INFILTRATION CIlARACTEKISTICS OF PRAIRIE … it is presur~led that they were at similar relative...
Transcript of INFILTRATION CIlARACTEKISTICS OF PRAIRIE … it is presur~led that they were at similar relative...
INFILTRATION CIlARACTEKISTICS OF PRAIRIE SOILS
Don PI. Gray, D . I . Norum and J . M. blurray'
A B S T R A C T
The p a p e r p r o v i d e s a b r i e f o u t l i n c of t h e has i .c f c tnda~nen ta l s
of t h e i n f i l t r a t i o n p r o c e s s i n c l u d i n g s e v e r a l a c c e p t e d t h e o r i - e s d e f i n i n g
t h e t i m e v a r i a t i o n i n t h e i n f i l t r a t i o n r a t e o f a soi .1. A co~npar i -son c f
t h e inass i n f i l t r a t i o n of s e v e r a l P r a i - r i e S o i l s d e r i v e d by two d i f f e r e n t
i n f i l t r a t i o n e q u a t i o n s u s i n g f i e l d d a t a i s p r e s e n t e d . The r e s u l t s
exempl i fy t h e i n h e r e n t danger of d i r e c t e x t r a p o l a t i o i l of t h e i n f i l t r a t i o n
r a t e s of " t e x t u r a l l y - simi.l.ar" s o i l s encoun te red on t h e P r a i r i e s .
C o n s i d e r a t i o n is a l s o g i v e n t o t h e i n f i l t r a t i o n p r o c e s s t o
f r o z e n s o i l s under P r a i r i e c o n d i t i o n s . D i f f e r e n c e s i n P r a i r i e ~n01.7paclis
compared t o Mountainous snowpaclis a s t h e y a f f e c t t h e p r o c e s s a r e d e s c r i b e d .
Measured v a l u e s o f t h e v o l u m e t r i c i n f i l t r a t i o n m o u n t s t o f r o z e n s o i l s
under d i f f e r e n t c o n d i t i o n s a r c g i v e n .
o on If. Gray, D. I . Norurn and J . K. K ~ ~ r r a y a r e r e s p e c t i v e l y ; P r o f e s s o r , A s s i s t a n t P ro fe . s so r and Assoclar .e P r o f c s s o r , Dcpcrtment of Agricu1.t-uual E n g i n e e r i n g , U n i v e r s i t y of Sz.skatchevan, Saska toon .
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I N T R O D U C T I O N
The p r o c e s s of i n f i l t r a t i o n i s by d e f i n i t i o n t h e e n t r y of
w a t e r i n t o t h e s o i l through t h e s o i l - a t m o s p h e r e i n t e r f ace . I n most
c a s e s , t h e amount of i n £ i l - t r a t i o n which o c c u r s d u r i n g any g i v e n r a i n -
f a l l . o r snownlelt e v e n t r e p r e s e n t s t h e major cotnponent l o s s of p r e c i p i -
t a t i o n t o s u r f a c e r u n o f f o r , t h e p o t e n t i a l amount of w a t e r which may
go t o groundwater r e c h a r g e . As i n d i c a t e d d i a g r a m n a t i c a l l y i n F i g u r e s
1 and 2 , depending on t h e i n t e n s i t y o f r a i n f a l l o r snowinelt r a t e ,
r e l a t i v e t o t h e i n f i l t r a t i o n r a t e , w a t e r may b e e n t i r e l y absorbed by
t h e s o i l o r may accuuiulate and f low from t h e a r e a a s s u r f a c e r u n o f f .
I n F i g u r e 1, t h e supp ly i n t e n s i t y , i, i s l e s s t h a n t h e maximum r a t e
a t which t h e s o i l i n i t s g i v e n c o n d i t i o n can a b s o r b w a t e r ( i = f )
and hence t h e t o t a l supp ly goes t o r e p l e n i s h i n g t h e s o i l m o i s t u r e
r e s e r v o i r and t o r e c h a r g i n g t h e groundwater supp ly ( n e g l e c t i n g evapor-
a t i o n and i n t e r c e p t i o n l o s s e s ) . I n F i g u r e 2 i n which i > f some w a t e r
accumula tes on t h e s u r f a c e and a p p e a r s a s s u r f a c e r u n o f f .
