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Hydrologic responsiveness of a Lower Fraser Valley lowland soil 1988

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HYDROLOGIC RESPONSIVENESS OF A LOWER FRASER VALLEY LOWLAND SOIL by KARIM ABBASPOUR A THESIS IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n FACULTY OF GRADUATE STUDIES Department of S o i l S c i e nce We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA October 1988 (c) Karira Abbaspour, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT S o i l d e g r a d a t i o n i n the lowland s o i l s of the Lower F r a s e r V a l l e y area i s an o f f - s e a s o n (September-April) problem. The le g a c y of the d e g r a d a t i o n process i s encountered every year i n the form of ponding which d e l a y s farming o p e r a t i o n s such as c u l t i v a t i o n and seeding. I t i s common f o r the lowland s o i l s i n west D e l t a t o be l e f t i n a bare, l o o s e , and t h e r e f o r e u n s t a b l e s t a t e i n the f a l l a f t e r h a r v e s t . As the r e s u l t of r a i n d r o p impact on such a s o i l , a d i s a g g r e g a t i o n process takes p l a c e which decreases 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 a t u r a t e d water content, the a i r e n t r y p r e s s u r e head, and the water r e l e a s i n g a b i l i t y of a s o i l . As a r e s u l t of these changes the h y d r o l o g i c r e sponsiveness of a s o i l w i l l decrease, d e c r e a s i n g i t s a b i l i t y t o al l o w r a p i d i n f i l t r a t i o n and drai n a g e . The o b j e c t i v e of t h i s t h e s i s was f i r s t l y , t o i n v e s t i g a t e the causes of the s o i l s t r u c t u r a l d e g r a d a t i o n and secondly, to use some of the s o i l s t r u c t u r a l parameters t o o p t i m i z e the responsiveness o f a s o i l and t h i r d l y , t o suggest a management model w i t h the o b j e c t i v e of improving the h y d r o l o g i c r e s p o n s i v e n e s s of a lowland s o i l . To f u l f i l l the above o b j e c t i v e s , i n the f i r s t chapter, the process of s o i l d e g r a d a t i o n was s t u d i e d on l a r g e u n d i s t u r b e d s o i l columns removed from two adj a c e n t l o c a t i o n s w i t h i n an area of Ladner i n west D e l t a , B r i t i s h Columbia. I t was found t h a t a d i s a g g r e g a t i o n process caused by the impact of r a i n d r o p s on a weakly aggregated s o i l was the main cause of a low h y d r o l o g i c responsiveness a t the b e g i n n i n g of the c u l t i v a t i o n season. As a r e s u l t of d e g r a d a t i o n of the s o i l s u r f a c e l a y e r , a s u r f a c e s e a l can form w i t h 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 i n the order of 9.7xl0"l° m s - ! . A s u r f a c e s e a l can e f f e c t i v e l y decrease the i n f i l t r a t i o n r a t e , l e a d i n g t o the f o r m a t i o n of a p e r s i s t e n t pond which w i l l make a s o i l u n t r a f f i c a b l e and unworkable. In the second chapter, a concept of "designer s o i l " was developed, where a s e t of "design h y d r o l o g i c parameters" were i d e n t i f i e d f o r a p a r t i a l l y h y p o t h e t i c a l s o i l . A s o i l p o s s e s s i n g h y d r o l o g i c parameters b e t t e r than the d e s i g n parameters would t h e r e f o r e d i s p l a y a c e r t a i n d e s i r e d h y d r o l o g i c r e s p o n s i v e n e s s . In the t h i r d c h apter, a d e s c r i p t i v e management model was suggested w i t h the o b j e c t i v e of a c h i e v i n g the d e s i g n parameters as i d e n t i f i e d i n the second chapter. i v TABLE OF CONTENT T I T L E PAGE ABSTRACT i i TABLE OF CONTENTS i v L I S T OF TABLES v i . L I S T OF FIGURES v i i L I S T OF SYMBOLS i x ACKNOWLEDGEMENT X INTRODUCTION . 1 CHAPTER 1 CAUSES OF A LOW HYDROLOGIC RESPONSIVENESS 1.1 ABSTRACT , 5 1.2 INTRODUCTION 6 1.3 MATERIALS AND METHODS 8 1.3.1 S o i l a n d S i t e D e s c r i p t i o n 8 1.3.2 S a m p l i n g P r o c e d u r e 10 1.3.3 L a b o r a t o r y P r o c e d u r e 14 1.3.4 P h y s i c a l P r o p e r t i e s o f t h e S o i l 19 i . D e t e r m i n a t i o n o f a g g r e g a t e s t a b i l i t y ... 19 i i . D e t e r m i n a t i o n o f o r g a n i c m a t t e r c o n t e n t 20 1.4 RESULTS AND DISCUSSION 20 1.4.1 E f f e c t o f D r y i n g a n d C o v e r C r o p on S e a l F o r m a t i o n 30 1.4.2 I n t e r n a l S e a l 33 1.5 SUMMARY AND CONCLUSIONS 38 REFERENCES 40 CHAPTER 2 USE OF S O I L STRUCTURE DESIGN PARAMETERS FOR OPTIMIZATION OF HYDROLOGIC RESPONSIVENESS 2.1 ABSTRACT 43 2.2 INTRODUCTION 44 2.3 MATERIALS AND METHODS 47 2.3.1 D e t e r m i n a t i o n o f t h e H y d r o l o g i c P a r a m e t e r s .. 50 a) F o r t h e t o p s o i l 50 b) F o r t h e s u b - s o i l 55 V TITLE PAGE 2.4 NUMERICAL MODEL 55 2.5 RESULTS AND DISCUSSION 59 2.5.1 Model Response 62 2.6 SUMMARY AND CONCLUSIONS 68 REFERENCES 70 CHAPTER 3 DESCRIPTIVE MODEL OF SEAL FORMATION AND MANAGEMENT MODEL 3.1 ABSTRACT 73 3.2 INTRODUCTION 73 3.3 MECHANISMS OF SURFACE AND INTERNAL SEAL FORMATION 75 3.4 DISCUSSION 83 3.5 SOIL MANAGEMENT 86 3.6 SUMMARY AND CONCLUSIONS 87 REFERENCES 89 CONCLUSIONS AND SUMMARY 91 v i L I S T OF TABLES CHAPTER 1 PAGE T a b l e 1: S o i l p r o f i l e d e s c r i p t i o n a n d some p e r t i n e n t i n f o r m a t i o n f o r t h e L a d n e r s o i l u n d e r c o n s i d e r a t i o n 11 CHAPTER 2 T a b l e 1: H y d r o l o g i c p a r a m e t e r s o f t h e s o i l 53 T a b l e 2: V a l u e s o b t a i n e d f o r some h y d r o l o g i c p a r a m e t e r s b y a s s u m i n g a r e a s o n a b l e b e h a v i o u r f o r t h e p a r t i a l r e t e n t i o n c u r v e s 54 T a b l e 3: S h o w i n g t i m e t o t r a f f i c a b i l i t y ( t t ) , r e s p o n s e t i m e ( r t ) , a n d t i m e t o w o r k a b i l i t y ( t w ) f o r a number o f p o s s i b l e h y d r o l o g i c p a r a m e t e r c o m b i n a t i o n s 63 v i i LIST OF FIGURES CHAPTER 1 PAGE F i g . 1: Schematic drawing of f i e l d sampling 12 F i g . 2: Schematic drawing of flow measurement system.. 15 F i g . 3: F i e l d measurement of water content vs. depth f o r a s o i l p r o f i l e under a ponded depression. S a t u r a t i o n i s i n d i c a t e d by s o l i d v e r t i c a l l i n e s . 22 F i g . 4: Pressure head vs. depth f o r the three s e t s of tensiometers i n column 'A'. S o l i d h o r i z o n t a l l i n e s show the l o c a t i o n of tensiometers. S o l i d curve shows the average value of pressure head..23 F i g . 5: T o t a l head vs. depth f o r column 'A'. Short h o r i z o n t a l l i n e s show the l o c a t i o n of tensiometers 25 F i g . 6: T o t a l head vs. depth f o r column 'C. Short h o r i z o n t a l l i n e s show the l o c a t i o n of tensiometers 26 F i g . 7: H y d r a u l i c c o n d u c t i v i t y r e d u c t i o n as a f u n c t i o n of r a i n f a l l events f o r d i f f e r e n t l a y e r s of column 'A'.'Zero event corresponds to a f r e s h l y c u l t i v a t e d s o i l s a t urated from below 27 F i g . 8a: T o t a l head vs. depth f o r column 'B' when c u l t i v a t i o n l a y e r was kept saturated. The gradient f o r the surface 5 mm and the c u l t , l a y e r was too small t o a l l o w K to be measured 31 F i g . 8b: T o t a l head vs. depth f o r column 'B' a f t e r formation of a t e n s i o n f o r c e at the surface .. 32 F i g . 9a: Resistance vs. r a i n f a l l f o r the l a b o r a t o r y column. Numbers i n the brackets show the r a i n f a l l i n t e n s i t y 34 F i g . 9b: H y d r a u l i c c o n d u c t i v i t y vs. r a i n f a l l event f o r an i n t e r n a l s e a l formed i n l a b o r a t o r y column f o r two assumed thicknesses of the s e a l 35 v i i i PAGE F i g . 10: The i n t e r n a l s e a l formed on top of the compacted pan i n l a b o r a t o r y column a: top view b: s i d e view 37 CHAPTER 2 F i g . 1: P a r t i a l water r e t e n t i o n c h a r a c t e r i s t i c curves f o r the top 30 cm l a y e r , and the bottom 90 cm l a y e r of s u b - s o i l 51 F i g . 2: P a r t i a l flow c h a r a c t e r i s t i c curves f o r the top 30 cm and the bottom 90 cm of s u b - s o i l ... 52 Fi g . . 3 : Mathematical model f o r one dimensional, v e r t i c a l unsteady i n f i l t r a t i o n or evaporation 57 F i g . 4: P a r t i a l water r e t e n t i o n curves at v a r i o u s stages of degradation 61 F i g . 5: Showing the process of d r y i n g from an i n i t i a l l y f looded c o n d i t i o n f o r case 3a i n Table 3 64 F i g . 6: Showing the process of w e t t i n g from an i n i t i a l l y t r a f f i c a b l e s t a t e (at t=7.6 days i n F i g . 5) i n response to a storm of 10 hr. d u r a t i o n and 2 cm day--*- i n t e n s i t y 65 CHAPTER 3 F i g . 1: D e s c r i p t i v e mechanisms of s e a l formation ...... 77 F i g . 2: Showing s i d e views of two surface s e a l s . A (a & b) coarse l a y e r at. the bottom of each s e a l topped w i t h f i n e r p a r t i c l e s can be i d e n t i f i e d . . 81 F i g . 3: A d e s c r i p t i v e management model :for Lower Fraser V a l l e y lowland s o i l s . Depicted at the centre i s an i d e a l s o i l and water management path 85 i x L I S T OF SYMBOLS S y m b o l U n i t A d r a i n a g e i n t e n s i t y s ~ l c s p e c i f i c w a t e r c a p a c i t y m ~ l h s o i l w a t e r p r e s s u r e h e a d m h a a i r e n t r y p r e s s u r e h e a d m h D h e i g h t o f w a t e r t a b l e a t m i d - p o i n t b e t w e e n d r a i n s m h 0 i n i t i a l h e i g h t o f w a t e r t a b l e a b o v e d r a i n d e p t h a t m i d - s p a c i n g m h t f i n a l h e i g h t o f w a t e r t a b l e a b o v e d r a i n d e p t h midway b e t w e e n d r a i n s m I r a i n f a l l i n t e n s i t y m s ~ l K h y d r a u l i c c o n d u c t i v i t y m s ~ l K s 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 m s ~ l K.E k i n e t i c e n e r g y j m mass o f o n e r a i n d r o p k g q>0 d r a i n a g e d i s c h a r g e m s ~ l q<0 r e c h a r g e m s ~ l r>0 r a i n f a l l r a t e m s ~ l r<0 e v a p o r a t i o n r a t e m s ~ l t t i m e s v v e l o c i t y m s ~ l z d e p t h ( p o s i t i v e u p w a r d ) m 8 h y d r o l o g i c p a r a m e t e r d e s c r i b i n g t h e s h a p e o f t h e w a t e r r e t e n t i o n c u r v e d i m e n s i o h l e s s 8 s o i l w a t e r c o n t e n t ra^ m"^ 6 S s a t u r a t e d w a t e r c o n t e n t m-* m"*3 X ACKNOWLEDGEMENTS I would l i k e t o thank Dr. Jan de V r i e s , my r e s e a r c h s u p e r v i s o r , f o r h i s guidance and support throughout the e n t i r e p r o j e c t . The c o n t r i b u t i o n and suggestions p r o v i d e d by the other members of my committee, Drs. M. D. Novak, A. Bomke, S. T.Chieng, and Mr. M. Driehuyzen are w e l l a p p r e c i a t e d . My thanks go t o Mr. J . Savage, whose farm was used as the study s i t e . S p e c i a l thanks are due t o Rosanne Rumley, Dr. Mensah Bonsu, T r e v o r M u r r i e , Babak Shapar, and Dr. Morteza Ghomshei f o r t h e i r c o n t r i b u t i o n s and h e l p f u l d i s c u s s i o n s throughout the e n t i r e p r o j e c t . I express my g r a t i t u d e t o my f a m i l y f o r t h e i r support and p a t i e n c e d u r i n g the course of t h i s work. 1 INTRODUCTION T h i s t h e s i s i s the product of an i n v e s t i g a t i o n c a r r i e d out to determine the causes of a low h y d r o l o g i c r e s p o n s i v e n e s s , manifested by widespread ponding d u r i n g the o f f - s e a s o n p e r i o d (September-March), i n a lowland s o i l i n the Lower F r a s e r V a l l e y (LFV) of B r i t i s h Columbia. The t h e s i s i s a composite of th r e e c h a p t e r s . Chapter 1 i s a r e p o r t of experiments designed t o i n v e s t i g a t e the processes which l e a d t o the d e g r a d a t i o n of the s u r f a c e l a y e r of an unprotected, f r e s h l y c u l t i v a t e d s o i l . These experiments were c a r r i e d out i n the l a b o r a t o r y on u n d i s t u r b e d s o i l columns 50 cm deep and 25 cm i n diameter.. I t was found t h a t a d i s a g g r e g a t i o n process caused by the impact of r a i n d r o p s on a weakly aggregated s o i l t r i g g e r s a d e g r a d a t i o n process which leads t o the fo r m a t i o n of a s u r f a c e s e a l . Formation of a s u r f a c e s e a l was i d e n t i f i e d as the main cause of a low h y d r o l o g i c r e s p o n s i v e n e s s . E x i s t e n c e of a h i g h r e s i s t a n c e l a y e r , r e f e r r e d t o as " i n t e r n a l s e a l " , was d e t e c t e d on top of a compacted pan approximately 10 cm below the s o i l s u r f a c e . Since t o the best of our knowledge an i n t e r n a l s e a l has never been r e p o r t e d i n the l i t e r a t u r e , f u r t h e r i n v e s t i g a t i o n i s necessary t o determine the mechanisms of i t s fo r m a t i o n i n the f i e l d . In the l a b o r a t o r y however, an i n t e r n a l s e a l was produced s e v e r a l times. T r a n s p o r t of v e r y f i n e sediment by co n d u c t i v e flow through a f r e s h l y c u l t i v a t e d , l o o s e l y s t r u c t u r e d s o i l was i d e n t i f i e d as the main mechanism f o r i t s f o r m a t i o n . 2 C h a p t e r 2 i s d e v o t e d t o t h e d e v e l o p m e n t o f a c o n c e p t w h i c h o p t i m i z e s t h e h y d r o l o g i c r e s p o n s i v e n e s s o f a s o i l b y u s i n g some d e s i g n s t r u c t u r a l p a r a m e t e r s . A s s o i l s t r u c t u r e d e g r a d e s a s a r e s u l t o f r a i n d r o p i m p a c t o r c o m p a c t i o n , s o i l r e s p o n s e t i m e i n c r e a s e s d u e t o c h a n g e s w h i c h o c c u r i n t h e s o i l h y d r o l o g i c c h a r a c t e r i s t i c f u n c t i o n s . S o i l h y d r o l o g i c c h a r a c t e r i s t i c f u n c t i o n s w h i c h a r e i n t i m a t e l y r e l a t e d t o t h e s o i l s t r u c t u r e c a n be d e f i n e d b y f o u r p a r a m e t e r s : 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 , s a t u r a t e d w a t e r c o n t e n t , a i r e n t r y p r e s s u r e h e a d a n d a p a r a m e t e r r e f e r r e d t o a s B w h i c h d e s c r i b e s t h e s h a p e o f t h e p a r t i a l w a t e r r e t e n t i o n c u r v e . As t h e d e g r a d a t i o n p r o c e s s i s s e t i n m o t i o n b y t h e d i s i n t e g r a t i o n o f t h e s u r f a c e a g g r e g a t e s , s o i l h y d r o l o g i c p a r a m e t e r s r e q u i r e new v a l u e s i n r e s p o n s e t o t h e c h a n g e s i n s o i l s t r u c t u r e . I n o r d e r t o d e t e r m i n e t h e s e n s i t i v i t y o f a s o i l s y s t e m t o t h e c h a n g e s i n t h e h y d r o l o g i c p a r a m e t e r s , s o i l r e s p o n s e t i m e was e v a l u a t e d a t d i f f e r e n t s t a g e s o f s o i l d e g r a d a t i o n b y u s i n g a m a t h e m a t i c a l m o d e l . I n c h a p t e r 3, a m e c h a n i s m i s o f f e r e d f o r t h e f o r m a t i o n o f a s u r f a c e a n d a n i n t e r n a l s e a l . T h i s m e c h a n i s m i s b a s e d o n t h e f i n d i n g s i n c h a p t e r 1, a n d w h at h a s b e e n r e p o r t e d i n t h e l i t e r a t u r e o n t h e t o p i c o f s u r f a c e s e a l i n g . A l s o p r e s e n t e d i n c h a p t e r 3 i s a d e s c r i p t i v e management m o d e l d e s i g n e d t o i m p r o v e t h e h y d r o l o g i c r e s p o n s i v e n e s s o f t h e L F V l o w l a n d s o i l s . I n t h i s m o d e l a s e t o f c r i t e r i a p r o v e n t o b e e s s e n t i a l i n s o i l a n d w a t e r management o f a l o w l a n d s o i l a r e s y s t e m a t i c a l l y s t a t e d . I t i s s u g g e s t e d t h a t a n y d e v i a t i o n 3 from the optimum management p r a c t i c e s should be remedied by c e r t a i n a p p r o p r i a t e a c t i o n s . T h i s model can be used as an a l g o r i t h m f o r development of an expert system which c o u l d be used by farmers and s o i l s c i e n t i s t s as a management t o o l . CHAPTER 1 CAUSES OF A LOW HYDROLOGIC RESPONSIVENESS 5 CAUSES OF A LOW HYDROLOGIC RESPONSIVENESS 1.1 ABSTRACT Research was conducted on 50 cm deep undisturbed s o i l columns removed i n A p r i l of 1987 from ponded depressions i n an i n t e n s i v e l y c u l t i v a t e d undrained f i e l d ( s i t e 1) i n west D e l t a . The degree of degradation of the s o i l was judged t o be maximum as i n d i c a t e d by ponding. The o b j e c t i v e was t o i n v e s t i g a t e the causes of a low h y d r o l o g i c responsiveness t o r a i n f a l l events. I n i t i a l i n v e s t i g a t i o n showed the exist e n c e of a surface s e a l w i t h 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 (K s) of 9 . 7 x l 0 ~ 1 0 m s ~ l f o r the 5 mm-layer t h i c k s u r f a c e . Also an i n t e r n a l s e a l w i t h a K s of 1.0x10"^ m s " l was detected on top of a compacted pan approximately 10 cm below the surface. The above s o i l was compared w i t h an u n c u l t i v a t e d s o i l column removed from an adjacent drained f i e l d ( s i t e 2). The degree of degradation f o r the s o i l was judged to be minimal. There were no se a l s present i n t h i s s o i l , which had an e f f e c t i v e K s of 2.8 x l 0 ~ 4 m s ~ l . Both s o i l s were subjected t o simulated c u l t i v a t i o n i n the l a b o r a t o r y . The process of surface s e a l formation was i n v e s t i g a t e d under c o n t r o l l e d c o n d i t i o n s of simulated r a i n f a l l . Surface and i n t e r n a l s e a l s were regenerated i n the s o i l columns from s i t e 1, however no s e a l formed on the column of s i t e 2. K s of the surface 5 mm f o r the s i t e 1 s o i l decreased from an i n i t i a l value of 2.1x10"^ a f t e r c u l t i v a t i o n 6 t o 1 . 4 x l 0 ~ y m s - i a f t e r an e f f e c t i v e s e a l had fo rmed , w h i l e t h a t o f the s i t e 2 d e c r e a s e d o n l y from 7 . 0 x l 0 ~ 5 t o 1 . 