h i s r a i n gauge was l o c a t e d on the r i d g e c r e s t , near G a l l i e Pond (see F i g u r e 5 ) . Mapping Maps of the study a r e a were c o m p i l e d from s e v e r a l s o u r c e s of d a t a : (1) Topographic maps a. Pemberton map sheet 92 J/7, 1970, 1:50,000, Surveys and Mapping Branch, Department of Energy Mines and Resources b. Pemberton map sheet 92 J 1:250,000, 1958, Surveys and Mapping Branch, Department of Mines and T e c h n i c a l Surveys c. Map of A l l i s o n ' s Bowl, c o m p i l e d by P. J o n e s ( l 9 8 l , u n p u b l i s h e d ) (2) Landsat f a l s e c o l o u r i n f r a r e d photographs (A37290, p r i n t s 2836,2837, and 2838) (3) Low l e v e l o b l i q u e and near v e r t i c a l photographs taken from a h e l i c o p t e r w i t h a c o n v e n t i o n a l 35mm camera (4) Plane t a b l e survey u s i n g a W i l d RK1 s e l f -r e d u c i n g a l i d a d e , and p l a n e t a b l e The t o p o g r a p h i c maps were used t o p l a c e the study a r e a i n F i g u r e s 3 and 4, and t o p r o v i d e g e n e r a l i z e d c o n t o u r s i n F i g u r e 4. The Landsat and low l e v e l a e r i a l photographs were used t o s e t the b o u n d a r i e s and the g e n e r a l d r a i n a g e p a t t e r n of the study b a s i n , w h i l e the p l a n e t a b l e s u r v ey was used t o 21 add p l a n i m e t r i c a l l y c o r r e c t d e t a i l t o F i g u r e 5, and a l s o t o l o c a t e s a m p l i n g s i t e s on a s i m p l e r e f e r e n c e map (not reproduced here) t h a t was used w h i l e the study was i n p r o g r e s s . S o i l s a m p l i n g No t e s t i n g of s o i l s was performed i n the f i e l d . Two methods of c o l l e c t i n g i n t a c t s o i l samples were used. In some cas e s p l a s t i c PCV p i p e , a p p r o x i m a t e l y 8.5 cm i n d i a m e t e r , was d r i v e n i n t o the ground. The sample was r e t r i e v e d by e x c a v a t i n g around the p i p e , and c a p p i n g both ends. In o t h e r c a s e s , a f r e e - s t a n d i n g column of s o i l , r o u g h l y 15 t o 20 cm i n d i a m e t e r was e x c a v a t e d . The column was wrapped i n f i b r e g l a s s c l o t h , which was s e c u r e d t o the column by p i n s , and then impregnated w i t h " C o l d Cure" epoxy (manufactured by I n d u s t r i a l F o r m u l a t o r s of Canada L t d . , 3824 W i l l i a m s S t r e e t , Burnaby, B.C.). T h i s p r o c e d u r e was d e v i s e d by G a l l i e ( p e r s o n a l communication,1980); i t i s p r e f e r r e d where a l a r g e d i a m e t e r sample i s r e q u i r e d , or where the former method i s d i f f i c u l t t o use - i n r o c k y s o i l s , f o r example. 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 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 was d e t e r m i n e d w i t h the a i d of a c o n s t a n t head permeameter c o n s t r u c t e d by the a u t h o r (see F r e e z e and C h e r r y , 1979, or any o t h e r g e n e r a l h y d r o l o g y t e x t , f o r a b a s i c d e s c r i p t i o n of t h i s 22 a p p a r a t u s ) . The g o v e r n i n g e q u a t i o n i s Darcy's Law: Q = A-KS'H/L Where: Q = the f l o w volume per u n i t time A = the c r o s s - s e c t i o n a l a r e a Ks = 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 H = the h y d r a u l i c head a c r o s s the sample L = the l e n g t h of the sample I t i s c l e a r t h a t : Ks = Q-L/(A-H) The v a l u e s of A, H, and L are a l l imposed; hence, o n l y Q, the f l o w volume per u n i t t i m e , need be measured i n o r d e r t o e s t i m a t e Ks, the s a t u r a t e d h y d r a u i c c o n d u c t i v i t y . Care was taken not t o d i s t u r b the samples t e s t e d . The upper l a y e r s of the s o i l were somewhat ' t u r f y ' , making i t p o s s i b l e t o c u t a d i s c of sample of the same d i a m e t e r as t h a t of the p l a s t i c sample h o l d e r . Epoxy was used t o s e a l the edge of the sample t o the i n s i d e s u r f a c e of the tube. Because many of the samples were w a t e r - r e p e l l e n t , the samples were s u b j e c t e d t o a p r e s s u r e head of a t l e a s t 10 cm to ensure s a t u r a t i o n . Water drop p e n e t r a t i o n time t e s t The water drop p e n e t r a t i o n time i s used as an index of w a t e r - r e p e l l e n c y : the l o n g e r the t i m e , the more r e p e l l e n t the sample. T h i s t e s t i s performed by t i m i n g the p e n e t r a t i o n of a drop of water i n t o an a i r d r i e d s o i l sample. The time 23 w i l l be l e s s than one second f o r v i r t u a l l y any n o n - r e p e l l e n t sample. In heterogeneous porous media, the c o n t a c t a n g l e -which i s a c o n v e n i e n t measure of the a f f i n i t y of the s o l i d f o r water - v a r i e s from p o i n t t o p o i n t . Hence, the water drop p e n e t r a t i o n time i s a s p a t i a l l y averaged measure of r e p e l l e n c y . To i s o l a t e the e f f e c t of the ' r e p e l l e n c y ' upon p e n e t r a t i o n t i m e , the p e n e t r a t i o n time of a one p e r c e n t s o l u t i o n of e t h y l a l c o h o l was d e t e r m i n e d . T h i s s o l u t i o n has n e a r l y the same l i q u i d - a i r i n t e r f a c i a l t e n s i o n as pure water (see Appendix B ) , but the s o l i d - l i q u i d i n t e r f a c i a l t e n s i o n drops such t h a t the c o n t a c t a n g l e approaches z e r o . In o t h e r words the t e s t s o l u t i o n behaves m u c h , l i k e water w i t h r e s p e c t t o a l l of i t s p h y s i c a l p r o p e r t i e s , except t h a t i t wets the s o l i d almost p e r f e c t l y . Thus, i f the s o l u t i o n of e t h y l a l c o h o l p e n e t r a t e s the sample p r o m p t l y , where pure water d i d n o t , then the slow p e n e t r a t i o n of the pure water must be due t o w a t e r - r e p e l l e n c y r a t h e r than a low h y d r a u l i c conduct i v i t y . 24 Chapter 4 - Hypotheses As noted i n the i n t r o d u c t i o n , t h r e e mechanisms of sto r m f l o w g e n e r a t i o n s h o u l d be c o n s i d e r e d : (1) H o r t o n i a n o v e r l a n d f l o w ( H o r t o n , 1933); (2) s a t u r a t i o n o v e r l a n d f l o w (Dunne, 1969); and (3) s u b s u r f a c e s t o r m f l o w (Whipkey, 1965). The f i r s t t a s k i s t o d e c i d e which of thes e mechanisms c o n t r i b u t e s q u a n t i t a t i v e l y s i g n i f i c a n t s t o r m f l o w i n the study a r e a . Some 16 mm of r a i n f e l l d u r i n g a p e r i o d of j u s t over s i x hours on September 12, 1980: an average r a t e of 7.4x10" 5 cm/s. O v e r l a n d f l o w was produced not o n l y upon bare bedrock o u t c r o p s , but a l s o upon more than o n e - h a l f of the ar e a of v e g e t a t e d s l o p e s w i t h i n the bowl s u r r o u n d i n g M i d d l e Pond; p r i n c i p a l l y on the a r e a s c o v e r e d by the Sedge, the Lu e t k e a - M o s s - L i c h e n , the Cassiope-Moss, and the Heather-Sedge-Forb A s s o c i a t i o n s . A number of e x c a v a t i o n s were made i n the a r e a s where o v e r l a n d f l o w was ob s e r v e d . Almost i n v a r i a b l y , the s o i l was u n s a t u r a t e d except f o r a t h i n l a y e r , no more than two c e n t i m e t r e s t h i c k , a t the s o i l s u r f a c e . The t h i c k n e s s of t h i s s a t u r a t e d s u r f a c e l a y e r d i d not i n c r e a s e d u r i n g a subsequent r a i n f a l l of 90 mm over the p e r i o d September 28 t o September 30, 1980. I t i s c l e a r from these s i m p l e o b s e r v a t i o n s t h a t the o v e r l a n d f l o w produced on the v e g e t a t e d s l o p e s cannot be c l a s s e d as s a t u r a t i o n o v e r l a n d f l o w , because the s o i l was o n l y s a t u r a t e d at the s u r f a c e . S u b s u r f a c e s t o r m f l o w was not q u a n t i t a t i v e l y s i g n i f i c a n t - most of the s t o r m f l o w was 25 g e n e r ated by the o v e r l a n d f l o w on the v e g e t a t e d s l o p e s and the bedrock o u t c r o p s , and by d i r e c t p r e c i p i t a t i o n on the stream c h a n n e l . I t i s e v i d e n t t h a t the o v e r l a n d f l o w was g e n e r a t e d as a r e s u l t of the r a i n f a l l r a t e e x c e e d i n g the i n f i l t r a b i l i t y . The q u e s t i o n , t h e n , i s what c o n t r o l s the i n f i I t r a b i 1 i t y . There are a number of p o s s i b l e c o n t r o l s on the i n f i l t r a b i l i t y . Three hypotheses a r e advanced i n i t i a l l y ; they a r e : (1) r e d u c t i o n of c a p i l l a r y g r a d i e n t s (2) t e x t u r a l c o n t r a s t s (3) a i r entrapment The t h e o r e t i c a l framework r e q u i r e d f o r e v a l u a t i o n of t h e s e hypotheses i s o u t l i n e d i n the pages f o l l o w i n g . R e d u c t i o n of c a p i l l a r y g r a d i e n t s In the i n t r o d u c t i o n , i t was noted t h a t H o r t o n ( l 9 3 3 ) a t t r i b u t e d the d e c l i n e i n i n f i l t r a b i l i t y t h a t u s u a l l y o c c u r s d u r i n g r a i n s t o r m s t o p h y s i c a l f a c t o r s such as compaction of the s o i l by r a i n d r o p i m p a c t s , the s w e l l i n g of c o l l o i d s , and the c l o g g i n g of pores by r e d i s t r i b u t i o n of f i n e s . However, as F r e e z e ( l 9 7 4 ) has p o i n t e d o u t , our u n d e r s t a n d i n g of the i n f i l t r a t i o n p r o c e s s has improved c o n s i d e r a b l y s i n c e the time of Horton's work. To H o rton's l i s t of p o s s i b l e causes of a d e c l i n e i n i n f i l t r a b i l i t y d u r i n g r a i n s t o r m s , one s h o u l d add t h a t the p o t e n t i a l g r a d i e n t due t o the a t t r a c t i o n of water by c a p i l l a r y s i z e d pores g r a d u a l l y d e c l i n e s t o z e r o , 26 l e a v i n g o n l y the g r a v i t a t i o n a l p o t e n t i a l g r a d i e n t t o d r i v e the i n f i l t r a t i o n p r o c e s s . Rubin and S t e i n h a r d t ( 1 9 6 2 ) have shown t h a t the d e c l i n e i n i n f i l t r a b i l i t y due t o r e d u c t i o n i n c a p i l l a r y g r a d i e n t s may be r i g o r o u s l y p r e d i c t e d f o r a s o i l w i t h known h y d r a u l i c p r o p e r t i e s . R u b i n ( l 9 6 6 ) demonstrated t h a t i n orde r f o r o v e r l a n d f l o w t o o c c u r , the f o l l o w i n g e q u a t i o n must be sat i s f i e d : R > Ks-(6*/6z + 6z/6z) Where: R = r a i n f a l l r a t e Ks = 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 6*/6z = p r e s s u r e p o t e n t i a l g r a d i e n t 6z/6z = g r a v i t a t i o n a l p o t e n t i a l g r a d i e n t The g r a v i t a t i o n a l p o t e n t i a l g r a d i e n t i s e q u a l t o 1.0 ( i n cm of water per cm); t h e r e f o r e the e q u a t i o n may be r e w r i t t e n a s : R > Ks•(6*/6z + 1) The p r e s s u