Because of t h e impor tance of t h e i n f i l t r a t i o n p r o c e s s i n
t h e h y d r o l o g i c c y c l e , t h e phenomenon d e s e r v e s s p e c i a l a t t e n t i o n and
s t u d y . I n t h e s e r e g a r d s , i t would b e expec ted t h a t a complete under-
s t a n d i n g of t h e p r o c e s s and t h e f a c t o r s a f f e c t i n g i t would a s s i s t t h e
h y d r o l o g i s t i n q u a n t i t a t i v e l y e v a l u a t i n g i n f i l t r a t i o n amounts and
h e n c e i n c r e a s e h i s c o n f i d e n c e and competence i n w a t e r balance, h y d r o l o g i c
d e s i g n and o t h e r s t u d i e s .
I n t h e i n f i l t r a t i o n p r o c e s s , w a t e r e n t e r s t h e s o i l s u r f a c e
due t o t h e combined i n f l u e n c e of g r a v i t y and c a p i l l a r y f o r c e s . Both
f o r c e s a c t i n t h e v e r t i c a l d i r e c t i o n t o c a u s e perco3.at-ion downward.
C a p i l l a r y f o r c e s a l s o a c t t o d i v e r t w a t e r l a t e r a l l y from l a r g e r p o r e s
( f e e d e r c a n a l s ) t o c a p i l l a r y p o r e s p a c e s which a r e much s m a l l e r i n .
dimens ion , b u t may b e v e r y numerous. A s t h e p r o c e s s c o n t i n u e s , t h e
c a p i l l a r y p o r e s p a c e s become f i l l e d and w i t h p e r c o l a t i o n t o g r e a t e r
d e p t h s t h e g r a v i t a t i o n a l wate.r normal ly e n c o u n t e r s i n c r e a s e d r e s i s t a n c e
t o f l o w due t o r educed e x t e n t o r d imension of £].ow c h a n n e l s , i n c r e ~ s e d
l e n g t h of c h a n n e l s , o r an impermeable b a r r i e r such a s rock o r c l a y .
A t t h e same t i m e t h e r e may b e i n c r e a s e d r e s i s t a n c e t o i n f l o w of w a t e r
a t t h e s o i l s u r f a c e due t o s u r f a c e s e a l i n g caused by t h e mechan ica l
a c t i o n of r a i n d r o p s i n b r e a k i n g down t h e s o i l a g g r e g a t e s and s u b s e q u e n t
inwash of t h e f i n e r s o i l p a r t i c l e s . The r e s u l t i s a r a p i d r e d u c t i o n o f
i n f i l t r a t i o n r a t e i n t h e f i r s t few h o u r s of a s t o r m , a f t e r which t h e
r a t e remains n e a r l y , c o n s t a n t f o r t h e remainder of t h e p e r i o d o f s t o r m
r a i n f a l l e x c e s s .
From t h i s q u a 1 i t a t i v e . d e s c r i p t i o n o f t h e i n f i l t r a t i o n p r o c e s s
i t can b e r e c o g n i z e d t h a t t h e p r o c e s s i n v o l v e s b o t h t r a n s m i s s i o n and
s t o r a g e o f w a t e r by t h e s o i l and t h a t t h e r a t e of i n f i l t r a t i o n may b e
governed by t h e s e p a r a t e p r o c e s s e s o f :
( a ) En t ry of w a t e r th rough t h e s u r f a c e l a y e r , and
(b) Downward movement o r p e r c o l a t i o n of w a t e r th rough t h e
s o i l p r o f i l e .
THEORY OF INFILTRATION INTO FROZEN SOILS
I n f i l t r a t i o n of w a t e r i n t o t h e s o i l , l i k e many o t h e r f low
p r o c e s s e s i n porous media , i s governed by t h e Richards s o i l m o i s t u r e
d i f f u s i o n equat i -on,
i n which €I = t h e v o l u m e t r i c m o i s t u r e c o n t e n t ,
k = t h e c a p i l l a r y c o n d u c t i v i t y , and
= t h e t o t a l p o t e n t i a l .