5 x l 0 ~ 5 m s ~ l . I t was found t h a t a d i s a g g r e g a t i o n p r o c e s s , which r e s u l t s i n the f o r m a t i o n of a s u r f a c e s e a l , was the main cause of the r e d u c t i o n i n the h y d r o l o g i c r e s p o n s i v e n e s s of the c u l t i v a t e d s o i l . The most impor tan t f a c t o r g o v e r n i n g the p r o c e s s of s u r f a c e s e a l f o r m a t i o n was i d e n t i f i e d as the s u r f a c e aggregate s t a b i l i t y . I t was a l s o found t h a t development of a s u c t i o n f o r c e a t the s u r f a c e was an e s s e n t i a l f a c t o r i n the f o r m a t i o n of a h y d r o l o g i c a l l y e f f e c t i v e s e a l . A c o v e r c r o p was found t o be an e x t r e m e l y e f f e c t i v e t o o l i n p r e v e n t i n g the f o r m a t i o n of a s u r f a c e and an i n t e r n a l s e a l . 1.2 INTRODUCTION The impor tance of s t a b l e s o i l aggrega tes t o c r o p growth i s w e l l documented (Baver e t a l . 1972) . T h i s impor tance i s l a r g e l y due t o a d e s i r a b l e h y d r o l o g i c r e s p o n s i v e n e s s d e f i n e d a s : the a b i l i t y o f a s o i l t o a l l o w r a p i d i n f i l t r a t i o n and d r a i n a g e . A s o i l w i t h good h y d r o l o g i c b e h a v i o u r has a s h o r t r e s p o n s e t i m e , d e f i n e d i n t h i s t h e s i s a s : the t ime t o r e a c h a workab le s t a t e f rom a f l o o d e d c o n d i t i o n w i t h the water t a b l e a t the s o i l s u r f a c e . Workable s t a t e i s a c o n d i t i o n where the s o i l water c o n t e n t i s l e s s than the lower p l a s t i c l i m i t ( H i l l e l 1980) . I t i s w i d e l y known t h a t s o i l aggrega tes become u n s t a b l e 7 and s l a k e upon r a p i d w e t t i n g (Emerson 1977). Also reported i s the d i s i n t e g r a t i o n of surface aggregates due t o raindrop impact (Mclntyre 1958; E l l i s o n 1944). The combined e f f e c t of these d i s a g g r e g a t i o n processes i s the c l o g g i n g of pore-necks (Uebler and Swartzendruber 1982) and r e d u c t i o n of p o r o s i t y i n the s o i l surface l a y e r . Pore-clogging at the surface i s a precursor of surface s e a l formation (Bonsu 1987). Surface s e a l s may range i n thic k n e s s from l e s s than 1 mm to g r eater than 5 cm (Tackett and Pearson 1965). They are normally more compact, harder and more b r i t t l e when dry than the s o i l j u s t beneath them. Surface s e a l formation i s an important process i n terms of i t s e f f e c t s on i n f i l t r a t i o n , e r o s i o n , s e e d l i n g emergence, and ponding. This phenomenon has been e x t e n s i v e l y s t u d i e d i n the l a s t four decades. Duley (1939), Lemos and Lutz (1957), and Mclntyre (1958) have proposed mechanisms of s e a l formation. T a r c h i t z k y et a l . (1984), and Evans and Buol (1968) s t u d i e d the micromorphology and s t r u c t u r e of s e a l s . Meyer and Monke (1965), Free (1952), and many others reported on runoff and, s o i l e r o s i o n . H i l l e l and Gardner (1969) examined the e f f e c t of s e a l i n g on i n f i l t r a t i o n . F a l a y i and Bouma (1975) s t u d i e d r e l a t i o n s h i p s between the h y d r a u l i c conductance of surface s e a l s and s o i l management. In the Lower Fraser V a l l e y (LFV), s o i l degradation i s an off-season process which begins e a r l y i n the f a l l . I t s legacy i s encountered every year at the beginning of the c u l t i v a t i o n season i n the form of ponding. Windshield 8 s u r v e y s , c o n d u c t e d i n t h e W e s t e r n r e g i o n o f t h e L F V s i n c e 1982 (de V r i e s , p e r s o n a l c o m m u n i c a t i o n 1 9 8 7 ) , show p o n d i n g t o be an i n c r e a s i n g p r o b l e m i n more t h a n 9 0 % o f t h e c u l t i v a t e d f i e l d s . T h i s o b s e r v a t i o n i n d i c a t e s t h a t s o i l s i n t h i s r e g i o n a r e i n an o n g o i n g s t a t e o f h y d r o l o g i c d e t e r i o r a t i o n , r e s u l t i n g i n a c o n t i n u o u s r e d u c t i o n i n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y . I n t h e e x t r e m e l y w e t s p r i n g o f 1 9 8 4 , c u l t i v a t i o n a n d s e e d i n g w e re d e l a y e d . b y two m o n t h s a s t h e r e s u l t o f p o n d i n g w h i c h r e n d e r e d t h e s o i l u n t r a f f i c a b l e ( w i n d s h i e l d s u r v e y s a n d c o m m u n i c a t i o n w i t h f a r m e r s ) . B e c a u s e o f i t s e f f e c t o n t h e t i m e l i n e s s o f f a r m i n g o p e r a t i o n s , p o n d i n g i s a m a j o r s o u r c e o f e c o n o m i c l o s s . Due t o t h e i m p o r t a n c e o f t h e L F V l o w l a n d s o i l s t o B r i t i s h C o l u m b i a ' s economy, t h e p r e s e n t w o r k was u n d e r t a k e n w i t h t h e f o l l o w i n g o b j e c t i v e s : 1) t o i n v e s t i g a t e t h e m e c h a n i s m s w h i c h c o n t r i b u t e t o t h e r e d u c t i o n o f t h e s o i l h y d r o l o g i c r e s p o n s i v e n e s s , 2) t o d e v e l o p a c o n c e p t o f o p t i m i z i n g t h e h y d r o l o g i c r e s p o n s i v e n e s s o f a s o i l b y u s i n g some d e s i g n s t r u c t u r a l p a r a m e t e r s 3) t o p r o p o s e a s o i l a n d w a t e r management m o d e l f o r t h e w e s t e r n l o w l a n d s o f t h e L F V a r e a . 1.3 MATERIALS AND METHODS 1.3.1 S o i l a n d S i t e D e s c r i p t i o n L a r g e u n d i s t u r b e d c o l u m n s w e r e r e m o v e d f r o m two a d j a c e n t s i t e s w i t h i n a n a r e a o f L a d n e r s o i l i n w e s t D e l t a . S i t e 1 was 9 l o c a t e d i n a ponded depression i n an undrained f i e l d . The s o i l a t t h i s s i t e has been under continuous c u l t i v a t i o n and the degree of s t r u c t u r e degradation i s at a maximum. In c o n t r a s t , s i t e 2 was lo c a t e d i n an unused p o r t i o n of a drained f i e l d (Boundary Bay research s t a t i o n ) . This s i t e was under permanent grass and the degree of s o i l s t r u c t u r e degradation was at a minimum. Ladner s o i l s are c l a s s i f i e d as Humic Luvic G l e y s o l (Luttmerding 1981). They have developed from moderately f i n e and some f i n e - t e x t u r e d s t o n e - f r e e , mixed marine and freshwater d e l t a i c d e p o s i t s which are u n d e r l a i n by sandy m a t e r i a l s at depths below 100 cm or more. Surface t e x t u r e s are mostly s i l t y c l a y loam w i t h v a r i a t i o n s to s i l t loam, sub-surfaces are u s u a l l y s i l t y c l a y and s u b - s o i l t e x t u r e s range from s i l t y c l a y loam to s i l t loam. Ladner s o i l s are moderately p o o r l y t o po o r l y drained. They are moderately t o s l o w l y pervious and have high water h o l d i n g c a p a c i t y and slow surface r u n o f f . High water t a b l e s are usual during the. winter and surface ponding i s common durin g and a f t e r heavy r a i n s . Ladner s o i l s g e n e r a l l y have a very dark gray, f i r m , s i l t y to cl a y e y , c u l t i v a t e d surface l a y e r between 15 and 20 cm t h i c k . I t i s u n d e r l a i n by 5 t o 10 cm of g r a y i s h , p a r t i a l l y leached, very f i r m , s i l t y t o clayey m a t e r i a l c o n t a i n i n g common, r e d d i s h - brown mot t l e s . T h i s , i n t u r n , i s u n d e r l a i n by a cl a y e y l a y e r about 30 cm t h i c k which i s g r a y i s h brown,, very f i r m and p l a s t i c , has str o n g , p r i s m a t i c s t r u c t u r e and contains many redd i s h or brownish mo t t l e s . Surface and sub-surface s o i l 10 r e a c t i o n i s v a r i a b l e depending on management p r a c t i c e s but i s u s u a l l y very s t r o n g l y or s t r o n g l y a c i d (Luttmerding 1981). The s o i l p r o f i l e f o r s i t e 1 i n A p r i l of 1987 p r i o r to c u l t i v a t i o n c o n s i s t e d of a surface s e a l , a c u l t i v a t i o n l a y e r , a compacted pan, and s u b - s o i l . S i t e 2 s o i l c o n s i s t e d of about 30 cm of top s o i l c o n t a i n i n g many s t a b l e worm holes u n d e r l a i n by s u b - s o i l of massive s i l t y m a t e r i a l c o n s i s t i n g of many s t a b l e o l d root holes. The c h a r a c t e r i s t i c s of s u b - s o i l i n both s i t e s were the same (Table 1). 1.3.2 Sampling Procedure The sampling method used was a m o d i f i c a t i o n of the method introduced by Bouma and Denning (1972). Three undisturbed columns 50 cm i n depth and 25 cm i n diameter encased i n 2 cm t h i c k concrete were removed i n A p r i l of 1987 from two ponded depressions 40 m apart and approximately 50 m2 i n area (columns A and B from the same depression, and C from the other one) w i t h well-formed surface s e a l s . One undisturbed column was taken from s i t e 2 (column D). A d e t a i l e d e x p l a n a t i o n of the sampling procedure i s o f f e r e d below: In the f i e l d , a c o n c e n t r i c p i t was dug and a c y l i n d r i c a l s o i l column 55 cm long and 25 cm i n diameter was c a r e f u l l y carved out ( F i g . 1). A c y l i n d r i c a l c a s i n g 50 cm long and 29 cm i n diameter was constructed using 0.75 mm t h i c k sheet metal. D i f f e r e n t l a y e r s i n the s o i l p r o f i l e ( i . e . c u l t i v a t i o n l a y e r and compacted pan) were c a r e f u l l y i d e n t i f i e d Table 1. Soil profile description and some pertinent information for the Ladner s o i l under consideration thickness aggregate st a b i l i t y (n=12) % bulk density (n=6) kg m -3 m s -1 particle size distribution %clay % s i l t %sand (s o i l type*) organic matter content (n=6) % Site 1 surface skin layer .05-.1 30±3 { seal compacted 1 - 2 4115 layer cultivation layer 7 - 1 0 47±4 compacted pan 10-17 1420±28 1105±33 1250±50 20.0 57.2 22.8 ( s i l t loam) 20.8 67.2 12.0 ( s i l t loam) 7.4±2.3 9.3±2.5 sub-soil - - - 10"6* Site 2 top layer 25 - 31 92±2.5 - 10"'*** 23.0 62.0 15.0 19.6±3.1 ( s i l t loam) sub-soil same as above *Single auger hole method, personal communication, S.T. Chieng, Bio-Resource Eng. Dept., U.B.C. 1988. **Personal communication, J. de Vries, Soil Science Dept., U.B.C. 1988. 12 FIG. 1 SCHEMATIC DRAWING OF FIELD SAMPLING 13 a n d t h e i r t h i c k n e s s e s w e r e m e a s u r e d , s i n c e two t e n s i o m e t e r s w e r e t o be f i t t e d i n e a c h l a y e r a t d i f f e r e n t d e p t h s a l o n g w i t h one t e n s i o m e t e r i n t h e s u b - s o i l . A t e a c h d e p t h t h r e e t e n s i o m e t e r s w e r e t o be f i t t e d a t 120 d e g r e e s a p a r t t o t r i p l i c a t e t h e p r e s s u r e h e a d r e a d i n g s . A p p r o p r i a t e l o c a t i o n s w e r e m a r k e d a l o n g t h e l e n g t h o f t h e c y l i n d r i c a l c a s i n g a n d 7 mm d i a m e t e r h o l e s w e r e d r i l l e d t o a l l o w t h e i n s t a l l a t i o n o f t h e t e n s i o m e t e r s . The t e n s i o m e t e r s u s e d w e r e p o r o u s c e r a m i c c u p s 5 cm l o n g , a n d o f 6 mm o u t s i d e d i a m e t e r w i t h a n a i r e n t r y v a l u e o f a b o u t 4 m e t e r s o f w a t e r . The t e n s i o m e t e r s w e r e c e m e n t e d t o 6 cm l o n g a n d 6 mm i n s i d e d i a m e t e r p l a s t i c t u b e s . To f a c i l i t a t e t r a n s p o r t a t i o n o f t h e c o l u m n s , f o u r c a s t i r o n r o d s 6 cm l o n g a n d 6 mm i n d i a m e t e r w e r e i n s t a l l e d i n t h e s h e e t m e t a l c y l i n d e r a t s y m m e t r i c a l p o s i t i o n s . T h e s e w e r e u s e d a s h a n d l e s f o r l i f t i n g t h e s o i l c o l u m n s . I n o r d e r t o r e i n f o r c e t h e h a n d l e s w i t h i n t h e c o n c r e t e c a s i n g , p i e c e s o f 5 cm b y 5 cm w i r e mesh w e r e p o s i t i o n e d a t t h e t i p o f t h e r o d s i n s i d e t h e c y l i n d r i c a l c a s i n g . B e a c h s a n d was p l a c e d a r o u n d t h e b o t t o m o f t h e c o r e t o g i v e a s m o o t h b a s e . The s h e e t m e t a l s l e e v e was l o w e r e d t o f i t e v e n l y a r o u n d t h e s o i l c o r e . A d r i l l b i t was u s e d t o make h o l e s i n t h e s o i l c o l u m n t o r e c e i v e t h e t e n s i o m e t e r s . T e n s i o m e t e r s w e r e t h e n f i t t e d i n t h e s a m p l e . A gypsum p a s t e was p o u r e d o n t o p o f t h e s a n d i n o r d e r t o p r e v e n t t h e c o n c r e t e f r o m s e e p i n g o u t . C o n c r e t e was t h e n p o u r e d a n d c o n t i n u o u s l y v i b r a t e d w i t h a r o d u n t i l t h e s p a c e 14 b e t w e e n t h e s o i l c o r e a n d t h e s h e e t m e t a l c a s i n g was f i l l e d . A r i m o f 6 cm d e e p a n d 27 cm i n d i a m e t e r s u p p o r t e d b y s h e e t m e t a l s c r e w s was p u s h e d a b o u t 2 cm d e e p i n t o t h e w e t c o n c r e t e a t t h e s u r f a c e t o c r e a t e s p a c e f o r p o n d i n g o f w a t e r . The c o n c r e t e was l e f t t o s e t i n t h e f i e l d , w i t h t h e s o i l c o r e a t t a c h e d t o t h e g r o u n d . A f t e r two d a y s , t h e c o r e was d e t a c h e d f r o m t h e b a s e , a n d t h e c o l u m n was r e m o v e d u s i n g a t r i p o d a n d t a c k l e a n d t r a n s p o r t e d t o t h e l a b o r a t o r y . To c o n s e r v e t h e c o l u m n s b e t w e e n t h e e x p e r i m e n t s , t h e y w e r e k e p t o u t s i d e i n p r e - d u g h o l e s w i t h t h e i r s u r f a c e e v e n w i t h t h e g r o u n d s u r f a c e a n d t h e i r b o t t o m k e p t m o i s t a t a l l t i m e s . 1.3.3 L a b o r a t o r y P r o c e d u r e I n t h e f i r s t e x p e r i m e n t , t o d e t e r m i n e t h e i n i t i a l s o i l c o n d i t i o n , t h e h y d r a u l i c c o n d u c t i v i t i e s o f v a r i o u s s o i l l a y e r s w e r e m e a s u r e d f o r t h e t h r e e c o l u m n s o f s i t e 1 ( A , B , a n d C ) , a s t h e y w e r e b r o u g h t i n f r o m t h e f i e l d . The h y d r a u l i c c o n d u c t i v i t y o f s e a l s was a ssumed t o be s a t u r a t e d due t o t h e l a r g e n e g a t i v e a i r e n t r y p r e s s u r e h e a d s , w h i l e t h o s e o f o t h e r l a y e r s w e r e u n s a t u r a t e d . The u p p e r b o u n d a r y f o r t h e c o l u m n s A a n d B h a d n o t c h a n g e d f r o m t h e f i e l d c o n d i t i o n , w h i l e t h e s u r f a c e o f c o l u m n C h a d d e v e l o p e d t e n s i o n c r a c k s . To c o n t r o l t h e l o w e r b o u n d a r y c o n d i t i o n , t h e c o l u m n s w e r e p l a c e d o n a c y l i n d r i c a l p a i l f i l l e d w i t h t e n s i o n - s a t u r a t e d medium s a n d , e n s u r i n g a p e r f e c t h y d r a u l i c c o n t a c t ( F i g . 2 ) . The s a n d s u r f a c e was k e p t w i t h i n t h e t e n s i o n s a t u r a t e d z o n e a t a l l 15 u Q) -P Q) E O •H CO c v /r c u l t . l a y e r comp . pan sub-- s o i l t e n s i o n s a t u r a t e d med. sand co n s t a n t head u n i t s i n k thermostat V (cm ) t (s) FIG. 2 SCHEMATIC DRAWING OF FLOW MEASUREMENT SYSTEM 16 t i m e s b y means o f a n o u t f l o w u n i t . A t h e r m o s t a t was u s e d t o k e e p t h e t e m p e r a t u r e c o n s t a n t a t 18°C. To c a r r y o u t f l o w m e a s u r e m e n t s , a c o n s t a n t h e a d was m a i n t a i n e d a t t h e s o i l s u r f a c e a n d t h e v o l u m e f l o w r a t e was d i r e c t l y - m e a s u r e d f r o m t h e o u t f l o w u n i t a t s t e a d y s t a t e . The g r a d i e n t was c a l c u l a t e d f r o m t h e t e n s i o m e t e r r e a d i n g s , e n a b l i n g c a l c u l a t i o n o f t h e h y d r a u l i c c o n d u c t i v i t y p r o f i l e b y u s i n g D a r c y ' s l a w . F o r t h e l a t t e r c a l c u l a t i o n i t was assumed t h a t a l l s e a l s w e r e 5 mm t h i c k , a n d t h a t t h e p r e s s u r e h e a d d i s t r i b u t i o n w i t h i n e a c h l a y e r was l i n e a r . A t t h i s s t a g e a n e f f e c t i v e h y d r a u l i c c o n d u c t i v i t y f o r c o l u m n D was a l s o d e t e r m i n e d u s i n g t h e same p r o c e d u r e . Due t o t e c h n i c a l p r o b l e m s w i t h t h e t e n s i o m e t e r s a h y d r a u l i c c o n d u c t i v i t y p r o f i l e c o u l d n o t be d e t e r m i n e d f o r t h i s c o l u m n . I n t h e s e c o n d e x p e r i m e n t a s u r f a c e s e a l was g e n e r a t e d i n t h e l a b o r a t o r y b y a p p l y i n g s i m u l a t e d r a i n f a l l w i t h c l o s e o b s e r v a t i o n o f t h e s e a l i n g p r o c e s s a n d c o n s t a n t m o n i t o r i n g o f t h e c h a n g e s i n t h e h y d r a u l i c c o n d u c t i v i t y o f d i f f e r e n t s o i l l a y e r s . The p r o c e d u r e was a s f o l l o w s : The c u l t i v a t i o n l a y e r o f c o l u m n s A a n d D w e r e l e f t t o d r y t o b e l o w t h e p l a s t i c l i m i t , a c o n d i t i o n n e c e s s a r y f o r c u l t i v a t i o n . The e f f e c t s o f d i s c i n g a n d p l o w i n g w e r e s i m u l a t e d i n t h e l a b o r a t o r y t o a d e p t h o f 10 cm b y u s e o f a k n i f e a n d a s p a d e . The s o i l was c u t w i t h a k n i f e p e r p e n d i c u l a r t o t h e s u r f a c e , a n d t h e n t u r n e d o v e r b y t h e s p a d e . N i n e s e t s o f h i g h i n t e n s i t y (120 mm h r " l ) a n d s h o r t d u r a t i o n (2 t o 5 m i n u t e s ) s i m u l a t e d r a i n f a l l s , e a c h s e t c o n t a i n i n g f i v e e v e n t s w e r e a p p l i e d t o t h e s u r f a c e . E a c h t i m e r a i n f a l l was a p p l i e d u n t i l a p o n d was f o r m e d . The p o n d was k e p t a t a c o n s t a n t d e p t h u n t i l s t e a d y s t a t e was r e a c h e d a n d t h e t e n s i o m e t e r r e a d i n g s w e r e r e c o r d e d . To p r o d u c e r a i n f a l l , a l a b o r a t o r y r a i n m a ker u n i t was u s e d w i t h h y p o d e r m i c n e e d l e s o f g a u g e 21 p l a c e d a t 2 cm b y 2 cm g r i d s p a c i n g . R a i n f a l l i n t e n s i t y was c o n t r o l l e d b y a m o v a b l e c o n s t a n t h e a d u n