SNOWMELT INFILTRATION TO FROZEN PRAIRIE SOILS

D.M. Gray and P.G. Landine

SNOWMELT INFILTRATION

TO FROZEN PRAIRIE SOILS

D.M. Gray

P.G. Landine

Divis ion o f Hydrology Univers i ty o f Saskatchewan

f o r

Research Management D iv is ion Alberta Env i ronment

RMD 83-34 AE Contract No. 84-0472

March 1984

TABLE OF CONTENTS

Page

LIST OF TABLES ........................................ i i i

LIST OF FIGURES ............................................ i v

ABSTRACT ................................................... v i

........................................... ACKNOWLEDGEMENTS v i i

1 . INTRODUCTION ..................................... 1

2 . SNOWMELT INFILTRATION TO FROZEN SOILS ............ 3 2.1 Review o f Fac to r s A f f e c t i n g Snowmelt I n f i l t r a t i o n . 3 2.2 Conceptual Model o f I n f i l t r a t i o n P o t e n t i a l ....... 4

. .................. 3 . INFILTRATION THE LIMITED CASE 9 ....................................... 3.1 F i e l d Data 9 ......................... 3.2 Mass I n f i l t r a t i o n Curves 12 3 3 Re la t i onsh ip Between I n f i l t r a t i o n . Snowcover Water

Equ iva len t and Frozen S o i l M o i s t u r e .............. 13 .......... 3.4 Es t imat ing t he Premel t M o i s t u r e Content 18

3.5 Sequencing I n f i l t r a t i o n Q u a n t i t i e s . The L im i t ed Case ............................................. 21

...................... 3.6 S n o m e l t I n f i l t r a t i o n Model 25

MODEL DEVELOPMENT. TESTING AND VERIFICATION ...... 27 General .......................................... 27 Water Balance C a l c u l a t i o n s ....................... 28 E v a l u a t i o n o f NWSRFSAppl iedtoCreightonWatershed 31

I n t r o d u c t i o n ................................... 31 Revised Program o f NWSRFS ...................... 33 Inpu t Parameters ............................... 35 Program Output ................................. 35

.................. 5 CONCLUSIONS AND RECOMMENDATIONS 41

6 . REFERENCES CITED ................................. 42

L l ST OF TABLES

Page

1. Coordinates o f dimensionless snowmelt i n f i l t r a t i o n curves f o r Advanced, Linear and Delayed pat te rns assuming con- t inuous mel t and a p e r i o d - o f - i n f i l t r a t i o n o f seven days ...... 24

2. Snowcover, land use, s o i l moisture and r u n o f f s t a t i s t i c s f o r t he Creighton T r i b u t a r y f o r the w in ters o f 1973-74 and 1974-75 and a comparison o f the volumes o f r u n o f f ca l cu la ted by the i n f i l t r a t i o n model w i t h those obta ined from recorded hydrographs .................................... 30

3. Comparison o f outputs f rom o r i g i n a l and rev ised NWSRFS model app l ied t o the Creighton watershed i n 1974 us ing the same inputs f o r each system. Un i t s are m i 1 1 imeters.. . . . . . 37

4. Comparison o f outputs from o r i g i n a l and rev ised NWSRFS model app l ied t o the Creighton watershed i n 1975 using the same inputs f o r each system. Un i ts are m i l l i m e t e r s ....... 39

i v

LIST OF FIGURES

Page

Conceptual model f o r c l a s s i f y i n g the i n f i l t r a t i o n p o t e n t i a l o f f rozen s o i l s : (a) Rest r ic ted ; (b) L imi ted and (c) Un l im i ted ................................................... 6

The r e s t r i c t e d case - photographs showing a l aye r o f i c e formed on the sur face o f a s o i l caused by mel t - f reeze events i n the pe r iod December through mid February, 1983-84 a t : (a) Outlook, Saskatchewan and (b) Asquith, Saskatchewan ........ 7

The u n l i m i t e d case - photographs showing the s i z e and con- t i nu i t y o f 1 arge cracks (macropores) which have developed i n a dry, l acus t r i ne , heavy c l a y (Sceptre >55% c lay ) under con- t inuous cropping ......................................... 8

Locat ion o f study s i t e s w i t h i n t h e brown and dark brown s o i l zones o f Saskatchewan. ................................ 10

Mass snowcover i n f i l t r a t i o n curves.......................... 12

Scat te r diagram o f i n f i l t r a t i o n (INF) p l o t t e d aga ins t snow- .... cover water equ iva len t (SWE) f o r uncracked P r a i r i e s o i l s 14

Scat te r diagram o f i n f i l t r a t i o n (INF) p l o t t e d aga ins t premel t f rozen water content o f the 0-30 cm s o i l l aye r (8 ) expressed

P as the r e l a t i v e d e g r e e o f s a t u r a t i o n (Rel. Sat.) ............ 15

Re la t ionsh ips between i n f i l t r a t i o n , snowcover water equiva- l e n t and premel t s o i l mo is tu re content described by Eqs. 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Re la t ionsh ips between premelt s o i l moisture content and f a l l s o i l moisture content f o r the s o i l l aye r 0-30 cm............ 20

Dimensionless curves o f the r a t i o : amount o f i n f i l t r a t i o n t o t ime t ( I N F ( ~ ) ) t o the t o t a l snowmelt i n f i l t r a t i o n ( INF) p l o t t e d w i t h the r a t i o o f t ime from the s t a r t o f m e l t ( t ) t o the length o f the i n f i l t r a t i o n p e r i o d ( t ( l N F ) ) assuming con- t inuous, un in te r rup ted snowmelt and i n f i l t r a t i o n ............ 23

Flow diagram o f "Proposed" I n f i l t r a t i o n Model t o Frozen S o i l - assuming d a i l y inputs and outputs ......................... 26

Schematic Diagram o f NWSRFS LAND subrout ine (Peck, 1976) .... 32

Schematic Diagram o f Subrout ine WLAND ....................... 34

Observed and s imul a ted s treamf low hydrog raphs from snowmel t f o r the Creighton Watershed, 1974 ........................... 38

15. Observed and simulated streamflow hydrographs from snowmelt fo r the Creighton Watershed, 1975 ........................... 4 0

ABSTRACT

The p r o j e c t has been d i rec ted t o the development o f an

a lgo r i t hm desc r ib ing the s n o w m e l t - i n f i l t r a t i o n i n t e r a c t i o n t o f rozen

P r a i r i e s o i l s . I t i s suggested t h a t f o r p r a c t i c a l purposes the i n f i l -

t r a t i o n p o t e n t i a l o f frozen s o i l s o f the Region may be grouped t o one

o f th ree c lasses: Rest r ic ted , L imi ted and Unl imi ted. Areas o f a

watershed classed as "Restr icted" a re considered impervious; those

classed as "Unl i m i ted" are capable o f absorbing a1 1 water o r i g i n a t i n g

from the snowcover. For s o i l s having "Limited" p o t e n t i a l i t i s shown

t h a t the amount o f i n f i l t r a t i o n can be est imated from the snowcover

water equ iva len t (SWE) and the soi 1 moisture content ( i c e content) o f

the s o i l l aye r , 0-30 cm, (0 ) a t the t ime o f mel t . Empir ica l expres- P

s ions r e l a t i n g these var iab les are presented. I n a d d i t i o n , the gene-

r a l format desc r ib ing the manner i n which the concept and empi r ica l

r e l a t i o n s h i p s may be used i n opera t iona l f o recas t ing systems fo r

p r e d i c t i n g streamflow runof f from snowmelt i s presented.