r e p o t e n t i a l g r a d i e n t d e v e l o p s i n response to the c a p i l l a r y f o r c e s a t , the w e t t i n g f r o n t . As the f r o n t p e n e t r a t e s f u r t h e r and f u r t h e r i n t o the s o i l , the p r e s s u r e p o t e n t i a l g r a d i e n t d e c l i n e s , g r a d u a l l y a p p r o a c h i n g z e r o . The g r e a t e r the r a t i o of the r a i n f a l l r a t e t o 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 , the s h o r t e r the time r e q u i r e d t o gen e r a t e o v e r l a n d f l o w . I t i s apparent t h a t the r a i n f a l l r a t e must exceed 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 i n o r d e r f o r o v e r l a n d f l o w t o occur - t h i s i s the minimum 27 r e q u i r e m e n t . A l t h o u g h Rubin was the f i r s t t o e x p l i c i t l y s t a t e the two c r i t e r i a e s s e n t i a l f o r o v e r l a n d f l o w p r o d u c t i o n due t o the d e c l i n e i n the i n f i l t r a b i l i t y of s o i l s , t he r o l e of c a p i l l a r y a t t r a c t i o n has been u n d e r s t o o d f o r much l o n g e r . In 1911, Green and Ampt d e r i v e d a v e r y s i m p l e e q u a t i o n r e l a t i n g the i n f i l t r a t i o n r a t e under j u s t - p o n d i n g c o n d i t i o n s t o a r e d u c t i o n i n the p r e s s u r e p o t e n t i a l g r a d i e n t as w e t t i n g proceeded. In or d e r t o d e v e l o p a s i m p l e a n a l y t i c a l s o l u t i o n , they were f o r c e d t o assume t h a t the h y d r a u l i c c o n d u c t i v i t y -t e n s i o n and water c o n t e n t - t e n s i o n r e l a t i o n s h i p s a r e s t e p -f u n c t i o n s . I n s p i t e of t h i s r e s t r i c t i o n , the Green and Ampt e q u a t i o n a d e q u a t e l y d e s c r i b e s the d e c l i n e i n i n f i l t r a b i l i t y as w e t t i n g proceeds f o r w e l l - s o r t e d , c o a r s e - g r a i n e d s o i l s . A b r i e f r e v iew of the Green and Ampt approach f o l l o w s . As noted above, Green and A m p t ( l 9 l l ) assumed t h a t the h y d r a u l i c c o n d u c t i v i t y - t e n s i o n and water c o n t e n t - t e n s i o n r e l a t i o n s h i p s were s t e p - f u n c t i o n s (see F i g u r e 7 ) . The h y d r a u l i c c o n d u c t i v i t y a t t e n s i o n s g r e a t e r than He (Ko i n F i g u r e 7) i s h e l d t o be n e g l i g i b l e compared w i t h 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 , and i t i s assumed t h a t the s o i l p r o f i l e i n i t i a l l y has a water c o n t e n t of 9 i everywhere. I t i s c l e a r t h a t under these a s s u m p t i o n s , w e t t i n g f r o n t s w i l l be p e r f e c t l y s h a r p , and t h a t the t e n s i o n a t the f r o n t w i l l always be a t the c r i t i c a l t e n s i o n , He. hydraul ic c o n d u c t i v i t y w a t e r c o n t e n t 29 I f a j u s t - p o n d i n g c o n d i t i o n i s m a i n t a i n t e d a t the s u r f a c e : i = Ks(Hc/L+1) Where: i = i n f i l t r a b i l i t y Ks = 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 He = c r i t i c a l t e n s i o n L = depth of p e n e t r a t i o n of the f r o n t I t i s c l e a r t h a t : I = L - A 6 Where: I = c u m u l a t i v e i n f i l t r a t i o n AG = change i n water c o n t e n t at He Combining t h e s e two e q u a t i o n s and i n t e g r a t i n g : Ks«T = I + Hc'Ae«ln(Ae«Hc/(I+Ae«Hc)) In t h e i r a n a l y s i s , Green and Ampt o n l y c o n s i d e r e d the case where a j u s t ponding c o n d i t i o n was m a i n t a i n e d from the o u t s e t of the i n f i l t r a t i o n e v e n t . However, t h e i r approach i s e a s i l y extended t o the case where the r a i n f a l l r a t e i s s p e c i f i e d , g i v e n t h a t i t exceeds 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 of the s o i l . I f the s o i l i s d r y i n i t i a l l y , t h e r e w i l l be a p e r i o d when a l l the r a i n f a l l w i l l be ac c e p t e d by the s o i l . D u r i n g t h i s p e r i o d , the t e n s i o n a t the s u r f a c e w i l l a d j u s t so t h a t the i n f i l t r a t i o n r a t e j u s t e q u a l s the r a t e of s u p p l y ; the g o v e r n i n g e q u a t i o n i s : i = Ks((Hc-Hs) - A 9/I+1) = R Where: Hs = t e n s i o n a t the s u r f a c e 30 R = r a i n f a l l r a t e The t e n s i o n a t the s u r f a c e can o n l y a d j u s t up t o the p o i n t where i t e q u a l s z e r o , a t which time o v e r l a n d f l o w w i l l b e g i n . The c u m u l a t i v e i n f i l t r a t i o n (Ip) up t o the time of ponding (Tp) i s g i v e n by: Ip = He•A9/(R/Ks-1) The time t o ponding i s : Tp = HcA6/(R- (R/Ks-1 ) ) I f j u s t - p o n d i n g c o n d i t i o n s a r e m a i n t a i n e d f o r tim e s g r e a t e r than the time t o ponding, then the g o v e r n i n g e q u a t i o n i s : Ks(T-Tp) = I - I p +Hc-A0-ln((Ip+AG-Hc)/(I+AG-Hc)) I t i s i n s t r u c t i v e t o compare the p r e d i c t i o n s of t h i s s i m p l e a n a l y s i s w i t h the more e x a c t p r e d i c t i o n s p r o v i d e d by n u m e r i c a l methods of s o l u t i o n . The m o i s t u r e c o n t e n t t e n s i o n r e l a t i o n s h i p assumed by R u b i n ( l 9 6 6 ) i s g i v e n i n F i g u r e 8, a l o n g w i t h the a p p r o x i m a t i o n of the f u n c t i o n t h a t i s used i n the m o d i f i e d Green and Ampt a n a l y s i s used h e r e . The change i n i n f i l t r a t i o n r a t e w i t h time g i v e n by Rubin f o r t h r e e r a i n f a l l r a t e s a r e g i v e n i n F i g u r e 9, a l o n g w i t h the r e s u l t s of the m o d i f i e d Green and Ampt a n a l y s i s . As can be seen, the agreement i s good i n a l l c a s e s . I t i s e v i d e n t t h a t the form of the s o l u t i o n i s s i m i l a r i n both c a s e s , even i f t h e r e a r e d i f f e r e n c e s i n d e t a i l . E r a i n f a l l r a t e - 0 . 0 6 c m / s V V Rubin \ \ \ \ \ \ \ s Green and Ampt \ N — r a i n f a l l r a t e - 0 . 0 3 c m / s t ime Figure 9 - C o m p a r i s o n of the Green and Ampt solution with a numerical s o l u t i o n ro 33 T e x t u r a l c o n t r a s t s A s e t of v e r t i c a l l y o r i e n t e d c a p i l l a r y t u b e s , a l l of r a d i u s r , would have s t e p - f u n c t i o n m o i s t u r e c o n t e n t t e n s i o n and h y d r a u l i c c o n d u c t i v i t y - t e n s i o n r e l a t i o n s h i p s , as assumed f o r the Green and Ampt a n a l y s i s of v e r t i c a l i n f i l t r a t i o n . The c r i t i c a l t e n s i o n i s r e l a t e d t o the r a d i u s of the tube by the c a p i l l a r y r i s e e q u a t i o n ( H i l l e l , 1971): He = 2tfcos(o)/pgr Where: He = c r i t i c a l t e n s i o n <s = s u r f a c e t e n s i o n a = c o n t a c t a n g l e p = d e n s i t y g = a c c e l e r a t i o n due t o g r a v i t y r = r a d i u s of tube I f a c o n t a c t a n g l e of z e r o degrees i s assumed, the r e l a t i o n s h i p between the c r i t i c a l t e n s i o n and the tube r a d i u s may be r e - e x p r e s s e d a s : He = 0.15/r f o r r i n cm and He i n cm of water 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 of the media i s r e l a t e d i n d i r e c t l y t o the r a d i u s by P o i s s e u i l l e ' s law, which d e s c r i b e s f l o w i n a c a p i l l a r y t u b e : q = ( i r r 2 / 8 v ) -6#/6z Where: q = f l o w i n tube v = dynamic v i s c o s i t y 34 6 * / 6 z = t o t a l p o t e n t i a l g r a d i e n t r = r a d i u s of the tube The a r e a of one tube i s n r 2 , but t h e r e are n tubes per u n i t a r e a ; t h e r e f o r e : r = n i r r 2 Where r i s the area of tube openings per u n i t a r e a . I t can be shown t h a t r i s n u m e r i c a l l y e q u i v a l e n t t o the s a t u r a t e d water c o n t e n t ( 6 s ) ; hence: n = G s / r r r 2 The f l o w per u n i t a r e a (q) i s : q = 6 s » r 2 • (6/>/6z)/8v Darcy's e q u a t i o n f o r f l o w i n s a t u r a t e d porous media i s : q = K s « 6 * / 6 z By i n s p e c t i o n , Ks = 9 s - r 2 / 8 i / Hence, one now has r a t i o n a l r e l a t i o n s h i p s between the c r i t i c a l t e n s i o n and pore r a d i u s , and between 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 and pore r a d i u s f o r media t h a t meet the assumptions of the Green and Ampt approach. The r a d i i of the pores i n the m a t r i x a r e r o u g h l y p r o p o r t i o n a l t o the s i z e of the s o l i d p a r t i c l e s , but the macropores t h a t occur i n s t r u c t u r e d s o i l s may be v i r t u a l l y independent of the s i z e of the s o l i d p a r t i c l e s . Hence, the e q u a t i o n s g i v e n above s h o u l d o n l y be a p p l i e d t o s t r u c t u r e l e s s media. Two v e r t i c a l c o n t r a s t s i n t e x t u r e may now be d e f i n e d i n terms of c o n t r a s t s i n h y d r a u l i c p r o p e r t i e s ; i n p a r t i c u l a r : (1) Coarse over f i n e - t h i s i m p l i e s t h a t the upper 35 l a y e r has a h i g h e r 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 and a lower c r i t i c a l t e n s i o n than the lower l a y e r (2) F i n e over c o a r s e - t h i s i m p l i e s t h a t the upper l a y e r has a lower 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 and a h i g h e r c r i t i c a l t e n s i o n than the lower l a y e r An a n a l y s i s of i n f i l t r a t i o n b e h a v i o r f o r each of the s e cases f o l l o w s . Coarse over f i n e As noted p r e v i o u s l y , a c o a r s e over f i n e c o n t r a s t i n t e x t u r e i s ex p e c t e d t o produce a c h a r a c t e r i s t i c c o n t r a s t i n h y d r a u l i c p r o p e r t i e s (see F i g u r e 10). The v a l u e s of Ks1 and Ks2 p r o v i d e a n a t u r a l s c a l e f o r r a i n f a l l i n t e n s i t y ; t h e r e a r e t h r e e d i s t i c t p o s s i b i l i t i e s : (1) R > Ks1 > Ks2 (2) Ks1 > R > Ks2 (3) K s l > Ks2 > R I f case (3) a p p l i e s , no o v e r l a n d f l o w w i l l be g e n e r a t e d because the r a i n f a l l r a t e does not exceed 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 of e i t h e r l a y e r ( R u b i n , 1966). I f case (2) a p p l i e s , o v e r l a n d f l o w w i l l o c cur i f the r a i n f a l l event l a s t s l o n g enough, s i n c e the r a i n f a l l r a t e exceeds 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 of the lower l a y e r . However, the use of the Green and Ampt approach may be i n a p p r o p r i a t e f o r case ( 2 ) , depending upon how much g r e a t e r 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 i s than the r a i n f a l l •W1 - H 1 W •W1 c o a r s e K s - K s 1 H - H 1 W- f ine K s - K s 1 H f ine K s - K s 2 H -H2 W-•W2 c o a r s e K s - K s 2 H - H 2 w- - W 2 K s 1 > K s 2 K s 2 > K s l H 2 > H I H1 > H2 Figure 1 0 - T e x t u r a l c o n t r a s t s OA 37 r a t e . I f the contrast in rates i s very l a r g e , then there w i l l be no saturated f r o n t . In such cases, one must res o r t to s o l u t i o n s of Richard's equation using numerical methods to model the i n f i l t r a t i o n process adequately. The Green and Ampt approach i s u s e f u l i n analyzing i n f i l t r a t i o n for case ( 1 ) , since the r a i n f a l l r ate exceeds the saturated h y d r a u l i c c o n d u c t i v i t y of both l a y e r s . The pattern of overland flow production may be rather complex there are s e v e r a l p o s s i b i l i t i e s : ( 1 ) overland flow begins while the f r o n t i s i n the upper l a y e r . When the f r o n t reaches the boundary between the l a y e r s , the i n f i l t r a b i 1 i t y w i l l increase abruptly - a r e s u l t that i s counter-i n t u i t i v e . Depending upon the magnitude of the increase i n i n f i I t r a b i 1 i t y , overland flow may or may not cease. I f overland flow does cease, i t w i l l e v e n t u a l l y begin again, since the i n f i l t r a b i l i t y w i l l d e c l i n e as the front penetrates the second l a y e r . (2) overland flow does not begin while the front i s i n the upper l a y e r . Overland flow w i l l begin e v e n t u a l l y , as the f r o n t penetrates the lower l a y e r . The expected changes i n i n f i l t r a b i l i t y as a wetting front penetrates a coarse over f i n e p r o f i l e , and the consequences for overland flow w i l l be discussed in more d e t a i l i n the pages f o l l o w i n g . 