i t . S i m u l a t e d r a i n f a l l was a p p l i e d f r o m a d i s t a n c e o f 30 cm a b o v e t h e s o i l s u r f a c e . I n t h e t h i r d e x p e r i m e n t t h e e f f e c t o f f o r m a t i o n o f a s u c t i o n f o r c e a t t h e b o t t o m o f t h e s u r f a c e l a y e r u p o n s u r f a c e s e a l i n g was e x a m i n e d . The w a t e r t a b l e was g r a d u a l l y r a i s e d f r o m b e l o w t o t h e t o p o f a f r e s h l y c u l t i v a t e d c o l u m n B a n d m a i n t a i n e d t h e r e . R a i n f a l l was a p p l i e d a s b e f o r e . A f l o o d e d c o n d i t i o n q u i c k l y d e v e l o p e d . U s i n g an i n f l o w u n i t a c o n s t a n t 3.5 cm d e p t h o f p o n d was m a i n t a i n e d a n d t h e o u t f l o w u n i t was s l o w l y l o w e r e d t o t h e b o t t o m o f t h e c o l u m n t o c r e a t e a g r a d i e n t . The p u r p o s e o f t h i s p r o c e d u r e was t o a v o i d t h e f o r m a t i o n o f a l a r g e s u c t i o n f o r c e a t t h e s u r f a c e . H y d r a u l i c c o n d u c t i v i t y o f d i f f e r e n t l a y e r s was c a l c u l a t e d a s b e f o r e . F i v e r a i n f a l l e v e n t s w e r e a p p l i e d , e a c h t i m e a l l o w i n g t h e f r e e w a t e r t o j u s t w i t h d r a w b e l o w t h e s u r f a c e b e t w e e n t h e r a i n f a l l e v e n t s . T h e r e f o r e , t h e c u l t i v a t i o n l a y e r was k e p t s a t u r a t e d a t a l l t i m e s . A f t e r t h e f i f t h r a i n f a l l e v e n t t h e f r e e w a t e r was a l l o w e d t o w i t h d r a w b e l o w t h e s u r f a c e u n t i l t h e t e n s i o m e t e r s i n t h e c u l t i v a t i o n l a y e r i n d i c a t e d n e g a t i v e r e a d i n g s , a n o t h e r r a i n f a l l e v e n t was a p p l i e d a n d h y d r a u l i c 18 c o n d u c t i v i t i e s c a l c u l a t e d a g a i n . The e f f e c t o f a c o v e r c r o p o n t h e f o r m a t i o n o f t h e s u r f a c e s e a l was s t u d i e d i n t h e f o u r t h e x p e r i m e n t . C o l u m n C was c u l t i v a t e d a n d p l a n t e d t o w h e a t . When t h e w h e a t was 5 cm l o n g , s i m u l a t e d r a i n f a l l was a p p l i e d a n d h y d r a u l i c c o n d u c t i v i t y o f d i f f e r e n t l a y e r s c a l c u l a t e d a s b e f o r e . I n t h e f i r s t e x p e r i m e n t a h i g h r e s i s t a n c e was d i s c o v e r e d t o e x i s t o n t o p o f t h e c o m p a c t e d p a n . D u r i n g t h e c o u r s e o f t h e s e c o n d e x p e r i m e n t a h i g h r e s i s t a n c e d e v e l o p e d a t t h e c u l t i v a t i o n l a y e r - c o m p a c t e d p a n b o u n d a r y , s i m u l t a n e o u s t o t h e f o r m a t i o n o f a s u r f a c e s e a l . To i n v e s t i g a t e t h i s e f f e c t i n more d e t a i l , a s o i l c o l u m n was made up i n t h e l a b o r a t o r y i n a 20x15 cm a c r y l i c p l a s t i c b o x . T h i s c o l u m n c o n s i s t e d o f a p o r o u s p l a t e made o f c a r b o r u n d u m a t t h e b o t t o m , f o r t h e p u r p o s e o f c o n t r o l l i n g t h e l o w e r b o u n d a r y c o n d i t i o n . On t o p o f t h e p o r o u s p l a t e a c o m p a c t e d p a n w i t h a b u l k d e n s i t y o f 1260 k g m~ 3 a n d a d e p t h o f 10 cm was a r t i f i c i a l l y f o r m e d u s i n g a i r d r y p a n m a t e r i a l w h i c h h a d b e e n p a s s e d t h r o u g h a 0.5 mm s i e v e . S o i l r e m o v e d f r o m t h e c u l t i v a t i o n l a y e r o f s i t e 1 was d i r e c t l y p l a c e d o n t o p o f t h e p a n a n d t h e c o l u m n g e n t l y v i b r a t e d a t t h e s i d e s u n t i l a d e p t h o f 10 cm was r e a c h e d . The b u l k d e n s i t y o f t h e c u l t i v a t i o n l a y e r was c a l c u l a t e d t o be 1100 k g m - 3. F o u r t e n s i o m e t e r s w e r e i n s t a l l e d , two w i t h i n t h e p a n ( a t 1 a n d 5 cm b e l o w t h e p a n s u r f a c e ) , a n d two w i t h i n t h e c u l t i v a t i o n l a y e r ( a t 1 a n d 5 cm a b o v e t h e s u r f a c e o f t h e p a n ) . S i m u l a t e d r a i n f a l l e v e n t s o f d i f f e r e n t i n t e n s i t i e s ( 5 5 , 60, 1 3 9 , 1 7 3 , 175 mm h r - 1 ) w e r e a p p l i e d a t t h e s u r f a c e , e a c h 19 t i m e u n t i l a p o n d was f o r m e d . The p o n d was k e p t a t a c o n s t a n t d e p t h o f 2 cm a n d a f t e r s t e a d y s t a t e h a d b e e n r e a c h e d t h e f l o w r a t e was m e a s u r e d a n d t h e p r e s s u r e h e a d s w e r e r e c o r d e d . U s i n g D a r c y ' s e q u a t i o n , r e s i s t a n c e was c a l c u l a t e d b e t w e e n t h e two t e n s i o m e t e r s 1 cm a b o v e a n d 1 cm b e l o w t h e p a n s u r f a c e a s a f u n c t i o n o f r a i n f a l l e v e n t s . 1.3.4 P h y s i c a l P r o p e r t i e s o f t h e S o i l i . D e t e r m i n a t i o n o f a g g r e g a t e s t a b i l i t y A w e t - s i e v i n g m e t h o d was u s e d t o d e t e r m i n e t h e a g g r e g a t e s t a b i l i t y o f t h e s o i l i n e a c h c o l u m n a f t e r t h e f i r s t e x p e r i m e n t . T h i s m e t h o d i s d e s c r i b e d b y Kemper ( 1 9 6 5 ) . The w e t - s i e v i n g e q u i p m e n t c o n s i s t e d o f a m o t o r - d r i v e n m e c h a n i c a l d e v i c e t h a t w o u l d r a i s e a n d l o w e r t h e s i e v e h o l d e r t h r o u g h a d i s t a n c e o f 2.5 cm, a n d a t a f r e q u e n c y o f 30 s t r o k e s p e r m i n u t e . The m o t i o n o f t h e s y s t e m h a s b o t h a n u p w a r d s t r o k e a n d a n o s c i l l a t i n g a c t i o n t h r o u g h a n a n g l e o f 30°. A s p e c i a l s i e v e h o l d e r c a p a b l e o f r e c e i v i n g 12 " s e p a r a t e s i e v e s i n e a c h d e t e r m i n a t i o n was u s e d . The s i e v e s h a d a n i n s i d e d i a m e t e r o f 7.5 cm a n d 0.25 mm mesh o p e n i n g s . 4 g o f a i r d r y a g g r e g a t e s ( 1 - 2 mm) p l a c e d o n t h e s i e v e s w e r e p r e - w e t t e d w i t h a n a t o m i z e r s p r a y ( B a v e r e t a l . 1 9 7 2 ) . The s a m p l e s w e r e w e t - s i e v e d w i t h t h e w h o l e s e t o f s i e v e s c o m p l e t e l y i m m e r s e d i n a b a s i n o f w a t e r f o r 10 m i n u t e s . The a g g r e g a t e s r e t a i n e d o n t h e s i e v e a f t e r w e t - s i e v i n g w e r e o v e n - 20 d r i e d a t 105°C f o r 24 h o u r s a n d w e i g h e d . The a g g r e g a t e s t a b i l i t y was e x p r e s s e d a s t h e r a t i o o f t h e o v e n - d r y mass o f t h e s t a b l e a g g r e g a t e s a f t e r w e t - s i e v i n g t o t h e o v e n - d r y mass o f a 4 g s u b - s a m p l e . i i . D e t e r m i n a t i o n o f O r g a n i c M a t t e r C o n t e n t The o r g a n i c m a t t e r c o n t e n t was d e t e r m i n e d b y i g n i t i n g 2 g o f o v e n - d r y a g g r e g a t e s ( 0 . 5 - 2 mm) i n a m u f f l e f u r n a c e a t a t e m p e r a t u r e o f 400°C f o r 8 h o u r s . The l o s s - o n - i g n i t i o n was t a k e n a s a m e a s u r e o f t h e o r g a n i c m a t t e r c o n t e n t . The m a j o r d r a w b a c k o f u s i n g l o s s - o n - i g n i t i o n a s an e s t i m a t e o f o r g a n i c m a t t e r o f a n o n c a l c a r e o u s s o i l i s t h e e r r o r a s s o c i a t e d w i t h l o s s o f c l a y m i n e r a l w a t e r ( B a l l 1 9 6 4 ) . T h i s l o s s o f a d h e r e d w a t e r i s i m p o r t a n t i n t h e t e m p e r a t u r e r a n g e o f 450-600°C ( B a l l 1 9 6 4 ) . T h u s , p r o v i d e d t h e t e m p e r a t u r e i s k e p t b e l o w 450°C, t h e l o s s - o n - i g n i t i o n m e t h o d i s s u f f i c i e n t l y a c c u r a t e f o r e s t i m a t i n g o r g a n i c m a t t e r o f a n o n c a l c a r e o u s s o i l ( B a l l 1 9 6 4 ) . The o r g a n i c m a t t e r c o n t e n t was e x p r e s s e d a s t h e r a t i o o f t h e mass l o s t u p o n i g n i t i o n t o t h e mass o f t h e s o l i d s . 1.4 RESULTS AND DISCUSSION E f f e c t i v e h y d r a u l i c c o n d u c t i v i t y m e a s u r e d i n t h e l a b o r a t o r y f o r c o l u m n D was 2 . 8 x l 0 ~ 4 m s--*-. T h i s d a t u m a g r e e s w i t h t h e v a l u e m e a s u r e d b y de. V r i e s i n 1983 ( p e r s o n a l 21 c o m m u n i c a t i o n 1988) ( T a b l e 1 ) . T h i s a g r e e m e n t shows t h e s t a b i l i t y o f t h e s o i l i n s i t e 2. F i g . 3 shows t h e f i e l d w a t e r c o n t e n t d i s t r i b u t i o n a t t h e t i m e o f s a m p l i n g a t t h e l o c a t i o n w h e r e c o l u m n A was r e m o v e d . W i t h t h e e x c e p t i o n o f t h e s u r f a c e s e a l t h e r e s t o f t h e p r o f i l e i s u n s a t u r a t e d . I t i s a ssumed t h a t u n d e r a p o n d t h e s e a l r e m a i n s s a t u r a t e d due t o a l a r g e n e g a t i v e a i r e n t r y v a l u e . F i g . 3 i s a g r a p h i c a l d e m o n s t r a t i o n o f a h y d r o l o g i c e v e n t r e f e r r e d t o a s " p o n d i n g " , w h e r e f r e e w a t e r i s p r e s e n t a t t h e s u r f a c e w h i l e t h e s o i l b e l o w t h e s u r f a c e i s u n s a t u r a t e d down t o t h e w a t e r t a b l e . T h i s phenomenon i s v e r y i n t e r e s t i n g i n t h a t , t h e s o i l c a n r e m a i n u n s a t u r a t e d w h i l e s i t t i n g u n d e r 10 t o 15 c e n t i m e t e r s o f p o n d f o r a number o f m o n t h s . F i g . 4 shows t h e p r e s s u r e h e a d d i s t r i b u t i o n f o r c o l u m n A i n t h e f i r s t e x p e r i m e n t . T h i s i s a t y p i c a l p r e s s u r e h e a d d i s t r i b u t i o n r e c o r d e d f r o m t h e t h r e e s e t s o f t e n s i o m e t e r s i n s t a l l e d i n a l l c o l u m n s . The w i d e r a n g e o f v a l u e s shows t h e c o m p l e x i t y o f f l o w p a t h s i n a s t r u c t u r e d s o i l . T h i s i n d i c a t e s t h a t c a u t i o n s h o u l d be e x e r c i s e d when u s i n g t e n s i o m e t r y u n d e r a n a t u r a l s e t t i n g . I f a s o i l c o n t a i n s p r e f e r r e d f l o w p a t h s s u c h a s worm h o l e s a n d r o o t c h a n n e l s , t h e n d e p e n d i n g on w h e t h e r o r n o t a t e n s i o m e t e r i n t e r c e p t s one o f t h e s e c h a n n e l s , a l a r g e o r s m a l l p r e s s u r e h e a d r e a d i n g w o u l d be r e c o r d e d . T h e r e h a v e b e e n i n s t a n c e s r e c o r d e d w h e r e t h e a v e r a g e t o t a l p r e s s u r e p r o f i l e i n d i c a t e d " c o u n t e r - f l o w " g r a d i e n t . T h i s p o i n t r a i s e s a q u e s t i o n a s t o t h e d e g r e e o f a c c u r a c y p r o v i d e d w i t h t h e t h r e e s e t s o f t e n s i o m e t e r s . I t s h o u l d h o w e v e r be 22 0.00 -10.00 surf. seal r cult. layer / / E -20.00 •>—' X r-Q_ UJ Q -30.00 -40.00 H comp. pan sub-soil —50.00 "| i i i i i i i i i | i i i i i i i i i | i i i i V 0.20 0.30 0.40 i i i I i i i i i i i i i i 0.50, 0.60 WATER CONTENT (cm/cm) v 3 FIG. 3 FIELD MEASUREMENT OF WATER CONTENT VS. DEPTH FOR A SOIL PROFILE UNDER A PONDED DEPRESSION. SATURATION IS INDICATED BY SOLID VERTICAL LINES. 23 0.00 n -10.00 - -20.00 - Q. UJ Q - 3 0 . 00 - -40.00 - - 5 0 . 00 - 4 0 i i i i i i i 1 1 1 1 11 i i i i i 11 i i i i i 11 i i 11 i i i i i i i i ,00 -30.00 -20.00 -10.00 0.00 PRESSURE HEAD (cm) 11 i i i i i i i | 10.00 FIG. 4 PRESSURE HEAD VS. DEPTH FOR THE THREE SETS OF TENSIOMETERS IN COLUMN *A'. SOLID HORIZONTAL • LINES SHOW THE LOCATION OF TENSIOMETERS. SOLID CURVE SHOWS THE AVERAGE VALUE OF PRESSURE HEAD. kept i n mind that although the o v e r a l l d i r e c t i o n of an unsaturated flow may be downward, t h i s does not exclude the p o s s i b i l i t y of an upward flow at some l o c a t i o n s w i t h i n the s o i l p r o f i l e . F i g . 5 shows the curve of t o t a l head versus depth f o r column A as i t was brought i n from the f i e l d . A l s o shown are h y d r a u l i c c o n d u c t i v i t i e s f o r the d i f f e r e n t l a y e r s . The t o t a l head values are an average of the three tensiometers at each depth ( F i g . 4). Values f o r column B i n the f i r s t experiment (not shown) are s i m i l a r to those of column A. F i g . 5 shows Ladner s o i l at i t s worst c o n d i t i o n . Since the gradient through the surface s e a l i s equal to 15 a simple c a l c u l a t i o n shows th a t i t would take almost 40 days f o r a 5 cm pond to recede i f there were no evaporation t a k i n g p l a c e . Surface s e a l i n g , t h e r e f o r e , can be regarded as the main cause of ponding. F i g . 6 shows the t o t a l head as a f u n c t i o n of depth i n experiment 1 f o r column C which had developed t e n s i o n cracks at the surface on a r r i v a l at the l a b . This process a l s o takes place i n the f i e l d upon d r y i n g . A s i g n i f i c a n t i n c r e a s e i n the h y d r a u l i c c o n d u c t i v i t y of the surface 5 mm can be a t t r i b u t e d t o the d i s r u p t i o n of the surface s e a l . A l l three columns from the c u l t i v a t e d f i e l d showed evidence of a high r e s i s t a n c e at the bottom of the c u l t i v a t i o n l a y e r . F i g . 7 shows the r e s u l t of regenerating the surface s e a l a f t e r c u l t i v a t i o n i n experiment 2. The h y d r a u l i c c o n d u c t i v i t y of d i f f e r e n t s o i l l a y e r s i n column A and the e f f e c t i v e 0.00 10.00 20.00 - 30.00 - 40.00 - 50.00 surface seal K = 9 . 7 x l O - 1 0 (m s _ 1 ) /— cu l t , layer K = 1.2x10" / i n t . seal K = l . O x l O ~ 9 / / L comp. pan K = l . l x l O " 8 I— / / sub-soil K = 1.0x10"6 / / / / T — \ — i — i — i — i — i — i — i — | — i — i — \ — i — i — i — i — i — i — \ — i — i — i — r -100.00 -60.00 -20.00 TOTAL HEAD (cm) i ' | l 20.00 FIG. 5 TOTAL HEAD VS. DEPTH FOR COLUMN 'A'. SHORT HORIZONTAL LINES SHOW THE LOCATION OF TENSIOMETERS. 0.00 •10.00 s u r f a c e 5 mm K : i . i x i O - 7 (m s - 1 ) c u l t , l a y e r K = 9. 3x10' 4- 20.00 - 30.00 - 40.00 - 50.00 i n t . s e a l K = 3 . 3 x i O - 6 / comp. p a n K = 8 . 5 x l O ~ 7 / * / / / / : : 1 ;—— / s u b - s o i l Krl.9X10" 6 / I I "I I I I I I I I I I I I I I I I I I I 1 I I I I I I I I I 'I I I I I I I I I I I I I I I I I I -80.00 -60.00 -40.00 -20.00 0.00 20.00 TOTAL HEAD (cm) FIG. 6 TOTAL HEAD VS. DEPTH FOR COLUMN «C SHORT HORIZONTAL LINES SHOW THE LOCATION OF TENSIOMETERS. 27 10 -*? 1 0 ~H 1 1 1 1 r 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 10 SETS OF RAINFALL EVENT FIG.7 HYDRAULIC CONDUCTIVITY REDUCTION AS A FUNCTION OF RAINFALL EVENTS FOR DIFFERENT LAYERS OF COLUMN 'A'. ZERO EVENT CORRESPONDS TO A FRESHLY CULTIVATED SOIL SATURATED FROM BELOW. 28 h y d r a u l i c c o n d u c t i v i t y o f c o l u m n D a r e p l o t t e d a s a f u n c t i o n o f r a i n f a l l e v e n t s . An i n t e r e s t i n g o b s e r v a t i o n i n F i g . 7 i s t h e p r o x i m i t y o f t h e f l u x t o t h e u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s o f t h e c u l t i v a t i o n l a y e r a n d t h e c o m p a c t e d p a n . T h i s shows t h a t t h e u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y o f t h e s o i l b e l o w t h e s e a l a d j u s t s i t s e l f t o t h e f l o w r a t e t h r o u g h t h e s e a l ( H i l l e l a n d G a r d n e r 1 9 6 9 ) . C o m p a r i s o n o f F i g s . 5 a n d 7 show t h a t t h e f i n a l h y d r a u l i c c o n d u c t i v i t y o f d i f f e r e n t l a y e r s i n c o l u m n A a r e a p p r o x i m a t e l y e q u a l t o t h e v a l u e s m e a s u r e d i n e x p e r i m e n t 1 w h i l e t h e c o l u m n was i n i t s o r i g i n a l s t a t e , d e s p i t e t h e h i g h i n t e n s i t y r a i n f a l l a p p l i e d i n t h e l a b o r a t o r y . Many i n v e s t i g a t o r s ( e . g . , M c l n t y r e 1 9 5 8 , F a l a y i a n d Bouma 1975) who c o m p a r e d h y d r o l o g i c p r o p e r t i e s o f s u r f a c e s e a l s f o r m e d u n d e r h i g h i n t e n s i t y s i m u l a t e d r a i n f a l l , w i t h t h o s e f o r m e d n a t u r a l l y i n t h e f i e l d , c o n c l u d e d c l o s e s i m i l a r i t i e s b e t w e e n t h e t w o . B e r t r a n d a n d S o r ( 1 9 6 2 ) , h o w e v e r , s t u d i e d t h e e f f e c t s o f d i f f e r e n t r a i n f a l l e n e r g i e s o n s o i l p e r m e a b i l i t y . T h e y c o n c l u d e d t h a t t h e e f f e c t s o f t h e 70 a n d 100 mm h r ~ l r a i n f a l l i n t e n s i t i e s w e r e o f t h e same m a g n i t u d e , w h i l e t h e e f f e c t o f a 40 mm h r - ^ - r a i n f a l l was l e s s . I n g e n e r a l , i t c a n be c o n c l u d e d t h a t b e y o n d a c e r t a i n r a i n f a l l i n t e n s i t y t h r e s h o l d a n y i n c r e a s e i n t h e i n t e n s i t y o f t h e r a i n f a l l w i l l n o t c h a n g e t h e h y d r a u l i c c o n d u c t i v i t y o f t h e s u r f a c e l a y e r . T h i s t h r e s h o l d i n t e n s i t y i s u n d o u b t e d l y a f u n c t i o n o f t h e a g g r e g a t e s t a b i l i t y o f t h e s o i l s u r f a c e a n d t h e k i n e t i c e n e r g y o f t h e r a i n f a l l . K i n e t i c e n e r g y o f t h e s i m u l a t e d r a i n f a l l a p p l i e d i n t h e s e c o n d e x p e r i m e n t was c a l c u l a t e d f o r one d r o p u s i n g : K.E = 1/2 m v 2 (1) w h e r e : K.E = k i n e t i c e n e r g y ( j ) , m = mass o f one d r o p = 10" 4' ( k g ) , a n d v = v e l o c i t y (m s ~ l ) . The v e l o c i t y o f a 3 mm d r o p f a l l i n g a d i s t a n c e o f 0.3 m i s g i v e n b y Laws ( 1 9 4 1 ) t o be e q u a l t o 2.3 m s ~ l . The e q u i v a l e n t i n t e n s i t y o f a n a t u r a l r a i n f a l l was c a l c u l a t e d f r o m (Laws a n d P a r s o n 1 9 4 3 ) : K.E = 11.89 + 8.74 l o g I (2) w h e r e : K.E = k i n e t i c e n e r g y ( j mm - 1 m ~ 2 ) , I = i n t e n s i t y ( mm h ~ l ) , w i t h t h e v a l u e o f k i n e t i c e n e r g y c a l c u l a t e d f r o m ( 1 ) . I t was f o u n d t h a t t h e s i m u l a t e d r a i n f a l l a p p l i e d i n t h e l a b o r a t o r y was e q u i v a l e n t i n t e r m s o f i t s e n e r g y i n p u t , t o a n a t u r a l r a i n f a l l i n t e n s i t y o f 27 mm d a y ~ l w h i c h i s a common r a i n s t o r m f o r t h e L F V a r e a . A c o m p a r i s o n o f t h e . r e d u c t i o n i n t h e h y d r a u l i c c o n d u c t i v i t i e s o f c o l u m n s A a n d D i n F i g . 