Synthesized streamflows from snowmelt on a small watershed

i n western Saskatchewan generated w i t h the National Weather Service

River Forecast ing System - Sacramento Model (NWSRFS) i n i t s o r i g i n a l

form and m o d i f i e d i n accordance w i t h the above concepts and f i nd ings

o f the i n f i l t r a t i o n process were compared w i t h measured o u t f l o w hydro-

graphs. I t i s shown t h a t the volumes o f runo f f , r u n o f f ra tes and t ime

elements o f t he hydrograph obtained w i t h the "modified" system are i n

c lose r agreement w i t h measured hydrographs than those obta ined w i t h

the unmodif ied NWSRFS. I t i s suggested t h a t a d d i t i o n a l t e s t i n g o f the

i n f i l t r a t i o n a l g o r i t h m i n an opera t iona l f o recas t ing system app l ied t o

l a rge watersheds i s warranted. Two important q u a l i t i e s o f the i n f i l -

t r a t i o n model a r e i t s s i m p l i c i t y and phys ica l i n t e g r i t y .

ACKNOWLEDGEMENTS

The w r i t e r s wish t o express t h e i r s incere thanks and g r a t i -

tude t o personnel o f t he D i v i s i o n o f Hydrology who p a r t i c i p a t e d i n the

p ro jec t . To: T. Brown f o r the design o f the s o l i d s t a t e sca le r t imer

u n i t s , the valuable c o n t r i b u t i o n s t o the opera t ion and c a l i b r a t i o n o f

equipment and the design o f data storage and r e t r e i v a l systems; W.

Stankewich f o r the cons t ruc t i on o f the e l e c t r o n i c equipment and par-

t i c i p a t i o n i n the f i e l d measurement program; R.J. Granger f o r the

con t r i bu t i ons t o the f i e l d data c o l l e c t i o n program and the a n a l y s i s

and i n t e r p r e t a t i o n o f data and f o r h i s assis tance t o the development

o f empi r ica l r e l a t i o n s h i p s and i n f i l t r a t i o n concepts; D. Bayne and M.

Jamieson f o r t h e i r e f f o r t s i n c o l l e c t i n g the f i e l d data, much o f which

was acquired under adverse and inclement cond i t ions .

Funding support provided by the Research Management D i v i s i o n ,

A1 be r ta Environment (Contract AE 84-0472) ; the Farmlab Program, Sask-

atchewan Department o f Ag r i cu l tu re ; Natura l Sciences and Engineering

Research Counc i 1 and Water Research Support Program, l nland Waters

D i rec tora te , Environment Canada f o r d i f f e r e n t aspects o f the study

program i s g r a t e f u l l y acknowledged.

1. INTRODUCTION

Most systems i n present use f o r forecast ing o r synthesizing

streamflow from snome l t were developed f rom data obtained from moun-

tainous regions which are characterized by deep snowpacks and frozen

so i ls . As a r e s u l t they cannot be used w i t h confidence i n large areas

o f cent ra l and northern Canada, where snowcover and ground condi t ions

d i f f e r vas t l y from those encountered i n mountains, w i thout major

rev i s i m s and changes t o the "snowcover accumu 1 a t ion and ab la t ion" and

"land phase" subroutines.

I n a previous study the D i v i s i o n o f Hydrology, Univers i ty o f

Saskatchewan (1977) reported the r e s u l t s o f an examination o f the U.S.

Na t i onal Weather Serv i ce River Forecast Sys tem (NWSRFS) snow accumu 1 a-

t i o n and ab la t ion model under P r a i r i e condi t ions. The main recornmen-

dations from t h i s work concern co r re l a t i ons between the 6-h mean tem-

perature and d a i l y maximum temperature, the use of OOC as the temper-

a ture which determines whether p r e c i p i t a t i o n occurs i n the form o f

r a i n or snow and as the base temperature above which snow melts; the

use o f a catch def ic iency factor o f 1.1 f o r the Nipher gauge; the use

o f a var iable mel t f ac to r (HF) i n the range 0.03 < MF < 0.15 which

should be adjusted dai l y i n response t o c l ima t i c conditions; a 1 i m i t

t o the l i q u i d water holding capacity of l ess than 0.05 and the assump-

t i o n that the d a i l y ground heat f l u x i s zero. To the wr i te r ' s know-

ledge s im i la r invest igat ions o f the a p p l i c a b i l i t y o f the land phase

rout ine o f the NWSRFS, o r other d i g i t a l systems, i n synthesizing snw-

melt i n f i l t r a t i o n have not been reported, p a r t i c u l a r l y for the P r a i r i e

region. A p laus ib le reason f o r t h i s def ic iency i s the lack o f physi-

cal data t o e f f e c t a rigorous eva luat ion of the di f ferent algorithms.

As a consequence, i n most models i n f i l t r a t i o n i s estimated from empi-

r i c a l (and o f ten synthet ic) re la t ionsh ips based on dif ferent moisture

storage zones i n a s o i l p r o f i l e and groundwater storage propert ies.

Several aspects o f these procedures t o c a l c u l a t i n g i n f i l t r a t i o n , hence

runoff, are important: (1) they lack a phys ica l base; (2) the calcu-

lated i n f i l t r a t i o n amounts and runof f ra tes and volumes, are extremely

sensi t ive t o small changes i n the moi s t u r e storage terms; (3) many

streams o f the P r a i r i e reg ion are "ephemeral", i .e. flow on ly occurs

fo l l ow ing a r a i n f a l l o r snowmelt event and therefore the recession

c h a r a c t e r i s t i c s can not be used t o index i n f i l t r a t i o n p o t e n t i a l o f a

watershed p r i o r t o the occurrence o f a runoff-producing event and (4) no attempt i s made t o d i s t i n g u i s h d i f f e rences i n i n f i l t r a t i o n t o

f rozen and unfrozen s o i l s . Because o f these f a c t o r s and o the rs i t i s

unreasonable t o expect the rou t ines t o provide r e l i a b l e est imates o f

s n o m e l t i n f i l t r a t i o n under d i f f e r e n t na tura l physiograhpic c o n d i t i o n s

from which they were developed. The primary o b j e c t i v e o f the study

reported here in was t o develop and t e s t an a lgo r i t hm o f snowmelt

i n f i l t r a t i o n t o f rozen P r a i r i e s o i l s which could be used i n opera t i ona l

streamf low forecast ing models.