38 O v e r l a n d f l o w w i l l b e g i n w h i l e the f r o n t i s i n the upper l a y e r i f : L1 > H1•(R/Ks1 - 1) Where: HI = c r i t i c a l t e n s i o n a t the f r o n t f o r l a y e r 1 Up t o the time when the f r o n t reaches the boundary, the a n a l y s i s i s i d e n t i c a l t o t h a t g i v e n p r e v i o u s l y f o r the r a i n f a l l s p e c i f i e d c a s e . Hence, up t o the time when o v e r l a n d f l o w b e g i n s : I = R-T When o v e r l a n d f l o w j u s t b e g i n s : Ip = H1-A01/(R/KS1 - 1) And: Tp = H1•AG 1/(R•(R/Ks1 - 1)) Where: Ip = c u m u l a t i v e i n f i l t r a t i o n when ponding b e g i n s Tp = time t o ponding A f t e r o v e r l a n d f l o w b e g i n s , the g o v e r n i n g e q u a t i o n i s : Ks1•(T-Tp) = I - Ip +H1-A01-ln((Ip+H1-A01)/(l+H1-A61)) T h i s e q u a t i o n h o l d s u n t i l the f r o n t reaches the boundary between l a y e r s . When the f r o n t j u s t reaches the boundary between l a y e r s 1 and 2, the t e n s i o n a t the f r o n t i n c r e a s e s from H1 t o H2. When o n l y an i n f i n i t e s s i m a l amount of water has p e n e t r a t e d the second l a y e r , a l l of the medium beh i n d the f r o n t has a c o n d u c t i v i t y of Ks1. 39 Hence, the g o v e r n i n g e q u a t i o n i s : i = Ks1•((H2-Hs)/(L1+1)) Where: Hs = t e n s i o n a t the s u r f a c e of the p r o f i l e The t e n s i o n a t the s u r f a c e can o n l y f a l l as f a r as z e r o i f t h e r e i s no ponding; t h e r e f o r e , the i n f i l t r a b i 1 i t y a t the time the f r o n t reaches the boundary between l a y e r s i s : i(max) = Ks1•(-H2/L1 + 1) J u s t b e f o r e the f r o n t reached the boundary, the i n f i l t r a b i l i t y was: i(max) = Ksl•(H1/L1+1) S i n c e H2 i s g r e a t e r than H1, the i n f i l t r a b i 1 i t y i n c r e a s e s when i t reaches the l o w e r , f i n e r l a y e r . In f a c t , o v e r l a n d f l o w w i l l cease u n l e s s : R > Ksl•(H2/L1 + 1) As the f r o n t p e n e t r a t e s the second l a y e r , the i n f i l t r a b i l i t y w i l l d e c l i n e r a p i d l y ; i f o v e r l a n d f l o w ceased when the f r o n t reached the boundary, i t w i l l e v e n t u a l l y resume. For the case where o v e r l a n d f l o w b e g i n s w h i l e the f r o n t i s i n the upper l a y e r , and does not cease when the f r o n t reaches the second l a y e r , the f o l l o w i n g e q u a t i o n s h o l d : (1) i = Ks1•(Hb/L1 + 1) (2) i = Ks2-(H2-Hb)/(L-L1) + 1) (3) I + A92«(L-L1) = A91-L1 Where: Hb = t e n s i o n a t the boundary 40 S u b s t i t u t i n g : ( ( A + B - I ) / ( C + D - I ) ) - d l = dT Where: A = L1•(AG2-Ks2/Ks1 - AG 1 ) B = 1 C = Ks2-(H2-AG2 - L1(A01~A62)) D = Ks2 I n t e g r a t i n g : T-Tf = B- ( I - I f ) / D + ( (AD-BO/D 2 ) .ln(C+D«I )/(c+D»If ) ) Where: Tf = time f r o n t reaches boundary , I f = c u m u l a t i v e i n f i l t r a t i o n when f r o n t reaches boundary I f o v e r l a n d f l o w d i d not b e g i n w h i l e the f r o n t was i n the upper l a y e r , or i f i t ceased when the f r o n t reached the boundary, then t h e r e w i l l be a p e r i o d when the i n f i l t r a b i l i t y exceeds the r a i n f a l l r a t e , but t h i s i s a temporary c o n d i t i o n . Once o v e r l a n d f l o w b e g i n s a g a i n , the g o v e r n i n g e q u a t i o n i s almost i d e n t i c a l t o t h a t g i v e n above: T-Tp = B - ( I - I p ) / D + ( (AD-BO/D 2 ) - l n ( (C+D-I )/(C+D-Ip) ) Where: Tp = time t o ponding Ip = c u m u l a t i v e i n f i l t r a t i o n t o the time of ponding The parameters A, B, C, and D are a l l as g i v e n p r e v i o u s l y . 41 The p a t t e r n s of changes i n i n f i l t r a t i o n r a t e t h a t occur where t h e r e i s a c o a r s e over f i n e t e x t u r a l c o n t r a s t d i s c u s s e d above are i l l u s t r a t e d i n F i g u r e 11. F i n e over c o a r s e The l a y e r i n g sequence i s d e p i c t e d i n F i g u r e 10. When the f r o n t . i s i n the upper l a y e r , the a n a l y s i s of i n f i l t r a t i o n b e h a v i o r i s s i m p l y t h a t a p p l i e d t o ' i n f i I t r a t i o n i n t o any homogeneous s o i l . O v e r l a n d f l o w may or may not occur w h i l e the f r o n t i s w i t h i n the upper l a y e r . As b e f o r e , the c o n d i t i o n s a r e : (1) O v e r l a n d f l o w does not occur i f (R/Ks1-1)>H1/L1 (2) O v e r l a n d f l o w w i l l o c cur i f H1/L1>(R/KS1-1) When the f r o n t reaches the boundary between l a y e r 1 and l a y e r 2: i = Ks1(H2/L1+1) S i n c e H2 i s l e s s than H1, the i n f i l t r a b i l i t y d r o p s . I f o v e r l a n d f l o w d i d not be g i n w h i l e the f r o n t was i n the upper l a y e r , i t w i l l s t a r t a t the boundary i f : R > Ks1(H2/L1+1) O t h e r w i s e , i t w i l l b e g i n when the f r o n t i s i n the second l a y e r . I f o v e r l a n d f l o w d i d be g i n when the f r o n t was i n the upper l a y e r , then i t w i l l i n c r e a s e a b r u p t l y i n response t o the a b r u p t d e c l i n e i n i n f i l t r a b i l i t y t h a t o c c u r s when the f r o n t reaches the boundary. H2 r a i n f a l l r a t e Ifront r e a c h e s b o u n d a r y t i m e c t i m e Figure 11 I n f i l t r a t i o n rate, c o arse over fine c o n t r a s t 43 I f o v e r l a n d f l o w d i d not b e g i n when the f r o n t reaches the boundary, then i t w i l l b e g i n a f t e r some f i n i t e p e n e t r a t i o n of the f r o n t i n t o the second l a y e r . At t h i s p o i n t : (1) R = Ksl(Hb/LI+1) (2) R = Ks2((H2-Hb)/((L-L1)+1) (3) 1 = (L-L1)-AG2 + L1•AGT Combining th e s e r e q u i r e m e n t s : ( ( A + B - I ) / ( C + D - I ) ) - d l = dT Where: A = L1•(AG2-KS2/KS1 - AG 1) B = 1 C = Ks2(H2-AG2 - L1 (AG 1-AG2)) D = Ks2 T h i s i s the same e q u a t i o n t h a t a p p l i e s f o r the case where a c o a r s e s o i l l i e s over a f i n e r s o i l . Hence, the g o v e r n i n g e q u a t i o n f o r i n f i l t r a t i o n when the f r o n t i s i n the second l a y e r and o v e r l a n d f l o w has commenced i s : T-Tp = B ( I - I p ) / D + ((AD-BC)/D 2)ln((C+D-I)/(C+D-Ip)) Where: Tp = time t o ponding Ip = c u m u l a t i v e i n f i l t r a t i o n when ponding b e g i n s However, as w i l l be shown , t h i s s o l u t i o n i s not v a l i d i n some ca s e s where a f i n e l a y e r l i e s over a c o a r s e r l a y e r . For i n f i l t r a t i o n i n t o the l o w e r , c o a r s e l a y e r a f t e r o v e r l a n d f l o w has commenced, we must s a t i s f y the e q u a t i o n s : 44 (1) i = Ks1(Hb/L1+1) (2) i = Ks2((H2-Hb)/(1-L1)+1) Thus: (Ks2-H2/Ks1 •(L-L1)+Ks2/Ks1 - 1) Hb = (Ks2/Ks1•(L-L1) + 1/L1) For s t a b i l i t y , Hb must always be l e s s than H2 (which i s lower than H1), or t h e r e w i l l be a tendency f o r d e w a t e r i n g t o o c cur a t the boundary ( r e c a l l t h a t H denotes t e n s i o n r a t h e r than p r e s s u r e ) . Thus, our c r i t e r i a f o r s t a b i l i t y w i l l be: ( 1 ) Hb < H2 S t a b l e (2) Hb > H2 U n s t a b l e I t can be shown t h a t Hb i s g r e a t e r than H2 i f : (Ks2/Ks1- 1)/H2 > 1/L1 In g e n e r a l , t h e n , i n s t a b i l i t y i s promoted by the f o l l o w i n g f a c t o r s : (1) Ks2/Ks1 i s l a r g e . The g r e a t e r the c o n t r a s t i n t e x t u r e , the more l i k e l y i t i s t h a t an i n s t a b i l i t y w i l l d e v e l o p i n the c o a r s e l a y e r . (2) H2 i s s m a l l . T h i s c r i t e r i o n i m p l i e s t h a t Ks2 i s l a r g e . In o t h e r words, i n s t a b i l i t y i s more l i k e l y t o d e v e l o p where the lower c o a r s e l a y e r i s v e r y c o a r s e . (3) L1 i s l a r g e . The t h i c k e r the o v e r l y i n g f i n e l a y e r i s , the more l i k e l y i s the f l o w i n the lower l a y e r t o become u n s t a b l e . H i l l and P a r l a n g e ( 1 9 7 2 ) have c o n s i d e r e d the q u e s t i o n of the i n s t a b i l i t y of w e t t i n g f r o n t s both e m p i r i c a l l y and 45 t h e o r e t i c a l l y . They found that the i n s t a b i l i t y l e d to the formation of saturated ' f i n g e r s ' , which subsequently c a r r i e d most of the flow. Where the flow does occur as f i n g e r s , i t i s evident that t h i s phenomenon must be analyzed as a two dimensional problem - a simple one-dimensional a n a l y s i s w i l l not s u f f i c e . In f a c t , because i t i s , u l t i m a t e l y , a problem i n dynamic i n s t a b i l i t y , i t i s not subject to exact a n a l y s i s . I t i s c l e a r , however, that the i n f i l t r a t i o n rate w i l l d e c l i n e to Ks1, since g r a v i t y w i l l be the only s i g n i f i c a n t d r i v i n g force for very deep penetration of the f r o n t . A i r entrapment In a s o i l that i s well-vented, or for cases where the r a i n f a l l rate i s below the saturated 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 , the e f f e c t s of a i r movement can probably be ignored when ana l y z i n g i n f i l t r a t i o n . However, where the s o i l i s poorly vented, and the r a i n f a l l rate greater than the saturated 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 , the i n f i l t r a t i o n rate may be lower than expected, due to a i r entrapment. There are two ways i n which the i n f i l t r a t i o n f a t e may be diminished as a r e s u l t of a i r entrapment: (1) A i r i s compressed i n advance of the f r o n t , e f f e c t i v e l y reducing the c a p i l l a r y tension at the f r o n t , and, hence, reducing the c a p i l l a r y g radient. (2) A i r may become trapped i n pores behind the f r o n t , e f f e c t i v e l y reducing the s a t u r a t i o n and the 46 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 (Green, Hasson, Weinaug, and P r i l l , 1 9 7 0 ; and McWhorter,1971). The c ompression of a i r ahead of an a d v a n c i n g f r o n t has been documented i n the f i e l d and the l a b o r a t o r y ( B i a n c h i a n d H a s k e l l , 1 9 6 6 ; Green, et a l , l 9 7 0 ; and McWhorter,1971). For s i g n i f i c a n t compression t o o c c u r , the s u r f a c e must be wetted over a l a r g e a r e a , and t h e r e must be a s h a l l o w impeding l a y e r . T h i s impeding l a y e r may be a l a y e r of i n t r i n s i c a l l y low p e r m e a b i l i t y w i t h r e s p e c t t o a i r , or i t may be the water t a b l e . McWhorter(1971) has c o n s i d e r e d the problem i n some d e t a i l . In the case where t h e r e i s an impeding l a y e r a t de p t h , McWhorter found t h a t the i n f i l t r a t i o n r a t e d e c l i n e d t o l e v e l s below 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 due t o compression of a i r between the f r o n t and the impeding l a y e r . However, when the a i r p r e s s u r e reaches some c r i t i c a l v a l u e , the a i r w i l l b e g i n t o f i n d p a t h s t o the s u r f a c e - a i r bubbles a t the s u r f a c e were o b s e r v e d t o form a t t h i s s tage i n McWhorter's e x p e r i m e n t s . There" i s then a r a p i d d e c r e a s e i n a i r p r e s s u r e i n advance of the f r o n t , and a r a p i d i n c r e a s e i n the i n f i l t r a b i l i t y t o l e v e l s t h a t u s u a l l y exceed 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 . The i n f i t r a t i o n r a t e s l o w l y d e c l i n e s towards a v a l u e t h a t i s l e s s than 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 . T h i s d e c l i n e t o a v a l u e t h a t i s below 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 i s e x p e c t e d , s i n c e the zone above the f r o n t i s no l o n g e r c o m p l e t e l y s a t u r a t e d , due t o the paths t h a t have been 47 c r e a t e d by the c o u n t e r f l o w of a i r . The complex changes i n i n f i l t r a t i o n r a t e d e s c r i b e d above are i l l u s t r a t e d i n F i g u r e 12. Summary In t h i s c h a p t e r , s e v e r a l mechanisms by which o v e r l a n d f l o w might be g e n e r a t e d have been r e v i e w e d ; i n p a r t i c u l a r : (1) R e d u c t i o n of c a p i l l a r y g r a d i e n t s - the i n f i l t r a b i 1 i t y of the s o i l may drop below the r a i n f a l l r a t e d u r i n g a storm due e i t h e r t o the changes i n p h y s i c a l p r o p e r t i e s of the s o i l s u ggested by H o r t o n ( 1 9 3 3 ) , or by the r e d u c t i o n i n the c a p i l l a r y g r a d i e n t of s o i l s t h a t have a 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 e s s than the r a i n f a l l r a t e , as suggested by R u b i n ( l 9 6 6 ) . (2) T e x t u r a l c o n t r a s t s - as shown i n t h i s c h a p t e r , b o th c o a r s e , over f i n e , and f i n e over c o a r s e l a y e r i n g may produce a complex sequence of changes i n the i n f i l t r a t i o n r a t e , depending upon the e x a c t p r o p e r t i e s of the s o i l s , and upon the r a i n f a l l r a t e . In some c a s e s , i n f i l t r a t i o n i n t o a s o i l t h a t has c o a r s e l a y e r l y i n g over a f i n e r l a y e r i s i n h e r e n t l y u n s t a b l e , and i s not s u b j e c t t o ex a c t a n a l y s i s . (3) A i r entrapment - where t h e r e i s an impeding l a y e r a t moderate depth i n a p o o r l y v e n t e d s o i l , the c o u n t e r f l o w of a i r d u r i n g i n f i l t r a t i o n must be c o n s i d e r e d . McWhorter(1971) has demonstrated the 49 r a t h e r complex changes i n i n f i l t r a t i o n r a t e t h a t may o c c u r . In the f o l l o w i n g c h a p t e r , each of these hypotheses w i l l be c r i t i c a l l y examined w i t h r e s p e c t t o the o b s e r v e d r u n o f f e v e n t s at the f i e l d s i t e . 50 Chapter 5 - E v a l u a t i o n of hypotheses In the p r e v i o u s c h a p t e r , the t h e o r e t i c a l framework n e c e s s a r y f o r e v a l u a t i o n of the hypotheses advanced was d e v e l o p e d . Each of these hypotheses w i l l be e v a l u a t e d c a r e f u l l y i n t h i s c h a p t e r . R e d u c t i o n of c a p i l l a r y g r a d i e n t s The mechanisms advanced by H o r t o n ( l 9 3 3 ) t o e x p l a i n the d e c l i n e i n i n f i l t r a b i l i t y d u r i n g the c o u r s e of r a i n f a l l e v e n t s i n c l u d e d : (1) the p a c k i n g of s u r f a c e m a t e r i a l t h r o u g h r a i n f a l l impact (2) the s w e l l i n g of c o l l o i d s ; and (3) the c l o g g i n g of s u r f a c e pores w i t h m a t e r i a l r e d i s t r i b u t e d by r a i n d r o p s . The f i r s t and t h i r d f a c t o r s c i t e d above a r e u n l i k e l y t o be i m p o r t a n t i n the a r e a s where o v e r l a n d f l o w was g e n e r a t e d , s i n c e these a r e a s were almost i n v a r i a b l y w e l l p r o t e c t e d by the v e g e t a t i v e c o v e r . D u r i n g the r a i n f a l l e v e n t s o b s e r v e d , t h e r e was no n o t i c e a b l e r e d i s t r i b u t i o n of sediment at the s u r f a c e . The low sediment y i e l d s i n the a r e a support t h i s c o n t e n t i o n (Slaymaker,1977). T e n s i o n c r a c k s were common, s u g g e s t i n g t h a t the s h r i n k i n g and s w e l l i n g of c o l l o i d s may be i m p o r t a n t . However, t h e i r s i z e and d i s t r i b u t i o n d i d not appear t o change g r e a t l y d u r i n g the r a i n f a l l e v e n ts o b s e r v e d , and they were o n l y a few m i l l i m e t r e s i n depth. I t i s p r o b a b l e t h a t d u r i n g most e v e n t s they s i m p l y a c t t o i n c r e a s e the d e p r e s s i o n s t o r a g e c a p a c i t y s l i g h t l y . As shown by R u b i n ( l 9 6 6 ) , o v e r l a n d f l o w r e s u l t i n g from 51 the d e c l i n e i n c a p i l l a r y g r a d i e n t s as a s o i l p r o f i l e 'wets up' w i l l o c cur o n l y i f the r a i n f a l l r a t e i s g r e a t e r than 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 , and then o n l y a f t e r enough water has p e n e t r a t e d the s o i l t o s u f f i c i e n t l y reduce the c a p i l l a r y g r a d i e n t . L a b o r a t o r y e x p e r i m e n t s showed t h a t 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 exceeds 10" 3 cm/s f o r u n d i s t u r b e d samples. S i n c e r a i n f a l l r a t e s were l e s s than 10"" cm/s when o v e r l a n d f l o w was o b s e r v e d , the ' p r e c i p i t a t i o n e x c e s s ' h y p o t h e s i s o u t l i n e d p r e v i o u s l y i s r e j e c t e d . T e x t u r a l c o n t r a s t s For i n f i l t r a t i o n i n t o a s o i l where a c o a r s e l a y e r l i e s over a f i n e l a y e r , t h e r e a re t h r e e p o s s i b i l i t i e s : (1) R>Ks1>Ks2 (2) Ks1>R>Ks2 and (3) Ks1>Ks2>R. In the p r e v i o u s s e c t i o n , i t was noted t h a t 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 of the s o i l h o r i z o n s g r e a t l y exceeds the observed r a i n f a l l r a t e s ; t h e r e f o r e , o n l y case (3) c o u l d p o s s i b l y a p p l y . O v e r l a n d f l o w w i l l not occur under such c o n d i t i o n s . F u r t h e r m o r e , i t i s g e n e r a l l y the case t h a t r e l a t i v e l y f i n e l o e s s l i e s over c o a r s e r v o l c a n i c a s h . However, a g r e a t e r d e n s i t y of r o o t s i n the upper p a r t of the l o e s s may g e n e r a t e , e f f e c t i v e l y , a c o a r s e over f i n e c o n t r a s t . As noted p r e v i o u s l y , though, 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 of the m a t e r i a l appears t o be too h i g h ; and even i f i t weren't, the w e t t i n g f r o n t s h o u l d advance a t a reduced r a t e , r a t h e r than h a l t i n g a t a f i x e d 52 depth as observed. The l o e s s - a s h c o m b i n a t i o n r e p r e s e n t s a f i n e over c o a r s e c o n t r a s t ; but a g a i n , measured s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s appear t o be f a r too h i g h t o g e n e r a t e o v e r l a n d f l o w by t h i s mechanism (see Chapter 4 ) . In the p r e v i o u s c h a p t e r a s i m p l e c r i t e r i o n of s t a b i l i t y i n the case of a f i n e l a y e r l y i n g over a c o a r s e l a y e r was d e v e l o p e d ; i t may be r e w r i t t e n a s : Ks2/Ks1 > H2/L1 + 1 For the upper ash l a y e r , H2 was on the o r d e r of 5 cm. (see Chapter 2 ) . The depth of the l o e s s i s on the o r d e r of 5 cm. T h e r e f o r e , a c o n t r a s t i n 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 i e s on the o r d e r of two t o one or g r e a t e r s h o u l d r e s u l t i n u n s t a b l e f l o w - t h a t i s , i n f i n g e r i n g . S i n c e 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 i s p r o p o r t i o n a l t o the square of the c h a r a c t e r i s t i c pore s i z e , the t e x t u r a l c o n t r a s t r e q u i r e d t o produce u n s t a b l e f l o w i s even l e s s than 2.0 (about 1.4). Hence, f l o w i n the ash l a y e r would almost c e r t a i n l y be u n s t a b l e . The c o n t r a s t between the l o e s s and ash l a y e r s cannot, however, be i m p o r t a n t i n a l l a r e a s , s i n c e the w e t t i n g f r o n t appears t o ' s t a l l ' w e l l above t h i s boundary. As w e l l , 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 of the f i n e l a y e r appears t o be g r e a t e r than the r a i n f a l l r a t e . Hence, o v e r l a n d f l o w s h o u l d not o c c u r . T h i s c o n t r a s t i n p r o p e r t i e s cannot be r e s p o n s i b l e f o r a l l of the f i e l d o b s e r v a t i o n s . 