7 shows t h e i m p o r t a n c e o f a g g r e g a t e s t a b i l i t y i n t h e f o r m a t i o n o f t h e s u r f a c e s e a l . The e f f e c t i v e h y d r a u l i c c o n d u c t i v i t y o f t h e s o i l f r o m s i t e 2 [ a g g . s t a b i l i t y = (92 ± 2 . 5 ) % ] , a f t e r l a b o r a t o r y c u l t i v a t i o n a n d a p p l i c a t i o n o f t h e same number o f r a i n f a l l e v e n t s a s f o r c o l u m n A [ a g g . s t a b i l i t y = (47 + 4 ) % ] , d e c r e a s e d o n l y f r o m 7.0 x 1 0 " ^ t o 1.5 x 1 0 ~ 5 m s ~ l , w i t h o u t 30 a n y s i g n o f a s u r f a c e s e a l b e i n g p r e s e n t a t t h e e n d o f t h e e x p e r i m e n t . 1.4.1 E f f e c t o f D r y i n g a n d C o v e r C r o p on S e a l F o r m a t i o n F i g u r e s 8a a n d b show t h e r e s u l t o f d r y i n g o n s u r f a c e s e a l f o r m a t i o n . I n F i g . 8a d r y i n g was p r e v e n t e d b y k e e p i n g t h e c u l t i v a t i o n l a y e r s a t u r a t e d a t a l l t i m e s . A t t h e e n d o f t h e e x p e r i m e n t t h e s u r f a c e , w h i l e h a v i n g t h e a p p e a r a n c e o f a s e a l , was e x t r e m e l y i n e f f e c t i v e i n r e d u c i n g t h e f l o w r a t e . The r e s i s t a n c e o f t h e s u r f a c e l a y e r t o f l o w was n e a r z e r o , w h i l e a n i n t e r n a l s e a l h a d f o r m e d a s b e f o r e . F i g . 8b shows t h e r e s u l t o f d r y i n g . The s u r f a c e 5 mm a f t e r d r y i n g h a d a h y d r a u l i c c o n d u c t i v i t y i n t h e o r d e r o f 1 0 " y m s " ^ . The i m p o r t a n c e o f d r y i n g o n t h e f o r m a t i o n a n d e f f i c i e n c y o f t h e s u r f a c e s e a l i s i n t h e c r e a t i o n o f a s u c t i o n f o r c e a t t h e b o t t o m o f t h e s u r f a c e l a y e r . As p o i n t e d o u t b y M o r i n e t a l . ( 1 9 8 1 ) , a n d T a c k e t t a n d P e a r s o n ( 1 9 6 5 ) , c r e a t i o n o f t h i s s u c t i o n f o r c e c a u s e s o r i e n t a t i o n o f t h e c l a y p a r t i c l e s i n t o a c o n t i n u o u s a n d d e n s e l a y e r . The e f f e c t o f a c o v e r c r o p o n s u r f a c e s e a l i n g was s t u d i e d i n t h e f o u r t h e x p e r i m e n t . I n i t i a l l y , t h e s u r f a c e 5 mm h a d a h y d r a u l i c c o n d u c t i v i t y o f 3 . 1 x 1 0 " ^ m s " 1 . A t t h e e n d o f t h e e x p e r i m e n t , t h i s v a l u e was r e d u c e d o n l y b y a f a c t o r o f two t o 1 . 6 x l 0 ~ 6 m s--'-, a l s o , w i t h no s i g n o f a n i n t e r n a l s e a l b e i n g p r e s e n t . I t s h o u l d be m e n t i o n e d t h a t t h e w h e a t p r o v i d e d f u l l c o v e r a g e o f t h e s u r f a c e . I t w o u l d be o f m a j o r i n t e r e s t , 31 0.00 -10.00 E -20.00 - UJ o -30.00 -40.00 - -50.00 - 7 0 surface 5 mm cult, layer Int. seal comp. pan sub-soil K = l . 3 x l 0 - 9 (m s _ 1 ) / K=1.8xl0-fi ' / / / / * : r=4.6xl0" 6 1 i i i i i i i i l i i i i i i i i i i | i i i i i i i i ; | i i i i I i i i i | 00 -50.00 -30.00 -10.00 10.00 TOTAL HEAD (cm) FIG. 8o TOTAL HEAD VS. DEPTH FOR COLUMN 'B* WHEN CULTIVATION LAYER WAS KEPT SATURATED. THE GRADIENT FOR THE SURFACE 5 mm AND THE CULT. LAYER WAS TOO SMALL TO ALLOW K TO BE MEASURED. 32 0.00 -10.00 - - i n t . s e a l £ —20.00 - j comp . p a n o UJ Q -30.00 -40.00 - -50.00 s u r f a c e s e a l K = 1 . 6 x i O - 9 (m s _ 1 ) / /— / / _ )' c u l t , l a y e r K = l . E x l O - 8 K : 1 . 6 X l O - 9 / / /* K = l . l X l O - 6 / / / <: H s u b - s o i l K = 1 . 9 x l O - 8 / / / / / i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | -70.00 -50.00 -30.00 -10.00 10.00 TOTAL HEAD (cm) FIG. 8b TOTAL HEAD VS. DEPTH FOR COLUMN 'B' AFTER FORMATION OF A TENSION FORCE AT THE SURFACE. h o w e v e r , t o d e t e r m i n e t h e minimum r e q u i r e d c o v e r a g e n e c e s s a r y f o r p r e v e n t i n g t h e i n f i l t r a t i o n r a t e t o d e c r e a s e b e l o w a c e r t a i n a c c e p t a b l e v a l u e . 1.4.2 I n t e r n a l S e a l C a l c u l a t i n g t h e h y d r a u l i c c o n d u c t i v i t y p r o f i l e f o r t h e s o i l c o l u m n s t a k e n f r o m t h e p o n d e d d e p r e s s i o n p r i o r t o t h e l a b o r a t o r y c u l t i v a t i o n , r e v e a l e d a n a n o m a l y w h i c h was l a t e r a t t r i b u t e d t o t h e e x i s t e n c e o f a n i n t e r n a l s e a l o n t o p o f t h e c o m p a c t e d p a n . T h i s a n o m a l y was t h a t t h e h y d r a u l i c c o n d u c t i v i t y o f t h e l a y e r b e t w e e n t h e two t e n s i o m e t e r s i n s t a l l e d b e l o w a n d a b o v e t h e s u r f a c e o f t h e p a n was s m a l l e r t h a n t h e h y d r a u l i c c o n d u c t i v i t y o f t h e p a n i t s e l f . F i g . 9a shows t h e e f f e c t i v e r e s i s t a n c e b e t w e e n t h e t e n s i o m e t e r s i n s t a l l e d a t one c e n t i m e t e r b e l o w a n d a b o v e t h e p a n s u r f a c e i n t h e f i f t h e x p e r i m e n t . T h i s r e s i s t a n c e i n c r e a s e d w i t h e v e r y r a i n f a l l a p p l i c a t i o n . An i n t e r e s t i n g o b s e r v a t i o n i s t h a t t h e i n c r e a s e i n r e s i s t a n c e i s r o u g h l y p r o p o r t i o n a l t o t h e r a i n f a l l i n t e n s i t y . S i n c e r e s i s t a n c e i s a d d i t i v e , a f t e r s u b t r a c t i o n o f t h e r e s i s t a n c e s o f f e r e d b y t h e c u l t i v a t i o n l a y e r a n d t h e c o m p a c t e d p a n , t h e r e s i s t a n c e due t o t h e i n t e r n a l s e a l a l o n e was c a l c u l a t e d a n d i s a l s o shown i n F i g . 9 a . The t h i c k n e s s o f t h e i n t e r n a l s e a l was m e a s u r e d w i t h a r u l e r t o v a r y f r o m 2 t o 5 mm. B a s e d on t h e s e two f i g u r e s t h e h y d r a u l i c c o n d u c t i v i t y o f t h e i n t e r n a l s e a l was c a l c u l a t e d a s shown i n F i g . 9b. The l a s t r a i n f a l l e v e n t was a p p l i e d 34 FIG. 9a RESISTANCE VS. RAINFALL FOR THE LABORATORY COLUMN. NUMBERS IN THE BRACKETS SHOW THE RAINFALL INTENSITY. 35 10 - 7n RAINFALL EVENT FIG. 9b HYDRAULIC CONDUCTIVITY VS. RAINFALL EVENT FOR AN INTERNAL SEAL FORMED IN LABORATORY COLUMN FOR TWO ASSUMED THICKNESSES OF THE SEAL 36 a f t e r a prolonged d r y i n g p e r i o d . I t i s b e l i e v e d t h a t the r e d u c t i o n i n the r e s i s t a n c e of the i n t e r n a l s e a l i s due to c r a c k i n g of the s e a l . A f t e r the experiment, the l a b o r a t o r y s o i l column was taken a p a r t and the s u r f a c e of the pan was photographed. F i g s . 10a and b show c l o s e - u p p i c t u r e s of the i n t e r n a l s e a l . A s e a l can always be d i s t i n g u i s h e d by a s h i n y s u r f a c e when viewed from the top, and by a dense and t h i n band a t the s u r f a c e , when viewed from the s i d e . During the course of t h i s experiment, movement of water i n t o t h e , s o i l p r o f i l e was c l o s e l y observed on a p p l i c a t i o n of each r a i n f a l l event. I t was seen t h a t a l a r g e amount of f i n e m a t e r i a l moved through the s o i l p r o f i l e along the c r a c k s and macro-pores, s e t t l i n g a t the bottom of c l o s e d c a v i t i e s , or moving through and e v e n t u a l l y s e t t l i n g on top of the compacted pan. In the f a l l of 1987 the s o i l a t s i t e 1 was examined f o r any evidence of an i n t e r n a l s e a l . T h i s examination, however, d i d not r e v e a l the e x i s t e n c e of any i n t e r n a l s e a l i n a l o c a t i o n where . the s o i l s u r f a c e had l i q u e f i e d . I t can be concluded t h a t i f the s u r f a c e s t r u c t u r e c o l l a p s e s p r i o r t o the fo r m a t i o n of an i n t e r n a l s e a l , then t h i s w i l l prevent i t s fo r m a t i o n . S t r u c t u r e of the s o i l s u r f a c e can c o l l a p s e d u r i n g the v e r y f i r s t r a i n f a l l event due to a severe s t r u c t u r a l i n s t a b i l i t y . I t i s of i n t e r e s t t o mention t h a t the f i e l d under i n v e s t i g a t i o n had been s u b - s o i l e d w i t h a c o n v e n t i o n a l s u b - s o i l e r i n the p r e v i o u s f a l l a f t e r h a r v e s t . S u b - s o i l i n g FIG. 10 THE INTERNAL SEAL FORMED ON TOP OF THE COMPACTED PAN IN LABORATORY COLUMN top view s i d e view a) b) 38 has the u n d e s i r a b l e s i d e e f f e c t of b r i n g i n g t o the s u r f a c e the weakly aggregated m i n e r a l s u b - s o i l . When exposed at the s u r f a c e , t h i s m a t e r i a l can e a s i l y s l a k e as the r e s u l t of a r a i n f a l l event. An e s s e n t i a l process i n the fo r m a t i o n of the i n t e r n a l s e a l i s the c o n v e c t i v e t r a n s p o r t of f i n e sediments downward through the open c u l t i v a t i o n l a y e r . I f such a flow i s prevented, the fo r m a t i o n of the i n t e r n a l s e a l w i l l a l s o be prevented. T h i s argument i s not c o n t r a d i c t e d by the r e s u l t i n F i g . 7 of the decrease i n the h y d r a u l i c c o n d u c t i v i t y of the s u r f a c e and the i n t e r n a l s e a l as a f u n c t i o n of r a i n f a l l e vents. A d e s c r i p t i v e model of s e a l f o r m a t i o n i s d i s c u s s e d i n chapter 3 as an e x t e n s i o n of the mechanisms of s u r f a c e and i n t e r n a l s e a l f o r m a t i o n a l r e a d y d i s c u s s e d i n the pr e s e n t chapter. 1.5 SUMMARY AND CONCLUSIONS A s e r i e s of l a b o r a t o r y experiments were conducted i n order to i n v e s t i g a t e the mechanisms t h a t reduce the h y d r o l o g i c r e s p o n s i v e n e s s of a Ladner s o i l . I t has been shown t h a t the c u l t i v a t e d Ladner s o i l s u f f e r s from a low aggregate s t a b i l i t y , making i t prone t o d i s a g g r e g a t i o n i n response t o r a i n f a l l e vents. The presence of a s u r f a c e s e a l , due to i t s extremely low h y d r a u l i c c o n d u c t i v i t y , i s the main cause of ponding which renders s o i l u n t r a f f i c a b l e a t the be g i n n i n g of the c u l t i v a t i o n season. 39 Development of a s u c t i o n f o r c e a t the s o i l s u r f a c e was shown to p l a y an important r o l e i n the c r e a t i o n and e f f i c i e n c y of the s u r f a c e s e a l s . A cover crop was shown t o be an important management t o o l i n p r e v e n t i n g the f o r m a t i o n of both s u r f a c e and i n t e r n a l s e a l s , thereby p r e v e n t i n g s o i l h y d r o l o g i c d e g r a d a t i o n as the r e s u l t of r a i n d r o p impact. An i n t e r n a l s e a l was observed t o form i n the l a b o r a t o r y on top of a compacted pan. A l a b o r a t o r y i n v e s t i g a t i o n demonstrated one mechanism f o r i t s f o r m a t i o n . T h i s mechanism was the t r a n s p o r t of f i n e p a r t i c l e s by c o n v e c t i v e flow through the macropores and the e v e n t u a l d e p o s i t i o n of the f i n e s on top of the compacted pan. 40 REFERENCES B a l l , D.F. 1964. L o s s - o n - i g n i t i o n a s a n e s t i m a t e o f o r g a n i c m a t t e r a n d o r g a n i c c a r b o n i n n o n - c a l c a r e o u s s o i l s . J . S o i l S c i . 15: 8 4 - 9 2 . Baver, L.D., Gardner, W.H, and Garner, W.R. 1972. S o i l P h y s i c s . 4 t h E d i t i o n , J o h n W i l e y a n d S o n s , I n c . New Y o r k , pp. 1 4 0 - 1 4 6 ; 1 7 9 - 1 8 0 . B e r t r a n d , A.R. and Sor, K. 1962. The e f f e c t o f r a i n f a l l i n t e n s i t y o n s o i l s t r u c t u r e a n d m i g r a t i o n o f c o l l o i d a l m a t e r i a l s i n s o i l s . S o i l S c i . S o c . Am. P r o c . 26: 2 9 7 - 3 0 0 . Bonsu, M. 1987. S t r u c t u r a l s t a b i l i t y a n d s u r f a c e s e a l i n g as r e l a t e d t o o r g a n i c m a t t e r d e p l e t i o n o f a s h a l l o w o r g a n i c s o i l . Ph.D. T h e s i s , F a c u l t y o f G r a d u a t e S t u d i e s , The U n i v e r s i t y o f B r i t i s h C o l u m b i a . Bouma, J . and Denning, J.L. 1972. F i e l d m e a s u r e m e n t o f u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y b y i n f i l t r a t i o n t h r o u g h gypsum c r u s t s . S o i l S c i . S o c . Am. P r o c . 36: 8 4 6 - 8 4 7 . Duley, F.L. 1939. S u r f a c e f a c t o r s a f f e c t i n g t h e r a t e o f i n t a k e o f w a t e r b y s o i l s . S o i l S c i . S o c . Am. P r o c . 4: 60-64 . E l l i s o n , W.D. 1944. S t u d i e s o f r a i n d r o p e r o s i o n . A g r i c . E n g . 25: 1 3 1 - 1 3 6 . Emerson, W.W. 1977. P h y s i c a l p r o p e r t i e s a n d s o i l s t r u c t u r e . I n : ' S o i l F a c t o r s i n C r o p P r o d u c t i o n i n a s e m i - a r i d E n v i r o n m e n t ' ( e d . R u s s e l l a n d G r e a c e n ) . p p . 7 8 - 1 0 4 , U n i v e r s i t y o f Q u e e n s l a n d P r e s s . Evans, D.D. and B u o l , S.W. 1968. M i c r o m o r p h o l o g i c a l s t u d i e s o f s o i l c r u s t s . S o i l S c i . S o c . Am. P r o c . 32: 1 9 - 2 2 . F a l a y i , O. and Bouma, J . 1975. R e l a t i o n s h i p s b e t w e e n t h e h y d r a u l i c c o n d u c t a n c e o f s u r f a c e c r u s t a n d s o i l management i n a T y p i c H a p l u d a l f . S o i l S c i . S o c . Am. P r o c . 39: 9 5 7 - 9 6 3 . Free, 6.R. 1952. S o i l movement b y r a i n d r o p s . A g r i c . E n g . 33: 4 9 1 - 4 9 4 . H i l l e l , D. and Gardner, W.R. 1969. S t e a d y i n f i l t r a t i o n i n t o c r u s t - t o p p e d p r o f i l e s . S o i l S c i . 108: 1 3 7 - 1 4 2 . Hooghoudt, S.B. 1938. B i j d r a g e t o t de k e n n i s v a n e n i g e n a t u u r k u n d i g e g r o o t h e d e n v a n d e n g r o n d . V e r s l . L a n d b . O n d e r z . , 43(13)B, 215 pp. 41 Kemper, W.D. 1965. Aggregate S t a b i l i t y . I n : Black, CA. ( E d i t o r ) . "Methods of S o i l A n a l y s i s " . Agronomy No. 9, part 1. Academic p r e s s , New York. pp. 511-519. Laws, J.O. 1941. Measurements of the f a l l v e l o c i t i e s of water drops and ra i n d r o p s . Trans. Am. Geophys. Union 22: 709-721. Laws, J.O. and Parsons, D.A. 1943. R e l a t i o n of raindrop s i z e to i n t e n s i t y . Trans. Am. Geophys. 24: 452-460. Lemos, P. and L u t z , J.F. 1957. S o i l c r u s t i n g and some f a c t o r s a f f e c t i n g i t . S o i l S c i . Soc. Am. Proc. 21: 485-491. Luttmerding, H.A. 1981. " S o i l of the Langley-Vancouver map area" V o l . 3. RAB B u l l e t i n 18. M c l n t y r e , D.S. 1958. S o i l s p l a s h and the formation of surface c r u s t s by r a i n drop impact. S o i l S c i . 85: 261-266. Meyer, L.D. and Monke, E.L. 1965. Mechanics of s o i l e r o s i o n by r a i n f a l l and overland flow. Trans. ASAE 8: 572-578. Morin, J . , Benyamini, Y., and M i c h a e l i , A. 1981. The e f f e c t of raindrop impact on the dynamics of s o i l surface c r u s t i n g and water movement i n the p r o f i l e . J o u r n a l of Hydrology 52: 321- 335. T a c k e t t , J.L. and Pearson, R.W. 1965. Some c h a r a c t e r i s t i c s of s o i l c r u s t s formed by simulated r a i n f a l l . S o i l S c i . 99(6): 407-413. T a r c h i t z k y , J . , Banin, A., Morin, J . , and Chen, Y. 1984. Nature, formation and e f f e c t s of s o i l c r u s t s formed by water drop impact. Geoderma, 33:135-155. Uebler, R.L. and Swartzendruber D. 1982. Flow of k a o l i n i t e and sewage suspensions i n sand and s a n d - s i l t : I . Accumulation of suspension p a r t i c l e s . S o i l S c i . Soc. Am. J . 46: 239-244. CHAPTER 2 USE OF SOIL STRUCTURE DESIGN PARAMETERS FOR OPTIMIZATION OF HYDROLOGIC RESPONSIVENESS 43 USE OF SOIL STRUCTURE DESIGN PARAMETERS FOR OPTIMIZATION OF THE HYDROLOGIC RESPONSIVENESS 2.1 ABSTRACT The concept of managing s o i l s t r u c t u r e a c c o r d i n g to a s e t of d e s i g n h y d r o l o g i c parameters i s c e n t r a l i n t h i s chapter. I t i s based on the i n t i m a t e c o n n e c t i o n between s o i l s t r u c t u r e and the shape of the s o i l ' s p a r t i a l h y d r o l o g i c f u n c t i o n s . A s e t of d e s i g n h y d r o l o g i c parameters i s used which c h a r a c t e r i z e s the shape of the p a r t i a l h y d r o l o g i c f u n c t i o n s . A s o i l t h a t i s managed a c c o r d i n g t o these d e s i g n parameters meets both predetermined h y d r o l o g i c response = requirements, and c o r r e s p o n d i n g time t o w o r k a b i l i t y requirements. To develop the concept, a one-dimensional, numerical model i n v o l v i n g flow through an i n t e g r a t e d s a t u r a t e d - u n s a t u r a t e d system r e p r e s e n t i n g a two-layered s o i l i s used t o determine the time t o t r a f f i c a b i l i t y and w o r k a b i l i t y f o r a Lower F r a s e r V a l l e y lowland s o i l . The s o i l i s s u b s u r f a c e - d r a i n e d . The s o i l s t r u c t u r e i s degraded i n s t e p s , from a f r e s h l y c u l t i v a t e d s t a t e by changing the d e s i g n h y d r o l o g i c parameters: s a t u r a t e d water content, 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 i r e n t r y p r e s s u r e head, and a f a c t o r 8 which d e s c r i b e s the shape of the r e t e n t i o n curve beyond the a i r e n t r y v a l u e . At each stage of d e g r a d a t i o n , the model c a l c u l a t e s the time t o t r a f f i c a b i l i t y and w o r k a b i l i t y . The e f f e c t of changing the parameters on the time t o t r a f f i c a b i l i t y and w o r k a b i l i t y i s examined t o a r r i v e 44 a t a b e t t e r understanding of the f a c t o r s which c o n t r o l the h y d r o l o g i c responsiveness of a s o i l . 