Pillsbury and Richards (1954) studied the effects on infiltration rates of different amounts of ammonium sulphate and organic matter added to the soil. They found that moderate application of ammonium sulphate resulted in significantly higher infiltration rates than did urea when combined with large amounts of organic matter. They also found that infiltration rates increased progressively as the amount of surface matter increased.
Johnson (1958) tested the effectiveness of chlorination of the applied water and found that when chlorination was stopped numbers of microorganisms increased rapidly and the infiltration rate dropped sharply. Each time chlorination was resumed the number of micro organisms declined and the infiltration rate increased.
Chlorine appeared to have no lasting effect on the soil other than the reduction of soil organic matter content and coincident destruction of soil structure.
McCalla (1945) investigated the influence of products of microbial activity on soil structure and infiltration rate. He found that
microbial decomposition of products of plant residues increased the in filtration rate of these soils. Johnson (1958) also evaluated the influence of the decomposition of organic residues on infiltration rate. He noted that infiltration rates were increased owing to an improvement in soil structure.
Li et al. (1942) determined the effect of different systems of soil management upon the physical characteristics of the soil and its moisture relationships. They found that different soil covers produced marked changes in soil organic matter level and in physical properties which
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i n t u m a l t e r e d t h e i n f i l t r a t i o n r a t e and m o i s t u r e s t a t u s o f t h e s o i l . C u l t i v a t i o n a l o n g w i t h a n n u a l c o v e r c r o p s r e d u c e d t h e p e r m e a b i l i t y as a more compact s u r f a c e l a y e r r e s u l t e d , g i v i n g r i s e t o p o o r e r i n f i l t r a t i o n c h a r a c t e r i s t i c s .
Musgrave (1935) compared t h e r a i n f a l l , s u r f a c e r u n o f f and i n f i l t r a t i o n on l a n d s u b j e c t t o p o o r and good g r a z i n g management. The r e s u l t s a r e shown i n F i g u r e 2 . 2 . G o o d g r a z i n g m a n a g e m e n t Eui nfc. l I 5 .0 c m/ h r Infiltration Po o r g r a z i n g m a n a g e m e n t R a i n f a l l 3.3 e m / h r Infi l tra t i o n M i n u t e s f r o m S t a r t F i g u r e 2 . 2 R a i n f a l l , s u r f a c e r u n o f f and i n f i l t r a t i o n on l a n d s s u b j e c t e d t o good and p o o r g r a z i n g management. ( A f t e r Musgrave 1 9 3 5 ) .
2 . 2 . 6 Summary
F o r e s t management p r a c t i c e s can a f f e c t i n f i l t r a t i o n r a t e s by c a u s i n g b o t h chan ge s i n t h e v e g e t a t i v e c o v e r and i n t h e s o i l p r o p e r t i e s . The e f f e c t s can be d r a m a t i c and even d e v a s t a t i n g i n t e r m s o f e r o s i o n i f t h e management p r a c t i c e a l l o w s t o t a l d e s t r u c t i o n o f t h e f o r e s t .
In g e n e r a l , t h e s t u d i e s o f i n f i l t r a t i o n i n f o r e s t s h av e b e e n e x p e r i m e n t a l i n n a t u r e and w h i l e p r o v i d i n g some b a s i s f o r q u a l i t a t i v e a s s e s s m e n t o f management p r e s c r i p t i o n s on a c a t c h m e n t b a s i s t h e y do n o t
s u g g e s t o r a s s i s t w i t h t h e q u a n t i t a t i v e p r e d i c t i o n o f e f f e c t s by i n c o r p o r a t i o n i n , f o r example h y d r o l o g i c models, f o r t h e s t u d i e s r e p o r t e d d e s c r i b e r a t h e r t h a n model t h e i n f i l t r a t i o n p r o c e s s i n f o r e s t e d
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CHAPTER I I I
THE THEORY OF INFILTRATION AND THE DEVELOPMENT OF FIELD MEASUREMENT TECHNIQUES
3 .1 INTRODUCTION
