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 .

- 2 -

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 ,

bO r: .rl d r-l C i

3 m 'cl . ,-< 0

7 - 1 7-:

0 U

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 .

LITERATURE CITED

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Gardner , W . and Wid t soe , J . A. 1921. The move-nient of s o i l m o i s t u r e . S o i l S c i . 11:215 - 232.

G i l l i e s , J . 1968 . I n f i l t r a t i o n t o f r o z e n p r a i r i e s o i l s . Unpubl ished M. Sc . T h e s i s . U n i v e r s i t y of Saskatchewan, Sas l ta toon.

Kirkham, D. and Feng, C. L. 1949. Some t e s t s of t h e d i f f u s i o n t h e o r y , and laws of c a p i l l a r y f low, i n s o i l s . S o i l S c i . 67: 29-40.

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P o s t , F. A. and D r e i b e l i s , F . R. 1942. Some i n f l u e n c e of f r o s t p e n e t r a t i o n and m i c r o c l i n ~ a t e on t h e w a t e r r e l a t i o n s h i p s o f woodland, p a s t u r e and c u l t i v a t e d s o i l s . P roc . S o i l S c i . Soc. Arner. 7: 95-104.

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