53 A i r entrapment We have a t l e a s t one of the n e c e s s a r y c o n d i t i o n s f o r the a i r entrapment h y p o t h e s i s o u t l i n e d i n the p r e v i o u s c h a p t e r : t h e r e i s an impeding l a y e r a t f a i r l y s h a l l o w depth (on the o r d e r of 30 cm.). The t i l l found at the base of the s o i l p r o f i l e i s r e l a t i v e l y impermeable t o a i r . In many ca s e s t h e r e i s a s h a l l o w zone of s a t u r a t i o n j u s t above the t i l l which w i l l a c t as a ve r y e f f e e t i v e - i m p e d i n g l a y e r . I t i s not c l e a r t h a t the o t h e r n e c e s s a r y c o n d i t i o n f o r s i g n i f i c a n t a i r compression by the adv a n c i n g f r o n t i s s a t i s f i e d - the s o i l may or may not be w e l l v e n t e d . However, i f the s o i l were w e l l - v e n t e d t h e r e would be no o p p o r t u n i t y f o r a i r t o be compressed, and i n f i l t r a t i o n would not be impeded. For the sake of argument, assume t h a t the s o i l i s p o o r l y v e n t e d . A i r entrapment may v i r t u a l l y h a l t the advance of the w e t t i n g f r o n t , l e a d i n g t o w i d e s p r e a d o v e r l a n d f l o w . T h i s , however, i s a temporary s i t u a t i o n . A c c o r d i n g t o McWhorter, d e s a t u r a t i o n of the upper l a y e r s h o u l d o c c u r , w i t h the w e t t i n g f r o n t a b r u p t l y b e g i n n i n g t o readvance, and o v e r l a n d f l o w c e a s i n g . However, the w e t t i n g f r o n t d i d not readvance a f t e r i t ' s t a l l e d ' , and o v e r l a n d f l o w d i d not cea s e . F u r t h e r m o r e , when h o l e s were e x c a v a t e d t o observe the advance of the w e t t i n g f r o n t , t h e r e was no readvance of the f r o n t . E x c a v a t i o n of a 30 cm deep t r e n c h t h a t i s s e v e r a l metres l o n g s h o u l d p r o v i d e adequate v e n t i n g l o c a l l y . On the b a s i s of the s e o b s e r v a t i o n s , the ' a i r entrapment' h y p o t h e s i s i s r e j e c t e d . 54 Chapter 6 - W a t e r - r e p e l l e n c y h y p o t h e s i s In the p r e v i o u s c h a p t e r , i t was proposed t h a t the low i n f i l t r a b i l i t y of the s o i l s i n the study a r e a i s due t o the presence of an ' a l t e r e d ' l a y e r near the s u r f a c e of the s o i l - ' a l t e r e d ' i n the sense t h a t the a f f i n i t y of the s o l i d phases f o r water i s g e n e r a l l y much l e s s than the maximum, or t h a t some, or a l l , of the s o l i d phases are a c t u a l l y h y d r o p h o b i c . The hydrophobic s u r f a c e s appear t e be caused by c o a t i n g of the c o n s t i t u e n t p a r t i c l e s by o r g a n i c m a t t e r . There appears t o be no s i n g l e source f o r the o r g a n i c m a t t e r , but Bozer, B r a n d t , and Hemwall(1968) have p o i n t e d but t h a t the c o a t i n g s must have be a m p h o p h i l l i c , t h a t i s the m o l e c u l e s i n v o l v e d must have both a hydrophobic and a h y d r o p h i l l i c p a r t . The h y d r o p h i l l i c m i n e r a l s u r f a c e s a t t r a c t the h y d r o p h i l l i c s i d e of the m o l e c u l e , l e a v i n g the hydrophobic p a r t d i r e c t e d outwards from the m i n e r a l s u r f a c e . Assume t h a t a l l the pores i n a g i v e n l a y e r have the same h y d r a u l i c p r o p e r t i e s , i n c l u d i n g c o n t a c t a n g l e . In these i d e a l i z e d s o i l s , a l a y e r i s c o n s i d e r e d t o be ' a l t e r e d ' i f the c o n t a c t a n g l e i s between 0 and 90 d e g r e e s , and 'water-r e p e l l e n t ' i f the a n g l e i s g r e a t e r than 90 d e g r e e s . A s i m p l e Green and Ampt s t y l e a n a l y s i s i s used i n both c a s e s . Case 1 - c o n t a c t a n g l e l e s s than 90 degrees The l a y e r i n g sequence i s i l l u s t r a t e d i n F i g u r e 13. I t i s assumed t h a t both l a y e r s have i d e n t i c a l p r o p e r t i e s , except f o r the c o n t a c t a n g l e . There i s an impermeable a l t e r e d " l a y e r n o r m a l s o i l K s - K s 1 H - H1 W - W 1 Ks - K s 1 H - H2 W - W 1 H2 > H1 Figure 13 - Pr o f i l e of an " a l t e r e d " soil U l U l 56 boundary a t the base. Two boundary c o n d i t i o n s a re c o n s i d e r e d : the j u s t - p o n d i n g c o n d i t i o n , a n d the r a i n f a l l s p e c i f i e d c o n d i t i o n . For the j u s t - p o n d i n g c o n d i t i o n the t e n s i o n a t the s u r f a c e i s m a i n t a i n e d a t z e r o . T h i s c o n d i t i o n might be found where o v e r l a n d f l o w i s b e i n g g e n e r a t e d on a h i l l s l o p e , where t h e r e i s a c o n t i n u i n g s u p p l y of water t o the p o i n t i n q u e s t i o n , but no p o s s i b i l i t y of ponding t o an a p p r e c i a b l e depth. The o t h e r boundary c o n d i t i o n c o n s i d e r e d , i s where the r a i n f a l l r a t e i s s p e c i f i e d . A l l r a i n f a l l i s ac c e p t e d up t o some c r i t i c a l p o i n t (assuming t h a t the r a i n f a l l r a t e i s g r e a t e r than 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 ) , a f t e r which a j u s t - p o n d i n g c o n d i t i o n w i l l be m a i n t a i n e d as o v e r l a n d f l o w i s ge n e r a t e d a t the s i t e . When the f r o n t i s i n the a l t e r e d l a y e r : dL/dt = (Ks/A6)•(H1/L + 1) I f L ( 0 ) = 0 and T(0) = 0, t h e n : T = A9/Ks(L + H 1 • l n ( H l / ( H 1 + L ) ) ) When the f r o n t reaches the lower 'normal' l a y e r : T = A9/Ks(L1 + H 1 • l n ( H l / ( H 1 + L ) ) ) Thus, the g o v e r n i n g e q u a t i o n i s : T = A9/Ks(L - L1 + H1-ln((H1+L1)/(H2+L1))) + Tp The change i n i n f i l t r a t i o n r a t e w i t h time f o r a s o i l w i t h an ' a l t e r e d ' s u r f a c e l a y e r , i s i l l u s t r a t e d i n F i g u r e 14. Where the r a i n f a l l r a t e i s ' s p e c i f i e d , o v e r l a n d f l o w w i l l b e g i n i f : L = Ks-Lp/(R-Ks) (see Chapter 4) Thus, o v e r l a n d f l o w w i l l b e g i n when the f r o n t i s i n the Ks t i m e Figure 14 - Infiltration rate for an altered soil under just-ponding conditions ^ 58 a l t e r e d l a y e r i f : L1 > K S - H 1 / ( R - K S ) When the f r o n t reaches the l o w e r , u n a l t e r e d l a y e r , the t e n s i o n a t the f r o n t w i l l i n c r e a s e , s i n c e the u n a l t e r e d s o i l has a g r e a t e r a f f i n i t y f o r the water than the a l t e r e d s o i l above. O v e r l a n d f l o w w i l l cease when the f r o n t reaches the boundary i f : K s-L2/(R-Ks) > L1 O v e r l a n d f l o w w i l l b e g i n b e f o r e the f r o n t reaches the impermeable base of the s o i l i f : L1 + L 2 > Rs-H2/(R-Ks) D u r i n g the p e r i o d s when o v e r l a n d f l o w i s o c c u r r i n g , p e n e t r a t i o n of the w e t t i n g f r o n t proceeds as f o r the j u s t -ponding c a s e , s i n c e a t e n s i o n of z e r o i s m a i n t a i n e d a t the s u r f a c e ( i t i s assumed t h a t t h e r e i s a s l i g h t s l o p e , and a l l o v e r l a n d f l o w 'runs o f f ' ) . When o v e r l a n d f l o w i s not o c c u r r i n g , a l l the r a i n i s a c c e p t e d , and the w e t t i n g f r o n t advances as : L = R-t/AG P o s s i b l e forms f o r the i n f i l t r a t i o n c u r v e s a re g i v e n i n F i g u r e 15. Note t h a t the form of the c u r v e s may be q u i t e complex. Case 2 - c o n t a c t a n g l e g r e a t e r than 90 degrees As noted p r e v i o u s l y , where the c o n t a c t a n g l e i s g r e a t e r than 90 d e g r e e s , the medium i s w a t e r - r e p e l l e n t . The j u s t -ponding case i s not c o n s i d e r e d here because an i d e a l i z e d U l 60 r e p e l l e n t s o i l w i l l , t h e o r e t i c a l l y , not a c c e p t any water u n l e s s t h e r e i s f i r s t ponding t o some c r i t i c a l d e p t h . The r a i n f a l l s p e c i f i e d boundary c o n d i t i o n must a l s o be m o d i f i e d water must pond t o the c r i t i c a l depth b e f o r e any i n f i l t r a t i o n o c c u r s . On a s i d e s l o p e t h e r e would be no i n f i l t r a t i o n ; a l l r a i n would s i m p l y f l o w a l o n g the s u r f a c e t o lower e l e v a t i o n s i t e s . When the water i s ponded t o some depth g r e a t e r than the c r i t i c a l depth r e q u i r e d : dL/dt = Ks/AG((Pd-Pc)/L + 1) Where: Pd = ponding depth Pc = c r i t i c a l ponding depth I f L ( 0 ) = 0, and T(0) = To, t h e n : T = ( A G / K S ) • ( L + ( P d - P c ) - l n ( ( P d - P c ) / ( P d - P c + L ) ) ) + To In the case where Pd=Pc, and L(0) = 0, and T(0) = 0: T = AG-L/Ks In F i g u r e 16, the c u r v e s f o r s e v e r a l ponding depths a re shown. For the case where the r a i n f a l l r a t e i s s p e c i f i e d , no i n f i l t r a t i o n w i l l o c c ur u n t i l water ponds t o a c r i t i c a l d e p t h . The time r e q u i r e d f o r t h i s t o occur i s : T1 = Pc/R When the c r i t i c a l depth i s reached, the f o l l o w i n g e q u a t i o n must be s a t i s f i e d : dP/dt = R - K s ( ( P d - P c ) / L + 1) P - O c m t i m e 16 - I n f i l t r a t i o n rate for w a t e r - r e p e l l e n t s o i l , ponding depth spec 62 Note t h a t : Pd - Pc = R-t - L•A0 Hence: dP/dt = R-Ks-(Rt-L-A9)/L + 1) T h i s e q u a t i o n has an a n a l y t i c a l s o l u t i o n . U n f o r t u n a t e l y , the s o l u t i o n a r r i v e d a t i s c o m p l i c a t e d , and can be e x p r e s s e d o n l y i n i m p l i c i t form. In p r a c t i c e , n u m e r i c a l s o l u t i o n of the e q u a t i o n i s much s i m p l e r . The s o l u t i o n f o r a p a r t i c u l a r example i s g i v e n i n F i g u r e 17. E v a l u a t i o n I t remains t o be d e c i d e d whether or not o n l y an a l t e r e d l a y e r i s r e q u i r e d t o e x p l a i n our o b s e r v a t i o n s , or whether t h e r e must be a w a t e r - r e p e l l e n t l a y e r near the s u r f a c e . When o n l y an a l t e r e d l a y e r e x i s t s , then i n f i l t r a t i o n i s slow i n i t i a l l y , but when the f r o n t reaches the lower 'normal' l a y e r , i n f i l t r a t i o n p roceeds as i f t h e r e were no a l t e r e d l a y e r . T h i s b e h a v i o r was not ob s e r v e d i n the f i e l d . When a r e p e l l e n t l a y e r e x i s t s on a h i l l s l o p e , water w i l l s i m p l y run o f f . S i n c e the c r i t i c a l ponding depth w i l l not be reached on a h i l l s l o p e , t h e r e w i l l be no i n f i l t r a t i o n i n t o an i d e a l i z e d s o i l , such as t h a t which i s presumed t o e x i s t f o r the purposes of t h i s a n a l y s i s . However, i n a r e a l s o i l , i t i s q u i t e p o s s i b l e t h a t not a l l pores w i l l be h y d r o p h o b i c , so t h a t some water may be conducted t h r o u g h the s o i l . The r e p e l l e n t l a y e r , i n such a c a s e , w i l l not rea c h s a t u r a t i o n , and thus w i l l have a h y d r a u l i c c o n d u c t i v i t y l e s s f r o n t r e a c h e s b o u n d a r y 4-» (fl k. • c o g r a p h s t a r t s w h e n c r i t i c a l p o n d i n g d e p t h r e a c h e d t i m e Figure 17 - Infitration rate for water-repellent soil, rainfall rate specified 64 than 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 . Hence, t h e r e w i l l be a permanent c o n t r a s t i n h y d r a u l i c p r o p e r t i e s . I t i s assumed t h a t t h e r e i s a 'normal' h o r i z o n a t the s u r f a c e , w i t h a t h i c k n e s s of no more than a few c e n t i m e t r e s , and then a w a t e r - r e p e l l e n t l a y e r of i n d e t e r m i n a t e t h i c k n e s s , w i t h 'normal', or s l i g h t l y a l t e r e d , s o i l s below t h a t . The h y p o t h e s i s t h a t a w a t e r - r e p e l l e n t l a y e r e x i s t s near the s u r f a c e was t e s t e d d i r e c t l y by the water drop p e n e t r a t i o n time (WDPT) t e s t . The r e s u l t s of the WDPT t e s t s a r e shown i n F i g u r e 18. Any p e n e t r a t i o n time g r e a t e r than about one second i n d i c a t e s w a t e r - r e p e l l e n c y . E s s e n t i a l l y , the g r e a t e r the p e n e t r a t i o n t i m e , the g r e a t e r the r e p e l l e n c y . Thus from F i g u r e 18, i t i s apparent t h a t a w a t e r - r e p e l l e n t l a y e r l a y e r of 1 t o 2 c e n t i m e t r e s t h i c k n e s s does e x i s t a t , or near, the s u r f a c e . S i n c e the e x i s t e n c e of such a l a y e r has a l r e a d y been shown t o be s u f f i c i e n t t o e x p l a i n the o b s e r v a t i o n s , the h y p o t h e s i s i s a c c e p t e d . 