Many w o r k e r s h av e i n v e s t i g a t e d t h e t h e o r y o f i n f i l t r a t i o n . Buck ingham (1907) p r o p o s e d a g e n e r a l t h e o r y o f w a t e r movement i n p o r o u s media w h ile Green and Ampt (1911) and G a rd n e r and W id sto e (1921) made e a r l y a t t e m p t s t o d e r i v e t h e o r e t i c a l l y t h e r a t e o r c u m u l a t i v e i n f i l t r a t io n as a f u n c t i o n o f t i m e . S i n c e t h e s e e a r l y w o rk e rs t h e i n f i l t r a t i o n p r o c e s s h a s b een i n v e s t i g a t e d i n a v e r y p r o g r e s s i v e way from q u a l i t a t i v e o b s e r v a t i o n s t o , i n more r e c e n t y e a r s , t h e f o r m u l a t i o n o f a w i d e l y a c c e p te d and a p p l i e d t h e o r y . 3 .2 INFILTRATION THEORY H i l l e l (1971) d e s c r i b e s i n f i l t r a t i o n , and F i g u r e s 3 . 1 , 3 . 2 and 3 . 3 r e s p e c t i v e l y i l l u s t r a t e an i n f i l t r a t i o n m o i s t u r e p r o f i l e , i n f i l t r a t i o n as a f u n c t i o n o f ti m e i n a u n i f o r m s o i l i n a more p o r o u s l a y e r and i n a s o i l c o v e r e d by a s u r f a c e c r u s t , and i n f i l t r a t i o n as a f u n c t i o n o f ti m e i n an i n i t i a l l y d r y and i n an i n i t i a l l y m o i s t s o i l . F i g u r e 3 . 3 show in g t h e i n f i l t r a t i o n r a t e s as d e p e n d e n t on m o i s t u r e c o n t e n t p o i n t s t o some o f t h e d i f f i c u l t i e s i n f o r m u l a t i n g a m a th e m a tic a l model o f t h e p r o
c e s s f o r as can be s e e n i t n o t o n l y d ep en d s on s o i l p h y s i c a l p r o p e r t i e s b u t a l s o on m o i s t u r e c o n t e n t . The m o i s t u r e c o n t e n t a t a p o i n t c h a n g e s o f c o u r s e as i n f i l t r a t i o n c o n t i n u e s and o v e r a l l t h e s o i l p r o p e r t i e s
T r o n t m u n o n zone W e t t i n g z o n e W e t t i n g f r o n t F i g u r e 3 . 1 -The i n f i l t r a t i o n m o i s t u r e p r o f i l e . At l e f t , a s c h e m a t i c s e c t i o n o f t h e p r o f i l e ; a t r i g h t , t h e w a t e r - c o n t e n t v s . d e p t h c u r v e . I n i t i a l l y d r y soi I I n i t i a l l y mo i s t soi I T i m e Fi-gure 3 . 2 I n f i l t r a t i o n as a f u n c t i o n o f t i m e . T i m e F i g u r e 3 . 3 I n f i l t r a t i o n as a f u n c t i o n o f t i m e : (a) i n a u n i f o r m s o i l (b) i n a s o i l w i t h a more p o r o u s u p p e r l a y e r ; and (c) i n s o i l c o v e r e d by a s u r f a c e c r u s t . S o u r c e : H i l l e l 1971 p p . 1 3 4 , 140, 144
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determining infiltration are in a state of flux.
Wissopakan (1977) as shown in Appendix 3.1 summarized the development of infiltration theory citing Green and Ampt (1911), Horton (1933),
Philip (1954), Parlange (1971) and Farrel and Larson (1972) as making significant contributions.
The work of Kostiakov (1932), Childs (1936) and Childs and Collis- George (1950) should also be noted. Kostiakov suggested the infiltration equation, discussed in Appendix 3.2, which has been the most widely used. Childs explicitly proposed the concept of soil water movement as a
diffusion phenomenon. Childs and Collis-George (1950) and others, confirmed that Darcy's law may hold for the flow of liquid water in un saturated media. The infiltration theory of Philip (1954) was adopted to simulate the infiltration process in the mathematical model of the whole catchment process, formulated as the main analytical tool of a programme initiated by the Australian Water Resources Council, to provide
a better basis for estimating the runoff from ungauged catchments and
at the same time a better understanding of the hydrologic cycle (Australian Water Resources Council, 1969, 1974).
Philip (1957a) presented a full analysis of infiltration based on the solution of the relevant concentration dependent diffusion equation for appropriate boundary conditions. He showed that when water is applied to a uniform soil at uniform initial water content the total infiltration can be expressed by a rapidly converging power series. He formulated the equation for total infiltration as
I = S t 4 + A(th}2 + BCt^)3 + . (2.1)
where I = the cumulative infiltration flux