Equa t ion 1, i s t h e c o n t i n u i t y e q u a t i o n f o r f low which h a s t h e f l u x ,
V, a t any p o i n t d e f i n e d by t h e Darcy e q u a t i o n ,
It is e v i d e n t from e q u a t i o n 2 t h a t t h e f l u x a t any p o i n t i n a s o i l
syst.em, i n c l u d i n g t h e s o i l s u r f a c e , i s p r o p o r t i o n a l t o t h e h y d r a u l i c
o r c a p i l l a r y c o n d u c t i v i t y , k , and t h e t o t a l p o t e n t i a l g r a d i e n t , Va.
T h e r e f o r e , t h e i n f i l t r a t i o n p r o c e s s w i l l be a f f e c t e d by any f a c t o r
which a f f e c t s e i t h e r of t h e s e two q u a n t i t i e s . A l i s t of t h e most
p e r t i n e n t f a c t o r s i s shown i n T a b l e 1.
Time V a r i a t i o n i n I n f i l t r a t i o n
Many e q u a t i o n s have been developed o r s u g g e s t e d t o d e f i n e t h e
mass o r d e p t h of w a t e r i n f i l t r a t e d , M f , a f t e r g i v e n t i m e , t . Some of
t h e most common of t h e s e e x p r e s s i o n s a r e t h e f o l l o w i n g :
Kos t i akov (1932) and Lewis (1937)
Gardner and Widtsoe (1921) and Horton (1940)
Kirkham and Feng (1949) ... h o r i z o n t a l i n r i l t r a t i o n
P h i l i p (1954)
A s i n d i c a t e d , most of t h e e q u a t i o n s t a k e t h e form of an e x p o n ~ n t i a l o r
power f u n c t i o n of t i n e i n which t h e c o n s t a n t s ( e . g . a and n o f e q u a t i o n
3 ) c h a r a c t e r i z e t h e a b i l i t y of s o i l i n i t s gi.ven c o n d i t i o i l t o a b s o r b
w a t e r .
Exper imenta l Program
During t h e p a s t s e v e r a l y e a r s , a s e r i e s of s t u d i e s have been
under taken on b o r d e r dyke i r r i g a t i o n sys tems t o g a i n an u n d e r s t a n d i n g
of t h e i n f i l t r a t i o n phenomenon and t o o b t a i n e x p e r i m e n t a l d a t a which
would b e a p p l i c a b l e t o t h e d e s i g n of s u r f a c e i r r i g a t i o n s y s t e m s . I n
t h e s e r e g a r d s , t h e i n f i l t r a t i o n r e s u l t s o b t a i n e d a r e p robab ly s i m i l a r
t o t h o s e which would b e measured by "Flooding-type" i n f i l t r o n i e t e r s -
a l t h o u g h t h e y do t a k e i n t o c o n s i d e r a t i o n t h e e f f e c t of t h e movement of
w a t e r o v e r t h e porous bed a s i t a f f e c t s i n f i l t r a t i o n . Converse ly , t h e
r e s u l t s do n o t r e f l e c t t h e e f f e c t of r a i n f a l l c h a r a c t e r i s t i c s ( impac t ,
e t c ) on i n f i l t r a t i o n and t h u s a r e p robab ly of h i g h e r v a l u e t h a n t h o s e
which would p r e v a i l under n a t u r a l r a i n f a l l - r u n o f f c o n d i t i o n s .
To d a t e , approx imate ly 70 t e s t s have been conducted on sys tems
l o c a t e d i n A l b e r t a and Saskatchewan. They encompass a wide r a n g e of
s o i l t y p e s , s l o p e s and vege ta , l cover . I n each t e s t t h e i n p u t d i s c h a r g e
r a t e , t h e r a t e of advance of t h e w e t t i n g f r o n t , t h e t i m e - r a t e o f
accumula t ion o f s u r f a c e s t o r a g e and t h e r a t e of r e c e s s i o n of t h e w e t t e d
f r o n t w e r e measured. U t i l i z i n g t h e s e d a t a , two approaches have been
a p p l i e d t o c h a r a c t e r i z e t h e advance and i n f i l t r a t i o n c h a r a c t e r i s t i c s
of t h e d i f f e r e n t s o i l s .