65 p e n e t r a t i o n t i m e ( m i n ) p e n e t r a t i o n t i m e ( m i n ) 0 1 2 3 4 5 6 E u a 3 <u -o 4 E a. -o - T 7 1 1 — — < ~ p e n e t r a t i o n t i m e ( m i n ) p e n e t r a t i o n t i m e ( m i n ) E a <u -a - I r 1 1 l — i E o a E u a p e n e t r a t i o n t i m e ( m i n ) p e n e t r a t i o n t i m e ( m i n ) E u Q. T5 Figure 18 - Water drop p e n e t r a t i o n times 66 Chapter 7 - D i s c u s s i o n I t has been demonstrated t h a t a w a t e r - r e p e l l e n t l a y e r e x i s t s over much of the study a r e a . A m p h o p h i l l i c substances are r e q u i r e d t o cause h y d r o p h o b i c i t y . Sources of such s u b s t a n c e s have been i d e n t i f i e d as the C h a p a r r a l v e g e t a t i o n i n s o u t h e r n C a l i f o r n i a (Krammes and Debano, 1965); as l i t t e r from J u n i p e r t r e e s ( S c h o l l , 1971); and as basidomycete f u n g i i n A u s t r a l i a (Bond and H a r r i s , 1964). S a v a g e ( l 9 6 8 ) has demonstrated t h a t a number of v a r i e t i e s of f u n g i produce n a t u r a l ' w a t e r - p r o o f i n g ' s u b s t a n c e s i n l a b o r a t o r y c u l t u r e s . F i r e s are of p a r t i c u l a r importance i n g e n e r a t i n g water-r e p e l l e n c y . They appear t o promote c e r t a i n r e a c t i o n s i n n a t u r a l l y o c c u r r i n g o r g a n i c matter t h a t i n c r e a s e s the water-r e p e l l e n c y by many t i m e s ; they a l s o promote the v o l a t i l i z a t i o n and r e d i s t r i b u t i o n of t h e s e s u b s t a n c e s (Debano, Mann, and H a m i l t o n , 1970; Savage, Osborne, L e t e y , and Heaton, 1972; and Reeder and J u r g e n s o n , 1979). S o i l s t h a t show n o t a b l e w a t e r - r e p e l l e n c y , even i n the absence of f i r e s , a re known t o o c c u r i n r e l a t i v e l y a r i d environments i n the s o u t h w e s t e r n U n i t e d S t a t e s , e s p e c i a l l y s o u t h e r n C a l i f o r n i a , and i n s o u t h w e s t e r n A u s t r a l i a (Krammes and Debano, 1965; H u s s a i n , Skau, B a s h i r , and Meeuwig, 1968; Corey and M o r r i s , 1968; S c h o l l , 1971; Bond, 1968; and R o b e r t s and Carbon, 1972). S i n c e the h y d r o p ho b i c l a y e r was d e v e l o p e d under d i f f e r e n t v e g e t a t i o n a s s o c i a t i o n s , i t may be t h a t the v a s c u l a r p l a n t s i n the a r e a are not the p r i m a r y s o u r c e s of 67 the w a t e r - p r o o f i n g a g e n t s . There i s no d i r e c t e v i d e n c e of r e c e n t f i r e s . F u r t h e r m o r e , i t i s u s u a l l y the case t h a t f i r e s o n l y i n t e n s i f y the r e p e l l e n c y , r a t h e r than c r e a t e i t . As mentioned p r e v i o u s l y , f u n g i have f r e q u e n t l y been i m p l i c a t e d as s o u r c e s of w a t e r - p r o o f i n g a g e n t s . For the c o o l m o i s t environment of the study a r e a , i t i s r e a s o n a b l e t o suppose t h a t f u n g i a re r e l a t i v e l y a c t i v e i n comparison t o the b a c t e r i a t h a t decompose the o r g a n i c compounds produced by the f u n g i . We know t h a t o r g a n i c m atter a c c u m u l a t i o n i s fa v o u r e d i n c o o l m oist environments (Jenny, 1941; and B i r k e l a n d , 1974). Hence, i t i s p o s s i b l e t h a t w a t e r - r e p e l l e n t s o i l h o r i z o n s a re not uncommon i n s i m i l a r s u b - a l p i n e environments i n the Coast M o u n t a i n s . There a r e a number of reasons why w a t e r - r e p e l l e n c y might be o v e r l o o k e d . There i s a commonly h e l d n o t i o n t h a t w a t e r - r e p e l l e n t c o a t i n g s a re f r e q u e n t l y 'washed away', and t h a t i t i s o f t e n a t r a n s i t o r y phenomenon o b s e r v e d o n l y a t the b e g i n n i n g of r a i n s t o r m s , a f t e r p r o l o n g e d d ry s p e l l s . L e t e y (1969). s u g gests t h a t the f a c t t h a t water drops e v e n t u a l l y p e n e t r a t e most w a t e r - r e p e l l e n t s o i l s i m p l i e s t h a t the water drop p e n e t r a t i o n time i s more a measure of the s t a b i l i t y of the hydrophobic c o a t i n g s than a measure of ' t r u e ' w a t e r - r e p e l l e n c y . I t i s assumed t h a t the i n i t i a l ' b a l l i n g up' of water drops on r e p e l l e n t samples demonstrates t h a t the c o n t a c t a n g l e i s g r e a t e r than 90 degr e e s . However, the f l a w i n t h i s argument i s t h a t a s i n g l e c o n t a c t a n g l e does not e x i s t when a drop r e s t s on a porous, 68 heterogeneous body because the drop i s a c t u a l l y i n c o n t a c t w i t h a number of d i s t i n c t s u r f a c e s , each w i t h t h e i r own c h a r a c t e r i s t i c c o n t a c t a n g l e . W e t t i n g may proceed through a s m a l l p r o p o r t i o n of the p o r e s . I f a c o n t i n u o u s t h r e a d of water reaches a w e t t a b l e p o c k e t , water may be drawn from the drop t o the pocket r a t h e r q u i c k l y . Hence, the e v e n t u a l , p o s s i b l y prompt, p e n e t r a t i o n of the drop may have n o t h i n g at a l l t o do w i t h any i n s t a b i l i t y of the hydrophobic c o a t i n g s . S i m i l a r l y , r e p e l l e n t s o i l l a y e r s may e v e n t u a l l y t r a n s m i t some water a f t e r becoming p a r t i a l l y w e t t e d , w i t h o u t any l o s s of the r e p e l l e n t c o a t i n g on the p o r e s . Another reason why w a t e r - r e p e l l e n c y might be o v e r l o o k e d i s t h a t the 'symptoms' a r e s i m i l a r t o those f o r some t e x t u r a l c o n t r a s t s , and f o r a i r entrapment. The i n f i l t r a t i o n c h a r a c t e r i s t i c s of s o i l s a r e f r e q u e n t l y i n v e s t i g a t e d by imposing j u s t - p o n d i n g c o n d i t i o n s , and o b s e r v i n g the change i n i n f i l t r a t i o n r a t e w i t h t i m e . Graph's of i n f i l t r a t i o n r a t e v e r s u s time a re g i v e n i n F i g u r e 19 f o r the ca s e s where: (1) t h e r e i s a c o a r s e l a y e r o v e r l y i n g a f i n e ( t h e o r e t i c a l ) ; (2) t h e r e i s a i r entrapment ( e m p i r i c a l , McWhorter, 1971); and (3) we have a hydrophobic l a y e r a t the s u r f a c e ( t h e o r e t i c a l ) . The parameters have been chosen t o produce graphs of s i m i l a r appearance, but i t i s assumed t h a t a l l the s o i l s have the same t e x t u r e , except f o r the f i n e l a y e r i n the f i n e over c o a r s e c a s e . There a r e s u b t l e d i f f e r e n c e s i n the form of these graphs, but i n p r a c t i c e i t i s not p o s s i b l e t o d i s t i n g u i s h between the mechanisms on the b a s i s of c time Figure 19 - Infiltration rate - time relationships 70 e x p e r i m e n t a l l y g e n e r a t e d c u r v e s . I t i s n e c e s s a r y t o employ the more d i r e c t t e s t s o u t l i n e d e a r l i e r i n t h i s r e p o r t ; w i t h o u t such t e s t s i t i s i m p o s s i b l e t o i d e n t i f y the mechanisms r e s p o n s i b l e c o n f i d e n t l y . In t h i s s t u d y , some v e r y s i m p l e m a t h e m a t i c a l models of i n f i l t r a t i o n i n t o a s o i l w i t h a w a t e r - r e p e l l e n t l a y e r were p r e s e n t e d . N u m e r i c a l methods would a l l o w the use of e m p i r i c a l l y d e t e r m i n e d water c o n t e n t - t e n s i o n and h y d r a u l i c c o n d u c t i v i t y - t e n s i o n r e l a t i o n s h i p s f o r the r e p e l l e n t s o i l s i n s o l u t i o n s of R i c h a r d ' s e q u a t i o n . R e p e l l e n t s o i l s s h o u l d show changes i n water c o n t e n t and h y d r a u l i c c o n d u c t i v i t y over n e g a t i v e t e n s i o n s ( p o s i t i v e p r e s s u r e s ) , i n c o n t r a s t t o 'normal', n o n - r e p e l l e n t s o i l s . One of the problems i n a p p l y i n g t h i s approach t o the s o i l s e n c o u n t e r e d i s t h a t the r e p e l l e n t l a y e r i s v e r y t h i n , and a p p a r a t u s d e s i g n e d e s p e c i a l l y f o r t h i s t a s k i s r e q u i r e d i f the s t r u c t u r e of the l a y e r i s t o be p r e s e r v e d . In f a c t , the r e s u l t s of t h i s study show c l e a r l y t h a t i t i s v e r y i m p o r t a n t t o c o n s i d e r c a r e f u l l y the s t r u c t u r e of the s o i l h o r i z o n s , and t h e i r r e l a t i o n s h i p s t o each o t h e r . Simply d e f i n i n g the h y d r a u l i c p r o p e r t i e s of the m a t r i x i s inadequate f o r s o i l s , w i t h w e l l - d e v e l o p e d s t r u c t u r e . The r e s u l t s of t h i s study have i m p l i c a t i o n s f o r the phenomenon of f l o w t h r o u g h 'macrochannels' ( r o o t h o l e s and the l i k e ) , another phenomenon t h a t i s not a c c o u n t e d f o r i f o n l y the p r o p e r t i e s of the m a t r i x a r e t a k e n i n t o a c c o u n t . One of the c o n c e p t u a l d i f f i c u l t i e s t h a t a r i s e s when one 71 presumes t h a t f l o w o c c u r s t h r o u g h macrochannels i n an u n s a t u r a t e d m a t r i x i s t h a t the m a t r i x s h o u l d n o r m a l l y have a s t r o n g a f f i n i t y f o r the water. Macrochannels ( i n e f f e c t , the l a r g e s t p o r e s ) s h o u l d f i l l l a s t . One s o l u t i o n i s t o assume t h a t the macrochannels f i l l i n a s a t u r a t e d zone near the s u r f a c e , and then conduct water r a p i d l y through the m a t r i x ; the m a t r i x absorbs some, but not a l l of the water from them. A w a t e r - r e p e l l e n t l a y e r near the s u r f a c e a l l o w s us t o g e n e r a t e a zone of s a t u r a t e d water n e c e s s a r y t o 'feed' the mac r o c h a n n e l s . Another p o s s i b i l i t y i s , of c o u r s e , t h a t the m a t r i x m a t e r i a l i t s e l f i s r e p e l l e n t . In a system where a l l pore s u r f a c e s were c o v e r e d w i t h e q u a l l y r e p e l l e n t s u b s t a n c e s , the l a r g e s t pores would f i l l f i r s t , s i n c e the l o w e s t p o s i t i v e p r e s s u r e s a r e r e q u i r e d t o f i l l them. Hence, f l o w t h r o u g h the l a r g e s t pore systems p r e s e n t would be f a v o u r e d . I f the l a r g e pore system has s u f f i c i e n t c a p a c i t y , or i f the p o s i t i v e p r e s s u r e s r e q u i r e d t o f i l l s m a l l e r pores a r e not reached, then water w l l f l o w t h rough macrochannels i n a permanently u n s a t u r a t e d m a t r i x . The ' l i n i n g ' of macrochannels might w e l l be p a r t i c u l a r l y r e p e l l e n t - the source of the a m p h o p h i l l i c compounds might be the decay p r o d u c t s from an o l d r o o t . B r a n d t ( l 9 6 8 ) has shown t h a t ped s u r f a c e s a r e sometimes c o a t e d w i t h r e p e l l e n t m a t e r i a l , which c o n t r i b u t e s t o t h e i r s t a b i l i t y . I f the s t r u c t u r e of the r o o t c h a n n e l i s p r e s e r v e d , i t i s c e r t a i n l y r e a s o n a b l e t o expect t h a t t h e r e i s some a l t e r a t i o n of the w a l l s of the c h a n n e l . 