2. SNOWMELT INFILTRATION TO FROZEN SOILS

2.1 REVIEW OF FACTORS AFFECTING SNOWMELT INFILTRATION

I n f i l t r a t i o n t o f rozen s o i l s involves the complex phenomenon

o f coupled heat and mass t r a n s f e r through porous media, t he re fo re t h e

process i s a f fec ted by many fac to rs . The most important inc lude: t h e

hydrophysical and thermal p r o p e r t i e s of the s o i l ; t he s o i l mo is tu re

and temperature regimes; t he r a t e of release of water from t h e snow-

cover and the energy content o f t he i n f i l t r a t i n g water. I n t h e ab-

sence o f major s t r u c t u r a l deformations i n a p r o f i l e , e.g. cracks o r

o the r macropores, t he major hydrophysical p roper ty of a f rozen s o i l

governing i t s a b i l i t y t o absorb and t ransmi t water i s i t s mo is tu re

content. Th i s a r i s e s because o f the reduct ion t o the hydraul i c con-

d u c t i v i t y caused by t h e c o n s t r i c t i o n o f blockage of the f l o w of water

by the i c e - f i 1 led pores and the e f f e c t s o f these pores on t h e to r tuos -

i t y and lengthening of the f lowpaths and the d i s t r i b u t i o n and c o n t i n -

u i t y o f the a i r - f i l l e d pores. The ex is tence o f an inverse r e l a t i o n -

sh ip between i n f i l t r a t i o n and f rozen s o i l moisture has been demonstr-

a ted o r pos tu la ted by many i n v e s t i g a t o r s ( W i l l i s e t a l . , 1961; Kuzik

and Bezrnenov, 1963; G i l l i es , 1968; Shipak, 1969; Romanov e t a l . , 1974;

Motovi lov, 1979; Granger and Dyck, 1980 and Kane, 1980). Haupt (1967)

and Har r i s (1972) have demonstrated the e f f e c t o f d i f f e r e n t types o f

f r o s t , as w e l l as t h e number and o r i e n t a t i o n o f connected macropores,

on the absorp t ion o f m e l t water.

The e f f e c t o f t he s o i l temperature regime on i n f i l t r a t i o n i s

less c l e a r than t h a t o f s o i l moisture. Par t o f a l l o f t he water

en ter ing a f rozen s o i 1 whose temperature i s below OOC w i 1 1 re f reeze;

t he amount being a f u n c t i o n o f t he energy s ta tus o f bo th the s o i l and

water, the amount (mass) o f me l t water a v a i l a b l e and the energy ex-

change between the med'ia. Refreezing i s most probable i n so i 1s w i t h

poor i n f i l t r a t i o n and dra inage c h a r a c t e r i s t i c s which a re f rozen a t a

low temperature (h igh thermal energy - negat ive) . McKay (1983) c i t e s

the work o f a number o f s c i e n t i s t s conf i rming t h a t the i n f i l t r a t i o n o f 0

meltwater ra ises the ground temperature toward 0 C by the t r a n s p o r t o f

sens ib le and l a t e n t heat. I t i s genera l l y accepted however, t h a t the

movement of water through capi 1 l a r y pores (as d i f f e r e n t i a t e d from

la rge non c a p i l l a r y pores) i s on l y poss ib le when the s o i l temperature

i s a t i t s f reez ing po in t , b u t no t below (Steenhuis e t al . , 1977).

Despite the complexity o f the i n f i l t r a t i o n process i n t o

f rozen s o i l s models have been developed t o descr ibe i t. For example,

Harlan (1972) developed a model of combined heat and mo is tu re f low i n

frozen and f reez ing so i 1s and Alexeev e t a l . (1972) presented equations

descr ib ing i n f i l t r a t i o n i n t o a frozen s o i l . These models and equations

a r e d i f f i c u l t t o so l ve and requ i re d e t a i l e d in format ion o f t he hydro-

phys ica l and thermal p roper t i es of a s o i l , thus they cannot be, app l ied

d i r e c t l y f o r s o l v i n g p r a c t i c a l opera t iona l problems. However, they

a r e usefu l t o an understanding o f the process and f o r e v a l u a t i n g the

r e l a t i v e importance o f d i f f e r e n t parameters a f f e c t i n g the process. At

t h e i r present s t a t e o f development they lend themselves more r e a d i l y

t o a n a l y t i c a l experiments. For example, Jame (1978) used Har lan 's

model t o c a l c u l a t e mois ture m ig ra t i on t o a f reez ing f r o n t i n a study

conducted on small so i 1 cores i n the labora tory . Motovi l o v (1978,

1979) adjusted Har lan ' s model t o a l l ow f o r the use o f approximations

o f the hydro log ica l and thermophysical c h a r a c t e r i s t i c s o f a s o i 1 and

app l ied h i s model s p e c i f i c a l l y t o i n f i l t r a t i o n i n t o f rozen s o i l . His

numerical c a l c u l a t i o n s a l lowed him t o e s t a b l i s h a r e l a t i o n between the

amount o f water i n the upper 30 cent imetres o f the s o i l and the c r i t i -

c a l f reez ing depth a t which a b lock ing laye r (impermeable i c e lens)

forms i n the s o i l under a constant average r a t e o f water re lease from

the snowcover. Although much progess has been made i n mode l l ing ,

i n f i l t r a t i o n i n t o f rozen s o i l s must s t i l l be est imated f rom empi r ica l

re la t i onsh ips i n s o l v i n g broadscale water management problems.

2.2 CONCEPTUAL MODEL OF INFILTRATION POTENTIAL

Based on approximately f i f t e e n years o f study o f t he snow

hydrology o f a P r a i r i e reg ion and the r e s u l t s o f s tud ies i n s i m i l a r

c l i m a t i c environments i n the USSR reported i n the l i t e r a t u r e (Popov, '

1972) i t i s pos tu la ted t h a t the i n f i l t r a t i o n p o t e n t i a l o f f rozen s o i l s

may be grouped t o th ree broad ca tegor ies , namely; r e s t r i c t e d , l i m i t e d

and u n l i m i t e d (see F ig . 1).

R e s t r i c t e d - i n f i l t r a t i o n i s impeded by an impermeable l aye r , such

as an i c e lense, a t the s o i l sur face (see Fig. 2) o r w i t h i n

the s o i l a t shal low depth. For p r a c t i c a l purposes the

amount o f meltwater i n f i l t r a t i o n can be assumed t o be neg-

l i g i b l e and most o f t he snowcover water equ iva len t goes t o

evaporat ion o r d i r e c t runo f f . Occurrences promoting t h i s

c o n d i t i o n inc lude r a i n f a l l o r snowmelt l a t e i n the f a l l near

freeze-up and m e l t ( r a i n f a l l ) - f reeze events dur ing w i n t e r

o r p r i o r t o continuous mel t . The c o n d i t i o n can u s u a l l y be

e a s i l y i d e n t i f i e d from d i r e c t f i e l d observat ions o r i n f e r r e d

from a review o f c l i m a t o l o g i c a l data.

L imi ted - i n f i 1 t r a t i o n i s governed p r i m a r i l y by the snowcover water

equ iva len t and the f rozen water content ( i c e content) o f a

sha l low layer o f s o i l ad jacent t o the s o i l surface.