2.2 INTRODUCTION I t was shown i n chapter 1 t h a t the s t r u c t u r e of a c u l t i v a t e d s o i l i s a dynamic e n t i t y . A f t e r h a r v e s t i n the f a l l , i t i s common f o r Lower F r a s e r V a l l e y lowland s o i l s t o be l e f t i n a bare and f r e s h l y c u l t i v a t e d s t a t e exposed to the d e s t r u c t i v e a c t i o n of r a i n d r o p impact. T h i s p r a c t i c e causes d e s t r u c t i o n of the s u r f a c e aggregates which i n c r e a s e s the bulk d e n s i t y , decreases the p o r o s 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 and the a i r e n t r y p r e s s u r e head.of the s u r f a c e l a y e r . The s t r u c t u r e of the c u l t i v a t i o n l a y e r w i l l a l s o change t o some ext e n t as a r e s u l t of c o n s o l i d a t i o n and m i g r a t i o n of f i n e p a r t i c l e s . As the r e s u l t of these changes i n the s o i l s t r u c t u r e , the h y d r o l o g i c c h a r a c t e r i s t i c s of the s o i l change, r e s u l t i n g i n an i n c r e a s e i n the s o i l response time. Response time i s d e f i n e d as the time r e q u i r e d f o r a s o i l t o reach a workable s t a t e from a f l o o d e d c o n d i t i o n w i t h the water t a b l e a t the s o i l s u r f a c e . A s o i l i s c o n s i d e r e d workable i f upon t i l l a g e i t crumbles e a s i l y and forms a l o o s e assemblage of r e l a t i v e l y s m a l l , s o f t c l o d s ( H i l l e l 1980). To c a r r y out farming o p e r a t i o n s such as manure sp r e a d i n g , a s o i l must be t r a f f i c a b l e ( i . e , a b l e to p r o v i d e t r a c t i o n without b e i n g damaged s t r u c t u r a l l y beyond l i m i t s f o r good crop growth), whereas w o r k a b i l i t y i s r e q u i r e d f o r c u l t i v a t i o n 45 a n d s e e d i n g . F rom a h y d r o l o g i c p o i n t o f v i e w , a s o i l i s o f g o o d s t r u c t u r e i f i t c a n p r o v i d e f a r m e r s w i t h a d e q u a t e b l o c k s o f t r a f f i c a b l e / w o r k a b l e d a y s a t t h e b e g i n n i n g o f t h e c u l t i v a t i o n s e a s o n . Much w o r k h a s b e e n done t o p r e d i c t t h e e f f e c t o f d i f f e r e n t f a c t o r s o n s o i l w o r k a b i l i t y a n d t r a f f i c a b i l i t y . W i n d ( 1 9 7 6 ) u s e d a m o d e l o f n o n - s t e a d y u n s a t u r a t e d f l o w o f m o i s t u r e i n a n a l o g a n d n u m e r i c a l m o d e l s t o i n v e s t i g a t e t h e i n f l u e n c e o f d r a i n a g e on w o r k a b i l i t y i n s p r i n g . P a u l a n d de V r i e s ( 1 9 8 3 a , b ) i n v e s t i g a t e d t h e e f f e c t s o f s u b s u r f a c e d r a i n a g e on s o i l t r a f f i c a b i l i t y . v a n W i j k a n d F e d d e s ( 1 9 8 6 ) d e v e l o p e d a m o d e l t o p r e d i c t t h e e f f e c t s o f c h a n g e s i n w a t e r management b y d r a i n a g e on t r a f f i c a b i l i t y a n d w o r k a b i l i t y i n s p r i n g . B u i t e n d i j k ( 1 9 8 5) c a l c u l a t e d t h e number o f w o r k a b l e d a y s f o r h a r v e s t o f s u g a r b e e t on a s a n d y l o a m s o i l i n t h e N e t h e r l a n d s b y a p p l i c a t i o n o f a p h y s i c a l m o d e l o f w a t e r movement i n s o i l s . I n a l l o f t h e a b o v e , a n d i n g e n e r a l i n a n y m o d e l o f t h e f l o w o f w a t e r i n a n u n s a t u r a t e d s o i l , a g o o d k n o w l e d g e o f t h e s o i l h y d r o l o g i c c h a r a c t e r i s t i c s i s r e q u i r e d . The p a r t i a l w a t e r r e t e n t i o n c u r v e c a n e a s i l y be m e a s u r e d i n t h e l a b o r a t o r y , b u t m e a s u r e m e n t o f t h e u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y i s r a t h e r d i f f i c u l t , p a r t i c u l a r l y f o r a s t r u c t u r e d o r a l o o s e l y a g g r e g a t e d s o i l . Many m e t h o d s h a v e t h e r e f o r e b e e n d e v e l o p e d t o r e p r e s e n t t h e p a r t i a l f l o w i n f o r m a t i o n o n t h e b a s i s o f a f u n c t i o n w h i c h d e s c r i b e s t h e s h a p e o f t h e p a r t i a l w a t e r r e t e n t i o n c u r v e ( v a n G e n u c h t e n 1980 ; C a m p b e l l 1 9 7 4 ) . 46 A f e a t u r e t h a t most of the h y d r o l o g i c f u n c t i o n s have i n common i s a s e t of f o u r parameters which can f u l l y d e s c r i b e the h y d r o l o g i c c h a r a c t e r i s t i c s of a s o i l . These are: s a t u r a t e d water content ( 6 S ) , 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 ( K s ) , a i r e n t r y p r e s s u r e head ( h a ) , and a parameter ((3) which can c h a r a c t e r i z e the water r e l e a s e a b i l i t y of a s o i l , and t h e r e f o r e d e f i n e the shape of the water r e t e n t i o n curve. These parameters, r e f e r r e d t o h e n c e f o r t h as h y d r o l o g i c parameters, are a l l interdependent and change as the s o i l s t r u c t u r e changes. The c e n t r a l o b j e c t i v e of t h i s paper i s t w o - f o l d : a) to model the e f f e c t of s o i l s t r u c t u r e d e g r a d a t i o n on the h y d r o l o g i c response time. L i t t l e a t t e n t i o n has been g i v e n to the f a c t t h a t h y d r o l o g i c parameters are dynamic i n nature, as most i n v e s t i g a t o r s have assumed a c o n s t a n t s e t of v a l u e s i n t h e i r models, b) To determine a s e t of d e s i g n h y d r o l o g i c parameters which produces a d e s i r e d response time. In other words, to d e s i g n a s o i l c h a r a c t e r i z e d by a s e t of d e s i g n parameters. Such a s o i l would d i s p l a y the d e s i r e d h y d r o l o g i c behaviour. The o b j e c t i v e of s o i l management can t h e r e f o r e be d e f i n e d as t o produce a s o i l w i t h s p e c i f i e d d e s i g n parameters. To achieve the above o b j e c t i v e s , the upper and lower l i m i t s of the h y d r o l o g i c parameters were est i m a t e d f o r a lowland s o i l a t two extreme s t r u c t u r a l s t a t e s , f r e s h l y c u l t i v a t e d (assumed t o be the l e a s t degraded) and the most degraded. F u r t h e r , d e g r a d a t i o n process was s i m u l a t e d by d e c r e a s i n g the 0 S of the f r e s h l y c u l t i v a t e d s o i l i n s m a l l i n c r e m e n t s . A s s u m i n g a r e a s o n a b l e s h a p e f o r t h e p a r t i a l r e t e n t i o n c u r v e a t e a c h s t a g e o f d e g r a d a t i o n e n a b l e d c a l c u l a t i o n o f t h e v a l u e s f o r B a n d h a . E a c h s e t o f p a r a m e t e r s w e r e c o m b i n e d w i t h a r a n g e o f K s ' s a n d t h e t i m e t o t r a f f i c a b i l i t y a n d w o r k a b i l i t y was c a l c u l a t e d a t e a c h s t a g e b y a n u m e r i c a l m o d e l b a s e d o n t h e f l o w e q u a t i o n t h r o u g h a two- l a y e r e d s u b s u r f a c e - d r a i n e d s o i l s y s t e m . 2.3 MATERIALS AND METHODS The s o i l u s e d i n t h i s w o r k c o n s i s t s o f two l a y e r s . A 30 cm c u l t i v a t e d s u r f a c e o n t o p o f a n i n f i n i t e l y d e e p s u b - s o i l . D r a i n s a r e l o c a t e d a t a d e p t h o f 120 cm. A l l t h e r e l e v a n t i n f o r m a t i o n s u c h a s K s a n d d r a i n a g e b e h a v i o u r i s t a k e n f r o m t h e s o i l i n s i t e 2 a s d e s c r i b e d i n c h a p t e r 1. The w o r k a b i l i t y c r i t e r i o n was s e t a t a w a t e r c o n t e n t e q u a l t o t h e l o w e r p l a s t i c l i m i t ( H i l l e l 1 9 8 0 ) . The p l a s t i c l i m i t o f t h e L a d n e r s o i l was d e t e r m i n e d t o be 0.28 k g k g - 1 ( n = 12, s t a n d a r d d e v i a t i o n = + 0 . 0 2 ) . Oh a v o l u m e t r i c b a s i s t h e p l a s t i c l i m i t was c a l c u l a t e d f o r t h e f r e s h l y c u l t i v a t e d s o i l t o be e q u a l t o 0.30 m 3 m" 3 f o r a b u l k d e n s i t y o f 1070 k g m" 3. T h i s v a l u e o f b u l k d e n s i t y was t h e l o w e s t v a l u e m e a s u r e d f o r t h e c u l t i v a t i o n l a y e r o n A p r i l o f 1987 ( c h a p t e r 1, T a b l e 1 ) . I t i s r e a l i z e d t h a t a s a s o i l d e g r a d e s a n d t h e b u l k d e n s i t y i n c r e a s e s , t h e p l a s t i c l i m i t e x p r e s s e d o n a mass b a s i s r e m a i n s t h e same, w h e r e a s e x p r e s s e d o n a v o l u m e t r i c b a s i s i t w i l l i n c r e a s e . I t c a n be c a l c u l a t e d t h a t f o r a p o r o s i t y o f 48 0.40 m~3 a n d a b u l k d e n s i t y o f 1590 k g m~3, t h e p l a s t i c l i m i t w o u l d be e q u a l t o 0.44 m~^. I t i s n o t h o w e v e r p h y s i c a l l y p o s s i b l e f o r a s a t u r a t e d s o i l t o be w o r k a b l e s i n c e i t w i l l p u d d l e a n d l i q u e f y u p o n b e i n g s t r e s s e d a n d v i b r a t e d b y t h e t r a c t o r w h e e l s a nd t h e t i l l a g e i m p l e m e n t s . T h i s i n d i c a t e s t h a t e x p r e s s i n g p l a s t i c l i m i t o n a v o l u m e t r i c b a s i s l o s e s p h y s i c a l m e a n i n g b e c a u s e i t e x c e e d s t h e p o r o s i t y . To a v o i d t h i s p r o b l e m , t h e w o r k a b i l i t y l i m i t f o r d i f f e r e n t s t a g e s o f d e g r a d a t i o n was assumed t o be e q u a l t o 0.30 m^ m"3. To s e t t h e t r a f f i c a b i l i t y c r i t e r i o n , a c o n d i t i o n d e t e r m i n e d b y P a u l a n d de V r i e s ( 1 9 7 9 ) i s u s e d i n w h i c h a s o i l i s c o n s i d e r e d t r a f f i c a b l e i f t h e p r e s s u r e h e a d i n t h e t o p 5 cm i s e q u a l t o o r l e s s t h a n -50 cm o f w a t e r . I t s h o u l d be k e p t i n m i n d t h a t t h i s c o n d i t i o n h o l d s t r u e o n l y i f t h e a i r e n t r y p r e s s u r e h e a d o f t h e s u r f a c e i s g r e a t e r t h a n ( l e s s n e g a t i v e ) -50 cm o f w a t e r . To c a l c u l a t e t h e p a r t i a l u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y f u n c t i o n a C a m p b e l l ( 1 9 7 4 ) t y p e a p p r o a c h was u s e d b e c a u s e o f i t s s i m p l i c i t y . I t s h o u l d h o w e v e r be m e n t i o n e d t h a t t h e f l o w o f w a t e r i n a s t r u c t u r e d s o i l i s more c o m p l e x t h a n w h a t i s i m p l i e d b y t h e a s s u m p t i o n s u n d e r l y i n g C a m p b e l l ' s e q u a t i o n . T h e s e a s s u m p t i o n s s t a t e t h a t f l o w o f w a t e r i n a s o i l i s c o n t r o l l e d b y t h e s m a l l e r o f two p o r e s i n a s e q u e n c e , o n l y p o r e s i n a d i r e c t s e q u e n c e c o n t r i b u t e t o t h e t o t a l h y d r a u l i c c o n d u c t i v i t y , a n d t h e p o r e s i n a p o r o u s medium f i t t o g e t h e r r a n d o m l y ( C h i l d s 1 9 6 9 ) . C a m p b e l l ( 1 9 7 4 ) s t a t e s t h a t i f t h e m o i s t u r e r e t e n t i o n f u n c t i o n c a n be r e p r e s e n t e d b y : 49 h=h a (e /e s ) -P ( i ) then the h y d r a u l i c c o n d u c t i v i t y i s g i v e n by: K = K s ( e / 6 s ) 2 3 + 3 (2) where: h = p r e s s u r e head (cm of water), h a = a i r e n t r y p r e s s u r e head (cm of water), 0 = water content (cm 3 cm" 3 ), &s = s a t u r a t e d water content (cm 3 c m - 3 ) , K = h y d r a u l i c c o n d u c t i v i t y (cm d a y - 1 ) , K s = 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 (cm d a y ~ l ) , B = a parameter r e f e r r e d t o as " f l a t n e s s f a c t o r " which d e s c r i b e s the shape of the p a r t i a l water r e t e n t i o n curve; 6 S, K s, h a , and B are the h y d r o l o g i c parameters. The u n i t s i n d i c a t e d i n b r a c k e t s are used i n the model f o r convenience. A major d i f f i c u l t y i n u s i n g equations 1 and 2 i s the d e t e r m i n a t i o n of the h y d r o l o g i c parameters at each stage of s o i l d e g r a d a t i o n . As the s t r u c t u r e of a s o i l changes, these parameters change. At every p h y s i c a l s t a t e of a s o i l , c l e a r l y t h e r e i s o n l y one s e t of v a l u e s t h a t can d e s c r i b e the c h a r a c t e r i s t i c s of t h a t s o i l . But the nature of the changes i n these parameters w i t h a change i n s t r u c t u r e i s not y e t c l e a r l y understood. 50 2.3.1 D e t e r m i n a t i o n o f t h e H y d r o l o g i c P a r a m e t e r s a) f o r t h e t o p s o i l The p a r t i a l w a t e r r e t e n t i o n c u r v e f o r t h e s u r f a c e l a y e r ( F i g . 1) was d e t e r m i n e d i n t h e l a b o r a t o r y w i t h a t e n s i o n t a b l e , f o r a f r e s h l y c u l t i v a t e d s o i l ( h i g h e s t © s ) . A c u r v e d e s c r i b e d b y E q . 1 was f i t t e d t o t h e m e a s u r e d r e t e n t i o n c u r v e a n d t h e v a l u e o f B d e t e r m i n e d f r o m i t ( T a b l e 1 ) . U s i n g a m e a s u r e d v a l u e o f K s = 864 cm day"-*- ( 1 0 ~ 4 m s " 1 ) ( c h a p t e r 1 ) , E q . 2 was u s e d t o c a l c u l a t e t h e p a r t i a l f l o w c u r v e ( F i g . 2) f o r t h e f r e s h l y c u l t i v a t e d s o i l . C o n d i t i o n s f o r t h e m o s t d e g r a d e d s t a t e w e r e s e t a t a s a t u r a t e d w a t e r c o n t e n t o f 0.40 c m 3 c m - 3 a n d a n a i r e n t r y p r e s s u r e h e a d o f -150 cm o f w a t 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 f o r t h e m o s t d e g r a d e d s t a t e was s e t e q u a l t o 8 . 6 4 x l 0 ~ 2 cm d a y " 1 ( 1 0 ~ 9 m s - 1 ) ( c h a p t e r I ) . C h a r a c t e r i s t i c c u r v e s f o r t h i s s t a t e o f t h e s o i l a r e a l s o shown i n F i g s . 1 a n d 2. V a l u e s o f t h e h y d r o l o g i c p a r a m e t e r s a r e shown i n T a b l e 1. U s i n g t h e c h a r a c t e r i s t i c c u r v e s f o r t h e l e a s t a n d t h e m o s t d e g r a d e d s o i l ( F i g s . 1 a n d 2) a s u p p e r a n d l o w e r l i m i t s , a n d a s s u m i n g a r e a s o n a b l e s h a p e f o r t h e p a r t i a l r e t e n t i o n c u r v e a t e a c h s t a g e o f d e g r a d a t i o n , a r e a s o n a b l e s e t o f v a l u e s was c a l c u l a t e d f o r B a n d h a ( T a b l e 2 ) . 0.70 q 0.60 z E y ^ 0 . 5 0 o LxJ o o 0.40 0.30 - 0.20 - most degraded top layer sub—sol freshly cultivated top layer 0.10 -j—i—i—i—i—i—i—i— i—i—|—i—i—i—i—i—i—i—i—i—|—i—i—r -150.00 -100.00 -50.00 PRESSURE HEAD (cm) T—I—I—I—I—| 0.00 FIG. 1 PARTIAL WATER RETENTION CHARACTERISTIC CURVES FOR THE TOP 30 cm LAYER, AND THE BOTTOM 90 cm LAYER OF SUB-SOIL. FIG. 2 PARTIAL FLOW CHARACTERISTIC CURVES FOR THE TOP 30 cm. AND THE BOTTOM 90 cm OF SUB-SOIL 53 Table 1. Hydrologic parameters of the s o i l ©s - Ks - h'a B cm0 cm 3 cm day 1 cm top 30 cm: freshly cultivated .6 864 -1.5 6.0 most degraded .4 .0086 -150 sub-soil .35 8.64 0.0 0.0 54 Table 2. Values obtained for some hydrologic parameters by assuming a reasonable behaviour for the partial retention curves *1 S _ o J cm •* cm P .59 -1.6 6.1 .58 -1.8 6.3 .57 -2.0 6.5 .56 -2.3 6.7 .55 -2.6 6.9 .54 -3.0 7.2 .53 -3.4 7.5 .52 -4.0 7.8 .51 -4.5 8.2 .50 -5.4 8.5 .49 -6.3 8.9 .48 -7.7 9.4 .47 -9.4 10.0 .46 -11.6 10.7 .45 -14.7 11.6 55 b) f o r t h e s u b - s o i l C h a r a c t e r i s t i c c u r v e s f o r t h e s u b - s o i l a r e a s s u m e d t o r e m a i n u n c h a n g e d d u r i n g t h e o f f - s e a s o n p e r i o d b e c a u s e i t s s t r u c t u r e d o e s n o t c h a n g e . T h e s e c u r v e s a r e shown i n F i g s . 1 a n d 2. The j u s t i f i c a t i o n f o r t h i s t y p e o f b e h a v i o u r i s b a s e d o n t h e o b s e r v a t i o n s , w h e r e : 1. when s a t u r a t e d , m o s t o f t h e f l o w i n t h e s u b - s o i l was o b s e r v e d t o t a k e p l a c e t h r o u g h t h e s t a b l e r o o t h o l e s a n d , 2. when u n s a t u r a t e d , t h e m a t r i x o f t h e s u b - s o i l was n o t e d t o h a v e a r e l a t i v e l y l o w h y d r a u l i c c o n d u c t i v i t y ( c h a p t e r 1, F i g . 5 ) . A l s o , s i n c e t h e c o n t r i b u t i o n o f t h e c o n d u c t i n g c h a n n e l s t o t h e p o r o s i t y i s n e g l i g i b l e , t h e w a t e r c o n t e n t c h a n g e s o n l y s l i g h t l y f r o m s a t u r a t e d t o u n s a t u r a t e d c o n d i t i o n . 2.4 NUMERICAL MODEL The s o i l w a t e r c o n s e r v a t i o n e q u a t i o n f o r a o n e - d i m e n s i o n a l , v e r t i c a l , u n s t e a d y , u n s a t u r a t e d , a n d r i g i d p o r o u s m a t e r i a l u n d e r i s o t h e r m a l c o n d i t i o n s c a n be w r i t t e n ( R i c h a r d s ' e q u a t i o n ) a s : c ( h ) 6 h / 6 t = 6 / 6 z [ K ( h ) 6 h / 6 z ] + 6 / 6 z [ K ( h ) ] (3) w h e r e : c ( h ) = dG/dh = s p e c i f i c w a t e r c a p a c i t y ( c m ~ l ) , h = s o i l w a t e r p r e s s u r e h e a d (cm o f w a t e r ) , t = t i m e ( d a y ) , G = v o l u m e t r i c w a t e r c o n t e n t (cm^ c m ~ 3 ) , K = h y d r a u l i c c o n d u c t i v i t y (cm d a y - 1 ) , z = depth (cm), p o s i t i v e upward w i t h z = 0 i n d i c a t i n g the surf a c e . For the s a t u r a t e d c o n d i t i o n K(h) = constant, and c(h) = 0, t h e r e f o r e Eq. 3 reduces t o : 6 2h/6z 2 = 0 (4) D e r i v a t i o n of these equations i s d e t a i l e d i n Freeze (1969). F i g . 3 taken from Freeze (1969) shows the mathematical model f o r continuous flow from the s o i l surface to the d r a i n s l o c a t e d at a depth of 120 cm below the surf a c e . The model c o n s i s t s of a column of n r a a x nodes numbered v e r t i c a l l y upward from the d r a i n l o c a t i o n . Most s o i l s e x h i b i t a negative a i r entry pressure head (h a) above which the values of h y d r a u l i c c o n d u c t i v i t y and water content are equal to t h e i r r e s p e c t i v e saturated values. . Eq. 3 thus holds f o r a l l values of h < h a and Eq. 4 f o r a l l values of h > h a . The boundary c o n d i t i o n at the e l e v a t i o n of the d r a i n i s : 6h/6z = [q/K(h)] - 1 (5) where: q > 0 represents the r a t e of drainage discharge, and q < 0 i n d i c a t e s a recharge of the system. The drainage discharge r a t e i s c a l c u l a t e d by: q = Ah D (6) 57 UPPER BOUNDARY: dh/dz CONDITION NODAL NUMBERS DRAINS AT 14 m APART nmax nmax-1 « r/K(h) 1 where r + ve = INFILTRATION r - ve = EVAPORATION GROUND SURFACE Z = 0 cm UNSATURATED ZONE EQUATION: c(h)dh/dt = d/dz[K(h)(dh/dz+l)] FUNCTIONAL RELATIONSHIPS: K = K(h) e * e(h) c = c(h) T ARBITRARY POSITION OF WATER TABLE (h =0) I SATURATED ZONE EQUATION: d 2h/dz 2 r 0 FUNCTIONAL RELATIONSHIPS: K = K s - constant 9 = © s = constant c r 0 BASAL BOUNDARY: dh/dz CONDITION BASE OF MODEL Z = -130 cm q + ve = DISCHARGE q - ve = RECHARGE = q/K(h) 1 where FIG. 3 MATHEMATICAL MODEL FOR ONE DIMENSIONAL, VERTICAL UNSTEADY INFILTRATION OR EVAPORATION. 