72 The presence of s a t u r a t e d ' f i n g e r s ' s u g g e s t s t h a t f l o w t h r ough macrochannels may not be the p r i m a r y mechanism by which lower zones of s a t u r a t i o n a r e f e d , a t l e a s t i n the s tudy a r e a . Bond and H a r r i s ( 1 9 6 4 ) , w o r k i ng i n A u s t r a l i a , found t h a t w i t h i n a r e a s w i t h r e p e l l e n t s o i l s , t h e r e were c e r t a i n a r e a s where the r e p e l l e n t l a y e r was reduced or non-e x i s t e n t . These a r e a s were s i t e s of i n c r e a s e d i n f i l t r a t i o n ; they c o n s t i t u t e d ' p r e f e r r e d pathways' f o r the the f l o w of water. In the study a r e a , t h e n , the d i s t r i b u t i o n of ' f i n g e r s ' may be d e t e r m i n e d by v a r i a t i o n s i n r e p e l l e n c y . I t i s known t h a t the r e p e l l e n c y depends not o n l y upon the p r o p e r t i e s of the c o a t i n g s on the p o r e s , but a l s o upon the pore s i z e : the s m a l l e r the p o r e , the g r e a t e r the p r e s s u r e r e q u i r e d t o f i l l i t . Thus, the v a r i a t i o n s i n r e p e l l e n c y may be r e l a t e d t o d i f f e r e n c e s i n the p r o p e r t i e s of the pore c o a t i n g s (which may depend upon the m i c r o b i a l f l o r a ) , or t o d i f f e r e n c e s i n t e x t u r e (pore s i z e ) . On a u n i f o r m h i l l s l o p e where o v e r l a n d f l o w i s o c c u r r i n g , a l a r g e p r e s s u r e head cannot d e v e l o p a t the s u r f a c e . However, i n d e p r e s s i o n s i n the l a n d s c a p e , water can c o l l e c t t o an a p p r e c i a b l e d e p t h . As the depth of ponding i n c r e a s e s , the p r e s s u r e i n c r e a s e s , and p o r e s of ever s m a l l e r d i a meter w i l l f i l l , assuming a u n i f o r m c o n t a c t a n g l e . Thus, i n a s o i l w i t h a f a i r l y narrow d i s t r i b u t i o n of pore s i z e s , t h e r e w i l l be a v e r y l a r g e change i n h y d r a u l i c c o n d u c t i v i t y and water c o n t e n t as the p r e s s u r e i n c r e a s e s t o the l e v e l r e q u i r e d t o f i l l the modal pore s i z e . Hence, i n some 73 d e p r e s s i o n s , a p r e s s u r e l a r g e enough t o f i l l most pores may d e v e l o p , l e a d i n g t o p r e f e r r e d i n f i l t r a t i o n a t such s i t e s . I f t h i s i s the c a s e , s a t u r a t e d f i n g e r s s h o u l d form p r e f e r e n t i a l l y below d e p r e s s i o n s i n the l a n d s c a p e . On a b a s i n s c a l e , the presence of a w a t e r - r e p e l l e n t l a y e r has r a t h e r i m p o r t a n t i m p l i c a t i o n s . I f the r e p e l l e n t l a y e r i s near the s u r f a c e , o v e r l a n d f l o w w i l l o c c u r even i n r a t h e r s m a l l r a i n s t o r m s . Y i e l d s s h o u l d be h i g h , s i n c e r e l a t i v e l y l i t t l e water i s s t o r e d i n the system f o r l a t e r e v a p o t r a n s p i r a t i o n . In the study a r e a , the 'normal' s u r f a c e l a y e r , which i s no more than a few c e n t i m e t r e s i n t h i c k n e s s , p r o v i d e s some s t o r a g e c a p a c i t y . T h i s s a t u r a t e d l a y e r s u p p l i e s r u n o f f t o the stream f o r a time a f t e r p r e c i p i t a t i o n c e a s e s . The a r e a l e x t e n t of the s a t u r a t e d l a y e r c o n t r a c t s a f t e r s torms, w i t h the l a s t a r e a s t o remain s a t u r a t e d b e i n g near the c h a n n e l s , and i n d e p r e s s i o n s . Presumably, i f a r a i n event began b e f o r e complete d r a i n a g e of the s u r f a c e l a y e r , the s a t u r a t e d zone would expand from the c h a n n e l margins and d e p r e s s i o n s outwards. T h i s i s s i m i l a r t o the p r o g r e s s i o n noted by D u n n e ( l 9 6 9 ) , but the mechanism i s not s a t u r a t i o n o v e r l a n d f l o w , as i t was i n Dunne's s t u d y . In t h i s case t h e r e i s a t r a n s f e r of water t o lower s a t u r a t e d zones through s a t u r a t e d f i n g e r s , and some t r a n s f e r by u n s a t u r a t e d f l o w . The h y d r o l o g i c a l p r o p e r t i e s of the s o i l s s t u d i e d have a b e a r i n g upon the way t h e s e s o i l s w i l l f a i l . For a g i v e n m a t e r i a l , s t r e n g t h i s a f u n c t i o n of the water c o n t e n t . At 74 h i g h pore water p r e s s u r e s the e f f e c t i v e s t r e s s on p a r t i c l e s i s reduced, l e a d i n g t o a much reduced apparent s t r e n g t h ( T e r z a g h i and Peck, 1967). As the s o i l i s dewatered, the s t r e n g t h t y p i c a l l y i n c r e a s e s w i t h i n c r e a s i n g t e n s i o n up t o some p o i n t , and then b e g i n s t o d e c l i n e as the c o n t i n u i t y of the water f i l m s t h a t were h o l d i n g the g r a i n s t o g e t h e r i s d i s r u p t e d . In a s o i l p r o f i l e , the p r o p e n s i t y t o f a i l w i l l v a r y as a f u n c t i o n o f : (1) the i n t r i n s i c s t r e n g t h of the h o r i z o n s ; (2) the water c o n t e n t of the h o r i z o n s ; (3) the depth ( g r a v i t a t i o n a l s t r e s s i n c r e a s e s w i t h d e p t h ) ; and (4) the t ype and d e n s i t y of r o o t i n g . The l a s t f a c t o r noted i s not a l t o g e t h e r independent of the h y d r a u l i c p r o p e r t i e s of the s o i l because the development of r o o t s depends, i n p a r t , upon the d i s t r i b u t i o n of m o i s t u r e w i t h i n the s o i l . There i s not enough i n f o r m a t i o n about the f o u r f a c t o r s l i s t e d above t o p r e d i c t the s o i l s t r e n g t h at any g i v e n p o i n t i n the s o i l p r o f i l e , but ' l o c a l minima' can be i d e n t i f i e d . A l o c a l minimum i s e x p e c t e d a t the base of the s o i l ( a t the t i l l b o u n d a r y ) , where t h e r e i s f r e q u e n t l y a s a t u r a t e d zone. As w e l l , the t i l l below i s q u i t e s t r o n g , and not l i k e l y t o f a i l , and the g r a v i t a t i o n a l l y i n d u ced s t r e s s i s g r e a t e r a t t h i s boundary than h i g h e r i n the s o i l p r o f i l e . A second minimum may occur i n the upper ash l a y e r . Because t h i s l a y e r i s composed of c o a r s e r m a t e r i a l than the s u r r o u n d i n g l o e s s l a y e r s , the c o n t i n u i t y of the water f i l m s t h a t t e n d t o b i n d i t t o g e t h e r may be' d i s r u p t e d a t a lower t e n s i o n than i n the l o e s s . The d e n s i t y of r o o t i n g i s a l s o much lower i n the 75 upper ash. In exhumed s o i l columns, t h e r e i s a s t r o n g tendency t o shear a c r o s s the upper ash l a y e r , c o n f i r m i n g our e x p e c t a t i o n s . 76 Chapter 8 - Summary The goal of t h i s study was to understand streamflow generation in a p a r t i c u l a r l o c a t i o n , i n t h i s case a sub-a l p i n e basin i n the Coast Mountains of B r i t i s h Columbia. Three mechanisms of stormflow generation should be considered: ( 1 ) Hortonian overland flow; ( 2 ) s a t u r a t i o n overland flow; and ( 3 ) subsurface stormflow. P r e l i m i n a r y f i e l d observations suggested that Hortonian overland flow was the p r i n c i p a l mechanism, o c c u r r i n g p r i m a r i l y upon the bare bedrock outcrops and many of the vegetated h i l l s l o p e s . I t i s not s u r p r i s i n g that overland flow was generated on bedrock surfaces; the question of i n t e r e s t i s what c o n t r o l s the i n f i l t r a b i 1 i t y on the vegetated slopes. The hypothesis that the overland flow was caused by the r a i n f a l l rate exceeding the saturated h y d r a u l i c c o n d u c t i v i t y was considered, but r e j e c t e d when l a b o r a t o r y t e s t s demonstrated that the saturated h y d r a u l i c c o n d u c t i v i t y was far higher than the r a i n f a l l r a tes encountered. As w e l l , t h i s mechanism would not account f o r the apparent h a l t i n the advance of the f r o n t . The p o s s i b i l i t y that a t e x t u r a l c o n trast was responsible for the h a l t in the advance of the fro n t was held to be tenable i n the cases where the front stopped i t s advance at the loess-ash boundary, but as noted p r e v i o u s l y , the front u s u a l l y stopped w e l l above t h i s boundary. Hence, a t e x t u r a l c o n t r a s t could hardly be the p r i n c i p a l cause. That a i r might be trapped, and then compressed in advance of the fr o n t was . considered, but 77 r e j e c t e d because the h a l t i n the advance of the f r o n t s h o u l d o n l y be temporary, which was not the c a s e . F u r t h e r m o r e , v e n t i n g of the s o i l l o c a l l y t h r ough e x c a v a t i o n of o b s e r v a t i o n p i t s d i d not a l l o w the f r o n t t o c o n t i n u e i t s advance. Hence, t h i s h y p o t h e s i s was a l s o r e j e c t e d . I t was then proposed t h a t a c o n t r a s t i n h y d r a u l i c p r o p e r t i e s e x i s t e d , but was due t o d i f f e r e n c e s i n the a f f i n i t y of pore s u r f a c e s f o r water, r a t h e r than a c o n t r a s t i n t e x t u r a l p r o p e r t i e s . As a s i m p l e t h e o r e t i c a l model f o r i n f i l t r a t i o n i n t o such s o i l s was d e v e l o p e d , i t became apparent t h a t our o b s e r v a t i o n s c o u l d be e x p l a i n e d o n l y i f the ' a l t e r e d ' l a y e r was a c t u a l l y w a t e r - r e p e l l e n t . D i r e c t t e s t s of r e p e l l e n c y demonstrated t h a t a w a t e r - r e p e l l e n t l a y e r was p r e s e n t w i t h i n the t o p few c e n t i m e t r e s of most of the samples. The c o o l m oist c o n d i t i o n s t h a t p r e v a i l may f a v o r the a c c u m u l a t i o n of the o r g a n i c compounds t h a t are r e s p o n s i b l e f o r the w a t e r - r e p e l l e n c y . There i s no apparent l i n k between the w a t e r - r e p e l l e n c y and any p r o c e s s unique t o the study a r e a , which suggests t h a t such a l a y e r might be a f e a t u r e of o t h e r s u b - a l p i n e and a l p i n e b a s i n s . The presence of a w a t e r - r e p e l l e n t l a y e r has i m p o r t a n t i m p l i c a t i o n s f o r watershed dynamics. Where such l a y e r s e x i s t , the response time of the b a s i n w i l l be much s h o r t e r than might o t h e r w i s e be e x p e c t e d . A r e a l v a r i a t i o n s i n r e p e l l e n c y , and t o p o g r a p h i c a l l y d e f i n e d d i f f e r e n c e s i n the p o s s i b l e depth of p o n d i n g , may produce p r e f e r r e d v e r t i c a l f l o w p a t h s . T h e r e f o r e , i n f i l t r a t i o n b e h a v i o r may show 78 c o n s i d e r a b l e s p a t i a l v a r i a b i l i t y , even on r e l a t i v e l y homogeneous s o i l - v e g e t a t i o n u n i t s . C a r e f u l f i e l d e x p e r i m e n t a t i o n i s r e q u i r e d t o d i f f e r e n t i a t e the mechanism of s t o r m f l o w p r o d u c t i o n o u t l i n e d i n t h i s s tudy from o t h e r p o s s i b l e mechanisms. 