Method I
Method I , proposed by Gray and Ahmed (1965) , u t i l i z e s a mass
b a l a n c e between t h e i n f l o w t o t h e b o r d e r s t r i p , t h e t ime r a t e of
accumula t ion o f s u r f a c e s t o r a g e and t h e volume of w a t e r i n f i l t r a t e d i n t o
t h e s o i l t o de te rmine t h e i n f i l t r a t i o n r a t e . The method assumes t h a t
(1 ) The d e p t h o f w a t e r i n f i l t r a t e d v a r i e s a s a power
f u n c t i o n w i t h t ime ( e q u a t i o n 3 ) .
( 2 ) The r a t e of advance of t h e w a t e r a l o n g t h e s t r i p can b e
e x p r e s s e d by a power f u n c t i o n a s ,
where L = l e n g t h of advance,
t = t ime s i n c e water was tu rned on t h e s t r i p , and
C,m = c o e f f i c i e n t and exponent, r e s p e c t i v e l y .
(3 ) The volume of water accumulated on t h e s t r i p w i th t ime
would be approximated by a second o rde r polynomial.
The mass ba l ance equa t ion t h a t was app l i ed was
where V = volume of i n f i l t r a t i o n i n t o t h e bo rde r i n f
t ime, t ,
q = i n f low r a t e t o t h e bo rde r , and
, s = s u r f a c e s t o r a g e .
U t i l i z i n g t h e above 'assumptions an exp re s s ion f o r t h e volume of i n f i l t r a -
t i o n t o t h e s t r i p of t h e fol1,owing form was developed.
where r i s t h e gamma func t ion .
By s u b s t i t u t i n g equa t ion 8 i n t o equa t ion 9 , va lues f o r a and n could
b e ob ta ined from t h e r e s u l t a n t equa t ion by s o l v i n g s imul taneous ly a t
two d i f f e r e n t t imes .
Method I1
Method 11, proposed by Norum (1969) u t i l i z e s an e x a c t s o l u t i o n
of t h e r a t e of advance e q u a t i o n developed by Lewis and Milne (1938) and
s o l v e d by P h i l i p and F a r r e l l (1964) . T h i s method assumes t h a t
( a ) The dep th of w a t e r i n f i l t r a t e d can b e d e s c r i b e d by
t h e two term power e q u a t i o n ( e q u a t i o n 6 ) .
(b ) The d e p t h of w a t e r f lowing down t h e b o r d e r is a
c o n s t a n t .
The r e s u l t i n g mathemat ica l s o l u t i o n is t rans formed i n t o a t h r e e
pa ramete r d i m e n s i o n l e s s form t h a t i s p l o t t e d a s a fami ly of c u r v e s on
l o g a r i t h m i c p a p e r . The c o e f f i c i e n t s , S and A , a r e o b t a i n e d by p l o t t -
i n g t h e r a t e of advance d a t a on s i m i l z r log- log p a p e r and matchi-ng t h e
d a t a t o t h e t h e o r e t i c a l c u r v e s .
R e s u l t s and D i s c u s s i o n
The r e s u l t s o b t a i n e d from 29 tests; 1 8 i n Saskatchewan and
11 i n A l b e r t a a r e t a b u l a t e d i n T s b l e s 2 and 3 and v a l u e s of t h e mass
i n f i l t r a t i o n a f t e r 3 h r i s de te rmined by t h e two methods o u t l i n e d
p r e v i o u s l y a r e p l o t t e d i n F i g u r e 3 . From a n a l y s e s of t h e s e r e s u l t s i t
is immediate ly obv ious t h a t ,
(1 ) The i n f i l t r a t i o n r a t e s of t h e A l b e r t a s o i l s are
s i g n i f i c a n t l y h i g h e r t h a n t h o s e f o r t h e Saskatchewan
* 0 0 9 " N W r m 9 N 0 0 d ' D d ' Q o m 0 * d 0 3 9 D F N 9 9 ~ d ' C O + M m 9 N N ~ d d o m d ~ m rr,o N ~ O ~ N . . . , . . . , . , . . o o o o o o o o a o o o a . d d o o o
N co Q d m m m m a N o D O r d + r n o N N m - p m M q N Q q f W I V t L n W m - l ' N N . . . , . . I .