58 w h e r e : A = d r a i n a g e i n t e n s i t y ( d a y - 1 ) ( W i n d a n d v a n D o o r n e 1 9 7 5 ) , a n d h D = h e i g h t o f w a t e r t a b l e a t m i d p o i n t b e t w e e n d r a i n s . E q . 6 i s b a s e d on H o o g h o u d t ( 1 9 3 8 ) , o n l y h o l d i n g m idway b e t w e e n two p a r a l l e l d r a i n s i f t h e d r a i n a g e r e s i s t a n c e (1/A) i s c o n s t a n t ( W i n d a n d v a n D o o r n e 1 9 7 5 ) . The v a l u e o f A u s e d i n t h e m o d e l was o b t a i n e d b y t h e m o d i f i e d t r a n s i e n t s t a t e Glover-Dumm e q u a t i o n : A = l n ( 1 . 1 6 h Q / h t ) / t (7) w h e r e : h Q a n d h t (cm) a r e t h e i n i t i a l a n d f i n a l h e i g h t s o f w a t e r t a b l e a b o v e d r a i n d e p t h midway b e t w e e n d r a i n s a t s t a r t a n d e n d o f t h e draw-down p e r i o d , r e s p e c t i v e l y , a n d t ( d a y ) i s t h e l e n g t h o f draw-down p e r i o d . D e r i v a t i o n o f t h i s e q u a t i o n i s d e t a i l e d i n P a u l a n d de V r i e s ( 1 9 8 3 ) . V a l u e s o f h Q a n d hj- w e r e o b t a i n e d f r o m w a t e r t a b l e r e c e s s i o n d a t a ( p r o v i d e d f r o m t h e B o u n d a r y B a y s t a t i o n p l o t s b y t h e B.C. M i n i s t r y o f A g r i c u l t u r e ) t a k e n d u r i n g a r a i n f r e e p e r i o d t , w h i c h f o l l o w e d a h e a v y r a i n f a l l i n M a r c h 1985. I t i s a s sumed i n t h e m o d e l t h a t t h e w a t e r t a b l e c a n r e c e d e t o a d e p t h o f n o t more t h a n 30 cm b e l o w t h e d r a i n s e a r l y i n t h e s p r i n g . The b o u n d a r y c o n d i t i o n a t t h e s u r f a c e i s : 6 h / 6 z = [ r / k ( h ) ] - 1 (8) 59 w h e r e : r = t h e f l o w r a t e (cm/day) a c r o s s t h e u p p e r b o u n d a r y . A p o s i t i v e r r e p r e s e n t s r a i n f a l l i n f i l t r a t i o n ; a n e g a t i v e r r e p r e s e n t s e v a p o r a t i o n . I n t h e a b s e n c e o f a n y p r e c i p i t a t i o n , a n e v a p o r a t i o n o f r = -0.1 cm d a y ~ l was a s s u m e d t o t a k e p l a c e e a r l y i n t h e s p r i n g . F u r t h e r i t was a ssumed t h a t t h e p r e s s u r e h e a d o f t h e w a t e r i n t h e s u r f a c e l a y e r d o e s n o t d e c r e a s e b e l o w - 1 0 ^ cm o f w a t e r . The p r o b l e m d e s c r i b e d a b o v e i s s o l v e d b y n u m e r i c a l a n a l y s i s , u s i n g a f i n i t e d i f f e r e n c i n g p r o c e d u r e s i m i l a r t o t h a t d e t a i l e d b y W h i s l e r a n d W a t s o n ( 1 9 6 8 ) . The o n l y d i f f e r e n c e i s t h a t a t t h e e n d o f e a c h t i m e s t e p ( 0 . 1 d a y ) t h e v a l u e o f t h e p r e s s u r e h e a d a t t h e s u r f a c e a n d a t t h e d r a i n d e p t h was c a l c u l a t e d a c c o r d i n g t o t h e b o u n d a r y c o n d i t i o n s d e s c r i b e d b y E q s . 5 a n d 8. To s o l v e t h e n u m e r i c a l p r o b l e m d e s c r i b e d a b o v e a c o m p u t e r programme was w r i t t e n i n C l a n g u a g e a n d i t was r u n on. a n IBM m i c r o c o m p u t e r . 2.5 RESULTS AND DISCUSSION B e f o r e p r e s e n t i n g a n y r e s u l t s , a d i s c u s s i o n o f t h e d a t a p r e s e n t e d i n T a b l e 2 i s w a r r a n t e d . I t was d i s c u s s e d i n c h a p t e r 1 t h a t a s . t h e r e s u l t o f r a i n d r o p i m p a c t on a f r e s h l y c u l t i v a t e d s o i l , a d i s a g g r e g a t i o n p r o c e s s t a k e s p l a c e w h i c h i n c r e a s e s t h e r e s p o n s e t i m e o f a s o i l . T h i s i n c r e a s e i n t h e r e s p o n s e t i m e i s due t o t h e c h a n g e s w h i c h t a k e s p l a c e i n t h e s o i l h y d r o l o g i c c h a r a c t e r i s t i c s , d e f i n e d b y t h e f o u r 60 h y d r o l o g i c parameters. As s o i l aggregates break down and f i l l the pore spaces, the p o r o s i t y of the s o i l and hence i t s s a t u r a t e d water content decreases. Furthermore, the diameter of the water c o n d u c t i n g pores w i l l decrease, r e s t r i c t i n g the flow of water and consequently lowering 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 . As a r e s u l t of the r e d u c t i o n i n pore diameter, the a i r e n t r y p r e s s u r e of the s o i l w i l l a l s o decrease along w i t h an accompanying i n c r e a s e i n the v a l u e of 8. The h y d r o l o g i c parameters change s i m u l t a n e o u s l y as a r e s u l t of s o i l s t r u c t u r e d e g r a d a t i o n w i t h r e l a t i o n s h i p s among them t h a t are not w e l l understood. To determine the e f f e c t of s o i l s t r u c t u r e changes on the h y d r o l o g i c parameters, the behaviour s t a t e d below i s assumed f o r the p a r t i a l water r e t e n t i o n curves i n response to s o i l d e g r a d a t i o n : As a s o i l degrades and the s a t u r a t e d water content d e c r e a s e s , the p a r t i a l water r e t e n t i o n curves f o l l o w the behaviour shown i n F i g . 4, where: a) the h o r i z o n t a l p a r t of each s u c c e s s i v e curve must i n t e r s e c t the p r e c e d i n g r e t e n t i o n curve b) each s u c c e s s i v e curve i n t e r s e c t s the curve f o r the most degraded s o i l , a t l e a s t , a t h = -150 cm and c) each s u c c e s s i v e curve does not c r o s s the p r e c e d i n g curve more than once ( i . e . , o n l y a t the h o r i z o n t a l p a r t ) . I t should be noted t h a t as 8 i n c r e a s e s the curves become f l a t t e r . The j u s t i f i c a t i o n f o r the above r e s t r i c t i o n s i s t h a t a t any g i v e n v a l u e of h, the water content of a more degraded s o i l should be h i g h e r than t h a t of a l e s s e r degraded s o i l f o r h < h a . T a b l e 2 shows the v a l u e s c a l c u l a t e d t o meet the above 0.70 q 0.60 K> E o 0.50 -m o 0.20 - 0.10 most degraded soil freshly cultivated soil T—i—i—i—i—i—i—i—i—|—i—i—i—i—i—i—i—i—i—|—i—i—i—i—i—i—i—i—i—| -1.50.00 -100.00 -50.00 0.00 PRESSURE HEAD (cm) FIG. 4- PARTIAL WATER RETENTION CURVES AT VARIOUS STAGES OF DEGRADATION. 62 c o n d i t i o n s . The m o d e l was r u n f o r some o f t h e c o n d i t i o n s i n T a b l e 2 c o m b i n e d w i t h a r a n g e o f s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s ( T a b l e 3 ) . 2.5.1 M o d e l R e s p o n s e Two d i f f e r e n t s c e n a r i o s w e r e r u n b y t h e m o d e l . F i r s t , f r o m a n i n i t i a l c o n d i t i o n o f f l o o d i n g w i t h t h e w a t e r t a b l e a t t h e s o i l s u r f a c e , t i m e t o t r a f f i c a b i l i t y a n d r e s p o n s e t i m e was c a l c u l a t e d w i t h a n e v a p o r a t i o n o f 0.1 cm d a y - 1 a n d z e r o p r e c i p i t a t i o n . S e c o n d , u s i n g t h e p r e s s u r e h e a d d i s t r i b u t i o n a t t h e t r a f f i c a b l e s t a t e o f t h e f i r s t r u n a s a n i n i t i a l c o n d i t i o n , t h e m o d e l was r u n f o r a t y p i c a l s t o r m o f 10 h o u r d u r a t i o n a n d 2 cm d a y - 1 i n t e n s i t y w h i c h made t h e s o i l u n t r a f f i c a b l e . The t i m e t o t r a f f i c a b i l i t y a n d w o r k a b i l i t y was c a l c u l a t e d a g a i n f r o m t h e t i m e t h a t t h e r a i n f a l l c e a s e d n e g l e c t i n g h y s t e r e s i s . R e s u l t s a r e shown i n T a b l e 3. F i g . 5 shows t h e r e s p o n s e o f t h e m o d e l t o d r y i n g f o r c o n d i t i o n 3a i n T a b l e 3 f r o m a f l o o d e d c o n d i t i o n t o a t r a f f i c a b l e s t a t e . F i g . 6 shows t h e r e s p o n s e o f t h e m o d e l t o t h e a b o v e s t o r m f r o m a n i n i t i a l l y t r a f f i c a b l e s t a t e . T a b l e 3 shows t h e r e s p o n s e t i m e a n d t i m e t o t r a f f i c a b i l i t y a n d w o r k a b i l i t y f o r d i f f e r e n t s o i l c o n d i t i o n s . I t c a n be s e e n t h a t a s s o i l s t r u c t u r e d e g r a d e s t h e r e s p o n s e t i m e i n c r e a s e s , w h e r e a s u s e o f p r e s s u r e h e a d a s a n i n d e x o f t r a f f i c a b i l i t y d o e s n o t r e s u l t i n a c o n s i s t e n t p a t t e r n . T h i s i n c o n s i s t e n c y r e s u l t s f r o m t h e d e f i n i t i o n o f t r a f f i c a b i l i t y o n t h e b a s i s o f Table 3. Showing tine to trafficability ( t t ) , response time (rt) , and time to workability ftw> for a number of possible hydrologic parameter combinations after a 10 hr. storm from ft flooded, condition of 2 cm day - 1 intensity s o i l h-based w.c-based status v.c-based case «s . ha (5 tt t t r t index tt tw n°t cm dav 1 cm3 cm 3 cm dav dav dav dav dav 1 864 .60 -1.5 6.0 6.4 6.5 10.6 1.0 2.4 5.8 la .60 -1.5 5.5 6.8 5.4 8.6 .8 1.6 4.6 lb .60 -1.5 6.5 6.2 8.0 12.4 1.2 3.7 7.8 lc .58 -1.8 6.3 6.7 7.6 12.1 1.1 1.8 6.4 Id .57 -2.0 6.5 6.2 7.7 12.8 1.2 1.9 6.9 le .56 -2.3 6.7 6.3 8.7 14.7 1.4 2.0 7.7 2 432 .58 -1.8 6.3 6.3 7.2 11.1 1.0 2.5 7-4 2a .57 -2.0 6.5 6.5 8.0 12.3 1.2 2.7 7.5 2b .56 -2.3 6.7 6.3 8.7 14.7 1.4 3.1 7.7 3 86.4 .58 -1.8 6.3 6.2 7.0 12.1 1.1 6.1 10.2 3a .57 -2.0 6.5 6.3 7.6 12.5 1.2 6.2 11.0 3b .56 -2.3 6.7 6.1 8.1 12.8 1.2 6.5 11.2 3c .54 -3.0 7.2 6.2 9.7 14.7 1.4 7.4 12.1 3d .52 -4.0 7.8 5.8 11.5 17.1 1.6 7.4 13.2 3e .51 -4.6 8.2 5.7 13.0 19.3 1.8 7.3 13.6 4 43.2 .56 -2.3 6.7 6.1 8.2 15.5 1.5 6.3 13.4 4a .54 -3.0 7.2 5.8 9.1 16.3 1.5 7.9 14.5 4b .52 -4.0 7.8 6.1 11.1 18.2 1.7 8.8 15.4 4c .50 -5.4 8.5 5.7 13.1 20.2 1.9 12.0 19.0 5 8.64 .52 -4.0 7.8 6.4 12.8 33.1 3.1 11.1 31.9 5a .50 -5.4 8.5 6.5 13.5 35.5 3.3 12.1 33.0 6 4.32 .53 -3.4 7.5 5.8 14.4 50.6 4.5 16.5 49.2 6a .52 -4,0 7,9 7.2 17.2 52.4 5.0 17.6 51,? *Based on a pressure potential of -50 cm at a depth of 5 cn. **Based on a volume trie water content (w.c) of 0.33 cm3 cn" 3 at a depth of 5 cn. 64 time = 7.6 6 2 0 days PRESSURE HEAD (cm) Fig 5. SHOWING THE PROCESS OF DRYING FROM AN INITIALLY FLOODED CONDITION FOR CASE 3a IN TABLE 3. 65 -100.00 -60.00 -20.00 20.00 PRESSURE HEAD (cm) FIG. 6 SHOWING THE PROCESS OF WETTING FROM AN INITIALLY TRAFF1 CABLE STATE (AT t=7.6 DAYS FIG. 5) IN RESPONSE TO A STORM OF 10 HR. DURATION AND 2 cm/day INTENSITY. 66 pressure head. In accordance w i t h the r e s t r i c t i o n s imposed on the behaviour of the r e t e n t i o n curves, as the s o i l degrades, the water content at which the pressure head reaches -50 cm of water i n c r e a s e s ( F i g . 4 ); hence the time t o t r a f f i c a b i l i t y may a c t u a l l y decrease f o r a more degraded s o i l . This f i n d i n g i n d i c a t e s t h a t u s i n g pressure head as an index of t r a f f i c a b i l i t y may not be adequate. A set of a l t e r n a t i v e values f o r time to t r a f f i c a b i l i t y was c a l c u l a t e d on the basis- of water content as shown i n Table 3. This water content, was assumed t o be 0.33 cm 3 cm - 3. Since a l l the h y d r o l o g i c parameters are interdependent and change simultaneously as a s o i l s t r u c t u r e degrades, i t would not make p h y s i c a l sense t o keep one parameter constant w h i l e changing the other ones. This f a c t r e s t r i c t s the way i n which the r e s u l t s i n Table 3 can be i n t e r p r e t e d . For. example a comparison of the c o n d i t i o n s l c and 2 i n d i c a t e s t h a t as K s i s reduced by h a l f , the response time decreases by 1 day. This i s c o n t r a d i c t o r y . A more meaningful comparison however would be between l c and 2a or 2b. There i s one important f a c t o r which leads t o over conservatism i n the r e s u l t s presented i n Table 3. This f a c t o r which i s not considered . i n the model, i s the formation of t e n s i o n cracks at the s o i l s u r f a c e . Formation of these cracks speeds up the process of d r y i n g by i n c r e a s i n g the evaporating surf a c e (Ross and Bridge 1984). A l s o , i n the case of a heavy r a i n f a l l event water can be conducted r a p i d l y i n the cracks bypassing the s o i l m atrix (Bouma and De Laat 1981). Therefore 67 a cracked s o i l c o u ld reach a workable stage at a much sho r t e r time than t h a t i n d i c a t e d In Table 3. S e t t i n g an a r b i t r a r y upper l i m i t of 20 days on the response time, the data i n Table 3 show t h a t the values of © s, K s, h a , and B should not be worse than 0.50, 43.2 cm day" 1 (5xl0~6 m s - 1 ) , -5.4 cm, and 8.5 r e s p e c t i v e l y . These are the so c a l l e d design values f o r the h y d r o l o g i c parameters. The o b j e c t i v e of a s o i l and water management programme f o r the Ladner s o i l should t h e r e f o r e be t o produce a s o i l w i t h the above h y d r o l o g i c parameters. To achieve t h i s , the e f f e c t of management p r a c t i c e s . such as c u l t i v a t i o n techniques and a d d i t i o n of organic matter on the h y d r o l o g i c parameters should be i n v e s t i g a t e d . I t should be emphasized t h a t the values of the design h y d r o l o g i c parameters should be chosen to optimize the response time and not t o minimize i t . P l a n t s need a s o i l w i t h good water r e t e n t i o n c a p a c i t y . The s o i l a l s o must have good water r e l e a s e a b i l i t y for. an optimum response time. By a s s i g n i n g u n i t y t o the response time of the f r e s h l y c u l t i v a t e d s o i l , an index f o r the h y d r o l o g i c s t a t u s of a s o i l can be d e f i n e d as shown i n Table 3. An index value of l a r g e r than 1.9 t h e r e f o r e , w i l l not meet the c r i t e r i o n of a 20 day l i m i t on the response time. I t was seen t h a t the e x i s t i n g t r a f f i c a b i l i t y and w o r k a b i l i t y c r i t e r i a are not adequate i n s i t u a t i o n s where s o i l s t r u c t u r e changes due to degradation. I t i s suggested t h a t f u r t h e r research should be done to determine how these 68 c r i t e r i a should change w i t h s o i l degradation. The concept of p l a s t i c l i m i t a l s o i s ambiguous w i t h regard t o degradation and warrants f u r t h e r research. In the l i g h t of the recent advances i n waste treatment p r a c t i c e s , many by-products are being considered f o r use as' s o i l amendments. A d d i t i o n of organic waste to the s o i l w i l l change i t s water r e l e a s e a b i l i t y and hence the value of B. Table 3 shows that i n c r e a s i n g the value of B by 0.50 f o r the f r e s h l y c u l t i v a t e d s o i l ( 0 S = 0.60), increases the response time by almost 2 days. Keeping i n mind th a t at the beginning of the c u l t i v a t i o n season, i t i s a block of workable days which i s important r a t h e r than a s i n g l e workable day, t h i s i n c r e a s e i n the response time i s very s i g n i f i c a n t . Therefore i t i s recommended t h a t the short and long term e f f e c t s of any organic waste being considered as a s o i l amendment, on the water r e l e a s i n g a b i l i t y of a s o i l be c a r e f u l l y i n v e s t i g a t e d . 2.6 SUMMARY AND CONCLUSIONS The c e n t r a l concept of t h i s paper i s q u a n t i f i c a t i o n of s o i l s t r u c t u r e and thereby s o i l management, i n terms of the h y d r o l o g i c behaviour of a s o i l . A mathematical model i s used t o determine the response time and time t o t r a f f i c a b i l i t y f o r a Ladner s o i l . A Campbell (1974) type approach i s used t o c a l c u l a t e the p a r t i a l flow f u n c t i o n from the p a r t i a l water r e t e n t i o n curve. Determination of the e f f e c t of s t r u c t u r e degradation on the h y d r o l o g i c parameters was c a r r i e d out by assuming a reasonable behaviour f o r the p a r t i a l water r e t e n t i o n curves i n response t o d e g r a d a t i o n . A s e t of d e s i g n parameters are determined f o r a d e s i r a b l e h y d r o l o g i c r e s p o n s i v e n e s s . The g o a l of s o i l management f o r the Ladner s o i l c o n s i d e r e d , should be t o achieve and maintain these d e s i g n parameters. F u r t h e r i n v e s t i g a t i o n i s r e q u i r e d t o determined the e f f e c t of d i f f e r e n t s o i l management p r a c t i c e s on the h y d r o l o g i c parameters. 70 REFERENCES Bouma, J . and De Laat, P.J.M. 1981. E s t i m a t i o n of the moisture supply c a p a c i t y of some s w e l l i n g c l a y s o i l s i n the Netherlands. J . Hydrol., 49: 247-259. B u i t e n d i j k , J . 1985. E f f e c t of w o r k a b i l i t y index, degree of mechanization and degree of c e r t a i n t y on the y i e l d of sugar beet. S o i l & T i l l a g e Res., 5: 247-257; Tech. B u l l . ICW 36. Campbell, G.S. 1974. A simple method f o r determining unsaturated c o n d u c t i v i t y from moisture r e t e n t i o n data. S o i l S c i . 117(6): 311-314. C h i l d s , E. C. 1969. An i n t r o d u c t i o n t o the p h y s i c a l b a s i s of s o i l water phenomena. Wiley ( i n t e r s c i e n c e ) , New York. pp. 179-201. Freeze, R.A. 1969. The mechanism of n a t u r a l ground-water recharge and discharge: 1. One dimensional, v e r t i c a l , unsteady, unsaturated flow above a recharging or d i s c h a r g i n g ground-water flow system. Water Resources Res. 5( 1 ) : .153-171. H i l l e l , D. 1980. Fundamentals of S o i l P h y s i c s . Academic Press, Inc. New York, pp. 347-351. P a u l , C.L. and de V r i e s , J . 1979. E f f e c t of s o i l water s t a t u s and s t r e n g t h on t r a f f i c a b i l i t y . Can. J . S o i l S c i . 