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And Debano, L.F., 1965, S o i l w e t t a b i l i t y : a n e g l e c t e d f a c t o r i n watershed management: Water  Resources Research, V.1, pp.283-286. L e t e y , J . , 1969, 'Measurement of c o n t a c t a n g l e , water drop p e n e t r a t i o n t i m e , and c r i t i c a l s u r f a c e t e n s i o n ' i n Debano, L.F. ( e d . ) , Water r e p e l l e n t s o i l s - P r o c e e d i n g s  of the symposium on water r e p e l l e n t s o i l s , U n i v e r s i t y of C a l i f o r n i a , R i v e r s i d e , Ca., May 6-10, 1968, pp.43-47. McWhorter, D.B., 1971, I n f i l t r a t i o n a f f e c t e d by f l o w of a i r : C o l o r a d o S t a t e Hydrology Paper No. 49, C o l o r a d o S t a t e U n i v e r s i t y , F o r t C o l l i n s , C o l o r a d o , 43p. Osipow, L . I . , 1977, S u r f a c e c h e m i s t r y ; t h e o r y and i n d u s t r i a l  a p p l i c a t i o n s , Robert E. K r i e g e r P u b l i s h i n g Company, New York, 473p. P i a t t , J.R., 1964, S t r o n g i n f e r e n c e : S c i e n c e , V.9, pp. 347-353. Reeder, C.J. And J u r g e n s o n , M.F., 1979, F i r e - i n d u c e d water r e p e l l e n c y i n f o r e s t s o i l s of upper M i c h i g a n : Canadian  J o u r n a l of F o r e s t R e s e a r c h , V.9, pp.369-373. R o b e r t s , F . J . And Carbon, B.A., 1972, Water r e p e l l e n c e i n sandy s o i l s of South Western A u s t r a l i a I I . Some c h e m i c a l c h a r a c t e r i s t i c s of the hy d r o p h o b i c s k i n s : A u s t r a l i a n J o u r n a l of S o i l R e s e a r c h , V.10, pp.35-42. Ru b i n , J . , 1966, Theory of r a i n f a l l uptake by s o i l s i n i t i a l l y d r i e r tha/i t h e i r f i e l d c a p a c i t y and i t s a p p l i c a t i o n s : Water Resources R e s e a r c h , V.2, pp.739-749. R u b i n , J . And S t e i n h a r d t , R., 1963, s o i l water r e l a t i o n s d u r i n g r a i n i n f i l t r a t i o n : I . Theory: S o i l S c i e n c e  S o c i e t y of America P r o c e e d i n g s , V.27, pp.246-251. 82 Savage, S.M., 1968, ' C o n t r i b u t i o n of some s o i l f u n g i t o water r e p e l l e n c y i n s o i l m a t e r i a l s ' i n Debano, L.F. ( e d . ) , Water r e p e l l e n t s o i l s - P r o c e e d i n g s of the symposium on water r e p e l l e n t s o i l s , U n i v e r s i t y of C a l i f o r n i a , R i v e r s i d e , Ca., May 6-10, 1968, pp.241-257. Savage, S.M., Osborne, J . , L e t e y , J . , and Heaton, C , 1972, Substances c o n t r i b u t i n g t o f i r e - i n d u c e d water r e p e l l e n c y i n s o i l s : S o i l S c i e n c e S o c i e t y of America  P r o c e e d i n g s , V.36, pp.674-678. S c h o l l , D.G., 1971, S o i l w e t t a b i l i t y i n Utah j u n i p e r s t a n d s : S o i l S c i e n c e S o c i e t y of America P r o c e e d i n g s , V.35, pp.344-345. Slaymaker, H.O., 1977, ' E s t i m a t i o n of sediment y i e l d i n temperate a l p i n e environments' i n E r o s i o n and s o l i d m a t t er t r a n s p o r t i n i n l a n d w a t e r s , p r o c e e d i n g s of the P a r i s symposium, I.A.S.H.-A.I.S.H. P u b l i c a t i o n No.  122. T e r z a g h i , K., and Peck, R.B., 1967, S o i l mechanics i n  e n g i n e e r i n g p r a c t i c e , John W i l e y and Sons, I n c . , New York, 729p. T e t i , P.A., 1979, The v a r i a b i l i t y of stream c h e m i s t r y i n a  c o a s t mountain watershed, B r i t i s h Columbia, u n p u b l i s h e d M.Sc. T h e s i s , U n i v e r s i t y of B r i t i s h Columbia, I85p. Whipkey, R.Z., 1965, S u b s u r f a c e s t o r m f l o w from f o r e s t e d s l o p e s : I n t e r n a t i o n a l A s s o c i a t i o n of S i e n t i f i c H y r o l o g y  Annual B u l l e t i n , V.10, pp.74-85. Woodsworth, G.J., 1977, Geology of the Pemberton (92J) map a r e a : G e o l o g i c a l Survey of Canada Open F i l e Report 482, 1 map s h e e t . Appendix A - Major v e g e t a t i o n a s s o c i a t i o n s of the Goat  Meadows Watershed Heather With Dwarf Trees C a s s i o p e m e r t e n s i a n a P h y l l o d o c e e m p e t r i f o r m i s , i n t e r m e d i a , q l a n u l i f o r a A b i e s l a s i o c a r p a L u e t k e a p e c t i n a t a Lycopodium s i t c h e n s e -G a u l t h e r i a humi f u s a C l a d o n i a sp. Heath e r , Sedge, Forb P h y l l o d o c e e m p e t r i f o r m i s  C a s s i o p e m e r t e n s i a n a  Luetkea p e c t i n a t a  Juncus p a r r y i  P i n u s a l b i c a u l i s  A b i e s l a s i o c a r p a  L u p i n u s l a t i f o l i u s  Anemone o c c i d e n t a l i s  Deschampsia a t r o p u r p u r e a  Carex s p e c t a b i l i s L u e t k e a , Moss, L i c h e n L u e t k e a pect i n a t a  R hasomitrium sudeticum  S t e r o c a u l o n sp. S o l o r i n a c r o c e a Tree I s l a n d s A b i e s l a s i c a r p a Tsuga m e r t e n s i a n a P i n u s a l b i c a u l i s V a c c i n i u m membranaccum  Rhododendron a l b i forum  P h y l l o d o c e sp. C a s s i o p e m e r t e n s i a n a  Leutkea p e c t i n a t a C a s s i o p e , Moss C a s s i o p e m e r t e n s i a n a  P o l y t r i c h u m s e x a n g u l a i e Sedge Carex n i g r i c a n s  Juncus drummondi i  Deschampsia a t r o p u r p u r e a  Carex s p e c t a b i l i s  P o l y t r i c h u m s e x a n g u l a i e Tolmie S a x i f r a g e S a x i f r a g e t o l m i e i  M a r s u p e l l a b r e v i s s i m a  L u z u l a p i p e r i . Juncus drummondii Sedge, F o r b , Moss Carex n i g r i c a n s , s p e c t a b i l i s Juncus drummondi i , m e r t e n s i a n u s  E p i l o b i u m a l p i n u m , l a t i f o l i u m  P h i l o n t i s f o n t a n a  deschampsia a t r o p u r p u r e a Sedge, Sphagnum Carex n i g r i c a n s , s p e c t a b i l i s  Sphagnum o t h e r mosses 86 Appendix B - The s u r f a c e t e n s i o n of e t h y l a l c o h o l - water  m i x t u r e s In t h i s s t u d y , a one p e r c e n t s o l u t i o n of e t h y l a l c o h o l i n water was used t o demonstrate t h a t slow p e n e t r a t i o n of drops of pure water i n t o s o i l samples was not due t o the sample h a v i n g a low h y d r a u l i c c o n d c u t i v i t y . I t i s assumed t h a t the p h y s i c a l p r o p e r t i e s of t h i s s o l u t i o n a r e v e r y s i m i l a r t o those of pure water, except f o r the s o l i d - l i q u i d i n t e r f a c i a l t e n s i o n . The s u r f a c e t e n s i o n of a m i x t u r e of two m i s c i b l e l i q u i d s i s g i v e n , a p p r o x i m a t e l y , by a s i m p l e a d d i t i v e law (Osipow, 1977); i n t h i s c a s e : tfin = <re*Xe + «w • ( 1 -Xe ) . Where tfm = s u r f a c e t e n s i o n of the m i x t u r e *e = s u r f a c e t e n s i o n of e t h y l a l c o h o l , 22 dynes/cm tfw = s u r f a c e t e n s i o n of water, 73 dynes/cm Xe = mole f r a c t i o n of e t h y l a l c o h o l The mole f r a c t i o n of a one p e r c e n t s o l u t i o n (v/v) of e t h y l a l c o h o l i s 0.003; t h e r e f o r e : tfin = 22 dynes/cm-0 . 003 + 73 dynes/cm-0 . 997 = 72.8 dynes/cm I t i s c l e a r t h a t the s u r f a c e t e n s i o n of a one p e r c e n t e t h y l a l c o h o l s o l u t i o n i s almost e x a c t l y the same as the s u r f a c e t e n s i o n of pure water. 87 Appendix C - P r e c i p i t a t i o n d a t a Date D a i l y t o t a l Maximum one hour August 17 1980 0.5 6.7X10-° 5 August 18 1 980 0.0 O.O August 19 1 980 0.0 O.O August 20 1 980 0.0 O.O August 21 1 980 0.0 0.0 August 22 1980 0.1 1.7X10' 0 5 August 23 1 980 0.0 O.O August 24 1 980 0.0 0.0 August 25 1 980 0.0 0.0 August 26 1 980 1 .0 3. 3 x 1 0 " 0 5 August 27 1 980 0.4 2 . 5 x l O " 0 5 August 28 1980 0.2 2.5X10-° 5 August 29 1 980 0.0 0.0 September 1 1 980 0.4 2.5X10- 0 5 September 2 1980 0.1 1 .7X10-° 5 September 4 1980 0.0 8.3X10" 0 6 averaged over a one hour p e r i o d . 88 Appendix D - l i s t of symbols A = c r o s s - s e c t i o n a l a rea g = g r a v i t a t i o n a l a c c e l e r a t i o n Hn = t e n s i o n a t which step-change i n h y d r a u l i c p r o p e r t i e s o c c u r s f o r m a t e r i a l i n l a y e r Hs = t e n s i o n a t s u r f a c e of s o i l Hb = t e n s i o n a t boundary between l a y e r s i = i n f i l t r a b i 1 i t y I = c u m u l a t i v e i n f i l t r a t i o n I f = c u m u l a t i v e i n f i l t r a t i o n when f r o n t reaches boundary between l a y e r s I p = c u m u l a t i v e i n f i l t a t i o n when ponding b e g i n s Ksn = 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 of l a y e r n L = l e n g t h of sample f o r 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 d e t e r m i n a t i o n , or depth of p e n e t r a t i o n of w e t t i n g f r o n t Ln = depth of l a y e r n Pc = c r i t i c a l ponding depth Pd = ponding depth q = f l o w per u n i t a r e a Q = f l o w r = r a d i u s of c a p i l l a r y tube or of s o i l pore R = r a i n f a l l r a t e T = time Tf = time f r o n t reaches boundary between l a y e r s Tp = time t h a t ponding b e g i n s water c o n t e n t ( f i g u r e s o n l y ) depth c o n t a c t a n g l e c r o s s - s e c t i o n a l a r e a of pores per u n i t a r e a water c o n t e n t v i s c o s i t y d e n s i t y s u r f a c e t e n s i o n t o t a l p o t e n t i a l p r e s s u r e p o t e n t i a l 

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