0 0 d d d d d d d d d 0 0 o o o o o
m M a d a c Q 0 0 W d D ad' m o m o o m a N N m ~ n * ~ l ' Q d c C m a m m m - i o Ln n) Q W d Y P N t - C O ~ m Ul4 U l + d * D . , . . * . . . . . . . . , . . 0 0 0 Q Q 0 0 0 - 4 0 0 0 0 0 0 0 " O
Ta-hle 3. Infiltration Data for Some Alber ta Soils
- - - -
Using values of a , n , S and A with t ime in minutes gives M in inches. f I
s o i l s . H e r e i n , i t must b e n o t e d t h a t u n f o r t u n a t e l y
t h e i n i t i a l s o i l m o i s t u r e c o n t e n t a t t h e b e g i n n i n g
* of each t e s t were n o t measured. Large d i f f e r e n c e s
i n s o i l m o i s t u r e c o n t e n t cou ld account f o r t h e
d i f f e r e n c e s i n i n f i l t r a t i o n . However, s i n c e a l l
t e s t s were conducted a t a t ime when t h e s o i l s r e q u i r e d
i r r i g a t i o n i t is presur~led t h a t t h e y were a t s i m i l a r
r e l a t i v e m o i s t u r e c o n t e n t s ( w i t h i n t h e i r a v a i l a b l e
m o i s t u r e r a n g e ) . On t h i s b a s i s , and because o f t h e
number of t e s t s conducted i t is concluded t h a t t h e
d i f f e r e n c e s i n t h e i n f i l t r a t i o n r a t e s o f t h e s o i l s
o f t h e two r e g i o n s a r e g r e a t e r t h a n can b e accounted
f o r by random d i f f e r e n c e s i n m o i s t u r e c o n t e n t .
A t p r e s e n t , t h e e x a c t r e a s o n why t h e i n f i l t r a t i o n
p r o p e r t i e s o f A l b e r t a s o i l s a r e h i g h e r t h a n f o r
. ,
Saskatchewan s o i l s i s n o t known. Needless- to-say
however, t h e r ' e s u l t s p o i n t o u t t h e i n h e r e n t danger
i n d i r e c t a p p l i c a t i o n of i n f i l t r a t i o n d a t a a s d e s i g n
c r i t e r i a from one a r e a t o a n o t h e r i n t h e absence o f
a d d i t i o n a l i n f o r m a t i o n .
(2) It i s a p p a r e n t t h a t w i t h i n r e g i o n s and between r e g i o n s
t e x t u r e is n o t a s u i t a b l e method f o r c l a s s i f y i n g t h e
i n f i l t r a t i o n c a p a c i t i e s of s o i l s . I n a l l p r o b a b i l i t y ,
d i f f e r e n c e s i n t e x t u r e a r e g r e a t l y o v e r shadowed by
d i f f e r e n c e s i n s o i l s t r u c t u r e , s o i l m o i s t u r e c o n t e n t ,
s o i l l a y e r i n g , e t c .