59: 313-324. P a u l , C.L. and de V r i e s , J . 1983a. S o i l t r a f f i c a b i l i t y i n s p r i n g . 1. S i m u l a t i o n of the drainage process. Can. J . S o i l S c i . 63: 15-26. P a u l , C.L. and de V r i e s , J . 1983b. S o i l t r a f f i c a b i l i t y i n s p r i n g . 2. P r e d i c t i o n and the e f f e c t of subsurface drainage. Can. J . S o i l S c i . 63: 27-35. Ross, P.J. and B r i d g e , B.J. 1984. MICCS: a model of i n f i l t r a t i o n i n t o c r a c k i n g c l a y s o i l s . I n : "The p r o p e r t i e s and u t i l i z a t i o n of c r a c k i n g c l a y s o i l s ' (ed. J.W. McGarity, E.H. Hoult, and H.B. So) pp. 155-163. van Genuchten, M. Th. 1980. A closed-form equation f o r p r e d i c t i n g the h y d r a u l i c c o n d u c t i v i t y of unsaturated s o i l s . S o i l S c i . Soc. Am. J . 44: 892-898. van Wijk, A.L.M. and Feddes, R.A. 1982. A model approach to the e v a l u a t i o n of drainage e f f e c t s . I n : "Land Drainage' (ed. M.J. Gardiner) pp. 131-149. W h i s l e r , F.D. and Watson, R.R. 1968. One-dimensional g r a v i t y drainage of uniform columns of porous m a t e r i a l s . J . Hydrol. 6: 277-296. 71 W i n d , G.P. 1 9 7 6 . A p p l i c a t i o n o f a n a l o g a n d n u m e r i c a l m o d e l s t o i n v e s t i g a t e t h e i n f l u e n c e o f d r a i n a g e o n w o r k a b i l i t y i n s p r i n g . N e t h . J . A g r i c . S c i . 24: 1 5 5 - 1 7 2 . W i n d , G.P. a n d v a n D o o r n e , W. 1 9 7 5 . A n u m e r i c a l m o d e l f o r t h e s i m u l a t i o n o f u n s a t u r a t e d v e r t i c a l f l o w o f m o i s t u r e i n s o i l s . J . H y d r . 2 4 : 1-20. 72 CHAPTER 3 DESCRIPTIVE MODEL OF SEAL FORMATION AND MANAGEMENT MODEL 73 DESCRIPTIVE MODEL OF SEAL FORMATION AND MANAGEMENT MODEL 3.1 ABSTRACT A d e s c r i p t i v e model f o r the fo r m a t i o n of a s u r f a c e and an i n t e r n a l s e a l i s o f f e r e d f o r a Lower F r a s e r V a l l e y (LFV) lowland s o i l . The s e a l f o r m a t i o n mechanism suggested i s a d e s c r i p t i o n of the dynamic changes " which occur i n the s t r u c t u r e of the s u r f a c e l a y e r of a bare s o i l as the r e s u l t of r a i n f a l l events. Some of the processes l e a d i n g t o the fo r m a t i o n of a s u r f a c e s e a l a re: s u r f a c e d i s a g g r e g a t i o n , s o i l s p l a s h , s e d i m e n t a t i o n i n ponded d e p r e s s i o n s , and development of a s u c t i o n f o r c e a t the bottom of the s u r f a c e l a y e r . A management model i s proposed w i t h the o b j e c t i v e of improving s o i l h y d r o l o g i c r e s p o n s i v e n e s s . The model comprises a s y s t e m a t i c c h e c k i n g of c o n d i t i o n s which l e a d t o the development of a s u r f a c e s e a l , and p r o v i d e s a p p r o p r i a t e management remedies. 3.2 INTRODUCTION Formation of a s u r f a c e s e a l i s an important process because of i t s adverse e f f e c t s on i n f i l t r a t i o n and s e e d l i n g emergence. Sur f a c e s e a l s are formed through the impact of water d r o p l e t s from . r a i n or s p r i n k l e r i r r i g a t i o n , and by s l a k i n g of the s u r f a c e aggregates i n a f l o o d e d c o n d i t i o n . S u r f a c e s e a l f o r m a t i o n i s a complex process and has been 74 e x t e n s i v e l y s t u d i e d d u r i n g the l a s t four decades. Mclntyre (1958 a & b) d e s c r i b e d the process of surface s e a l i n g and observed t h a t d e p o s i t i o n of f i n e p a r t i c l e s , the washing of these i n t o the s o i l m a t r i x , and compaction caused by r a i n drop impact were i n v o l v e d . He noted two d i s t i n c t l a y e r s at the s u r f a c e : a- s k i n s e a l 0.1 mm i n t h i c k n e s s , and a "washed-in" re g i o n 1.5 t o 2.5 mm t h i c k . Other i n v e s t i g a t o r s however, f a i l e d to v e r i f y the e x i s t e n c e of a washed-in l a y e r . Tackett and Pearson (1965), and Evans and Buol (1968) s t u d i e d surface s e a l s formed by simulated r a i n f a l l on a sandy loam. They found the s e a l s t o be very dense, and s t a t e d that c l a y p a r t i c l e o r i e n t a t i o n plays an important r o l e i n the s e a l i n g phenomenon. F a l a y i and Bouma (1975) i n v e s t i g a t e d the e f f e c t of d i f f e r e n t management p r a c t i c e s on the h y d r a u l i c c o n d u c t i v i t y of the surface s e a l . They found t h a t d i f f e r e n t t i l l a g e p r a c t i c e s , and crop r o t a t i o n had s i g n i f i c a n t e f f e c t on the conductance of the surface s e a l . They a l s o concluded t h a t conductances of surface s e a l s formed under short-term high- energy experimental r a i n f a l l were not s i g n i f i c a n t l y d i f f e r e n t from those formed under n a t u r a l c o n d i t i o n s . However, they n o t i c e d a s i g n i f i c a n t d i f f e r e n c e between the morphology of surface s e a l s formed under d i f f e r e n t c o n d i t i o n s . Morin et a l . (1981) hypothesized t h a t the s e a l i n g e f f i c i e n c y of a surface s e a l i s enhanced by s u c t i o n forces which cause the c l a y p a r t i c l e s t o be arranged i n t o a continuous dense s k i n . The s u c t i o n f o r c e s at the s o i l - s e a l 75 i n t e r f a c e are c r e a t e d as a r e s u l t of the l a r g e d i f f e r e n c e s i n h y d r a u l i c c o n d u c t i v i t y between the s e a l and the u n d e r l y i n g s o i l . The s u c t i o n mechanism accounts f o r the s t a b i l i t y of the s e a l and the s i m i l a r i t y i n v a l u e s of s e a l h y d r a u l i c c o n d u c t i v i t y between s o i l s v a r y i n g g r e a t l y i n t h e i r t e x t u r e and mineralogy. Bonsu (1987) o f f e r s a p h y s i c a l l y - b a s e d model d e s c r i b i n g a mechanism f o r s u r f a c e s e a l i n g i n the context of aggregate s t a b i l i t y . He shows 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 c u l t i v a t i o n l a y e r t o be p o s i t i v e l y c o r r e l a t e d w i t h the aggregate s t a b i l i t y and n e g a t i v e l y c o r r e l a t e d w i t h the m i n e r a l matter c o n t e n t . The o b j e c t i v e of t h i s chapter i s t o suggest a s i n g l e d e s c r i p t i v e model of the processes which c o n t r i b u t e t o the f o r m a t i o n o f a s u r f a c e and an i n t e r n a l s e a l . T h i s model i s based, i n p a r t on the experiments and o b s e r v a t i o n s of chapter 1 and those which have been r e p o r t e d i n the l i t e r a t u r e . A management model i s a l s o presented w i t h the o b j e c t i v e of p r e v e n t i n g f o r m a t i o n of the s e a l s and improving s o i l h y d r o l o g i c r e s p o n s i v e n e s s . 3.3 MECHANISMS OF SURFACE AND THE INTERNAL SEAL FORMATION In d e s c r i b i n g the mechanisms of s u r f a c e and i n t e r n a l s e a l f o r m a t i o n , the i n i t i a l c o n d i t i o n i s a c u l t i v a t e d bare s o i l a t the end of the c u l t i v a t i o n season. The process of s e a l i n g begins w i t h the f i r s t r a i n f a l l event and c o n t i n u e s throughout 76 t h e o f f - s e a s o n p e r i o d . T h e r e f o r e , we b e g i n w i t h w h a t i s c a l l e d a n o p e n s o i l m o d e l ( F i g . 1 ) . As t i m e p r o c e e d s a n d t h e s o i l c o n d i t i o n s c h a n g e , d i f f e r e n t m o d e l s a r e i n t r o d u c e d . T h e s e m o d e l s a r e r e f e r r e d t o a s t h e s u r f a c e s e a l m o d e l , i n t e r n a l s e a l m o d e l , a n d t h e c r a c k e d s o i l m o d e l . The t e r m m o d e l i s u s e d t o i n d i c a t e a s e t o f p r o c e s s e s , a n d t h e e n t i r e p i c t u r e i n F i g . 1 i s r e f e r r e d t o a s t h e m e c h a n i s m s o f s e a l f o r m a t i o n . I t i s a s s u m e d t h a t t h e m e c h a n i s m s o f s e a l f o r m a t i o n a r e t h e same i n t h e f i e l d a s i n t h e l a b o r a t o r y . I n t h i s c h a p t e r t h e r e s u l t s o f t h e e x p e r i m e n t s i n c h a p t e r 1, c o n d u c t e d i n t h e l a b o r a t o r y , a r e e x t e n d e d t o t h e f i e l d c o n d i t i o n . I n F i g . 1 e a c h b o x d e s c r i b e s a p r o c e s s . The l o w e r b o x s i m p l y shows some o f t h e i m p o r t a n t d e p e n d e n t v a r i a b l e s o f t h e u p p e r b o x . F o r e x a m p l e , t h e e f f e c t o f r a i n d r o p i m p a c t i s a f u n c t i o n o f t h e r a i n f a l l e n e r g y . 3.4 DISCUSSION The m e c h a n i s m o f s u r f a c e s e a l f o r m a t i o n c a n be d i v i d e d i n t o f i v e s t a g e s : 1. P r e - o v e r l a n d f l o w a s d e s c r i b e d b y T a r c h i t z k y e t a l . ( 1 9 8 4 ) . 2. P o s t - o v e r l a n d f l o w , f o r m a t i o n o f a p o n d i n t h e d e p r e s s i o n s . 3. S e t t l e m e n t o f t h e s u s p e n d e d l o a d i n t h e p o n d e d d e p r e s s i o n s . T h i s s t a g e may o r may n o t c o n t a i n e a r l y r e c e s s i o n o f t h e p o n d , d e p e n d i n g o n w h e t h e r o r n o t a n e f f e c t i v e s e a l i s a l r e a d y p r e s e n t . I f a n e f f e c t i v e s e a l i s n o t a l r e a d y i n p l a c e a n d t h e c u l t i v a t i o n l a y e r i s s a t u r a t e d , 77 £2 >OPENSOILMODEL RAIN DROP IMPACT - RAIN DROP ENERGY DISAGGREGATION INTO PRIMARY PARTICLES MOVEMENT OF SUSPENSION TOUA'Rrt nFPRFssTnw; PONDING - TOPOGRAPHY - RAIN INTENSITY - RAIN DURATION - INFILTRATION RATE SETTLEMENT OF SUSPENDED LOAD INTO A SIMPLE ORDER CONCENTRATION OF SUSPENSION - DEPTH OF POND FORMATION OF WASHED-OUT LAYER DEPOSITION OF CLAY PARTICLES BY MASS FLOW SURFACE SEAL MODEL DRYING INCREASE IN TENSION - W.C. OF - SOIL CULT. LAYER TEXTURE ORIENTATION AND ARRANGEMENT OF CLAY PARTICLES INTO A CONTINUOUS & DENSE LAYER CRACKING - DRYING PERIOD - E.T. - AGG. STABILITY - RAIN INTENSITY SOIL SPLASH - SOIL TEXTURE - AGG. STABILITY - RAIN INTENSITY FILLING OF MICRO-DEPRESSIONS - POROSITY - SURFACE ROUGHNESS - AGG. CLOD SIZE SURFACE COMPACTION POROSITY APPEARANCE OF MICRO-PONDS CONTAINING FINE SEDIMENTS - SOIL TEXTURE - RAIN INTENSITY - AGG• STABILITY HIGH ENERGY AIR BUBBLES ESCAPING - RAIN INTENSITY - POROSITY SURFACE SEAL MODEL INTERNAL SEAL MODEL WETTING | MOVEMENT OF SUSPENSION INTO CRACKS, CAVITIES & MACRO-PORES - POROSITY - BULK DENSITY - SURFACE ROUGHNESS ••sir-.. TRANSPORT OF FINES INTO SOIL PROFILE . BY CONVECTIVE FLOW POROSITY, PORE SIZE DIST. - CULT. PRACTICE CRACKED SOIL MODEL FIG. 1 DESCRIPTIVE MECHANISMS OF SEAL FORMATION 78 t h e n t h e f o u r t h s t a g e i s : 4. R e c e s s i o n o f t h e p o n d a n d t h e f o r m a t i o n o f a s u c t i o n f o r c e a t t h e b o t t o m o f t h e s u r f a c e l a y e r . 5. F o r m a t i o n o f t e n s i o n c r a c k s a s s o i l d r i e s f u r t h e r . I t was d i s c u s s e d i n t h e f i r s t two c h a p t e r s t h a t f o r m a t i o n o f t h e s e c r a c k s w i l l d r a m a t i c a l l y i m p r o v e t h e h y d r o l o g i c b e h a v i o u r o f t h e s o i l . I n t h e f i r s t s t a g e o f s u r f a c e s e a l f o r m a t i o n ( o p e n s o i l m o d e l ) two i m p o r t a n t m e c h a n i s m s a r e r e s p o n s i b l e f o r g e n e r a t i n g t h e s o u r c e m a t e r i a l f o r t h e f o r m a t i o n o f t h e s e a l s . One i s t h e d e s t r u c t i o n o f t h e a g g r e g a t e s b y t h e r a i n f a l l i m p a c t a n d t h e o t h e r i s t h e s o i l s p l a s h w h i c h d i s t r i b u t e s t h e f i n e p a r t i c l e s a c r o s s t h e s u r f a c e ( A l - D u r r a h a n d B r a d f o r d 1 9 8 2 ; B u b e n z e r a n d J o n e s 1 9 7 1 ) . As shown i n c h a p t e r 1, t h e m o s t i m p o r t a n t f a c t o r i n c o n t r o l l i n g t h e c o u r s e o f e v e n t s i n t h i s s t a g e i s t h e a g g r e g a t e s t a b i l i t y o f t h e s o i l . A g g r e g a t e d i s i n t e g r a t i o n a n d t h e c o n s e q u e n t s p l a s h l e a d t o t h e f i l l i n g o f m i c r o - d e p r e s s i o n s , c a u s i n g a r e d u c t i o n o f t h e s u r f a c e p o r o s i t y a n d c o n s e q u e n t l y c a u s i n g s u r f a c e c o m p a c t i o n due t o t h e r a i n d r o p i m p a c t ( T a c k e t t a n d P e a r s o n , 1 9 6 5 ; M i t c h e l l a n d J o n e s , 1 9 7 8 ) . C l o s e o b s e r v a t i o n o f t h e g e n e r a t i o n o f t h e s u r f a c e s e a l i n t h e s e c o n d e x p e r i m e n t o f c h a p t e r 1 r e v e a l e d t h a t a s i n f i l t r a t i o n r a t e d e c r e a s e s , m i c r o - p o n d s c o n t a i n i n g s u s p e n d e d f i n e p a r t i c l e s a p p e a r a t t h e s u r f a c e . As a l a y e r o f f r e e w a t e r c o v e r s t h e s u r f a c e , some o f t h e e n t r a p p e d a i r w i t h i n t h e s o i l p r o f i l e e s c a p e s i n a v i o l e n t manner ( b a s e d o n t h e o b s e r v a t i o n s o f t h e e x p e r i m e n t s i n c h a p t e r 1 ) , f u r t h e r b r e a k i n g up t h e 79 a g g r e g a t e s a n d p u t t i n g f i n e s e d i m e n t i n t o s u s p e n s i o n ( F i g . 1 ) . T h e s e p r o c e s s e s c r e a t e t h e s o u r c e m a t e r i a l n e c e s s a r y f o r t h e f o r m a t i o n o f a s e a l , a n d s e t i n m o t i o n t h e s e a l i n g p r o c e s s . The s e c o n d s t a g e c o n s i s t s o f t h e b e g i n n i n g o f o v e r l a n d f l o w a n d t h e c r e a t i o n o f a s u r f a c e p o n d i n t h e d e p r e s s i o n a r e a s ( w e t t i n g p r o c e s s o f s u r f a c e s e a l m o d e l , F i g . 1 ) . Much w o r k h a s b e e n done t o d e s c r i b e t h e p r o c e s s o f o v e r l a n d f l o w i n t e r m s o f i t s p o t e n t i a l f o r c a u s i n g e r o s i o n a n d t r a n s p o r t i n g f i n e p a r t i c l e s . T h i s p r o c e s s i s f u l l y d e s c r i b e d b y T a r c h i t z k y e t a l . ( 1 9 8 4 ) , G a b r i e l a n d M o l d e n h a u e r ( 1 9 7 8 ) , E p s t e i n a n d G r a n t ( 1 9 6 7 ) , a n d many o t h e r s . I n c h a p t e r 1 t h e s o u r c e m a t e r i a l f o r t h e g e n e r a t i o n o f t h e s u r f a c e s e a l ( e x p e r i m e n t 2) was c r e a t e d i n - s i t u a s t h e r e s u l t o f d i s i n t e g r a t i o n o f l o c a l a g g r e g a t e s a l o n e . I n t h e f i e l d , h o w e v e r , s o u r c e m a t e r i a l i s a l s o t r a n s p o r t e d i n t o t h e d e p r e s s i o n s , w h e r e t h e m o s t e f f e c t i v e s u r f a c e s e a l s a r e o f t e n f o r m e d . I n t h e t h i r d s t a g e , t h e i m p o r t a n t p r o c e s s o f p a r t i c l e s e t t l e m e n t i n t h e p o n d e d d e p r e s s i o n t a k e s p l a c e . The a u t h o r i s n o t a w a r e o f a n y s t u d i e s d e s c r i b i n g t h i s s t a g e o f t h e s u r f a c e s e a l f o r m a t i o n . O r i g i n a l l y t h e a r r a n g e m e n t o f t h e p a r t i c l e s i n t h e s o i l i s v e r y random. F i n e a n d c o a r s e p a r t i c l e s c o - e x i s t r a n d o m l y . The c o n g l o m e r a t i o n o f p a r t i c l e s i n t o a g g r e g a t e s c o n s i s t s o f f i n e a n d c o a r s e p a r t i c l e s t o g e t h e r . B u t u p o n r a i n f a l l , when t h e a g g r e g a t e s b r e a k up i n t o p r i m a r y p a r t i c l e s w h i c h a r e t h e n p u t i n t o s u s p e n s i o n , t h i s r a n d o m n e s s i n a r r a n g e m e n t i s l o s t . Upon s e t t l e m e n t , a s g o v e r n e d b y S t o k e ' s l a w , t h e l a r g e r 80 p a r t i c l e s s e t t l e f i r s t , f o l l o w e d b y f i n e r a n d f i n e r p a r t i c l e s . T h i s s i m p l e a r r a n g e m e n t , w h e r e f i n e r p a r t i c l e s b l o c k t h e p o r e s p a c e s i n b e t w e e n t h e c o a r s e r p a r t i c l e s , r e d u c e s t h e p o r o s i t y a n d t h e r e f o r e t h e i n f i l t r a t i o n r a t e . A l a y e r o f c o a r s e p a r t i c l e s was o b s e r v e d i m m e d i a t e l y b e l o w t h e s u r f a c e s e a l s , when v i e w e d u n d e r a m i c r o s c o p e . F i g s . 2a,b show two s u c h s e a l s , w h e r e t h e c o a r s e r l a y e r i s i d e n t i f i e d b y t h e l i g h t e r c o l o u r i n g . N e x t s t e p i s t h e d e p o s i t i o n o f c l a y p a r t i c l e s b y mass f l o w , a s t h e p o n d r e c e d e s , t o f o r m a s o - c a l l e d M c l n t y r e " s k i n l a y e r " ( M c l n t y r e 1958 a & b ) . S t u d y i n g m a g n i f i e d p i c t u r e s o f s u r f a c e s e a l s d i d n o t r e v e a l M c l n t y r e ' s " w a s h e d - i n " l a y e r . T h i s a l s o was t h e c o n c l u s i o n o f T a r c h i t z k y e t a l . ( 1 9 8 4 ) . F o r m a t i o n o f a s u c t i o n f o r c e i s t h o u g h t t o d e v e l o p i n t h e f o u r t h s t a g e o f t h e m e c h a n i s m o f t h e s u r f a c e s e a l f o r m a t i o n ( d r y i n g p r o c e s s o f s u r f a c e s e a l m o d e l , F i g . 1 ) . H o w e v e r , i n r e a l i t y a s u c t i o n d e v e l o p s a t t h e s u r f a c e d u r i n g t h e i n i t i a l s t a g e s o f t h e s e a l i n g p r o c e s s . I n a n o n - s a t u r a t e d c o n d i t i o n , t h i s s u c t i o n f o r c e a t t h e s u r f a c e i s d e t e r m i n e d b y t h e s u c t i o n a t t h e w e t t i n g f r o n t ( M o r i n e t a l . 1 9 8 1 ) . I n c h a p t e r 1 ( e x p e r i m e n t 3 ) , t h e f o r m a t i o n o f a s u c t i o n f o r c e was p r e v e n t e d f r o m o c c u r r i n g b y k e e p i n g t h e e n t i r e c u l t i v a t i o n l a y e r s a t u r a t e d a t a l l t i m e s . The r e s u l t shows t h a t a s u r f a c e s e a l d i d n o t f o r m u n t i l t h e s o i l was a l l o w e d t o d r y . I n t h e t h i r d e x p e r i m e n t o f c h a p t e r 1 t h e d r y i n g p r o c e s s o c c u r s d u r i n g t h e p e r i o d when t h e w a t e r t a b l e i s l o w e r e d t o t h e b a s e o f t h e c o l u m n a n d t h e p r e s s u r e h e a d d i s t r i b u t i o n FIG. 2 SHOWING SIDE VIEWS OF TWO SURFACE SEALS. A COARSE LAYER AT THE BOTTOM OF EACH SEAL TOPPED WITH FINER PARTICLES CAN BE IDENTIFIED. 