Un l im i ted - a s o i l i n t h i s c o n d i t i o n conta ins a h i g h percentage

o f la rge , a i r - f i l l e d , non -cap i l l a r y pores o r macropores a t

the t ime o f me l t and most o r a l l the snow water w i l l i n f i l -

t r a t e . Examples o f s o i l s e x h i b i t i n g these p rope r t i es would

be dry, heavi ly-cracked, heavy c lays (see Fig. 3) and coarse,

d ry sands. The c o n d i t i o n can be i d e n t i f i e d a t the t ime o f

freeze-up.

I n the above c l a s s i f i c a t i o n , i t i s obvious t h a t when evaporat ion

losses a r e neglected, the runof f c o e f f i c i e n t s t o be assigned t o s o i l s

whose i n f i l t r a t i o n p o t e n t i a l can be de f i ned as "Restr ic ted" o r "Un-

l im i ted " i n any model l ing scheme would be 1.0 o r 0 respect ive ly .

Thus, the problem remaining i s one o f d e f i n i n g the r e l a t i o n s h i p be-

tween i n f i l t r a t i o n , snowcover water equ iva len t and frozen s o i l mois-

t u r e content f o r the "Limited" case.

(a) Restricted: amount of snowmelt infiltration is low, high runoff

-- potential.

Decreasing Soil Moisture conten?

(b) Limited: amount of infiltration governed primarily by ice content of the soil layer 0-30 cm at the time of melt.

(c) Unlimited: soil has the capacity to infiltrate all or most of the snowcover water equivalent.

Figure 1. Conceptual model for classifying the infiltration potential of frozen soi 1s: (a) Restricted; (b) Limited and (c) Unl imited.

(a) Outlook

(b) Asquith

Figure 2. The r e s t r i c t e d case - photographs showing a layer o f i ce formed on the surface of a s o i l caused by melt- freeze events i n the period December through mid February, 1983-84 a t : (a) Out look, Saskatchewan and (b) Asqui th , Saskatchewan.

(a) Depth of crack ( top w i d t h -5 cm).

(b) Con t i nu i t y o f cracks (average depth = 33 cm) . Figure 3. The un l im i t ed case - photographs showing the s i z e and con-

t i n u i t y of la rge cracks (macropores) which have developed i n a dry, lacust r ine, heavy c l a y ( ~ c e p t r e >55% c lay) under con- tinuous cropping. P ic tures taken a t Richlea, Saskatchewan i n the f a l l o f 1983 near the t ime o f freeze-up. (a) Depth o f crack ( top w id th -5 cm) ; (b) Cont i nu; t y o f crack (average depth = 33 cm) .

3 INFILTRATION - THE LIMITED CASE

3.1 FIELD DATA

The f i e l d data used t o develop a r e l a t i o n s h i p f o r p r e d i c t i n g

snowmelt i n f i l t r a t i o n t o a f rozen s o i l c l a s s i f i e d as having a "Limited"

i n f i l t r a t i o n p o t e n t i a l were obta ined from a comprehensive f i e l d inves-

t i g a t i o n o f the phenomenon conducted by the D i v i s i o n o f Hydrology i n

the Brown and Dark Brown s o i l zones o f Saskatchewan i n the pe r iod

1978-1983 i n c l u s i v e (see Granger, e t a l . , 1984). F igure 4 shows the

geographical l o c a t i o n o f the study areas i n the province. D i r e c t , in -

s i t u measurements o f i n f i l t r a t i o n from snowmelt were made a t 90 s i t e s

and inc lude a range o f s o i l tex tures (sandy loam; -50% sand t o heavy

c lay; -63% c l a y ) , land use p rac t i ces ( fa1 low, grass and s tubb le ) and

c l i m a t i c cond i t i ons o f the a rab le farm lands o f the Canadian P r a i r i e s .

I n f i l t r a t i o n amounts were ca l cu la ted from readings made

throughout the snowmelt per iod us ing a t w i n probe dens i t y meter. Th is

equipment measures the wet dens i ty o f a s o i l (or the s o i l mo is tu re

content when the b u l k dens i t y i s known) ; therefore, readings taken on

successive dates g i v e the change i n s o i l dens i ty (o r s o i l mo is tu re) i n

the i n t e r v a l . Each s i t e consis ted o f two PVC tubes, w i t h an i n s i d e

diameter o f 5 cm and spaced 25 cm apar t , i n s t a l l e d v e r t i c a l l y i n t o the

s o i l t o a depth o f 160 cm. These tubes served as access tubes f o r the

equipment. The measurement procedure involves lowering a 5 m C i cesium

source t o a g i ven depth i n one tube and a s c i n t i l l a t i o n de tec to r t o

the same depth i n the o ther . A one-minute count i s then taken o f the

at tenuated r a d i a t i o n received by the de tec tor and s tored i n a p o r t a b l e

s o l i d s t a t e recorder . This procedure was repeated a t 2-cm increments

o f depth throughout the p r o f i l e . Complete d e t a i l s o f the t w i n probe

dens i ty meter and i t s use i n s o i l dens i t y and s o i l mo is tu re measure-

ments a r e repor ted i n numerous works ( f o r example see Trox le r , un-

dated; Smith e t a l . , 1967; Ligon, 1969; Ryhiner and Pankow, 1969;

Reginato and Jackson, 1971 and Jame and Norum, 1980). As i nd i ca ted

above, p r o f i l e s were obta ined p r i o r to , dur ing and immediately f o l l o w -

i ng the d i sappearance o f the snowcover (before s i gn i f i cant evaporat ion

Figure 4. Location o f study s i t e s w i t h i n the brown and dark brown s o i l zones o f Saskatchewan.

losses from the s o i 1 occurred). A1 1 s i t e s were located on landscapes

t h a t al lowed dra inage o f sur face runo f f ; t h a t i s they were f r e e from

ponded water and d i d n o t receive major c o n t r i b u t i o n s from the drainage

o f sur face water o r i g i n a t i n g on adjacent areas. S o i l mo is tu re changes

( i n f i 1 t r a t ion) between successive dates o f measurement were ca l cu la ted

from the dens i t y changes assuming the b u l k dens i t y o f the s o i l between

the tubes remained constant and a dens i t y o f water equal t o 1000 kg/m3.

The b u l k d e n s i t y and i n i t i a l mo is tu re p r o f i l e s were estab-

1 ished from a l abo ra to ry ana lys is o f s o i 1 p r o p e r t i e s o f cores (5.2 cm

i n diameter and 10 cm i n length) taken f rom each access ho le a t the

t ime the tubes were i n s t a l led. These measurements provided the base-

l i n e data from which moisture cond i t ions a t the t ime o f me l t were

ca lcu la ted (us ing the measured so i 1 dens i t y changes obta ined w i t h the

t w i n probe dens i t y meter).