(3) The mass i n f i l t r a t i o n s a f t e r 3 h r a s calcu1.ated by
I Method I1 (which u t i l i z e s t h e P h i l i p equa t i on -- \ \\ equa t i on 6) a r e i n agreement w i t h t h o s e amounts '\ \ c a l c u l a t e d on t h e b a s i s of a s imple power r e l a t i o n s h i p
( equa t i on 1 ) a s o u t l i n e d i n Method I . Although, t h e
t ime of i n f i l t r a t i o n of 3 h r was s e l e c t e d a r b i t r a r i l y ;
i t does cor respond t o approx imate ly t h e average l e n g t h
of t ime t h a t w a t e r was a p p l i e d i n each tes t . It i s
s i g n i f i c a n t t o n o t e t h a t t h e i n f i l t r a t i o n amounts
a f t e r t h i s t ime showed a t r e n d t o be h i g h e r when
c a l c u l a t e d by Method I1 ( s e e F igu re 3 ) . Th i s r e s u l t
would be expec ted inasmuch a s t h e shape of t h e i n f i l t r a -
t i o n r a t e cu rve which fo l l ows a s imp le power e q u a t i o n
( equa t i on 3) i s s t e e p e r and dec r ea se s a s y m p t o t i c a l l y
t o z e r o w i t h t ime whereas t h e s l o p e u s ing t h e two
term power e q u a t i o n ( equa t i on 6 ) i s n o t a s s t e e p and
approaches a f i x e d v a l u e equa l t o t h e c o n s t a n t , A ,
w i t h t ime. However, on t h e b a s i s of t h e s e f i n d i n g s
i t would appear t h a t equa t i on 3 cou ld be s u c c e s s f u l l y
employed t o d e f i n e t h e t ime r a t e of i n f i l t r a t i o n f o r
s h o r t term even t s (eg . i r r i g a t i o n de s ign and s h o r t
d u r a t i o n r a i n f a l l e v e n t s ) whereas t h e two tern1 power
equa t i on ( equa t i on 6 ) should b e a p p l i e d t o long
d u r a t i o n e v e n t s . However, e x t r a p o l a t i o n of t h i s
e q u a t i o n much beyond t h e test t ime of approxinia te ly
3 h r s would be i n a d v i s a b l e . Th is i s d e f i n i t e l y s o i n
t h e cases where t h e c o e f f i c i e n t A of equa t i on 6 h a s
n e g a t i v e v a l u e s . C a r r i e d t o t h e ex t reme ,
e x t r a p o l a t i o n would g i v e a n e g a t i v e v a l u e f o r
t h e mass i n f i l t r a t i o n .
INFILTRATION TO FROZEN SOILS
For Canadian c o n d i t i o n s , no d i s c u s s i o n of t h e i n f i l t r a t i o n
p r o c e s s would b e complete w i t h o u t some c o n s i d e r a t i o n of t h e p r o c e s s o f
i n f i l t r a t i o n t o f r o z e n s o i l s . A t Saska toon d u r i n g 1966-67, G i l l i e s
(1968) d i d c o n s i d e r a b l e s t u d y of t h e phenomenon f o r P r a i r i e C o n d i t i o n s ,
w i t h p a r t i c u l a r a t t e n t i o n t o t h e s p r i n g thawing p e r i o d . H i s f i n d i n g s
i n d i c a t e d t h a t t h e m o i s t u r e p r o f i l e under thawing c o n d i t i o n s , which
p r o v i d e e x c e s s m o i s t u r e a t t h e s u r f a c e , cou ld be d i v i d e d i n t o two
d i s t i n c t zones , ( a ) a zone of s a t u r a t i o n e x t e n d i n g dorm from t h e s o i l
s u r f a c e i n which t h e s o i l - w a t e r m a t r i x was comple te ly thawed and i t s
t e m p e r a t u r e was above 32'F, and, (b) an u n s a t u r a t e d zone e x t e n d i n g a
s h o r t d i s t a n c e below t h e thawed l a y e r i n which t h e l i q u i d - - i c e - s o i l
m a t r i x was below 32°F. F u r t h e r , h e found t h a t a s m e l t i n g p r o g r e s s e d
i t appeared t h a t t h e i n c r e a s e i n d e p t h o f p e n e t r a t i o n of t h e u n s a t u r a t e d
zone o r w e t t i n g f r o n t i n t o t h e f r o z e n s o i l was approx imate ly e q u a l t o
t h e e l o n g a t i o n of t h e thawed l a y e r . Ex tens ion o f t h e thawed zone i s
caused by an i n c r e a s e i n t h e h e a t supp ly t o t h e ground s u r f a c e . I n
t h e s e r e g a r d s i t shou ld b e recognized t h a t t h e energy t r a n s m i t t e d
th rough a snowpack f o l l o w s a s i m p l e e x t i n c t i o n c u r v e depending on t h e
d e n s i t y and dep th of t h e pack. Thus, i n s h a l l o w P r a i r i e snowpacks,
l i g h t waves may c o n ~ p l e t e l y p e n e t r a t e t h e snow cover and t h u s t h e energy
- 15 -
exchange a t t h e snow-land i n t e r f a c e may be a s i g n i f i c a n t f a c t o r
govern ing t h e i n f i l t r a t i o n p r o c e s s . T h e r e f o r e , under P r a i r i e c o n d i t -
/ i o n s , t h e i n f i l t r a t i o n p r o c e s s must b e a l s o c o n s i d e r e d from a thermo- i
dynamic a s p e c t t o i n c l u d e t h e coupled e f f e c t of h e a t t r a n s f e r and
mass m o i s t u r e m i g r a t i o n .