82 a p p r o a c h e s e q u i l i b r i u m . As d r a i n a g e commences, a s u c t i o n f o r c e d e v e l o p s a t t h e b o t t o m o f t h e s e a l , w h i l e t h e s e a l i t s e l f r e m a i n s t e n s i o n - s a t u r a t e d due t o i t s l a r g e ( n e g a t i v e ) a i r e n t r y v a l u e . D e v e l o p m e n t o f t h i s t e n s i o n f o r c e w i t h i n t h e s e a l c a u s e s : 1. a r r a n g e m e n t o f p a r t i c l e s i n t o a more c o n t i n u o u s a n d d e n s e r c o n f i g u r a t i o n a n d , 2. o r i e n t a t i o n o f c l a y p a r t i c l e s i n t o a more o r d e r e d a r r a n g e m e n t ( M o r i n e t a l . 1 9 8 1 / T a c k e t t a n d P e a r s o n 1 9 6 5 ) . E v a n s a n d B u o l ( 1 9 6 8 ) , s t u d i e d t h e m i c r o m o r p h o l o g i c a l f e a t u r e s o f s u r f a c e s e a l s f o r m e d u n d e r d i f f e r e n t c o n d i t i o n s . T h e y r e p o r t e d o b s e r v i n g numerous h o r i z o n t a l l y o r i e n t e d p l a t e - l i k e p a r t i c l e s i n t h e s u r f a c e s e a l s . M o r i n e t a l . ( 1 9 8 1 ) , a t t r i b u t e t h e s e a l i n g e f f i c i e n c y o f a s u r f a c e s e a l t o s u c t i o n f o r c e s w h i c h d e v e l o p a t t h e b o t t o m o f t h e s u r f a c e l a y e r . The t h i r d e x p e r i m e n t i n c h a p t e r 1 showed t h e d e v e l o p m e n t o f a s u c t i o n f o r c e t o be a n e s s e n t i a l f a c t o r i n t h e f o r m a t i o n o f a s u r f a c e s e a l . The p r o c e s s o f i n t e r n a l s e a l f o r m a t i o n c a n be d i v i d e d i n t o t w o s t a g e s : 1. Downward movement o f w a t e r c a r r y i n g f i n e p a r t i c l e s . 2. D e p o s i t i o n o f t h e f i n e p a r t i c l e s o n t o p o f a c o m p a c t e d p a n . I n t h e s i m u l a t e d s o i l c o l u m n ( f i f t h e x p e r i m e n t i n c h a p t e r 1 ) , i t was o b s e r v e d t h a t t h e downward f l o w o f w a t e r t h r o u g h a f r e s h l y c u l t i v a t e d s o i l c a r r i e d a l a r g e amount o f s u s p e n d e d l o a d t o t h e t o p o f t h e c o m p a c t e d p a n . T h e s e f i n e m a t e r i a l s w e r e g e n e r a t e d e i t h e r b y t h e d i s i n t e g r a t i o n o f t h e s u r f a c e a g g r e g a t e s due t o t h e r a i n d r o p i m p a c t , o r t h e y w e r e g e n e r a t e d 83 a s a r e s u l t o f t h e c u l t i v a t i o n p r a c t i c e . A t t h e t o p o f t h e p a n , f i n e s e d i m e n t p a r t i c l e s w e r e d e p o s i t e d f o r m i n g a n i n t e r n a l s e a l . I n f a c t , d u r i n g t h e s i m u l a t e d c u l t i v a t i o n ( s e c o n d e x p e r i m e n t i n c h a p t e r 1 ) , a n d a t t h e t i m e o f t h e d i s m a n t l i n g o f t h e s i m u l a t e d c o l u m n r e f e r r e d t o a b o v e , c l o s e o b s e r v a t i o n o f t h e s o i l s t r u c t u r e showed t h a t s m a l l a n d v e r y l o c a l i z e d m i c r o - s e a l s h a d f o r m e d t h r o u g h o u t t h e c u l t i v a t i o n l a y e r a t d e a d e n d p o r e s a n d a t t h e b o t t o m o f c l o s e d c a v i t i e s . B u t t h i s e f f e c t was much more p r o n o u n c e d a t t h e i n t e r f a c e b e t w e e n t h e t i l l e d l a y e r a n d t h e u n d i s t u r b e d c o m p a c t e d p a n . 3.5 S O I L MANAGEMENT The h y d r o l o g i c o b j e c t i v e o f s o i l management s h o u l d be t o p r e v e n t t h e f o r m a t i o n o f a n e f f e c t i v e s u r f a c e a n d i n t e r n a l s e a l . F o r t h i s p u r p o s e , a s e t o f " d e s i g n p a r a m e t e r s " ( c h a p t e r 2 ) , c o n t r o l l i n g t h e s o i l h y d r o l o g i c b e h a v i o u r , s h o u l d be s p e c i f i e d a c c o r d i n g t o t h e r e q u i r e m e n t s o f a r e g i o n . Management p r a c t i c e s s h o u l d t h e n be d e s i g n e d i n o r d e r t o meet t h e c r i t e r i a a s s p e c i f i e d b y t h e s e d e s i g n p a r a m e t e r s . I t was shown i n c h a p t e r 1 t h a t i f a s o i l i s l e f t i n a l o o s e , b a r e a n d t h e r e f o r e u n s t a b l e s t a t e a f t e r t h e f a l l h a r v e s t , i t s s t r u c t u r e w i l l d e g r a d e a s t h e r e s u l t o f r a i n d r o p i m p a c t . T h e r e a r e a number o f s t e p s t h a t c a n b e u n d e r t a k e n i n t h e a r e a o f s o i l management t h a t c o u l d s t a b i l i z e t h e s t r u c t u r e o f t h e s o i l s u r f a c e a n d t h e r e f o r e m i n i m i z e t h e r e d u c t i o n i n t h e h y d r o l o g i c r e s p o n s i v e n e s s . 84 T h e s e management p r a c t i c e s a r e d e s c r i b e d i n t h e m o d e l p r e s e n t e d i n F i g . 3 a n d a r e d i s c u s s e d b e l o w i n t e r m s o f p a t h w a y s n u m b e r e d 1 t o 7. E a c h p a t h w a y c o n s i s t s o f one o r more s o i l a n d w a t e r management p r o b l e m s a n d s u g g e s t e d r e m e d i e s . P a t h w a y 1. The m o d e l s t a r t s a t t h e t o p w i t h t h e q u e s t i o n , " I s f r e e w a t e r p r e s e n t a t t h e s o i l s u r f a c e ? " B e c a u s e o f t h e n a t u r e a n d f r e q u e n c y o f o f f - s e a s o n r a i n f a l l e v e n t s , i t i s n o t a l w a y s e c o n o m i c a l l y f e a s i b l e t o meet t h e d r a i n a g e r e q u i r e m e n t f o r a l l r a i n f a l l e v e n t s a n d t h e r e b y p r e v e n t f r e e w a t e r f r o m e v e r b e i n g p r e s e n t o n t h e s o i l s u r f a c e (B.C. A g r i c u l t u r a l D r a i n a g e M a n u a l , 1 9 8 6 ) . B u t f o r p a t h w a y 1. w h i c h i s t h e " i d e a l s o i l a n d w a t e r c o n d i t i o n p a t h w a y " , a f t e r a n e x c e p t i o n a l r a i n f a l l e v e n t t h a t c a u s e s f l o o d i n g , f r e e w a t e r o n t h e s u r f a c e w i l l r e c e d e q u i c k l y , a n d t h e w a t e r t a b l e w i l l d r o p q u i c k l y t o t h e d e s i g n d e p t h , r e t u r n i n g t h e s o i l t o a t r a f f i c a b l e s t a t e w i t h i n a f e w d a y s . I n s u c h a s o i l a f u n c t i o n i n g d r a i n a g e s y s t e m i s p r e s e n t a n d a p r o p e r c o v e r c r o p o r c r o p r e s i d u e f o r o f f - s e a s o n s u r f a c e p r o t e c t i o n e x i s t s . The s o i l i s o f g o o d t i l t h a n d h a s a s t a b l e s t r u c t u r e . T h i s o p t i m u m a n d t h e r e f o r e d e s i r a b l e p a t h w a y i s d e p i c t e d i n t h e c e n t r e o f F i g . 3, f r o m t o p t o b o t t o m . A n y d e v i a t i o n f r o m t h i s p a t h w a y c a n be r e m e d i e d a n d u l t i m a t e l y l o o p e d b a c k t o t h e i d e a l p a t h . P a t h w a y 2. I f f r e e w a t e r i s p r e s e n t o n t h e s u r f a c e a n d no d r a i n a g e s y s t e m i s p r e s e n t , t h e remedy i s t o i n s t a l l a d r a i n a g e s y s t e m . D e p e n d i n g o n t h e r e q u i r e m e n t s a n d c o n d i t i o n s o f a f i e l d , a s u r f a c e , o r s u b - s u r f a c e , d r a i n a g e s y s t e m o r 85 repair pump Install inter- mediate drains clean up •ysten system plugged spacing too vide need laterals new f placed drainage V too criteria shallow free water on surface will recede quickly check hydro l o g i c parameters © surface seal w i l l forts. to remedy provide: © provide appropriate cover crop subsoillng: promote earthworm population © add organic natter © farmer education FIG. 3 A DESCRIPTIVE MANAGEMENT MODEL FOR LOWER FRASER VALLEY LOWLAND SOILS. DEPICTED AT THE CENTRE IS AN IDEAL SOIL AND WATER MANAGEMENT PATH. 86 b o t h , may be c o n s i d e r e d . P a t h w a y 3..... R e l a t i v e t o t h i s p a t h , F i g . 3 i s s e l f e x p l a n a t o r y . I f a s u b - s u r f a c e d r a i n a g e s y s t e m i s p r e s e n t b u t i t i s n o t f u n c t i o n i n g , t h e c a u s e ( s ) m u s t b e d e t e r m i n e d b y a s y s t e m a t i c c h e c k i n g p r o c e d u r e a s i n d i c a t e d i n F i g . 3. C o n t r o l o f t h e w a t e r t a b l e i s a d i r e c t means o f c o n t r o l l i n g t h e t r a f f i c a b i l i t y o f l o w l a n d s o i l s ( P a u l a n d de V r i e s , 1 9 7 9 ) . P a t h w a y 4. T h e p r e s e n c e o f a c o v e r c r o p dampens t h e p o u n d i n g , a c t i o n o f t h e r a i n d r o p s , k e e p s t h e s o i l s u r f a c e o p e n , a n d h e l p s m e e t t h e i n f i l t r a t i o n r e q u i r e m e n t . T h i s r e q u i r e m e n t i s met i f r a i n f a l l d o e s n o t , r e s u l t i n p o n d i n g . I t s h o u l d be u s e d i n management a s a t o o l t o r e d u c e s o i l e r o s i o n a n d d e g r a d a t i o n . M o r e r e s e a r c h i s r e q u i r e d i n t h i s a r e a i n o r d e r t o d e t e r m i n e t h e t y p e a n d amount o f c o v e r c r o p n e e d e d t o p r e v e n t s u r f a c e s e a l f o r m a t i o n . I n t h e a b s e n c e o f a c o v e r c r o p , p r o p e r management o f c r o p r e s i d u e a f t e r h a r v e s t c a n p r o v i d e a n e f f e c t i v e a l t e r n a t i v e . P a t h w a y 5. U n d e r a c o n d i t i o n w h e r e a s u b - s u r f a c e d r a i n a g e s y s t e m i s i n p l a c e , b u t w h e r e a t r a f f i c p a n i s p r e s e n t , w a t e r may r e m a i n p e r c h e d o n t o p o f t h e p a n , o r o n t h e i n t e r n a l s e a l a f t e r r e c e s s i o n o f t h e w a t e r t a b l e t o t h e d e s i g n d e p t h . T h e management r e m e d y f o r t h i s c o n d i t i o n i s s u b - s o i l i n g . I t i s p r e f e r r e d t h a t s u b - s o i l i n g b e d o n e w i t h t h e p a r a p l o u g h , w h i c h o p e n s u p t h e n a t u r a l c r a c k s i n t h e s o i l w i t h o u t t r a n s f e r r i n g t h e s u b - s u r f a c e s o i l t o t h e s u r f a c e ( J . de V r i e s , p e r s o n a l c o m m u n i c a t i o n 1 9 8 7 ) . P a t h w a y 6. A p p r o p r i a t e c u l t i v a t i o n m e t h o d s a l o n g w i t h t h e 87 supply of o r g a n i c matter t o the s o i l as w e l l as maintenance of a cover crop and earthworm p o p u l a t i o n would improve the s o i l s t r u c t u r e . Pathway 7. When a s o i l of good t i l t h i s d r i v e n on and/or c u l t i v a t e d under u n t r a f f i c a b l e / u n w o r k a b l e c o n d i t i o n s , compaction/puddling w i l l r e s u l t , which i n t u r n w i l l r e s u l t i n the accumulation of r a i n water on the s o i l s u r f a c e and the t r i g g e r i n g of s u r f a c e s e a l f o r m a t i o n . For t h i s pathway, i n p a r t i c u l a r , the remedy i s e d u c a t i o n of the farmers. To ensure the e f f e c t i v e n e s s of the above management p r a c t i c e s , s o i l h y d r o l o g i c parameters should be measured a t the end of each season and compared w i t h the d e s i g n parameters. The above management model has the p o t e n t i a l t o be used as an a l g o r i t h m f o r d e v i s i n g an e x p e r t system f o r the purposes of s o i l management. An expert system i s a da t a base which c o n t a i n s the o p i n i o n s and experience of many d i f f e r e n t e x p e r t s . 3.6 SUMMARY AND CONCLUSIONS A d e s c r i p t i v e model i s pre s e n t e d showing the mechanisms t h a t c o n t r i b u t e t o the r e d u c t i o n of the h y d r o l o g i c r e s p o n s i v e n e s s i n a lowland s o i l . A new phenomenon r e f e r r e d t o as i n t e r n a l s e a l was i d e n t i f i e d as forming a t the top of the compacted pan, and one mechanism f o r i t s form a t i o n i s o f f e r e d based on l a b o r a t o r y experiments (chapter 1 ) . 88 The mechanism of, the s e a l f o r m a t i o n developed i n t h i s c hapter i s i n r e a l i t y a d e s c r i p t i o n of the dynamic changes which occur i n the s t r u c t u r e of a bare s o i l i n response to r a i n f a l l e vents. As s o i l s t r u c t u r e changes, s o i l parameters which govern the s o i l h y d r o l o g i c behaviour w i l l a l s o change. T h e r e f o r e , any model which i s designed t o p r e d i c t s o i l h y d r o l o g i c behaviour over a long p e r i o d of time, should take these changes i n t o account t o a v o i d s e r i o u s e r r o r s . To prevent f o r m a t i o n of the s u r f a c e and the i n t e r n a l s e a l , a management model i s presented. P r a c t i c i n g the recommendations made i n t h i s model w i l l ensure the maintenance of an optimum h y d r o l o g i c r e s ponsiveness and a s h o r t response time f o r the purposes of t i m e l y c u l t i v a t i o n and s e e d i n g / p l a n t i n g i n the s p r i n g . 89 REFERENCES Al-Durrah, M.M. and Bradford, J.M- 1982. Parameters f o r d e s c r i b i n g s o i l detachment due to s i n g l e water drop impact. S o i l S c i . Soc. Am. J . 46: 836-840. B.C. A g r i c u l t u r a l Drainage Manual. 1986. B.C. Min. of Agr. and Food. Engineering Branch. V i c t o r i a , B.C. Bonsu, M. 1987. S t r u c t u r a l s t a b i l i t y and surface s e a l i n g as r e l a t e d t o organic matter d e p l e t i o n of a shallow organic s o i l . Ph.D. Th e s i s , F a c u l t y of Graduate S t u d i e s , The U n i v e r s i t y of B r i t i s h Columbia. Bubenzer, G.D. and Jones, B.A. 1971. Drop s i z e and impact v e l o c i t y e f f e c t s on the detachment of s o i l under simulated r a i n f a l l . Trans. ASAE 14: 625-628. Epstein, E. and Grant, W.J. 1967. S o i l l o s s e s and c r u s t formation as r e l a t e d t o some p h y s i c a l p r o p e r t i e s . S o i l S c i . Soc. Am. Proc. 31: 547-550. Evans, D.D. and Buol, S.W. 1968. Micromprphological s t u d i e s of s o i l c r u s t s . S o i l S c i . Soc. Am. Proc. 32: 19-22. F a l a y i , O. and Bouma, J . 1975. R e l a t i o n s h i p s between the h y d r a u l i c conductance of surface c r u s t and s o i l management i n a Typic Hapludalf. S o i l S c i . Soc. Am. Proc. 39: 957-963. Gabriels, D. and Moldenhauer, W.C. 1978. S i z e d i s t r i b u t i o n of eroded m a t e r i a l from simulated r a i n f a l l : e f f e c t over a range of t e x t u r e . S o i l S c i . Soc. Am. J . 42: 954-958. Mclntyre, D.S. 1958a. P e r m e a b i l i t y measurements of s o i l c r u s t s formed by r a i n drop impact. S o i l S c i . 85: 185-189. Mclntyre, D.S. 1958b. S o i l s p l a s h and the formation of surface c r u s t s by r a i n drop impact. S o i l S c i . 85: 261-266. M i t c h e l l , J.K. and Jones, B.A. 1978. M i c r o - r e l i e f surface depression storage: changes d u r i n g r a i n f a l l events and t h e i r a p p l i c a t i o n to r a i n f a l l r u noff models. AWRA Water Resources B u l l . 14(4): 777-802. Morin, J . , Benyamini, Y., and Michaeli, A. 1981. The e f f e c t of r a i n d r o p impact on the dynamics of s o i l s urface c r u s t i n g and water movement i n the p r o f i l e . J o u r n a l of Hydrology 52: 321- 335. Paul, C L . and de V r i e s , J . 1979. E f f e c t of s o i l water s t a t u s and s t r e n g t h on t r a f f i c a b i l i t y . Can. J . S o i l S c i . 59: 313-324. 90 T a c k e t t , J.L, and Pearson, R.W. 1965. Some c h a r a c t e r i s t i c s o f s o i l c r u s t s f o r m e d b y s i m u l a t e d r a i n f a l l . S o i l S c i . 99(6): 4 0 7 - 4 1 3 . Tarchitzky, J . , Banin, A., Morin, J . , and Chen, Y. 1984. N a t u r e , f o r m a t i o n a n d e f f e c t s o f s o i l c r u s t s f o r m e d b y w a t e r d r o p i m p a c t . Geoderma, 3 3 : 1 3 5 - 1 5 5 . 91 CONCLUSIONS AND SUMMARY Lab o r a t o r y experiments were c a r r i e d out to determine the causes of a low h y d r o l o g i c responsiveness i n a lowland s o i l i n the Lower F r a s e r V a l l e y of B r i t i s h Columbia. The r e s u l t s are p r e s e n t e d i n t h r e e c h a p t e r s . In chapter 1, h y d r a u l i c c o n d u c t i v i t i e s were measured on u n d i s t u r b e d s o i l columns removed from a ponded d e p r e s s i o n i n a c u l t i v a t e d s o i l i n west D e l t a . The r e s u l t s were compared w i t h another s o i l column removed from an u n c u l t i v a t e d s o i l i n an a d j a c e n t f i e l d . I t was found t h a t the c u l t i v a t e d s o i l c o n t a i n e d h i g h r e s i s t a n c e l a y e r a t the s u r f a c e ( s u r f a c e s e a l ) and on top of the compacted pan ( i n t e r n a l s e a l ) . No s e a l s , however, were observed i n the u n c u l t i v a t e d s o i l . C u l t i v a t i o n was s i m u l a t e d on both columns and s i m u l a t e d r a i n f a l l was a p p l i e d i n an attempt t o regenerate the s e a l s under c l o s e o b s e r v a t i o n s . A d i s a g g r e g a t i o n p r o c e s s , l e a d i n g t o the f o r m a t i o n of the s e a l s was" i d e n t i f i e d as the main cause of the low h y d r o l o g i c r e s p o n s i v e n e s s . In chapter 2, the process of s o i l d e g r a d a t i o n observed i n chapter 1, was q u a n t i f i e d i n terms of the parameters which govern the h y d r o l o g i c behaviour of the s o i l . T h i s q u a n t i f i c a t i o n was used t o a s s i g n a s e t of d e s i g n parameters to s o i l s t r u c t u r e . The o b j e c t i v e of s o i l and water management f o r the Ladner s o i l i s t h e r e f o r e to achieve and m a i n t a i n a s o i l s t r u c t u r e as d e f i n e d by the a s s i g n e d parameters. In chapter 3, a d e s c r i p t i v e , mechanism of s e a l f o r m a t i o n i s 92 p r e s e n t e d along w i t h a management model. The mechanism of s e a l f o r m a t i o n based on the o b s e r v a t i o n s i n chapter 1 and a l i t e r a t u r e review, i s d i v i d e d i n t o f i v e s t a g e s . The management model d e p i c t s an i d e a l s o i l and water management path and o f f e r s a remedy f o r each d e v i a t i o n . T h i s model can be used as an a l g o r i t h m f o r development of an expert system f o r the s o i l and water management of the Lower F r a s e r V a l l e y lowland s o i l s .

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