I n a d d i t i o n t o moni tor ing changes i n s o i l dens i ty , measure-

ments were made o f t he depth and dens i t y o f the snowcover. Where

poss ib le these were taken throughout the accumulat ion pe r iod t o the

t ime o f a c t i v e snowmelt. Unfor tunate ly , a t some s i t e s , because o f

l o g i s t i c a l problems, i t was impossible t o cont inuous ly moni tor the

snow water equ iva len t . For these s i t e s , when a snowfal l event occurred

a f t e r the snow survey, the "on-s i te" measurements were updated o r

rev ised us ing Nipher gauge readings recorded a t a nearby c l i m a t o l o g i -

c a l s t a t i o n and an assumed snow r e t e n t i o n c o e f f i c i e n t (see Gray e t

a l . , 1979). I n most cases, however, adjustments t o the data were

small r e l a t i v e t o the t o t a l accumulated snowcover water equ iva len t .

Fur ther problems a r i s e i n es t imat ing water equ iva len t a t a s i t e a t the

beginning o f a c t i v e me l t as a r e s u l t o f mixed r a i n and snow events,

h igh wind speeds and snow t ranspor t (e ros ion and accumulat ion), con-

densat ion gains and sub l imat ion losses and l a t e r a l f l ow o f meltwater

through the snowcover which may occur du r ing me l t o r i n the per iod

from the date o f t he snow survey t o the t ime o f mel t .

3.2 MASS INFILTRATION CURVES

An examinat ion o f the mass i n f i l t r a t i o n curve (cumulat ive

i n f i l t r a t i o n p l o t t e d w i t h t ime) provldes an overview o f the s i m i l a r i -

t i e s o r d i s s i m i l a r i t i e s o f the e n t r y of water t o f rozen and unfrozen

s o i l s . Figure 5 shows three curves represent ing d i f f e r e n t premel t

s o i l moisture, snowcover and me l t cond i t ions .

DAYS O F SNOWMELT

Figure 5. Mass snowcover i n f i l t r a t i o n curves. Curve 1 - d ry s o i l (-14% mois ture by volume), deep snowcover and v a r i a b l e rates o f snowmelt; Curve 2 - d ry s o i l (-18% mois ture by volume), r a p i d me l t of snowcover and water ponds on sur face; and Curve 3 - wet so i I (-35% moisture by volume), r a p i d me1 t and an i c e l aye r forms a t t he s o i l sur face e a r l y i n the me l t period.

Curve 1 : i n f i l t r a t i o n t o a r e l a t i v e l y d ry so i 1 (-14% moisture content

by volume) r e s u l t i n g from the slow me l t o f a r e l a t i v e l y deep

snowcover (-50 cm). l n f i 1 t r a t i o n i s delayed by the movement and

storage o f mel twater i n the snowcover. A f t e r the cover r ipens,

water i s released almost cont inuously throughout the me l t per iod.

I n f i l t r a t i o n occurs a t v a r i a b l e ra tes w i t h a trend f o r the r a t e

t o increase w i t h t ime due t o an increase i n the mel t r a t e and the

thawing o f the s o i l by the i n f i l t r a t i n g water.

Curve 2: i n f i l t r a t i o n t o a r e l a t i v e l y d r y s o i 1 (-18% mois tu re c o n t e n t

by volume) caused by t h e r a p i d me1 t o f a r i p e snowcover and wa te r

ponds on t he s o i l s u r f a c e p r o v i d i n g a reasonably cons tan t supp ly .

The maximum i n f i l t r a t i o n r a t e occu rs e a r l y i n t he m e l t p e r i o d

w i t h s o i l mo is tu re s to rage requi rements be ing s a t i s f i e d a f t e r

approx imate ly n i ne days o f me l t . A f t e r t h i s t ime re f reez ing o f

wa te r w i t h i n the s o i l p r o f i l e r e s t r i c t s t h e e n t r y o f water .

Curve 3: i n f i l t r a t i o n t o a r e l a t i v e l y wet s o i 1 (-35% mois tu re c o n t e n t

by volume) r e s u l t i n g f r om the r a p i d m e l t o f a sha l low snowcover;

an i c e l a y e r formed a t t h e s o i l su r f ace and prevented i n f i l t r a -

t i o n u n t i l i t thawed on t h e f i f t h day o f me l t . The amount o f

i n f i l t r a t i o n i s low and t h e imperv ious i c e l a y e r impedes t he

e n t r y o f water.

F i g u r e 5 i l l u s t r a t e s t h a t t h e i n f i l t r a t i o n o f snowmelt t o f r ozen s o i l s

i s n o t s o l e l y a f u n c t i o n o f s o i l p r o p e r t i e s b u t i s s t r o n g l y dependent

on many o t h e r f a c t o r s i n c l u d i n g t he snowmelt r a t e , t he water t r a n s -

m i s s i o n and s torage p r o p e r t i e s o f t h e snowcover and the presence o f

impermeable i c e lenses on o r w i t h i n t h e s o i l p r o f i l e . Because o f t h e

numerous f a c t o r s a f f e c t i n g t h e phenomenon, snowmelt i n f i l t r a t i o n

curves may e x h i b i t many d i f f e r e n t shapes. S i m i l a r f i n d i n g s were re-

po r t ed by Gray e t a l . (1970) whose d i s c u s s i o n o f t he sub jec t makes

d i r e c t re fe rence t o the r e s u l t s o f s t u d i e s r e p o r t e d by Sov ie t s c i e n t -

i s t s . As discussed l a t e r t h i s v a r i a b i l i t y compl i ca tes t he s e l e c t i o n

o f an a p p r o p r i a t e sequence f o r d i s t r i b u t i n g i n f i l t r a t i o n amounts i n

t ime.

3.3 RELATIONSHIP BETWEEN INFILTRATION, SNOWCOVER WATER

EQUIVALENT AND FROZEN SOIL MOISTURE

F i e l d observat ions and t h e r e s u l t s of nurnerou's research

s tud ies , bo th f i e l d and l a b o r a t o r y , suppor t t h e p r o p o s i t i o n t h a t t h e

amount o f i n f i l t r a t i o n d u r i n g snowmelt v a r i e s d i r e c t l y w i t h t he

snowcover water equ iva len t and i n v e r s e l y w i t h t h e f r ozen water con ten t

o f the s o i l a t the t ime o f m e l t . These r e l a t i o n s h i p s p l o t t e d as

s c a t t e r diagrams, using da ta recorded d u r i n g t h e w i n t e r s and me1 t-

pe r i ods f rom 1979-1983 i n c l u s i v e , a r e shown i n F igs. 6 and 7

SNOW WATER E Q U I V A L E N T <mm>

Figure 6 . Scatter djagram of i n f i l t r a t i o n ( INF) plotted against snow- cover water equivalent (SWE) f o r uncracked Prai r i e soi 1 s.

respec t i ve l y . F igu re 6 shows a general t rend f o r i n f i l t r a t i o n (INF)

t o increase w i t h snowcover water equ iva lent (SWE) ; the t rend i s non

l i n e a r r e f l e c t i n g t h a t frozen s o i l s have a l i m i t e d capac i t y t o absorb

water and the l a r g e r the snowcover water equ iva lent t he g r e a t e r the

losses t o evaporat ion and runof f du r ing the me l t per iod. The wide

s c a t t e r o f t he data can be la rge ly a t t r i b u t e d t o d i f f e rences i n s o i l

moisture and temperature regimes a t t he t ime o f mel t ; d i f f e r e n c e s i n

the seasonal a b l a t i o n pat te rns and inaccuracies i n the est imates o f

snowcover water equ iva lent .