Many i n v e s t i g a t o r s have recognized t h a t t h e s o i l m o i s t u r e
c o n t e n t i s an i m p o r t a n t f a c t o r govern ing i n f i l t r a t i o n t o f r o z e n s o i l s .
For exainple, s e v e r a l Russia11 workers (Lark in 1962, Kuznik and Brezmenov,
1964) and o t h e r s ( P o s t and D r e i b e l i s , 1942) r e p o r t t h a t i f a s o i l i s
f r o z e n when i t s m o i s t u r e c o n t e n t i s g r e a t e r t h a n t h e f i e l d c a p a c i t y ,
i t s i n f i l t r a t i o n r a t e w i l l b e v e r y low and i f s a t u r a t e d , t h e i n t a k e
r a t e i s v i r t u a l l y z e r o . S i m i l a r l y , W i l l i s -- e t a l . (1961.) i n t h e i r
s t u d i e s on s m a l l p l o t s i n North Dakota r e p o r t t h a t a s much a s 90 per-
c e n t of t h e snowpaclc w a t e r i s l o s t a s s u r f a c e runof f when t h e p l o t s
were f r o z e n a t h i g h m o i s t u r e l e v e l s .
I n h i s s t u d y of i n f i l t r a t i o n t o f r o z e n g l a c i a l s o i l s ,
G i l l i e s (1968) found t h a t u n l i k e t h e c a s e of w a t e r e n t r y i n t o unf rozen
s o i l s i t was observed t h a t t h e advance o f t h e wet f r o n t i n a f r o z e n
s o i l was i n v e r s e l y r e l a t e d t o t h e s o i l m o i s t u r e c o n t e n t . That i s , t h e
lower t h e m o i s t u r e c o n t e n t , t h e f a s t e r t h e advance. Th is r e s u l t would
b e expec ted inasmuch a s an i n c r e a s e i n m o i s t u r e c o n t e n t would r e s u l t
i n ( a ) an i n c r e a s e i n t h e number of t l ie s o i l p o r e s b locked w i t h i c e ,
(b ) a n i n c r e a s e i n t h e s p e c i f i c h e a t of tlle s o i l - w a t e r m a t r i x and
consequen t ly more h e a t would b e needed p e r u n i t mass t o c a u s e thawing,
( c ) a d e c r e a s e i n t h e c a p i l l a r y g r a d i e n t . He found t h a t t h e v o l u m e t r i c
r a t i o o f t h e amount of w a t e r e n t e r i n g t h e f r o z e n s o i l and c o n t a i n e d i n
t h e upper 18- inch d e p t h o f t h e p r o f i l e t o t h e d e p t h o f a v a i l a b l e s u r f a c e
w a t e r o f t h e snowpaclc cou ld b e r e l a t e d t o t h e i n i t i a l s o i l m o i s t u r e
c o n t e n t of t h e 2-inch s u r f a c e l a y e r . T h i s r e l a t i o n s h i p , shown i n F i g u ~ e
4 , i n d i c a t e s t h a t t h e v o l u m e t r i c i n f i l t r a t i o n t o thawing s o i l s d e c r e a s e s
e x p o n e n t i a l l y w i t h t h e m o i s t u r e c o n t e n t o f t h e s u r f a c e l a y e r . I n t h e s e
e x p e r i m e n t s , when m e l t was i n i t i a t e d , t h e m o i s t u r e i n t h e s u r f a c c l a y e r
was f r o z e n and t h u s i t would b e expec ted t h a t i n f i l t r a t i o n would d e c r e a s e
w i t h i n c r e a s i n g m o i s t u r e b e c a u s e of t h e i n c r e a s e i n number of i c e - f i l l e d
p o r e s . Thus t h e amount of h e a t r e q u i r e d t o e s t a b l i s h and ex tend t h e
thawed s o i l zone was i n c r e a s e d .
Recogni t ion o f t h e dependence of t h e i n f i l t r a t i o n p r o c e s s
under f r o z e n c o n d i t i o n s on t h e s t a t e and amount of m o i s t u r e i n t h e
s u r f a c e l a y e r i s ex t remely i m p o r t a n t t o a c c u r a t e p r e d i c t i o n of s n o ~ . ~ m e l t
r u n o f f . It p o i n t s o u t t h e need t o t a k e t h e s e measurements a t t h e t i m e
o f / o r immediate ly p r e c e d i n g m e l t , and t h a t a s o i l m o i s t u r e s t a t u s
e v a l u a t e d a c o n s i d e r a b l e t i m e i n advance of t h e m e l t i n g p e r i o d may n o t
n e c e s s a r i l y r e f l e c t t h e r u n o f f p o t e n t i a l of a wa te r shed . Even though
t h e e a r l i e r measurements may i n d i c a t e t h e r u n o f f p o t e n t i a l of a water-
shed t o b e ve ry low, t h e s e c o n d i t i o n s may b e comple te ly changed by
r e f r e e z i n g of s m a l l amounts of m e l t w a t e r which o r i g i n a t e from minor
thawing of t h e snowpack p r i o r t o t h e major m e l t sequence I n t-he s u r f a c e
o f t h e s o i l .
I n manner o f summary, i t would a p p e a r t h a t t h e s h a p e o f t h e
, i n f i l t r a t i o n - r a t e c u r v e s o f a f r o z e n s o i l may adopt s e v e r a l d i s t i n c t
forrns dependent on c o n d i t i o n s which p r e v a i l a t t h e time o f f r e e z i n g o r
thawing.
1. An i n t a k e r a t e which i s r e a s o n a b l y c o n s t a n t w i t h
tirne a t a v e r y low v a l u e - a c o n d i t i o n which would
p r e v a i l i f f r o z e n w h i l e a t a h i g h m o i s t u r e c o n t e n t
o r when an impervious l a y e r deve lops a t t h e s u r f a c e
due t o r e f r e e z i n g o f t h e m e l t w a t e r a t t h e time of thaw
and l i t t l e o r no thaw o c c u r s . The f a i l u r e o f s o i l
thaw t o deve lop cou ld b e due t o snowpack c h a r a c t e r -
i s t i c s which i n h i b i t t h e passage of h e a t o r t o low
l e v e l s o f r a d i a n t energy ( c l o u d y ) .
2. An i n t a k e r a t e which d e c r e a s e s v e r y r a p i d l y w i t h t ime
from a reasonab ly-h igh i n i t i a l v a l u e t o n e a r z e r o - a
c o n d i t i o n which may p r e v a i l when a s o i l i s f r o z e n a t
a low m o i s t u r e ' con ten t b u t t h e s o i l t e m p e r a t u r e i s
below f r e e z i n g . Mel twater e n t e r i n g t h e s o i l i s f r o z e n
i n t h e p o r e s and movement i s i n h i b i t e d . Th is c o n d i t i o n
i s l i k e l y t o o c c u r under deep snowpacks which i n h i b i t
t h e f low of h e a t t o t h e s o i l s u r f a c e .
3 . An i n c r e a s e i n i n f i l t r a t i o n r a t e w i t h t i r n e - a c o n d i t i o n
which may e x i s t when t h e s o i l i s f r o z e n a t a h i g h mois t -
u r e c o n t e n t (70-80% f i e l d c a p a c i t y ) and where s u f f i c i e n t
h e a t r e a c h e s t h e s o i l th rough t h e m e l t i n g snowpack
t o produce s i g n i f i c a n t s o i l thaw. For t h i s c a s e ,
some of t h e m e l t w a t e r i s a b l e t o p e n e t r a t e t h e s o i l
and t h u s t r a n s f e r h e a t i s used t o m e l t t h e i c e -
f i l l e d p o r e s . P r o g r e s s i v e l y , a s t h e s o i l walms and
more p o r e s m e l t , t h e i n f i l t r a t i o n r a t e i n c r e a s e s .
Zavodchikov (1962) c i t e s examples i n which t h e
i n f i l t r a t i o n r a t e of a s o i l i n c r e a s e d 6-8 t i m e s i t s
i n i t i a l r a t e d u r i n g t h e m e l t i n g p e r i o d .
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