F igu re 7 shows i n f i l t r a t i o n p l o t t e d w i t h the average s o i l

moisture ( i c e ) content o f the 0-30 cm s o i l layer , expressed as the

r e l a t i v e degree o f sa tura t ion , j u s t p r i o r t o the occurrence o f a c t i v e

snowmelt. Although the data e x h i b i t considerable s c a t t e r , a t rend fo r

i n f i l t r a t i o n t o vary inverse ly w i t h the mois ture content i s ev ident .

The s o i l layer , 0-30 cm (herein r e f e r r e d t o as the "Zone o f I n f i l t r a -

t ion"), was se lec ted because numerous observat ions showed t h i s t o be a

rep resen ta t i ve va lue o f the depth increment i n which most o f the

i n f i 1 t r a t e d water t o "uncracked" P r a i r i e s o i 1s was conf ined dur ing the

me l t per iod. An ana lys i s o f data c o l l e c t e d a t 78 measurement s i t e s

showed t h a t t h e depth o f penet ra t ion ranged f rom 8 cm t o 54 cm; the

average f o r a l l s i t e s being 26 cm w i t h a standard d e v i a t i o n o f 10 cm.

The shal low depth o f penetrat ion can be a t t r i b u t e d t o r e f r e e z i n g o f

mel twater i n the f rozen s o i l thereby causing a blockage t o f low.

S i m i l a r f i n d i n g s have been reported i n the Sov ie t Union by Motov i lov

(1979) who suggested the c r i t i c a l f reez ing depth a t which a b lock ing

(impermeable) l a y e r forms i n a f rozen s o i l , having an average water

content (115 mm i n 0-30 cm layer) o r h igher , under a constant average

r a t e o f re lease o f snowcover water was 30-40 cm. To support h i s

f i nd ings Motovi l o v c i tes the works o f Apol l o v e t a1 . (1964) and the

l n s t r u c t ions on Hydrolog i c a l Forecast ing (1963) ; both based on hydro-

l o g i c a l analyses o f spr ing f loods i n r i v e r basins i n the USSR, where

i t was observed, " the s o i l i s capable t o absorb meltwater when f rozen

t o a depth o f 15-30 cm o r less". Motov i lov showed a r a p i d increase i n

the c r i t i c a l f r e e z i n g depth o f the b lock ing laye r when the amount o f

water i n the 0-30 cm layer was less than -100 mm; a t rend t h a t could

not be noted i n the f i e l d data used i n the analys is . This discrepancy

might be a t t r i b u t e d t o d i f f e rences i n the frozen s o i l mo is ture and

temperature regimes and the c y c l i c pa t te rns o f snowmelt release.

Likewise, W i l l i s e t a l . (1961) i n the USA suggested t h a t the amount o f

runo f f [hence i n f i 1 t r a t ion) may be governed t o some ex tent by so i 1

moisture cond i t ions i n the sur face 12-15 inches (30-45 cm).

Also shown i n F ig . 7 i s the l i n e representing 100% satu-

r a t i o n o f the Zone o f I n f i l t r a t i o n . I t can be observed t h a t i n f i l -

t r a t i o n r e s u l t s i n s a t u r a t i o n o f the Zone on ly i n s o i l s t h a t a re

i n i t i a l l y very wet (8 > -95) . The t rend i s f o r d r i e r s o i l s t o reach P

moisture leve ls much less than sa tu ra t i on . I t i s suggested t h a t t h i s

occurs because dry , f rozen s o i l s con ta in a l a rge number o f a i r - f i l l e d

micropores which a r e e i t h e r i n i t i a l l y blocked o r become blocked w i t h

i c e as water re f reezes i n the s o i l . The envelope curve, which def ines

the maximum amount o f i n f i l t r a t i o n , can be approximated by the s t r a i g h t

1 ine INF(max) = 100 (1-8 ) . Th is provides a simple y e t use fu l method P

f o r est imat ing the i n f i l t r a t i o n p o t e n t i a l when 8 i s known. P

The data g iven i n Figs. 6 and 7 form the basis f o r develop-

ment o f a simple r e l a t i o n s h i p f o r es t imat ing i n f i l t r a t i o n . Applying a

regression ana lys i s t o the data the "best f i t " r e l a t i o n between the

th ree var iables was c a l c u l a t e d as

INF = o . ~ ~ o ( s w E / ~ ) 0 . 6 5 9 , P

( 1

w i t h a c o r r e l a t i o n c o e f f i c i e n t r = 0.80 and a standard d e v i a t i o n o f

I n sd = 0.357 mm. I n Eq. 1 INF and SWE are i n mm and 8 i s i n P

cm3/cm3. Another express i on t h a t may a l s o be used t o descr ibe the

i n te rac t i on i s

Figures 8a and 8b a r e p l o t s o f Eqs. 1 and 2 respect ive ly . I n comparing

the two f i gu res i t i s ev ident the r e l a t i o n s h i p s d i f f e r . The d i f f e r e n c e s

can be a t t r i b u t e d main ly t o two fac tors : (a) Eq. 1 provides a non l i n -

ear change i n i n f i l t r a t i o n w i t h moisture content whereas Eq. 2 assumes

the r e l a t i o n s h i p between the two va r iab les t o be l i n e a r , as suggested

by the envelope curve o f F ig . 7 and (b) an imbalance i n t h e d i s t r i b u -

t i o n of the data p o i n t s throughout the moisture range 8 P = O S 3 ' eP =

0.8 which were used t o d e r i v e Eq. 1 from a regression ana lys i s . For

p r a c t i c a l a p p l i c a t i o n Eq. 2 (Fig. 9) i s recomnended; however, i t i s

l i k e l y t h a t the r e s u l t s obta ined w i t h Eq. 1 (Fig. 8) would n o t d i f f e r

appreciably from the former as the amounts o f i n f i l t r a t i o n c lacu la ted

from the two r e l a t i o n s h i p s a re i n reasonable agreement f o r 8 -values P

i n the range o f 0.3 -t 0.5 which inc lude moisture l e v e l s normal ly en-

countered i n the f i e l d and a t the h igher 8 -values, INF i s smal l . P

3.4 ESTIMATING THE PREMELT MOISTURE CONTENT

I t i s expected t h a t any opera t iona l u n i t , whose respons ib i l -

i t y i s f o recas t ing streamflow from snowmelt, would have snowcover data

ava i lab le . Hence the major l i m i t a t i o n t o the use of Eqs. 1 o r 2 f o r

es t imat ing i n f i l t r a t i o n i s they requ i re an est imate o f t h e premel t

moisture content, 8 . Gray e t a l . (1983) reported t h a t s i g n i f i c a n t P

changes may occur i n t h e moisture regime o f a s o i l over w i n t e r because

o f moisture t r a n s f e r as a vapor across the s o i l / a i r o r s o i l / snow

in ter faces, the i n f i l t r a t i o n o f water from mid-winter m e l t o r r a i n

events and the m i g r a t i o n o f water i n response t o the f r e e z i n g ac t i on .

Usual ly, losses (decreases i n the s o i l moisture content measured a t

the time o f freeze-up) a r e common from the 0-30 cm s o i l l aye r whereas

the frozen s o i l p r o f i l e below 30 cm shows e i t h e r gains o r l i t t l e

change i n mois ture content , depending on s o i l moisture cond i t ions .

For p r a c t i c a l purposes however the s o i l moisture content o f the 0-30

cm layer i n the f a l l (Bf) can be used t o index the mois ture content a t

the t ime o f m e l t (8 ) . F igure 9 shows the re la t i onsh ips between these P

values f o r fa1 low (Fig. 9a) and s tubb le (Fig. 9b). The "best- f i t"

regression equations ca l cu la ted from the data were:

Fa1 low: P

= - 5.08 + 1.05 o f , and

Stubble: 8 = 0.294 + 0.957 O f , P

o 30 SO 00 120 is0 i eo

SNOW WATER EQUIVALENT <mm>

-659 <a> 1 NF = 0. 980 <SWE/Bp>

SNOW WATER EQUIVALENT <mm>

,584 <b> INF = 5*<1-8p> *SWE

Figure 8. Relationships between infiltration, snowcover water equiva- lent and premelt soil moisture content described by Eqs. 1

and 2: (a) INF = 0.980(SW~/B ) . 6 5 9 and (b) 5(1-8 ) s w E . ~ ~ ~ . P P

Fall Soil Moist. <XVol> <a> Fallow

Fall Soil Moiet. <XVol> <b) Stubble

Figure 9. Relationships between premelt soil moisture content and fall soil moisture content for the soi 1 layer 0-30 cm: (a) Fallow and (b) Stubble.

i n which 0 and 0 a r e expressed as a percent mo is tu re by volume. Each P f

expression has a c o r r e l a t i o n c o e f f i c i e n t o f approximate ly 0.9 w i t h a

standard d e v i a t i o n from regression o f approximate ly 3.33% by volume.

I n review o f Figs. 9a and 9b and Eqs. 3a and 3b i t i s ev ident t h a t the

overwin ter s o i l mo is tu re losses from s i t e s l oca ted i n f a l l o w a r e

g rea te r than those under s tubble. This i s a t t r i b u t e d main ly t o d i f f -

erences i n snowcover cond i t i ons on the land uses; i n f a c t , i t i s

l i k e l y t h a t some o f the f a l l o w s i t e s used i n the analyses were no t

snowcovered dur ing the e n t i r e w in te r period. For p r a c t i c a l app l i ca -

t i o n s 0 can be taken equal t o O f f o r stubble. P

3.5 SEQUENC ING I NF I LTRA'TION QUAN'TITI ES - THE L l M ITED CASE

The above prov ides d e t a i l s on procedures and r e l a t i o n s h i p s

which can be app l i ed i n es t ima t ing i n f i l t r a t i o n t o f rozen s o i l s o f

d i f f e r e n t i n f i l t r a t i o n p o t e n t i a l a t the t ime o f m e l t . For these data

t o be employed i n an opera t iona l fo recas t ing system they requ i re i n -

formation on the v a r i a t i o n i n i n f i l t r a t i o n r a t e w i t h t ime dur ing the

me1 t sequence. As discussed prev ious ly (see Sect ion 3.2), the shape

o f the curve may vary w ide l y depending on such f a c t o r s as; the r a t e

and p a t t e r n o f snowmelt, the release o f mel twater f rom the snowcover,

the fo rmat ion o f impermeable layers w i t h i n the s o i l o r snowcover, t he

content and d i s t r i b u t i o n o f i c e i n the s o i l a t t h e t ime of me l t and

others.

Two approaches t o sequencing i n f i 1 t r a t i o n amounts (a1 so

snowcover r u n o f f ) i n an a lgo r i t hm f o r i n f i l t r a t i o n seem p laus ib le :

(1) Assume no d i r e c t sur face runo f f occurs be fore the i n f i l-

t r a t i o n p o t e n t i a l o f the f rozen s o i l , as ca lcu la ted by Eqs.

1 o r 2, has been s a t i s f i e d . That i s , d u r i n g the e a r l y

per iods o f the me l t sequence, the s o i l i n f i l t r a t i o n r a t e i s

taken equal t o the r a t e a t which me1 twa te r i s released from

the snowcover. I n t h i s fo rmula t ion the t ime o f snowcover

r u n o f f t o a stream channel, neg lec t i ng de tent ions and de-

pressional storage, i s es tab l ished by the snowcover accumu-

l a t i o n and a b l a t i o n subrout ine o f t he fo recas t i ng system.

Th i s approach i s considered f e a s i b l e under cond i t ions where

the mel t r a t e o r c o r r e c t l y , the snowcover discharge ra te ,

increases progress ive ly over the m e l t sequence.

(2) Develop "cha rac te r i s t i c " i n f i l t r a t i o n - r a t e curves f o r d i f f -

eren t snowcover, snowmel t and "premel t." so i 1 mi s t u r e con-

d i t i o n s - w i t h f u l l r ecogn i t i on o f the wide v a r i a b i l i t y

which may occur i n shapes o f these curves as a r e s u l t o f the

f a c t o r s discussed above.

G iv ing cons idera t ion t o the l a t t e r ( i tem 2 above) one app-

roach t o developing "cha rac te r i s t i c " i n f i l t r a t i o n curves, which reduces

the e f f e c t s o f snowmelt and snowcover d ischarge ra tes on shape, would

be t o analyze the a v a i l a b l e data w i t h the assumption t h a t i n f i l t r a t i o n

occurred un in ter rupted and cont inuously throughout the per iod o f

i n f i l t r a t i o n . Under these cond i t ions one might expect w i t h a r a p i d

mel t , which produces me l t volumes i n excess o f the volumes t h a t w i l l

en te r the s o i l , the i n f i l t r a t i o n r a t e w i l l depend l a r g e l y on the water

t ransmission c h a r a c t e r i s t i c s of the f rozen s o i l and tend t o decrease

w i t h t ime. On the o the r hand, when the m e l t r a t e i s less than the

i n f i l t r a t i o n r a t e a t the s t a r t o f the mel t sequence and progress ive ly

increases throughout the mel t per iod due t o increas ing r a d i a t i v e and

sens ib le energy f luxes, one might expect the i n f i l t r a t i o n r a t e t o be

dominated by the mel t process and increase w i t h t ime. Figure 10

shows non-dimensional graphs o f the r a t i o , t he amount o f i n f i l t r a t i o n

occu r r i ng a t g iven t ime fo l l ow ing the s t a r t of i n f i 1 t r a t i o n ( l ~ F ( t ) )

t o the t o t a l amount o f i n f i l t r a t i o n (INF) p l o t t e d w i t h the r a t i o o f

the t ime from the beginning o f i n f i l t r a t i o n t o the length o f the

i n f i l t r a t i o n per iod which were constructed f rom f i e l d data c o l l e c t e d

dur ing the 1978, 1979 and 1980 a b l a t i o n per iods assuming continuous,

un in te r rup ted i n f i l t r a t i o n . As expected, the data tend t o e x h i b i t

considerable scat te r . Nevertheless, regardless o f the sca t te r two

general trends emerge which charac ter ize the i n f i l t r a t i o n sequence.

That i s , an "advanced" p a t t e r n represent ing a r a p i d me l t i n which

most o f the i n f i l t r a t i o n p o t e n t i a l i s s a t i s f i e d e a r l y i n the mel t

per iod and a "delayed" sequence which charac ter izes the accumulation

o f i n f i l t r a t i o n w i t h t ime under an extended, prolonged mel t . Using

these data representa t ive curves were developed f o r the two cases.

Rat io : Time from s t a r t o f i n f i l t r a t i o n ( t ) / l e n g t h o f i n f i l t r a t i o n per iod ( ~ ( I N F ) ) .

F igure 10. Dimensionless Curves of the Rat io: amount o f i n f i l t r a t i o n t o t ime t ( I N F ( ~ ) ) t o the t o t a l snowmelt i n f i l t r a t i o n (INF) p l o t t e d w i t h the r a t i o o f t ime from the s t a r t o f m e l t ( t ) t o the l eng th of the i n f i l t r a t i o n pe r iod ( t ( l N F ) ) assuming cont inuous, un in ter rup ted snowmelt and i n f i l t r a t i o n .

On review o f the f i e l d data i t was found t h a t under the

assumption o f cont inuous i n f i l t r a t i o n the " p e r i o d - o f - i n f i l t r a t i o n "

f e l l i n the range from 5-9 days. The sho r te r per iods being associated

w i t h r a p i d m e l t o f a shal low snowcover; the longer per iods associated

w i t h the slow m e l t o f a re la t i ve l y -deep snowcover. Based on these

f i nd ings i t i s suggested f o r design purposes an average va lue o f 7 d

be taken as the length o f the per iod . The recommendation i s based on

two major cons idera t ions : (a) i t i s h i g h l y improbable t h a t a design

snowcover, capable o f producing maximum f l o w ra tes and volumes, would

completely a b l a t e i n a pe r iod l ess than seven days f o l l o w i n g the

i n i t i a t i o n o f me l t and (b) the i n f i l t r a t i o n per iod can be e a s i l y

extended t o longer durat ions t o account f o r i n t e r r u p t e d m e l t and

re lease pa t te rns i n the i n f i l t r a t i o n a lgor i thm. Table 1 summarizes

values o f r a t i o l ~ F ( t ) / l NF corresponding t o d i f f e r e n t va lues o f

t / t (INF) f o r Advanced, Uniform and Delayed pat te rns .

Table 1. Coordinates o f dimensionless snowmelt i n f i l t r a t i o n curves f o r Advanced, L inear and Delayed pa t te rns assuming cont in - uous me l t and a p e r i o d - o f - i n f i l t r a t i o n o f seven days.

I N F ( ~ ) / I N F

t / t ( l ~ ~ ) Advanced Uni forma Delayed

a Uni form assumes an equal amount o f i n f i l t r a t i o n each day u n t i l i n f i l t r a t i o n p o t e n t i a l has been s a t i s f i e d .

3 . 6 SNOWMELT INFILTRATION MODEL

The above r e s u l t s and discussions presented i n Sect ion 3

serve as a bas is f o r the development o f an a l g o r i t h m desc r ib ing i n f i l -

t r a t i o n t o f rozen s o i l s . F igure 1 1 i s a f l ow diagram o f one model

which w i l l be tes ted i n subsequent ca l cu la t i ons ; f o r convenience d a i l y

inputs and outputs a r e assumed. As shown the model makes use o f

empi r ica l r e s u l t s and r e l a t i o n s h i p s which have been der ived. I n the

example the i n f i l t r a t i o n amounts a r e sequenced accord ing t o a selected

pat te rn . Note, when the amount o f snowcover r u n o f f (DsRO) i s less than

the q u a n t i t y o f i n f i l t r a t i o n i t i s assumed the e f f e c t can be compen-

sated: by reducing the t o t a l i n f i l t r a t i o n (INF) through an increase i n

t he premel t mo is tu re content (8 ) and a decrease i n t he snowcover P

water equ iva len t (SWE) i n amounts equivalent t o DSRO; and by s h i f t i n g

the curve f o r cont inuous i n f i l t r a t i o n the equ iva len t o f one time step.

I den t i f y and Classify areas o f a Watershed according t o the1 r

l n f i l t r r t l o n Po ten t l r l

Unl lmited ?l Llrni ted a Crlculate Premelt Moisture, -1

from Snowcover Water Equlvalent, SUE and0 ... Eq.2

Select l n f l l t r a t i o n Pattern (Flg. 10) and Dis t r ibu te INF t o Daily Totals, DlNF

See Table 1

Calculated Dai ly Snowcover

Restricted ?l

Runoff or ~ n o & l t Amounts f ran Snow Ablation Routine

I DSRO

S h i f t I n f i l t r a t i o n Pattern One Day h I

Recalculate 0 SUE and INF .. Eq. 2 ''

Bp - 8 + DSRO

SUE - SUE - DSRO I I ( 1 Compare DlNF and DSRO I I

RUNOFF, DRO - 0

Figure 1 1 . Flow diagram of "Proposed" Infiltration Model to Frozen Soil - assuming daily inputs and outputs.

MODEL DEVELOPMENT, TESTING AND VERIFICATION

4.1 GENERAL

The i n i t i a l t e s t s d i rec ted t o the development o f an a lgo r -

i thm f o r descr ib ing the s n o w m e l t - i n f i l t r a t i o n i n t e r a c t i o n i n f rozen

s o i l s i n opera t iona l models were undertaken w i t h the U.S. Nat ional

Weather Serv ice R iver Forecast i ng System - Sacramento Model (NWSRFS)

appl ied t o a small watershed, the Creighton T r i b u t a r y , i n the Bad Lake

watershed located i n the semi-ar id region o f western Saskatchewan.

The s e l e c t i o n o f the NWSRFS and Creighton T r i b u t a r y f o r analyses was

based on several f a c t o r s o f which the most impor tan t include:

1. The D i v i s i o n o f Hydrology has had p rev ious experience w i t h

the NWSRFS and the system i s o p e r a t i o n a l on the HP 1000

microcomputer system a v a i l a b l e t o t h e D i v i s i o n , and

2. Throughout the past 12 years members o f t he D i v i s i o n have

conducted numerous s tud ies i n the Bad Lake watershed; f o r

example those i nves t i ga t i ons concerned w i t h snowcover accu-

mulat ion, a b l a t i o n and runo f f , energy budget, s o i l moisture,

evapot ransp i ra t ion and others, du r ing which a vast amount o f

experience has been gained on the hydro logy of the region

and a reasonably comprehensive da ta base o f the parameters

requ i red f o r the development o f t h e i n f i l t r a t i o n model have

been es tab l ished and stored i n d i g i t a l form. In add i t i on ,

the Creighton watershed does n o t c o n t a i n l a rge amounts o f

depressional storage and thus the e f f e c t s o f t h i s f a c t o r on

runo f f can be neglected and the gross area can be taken as

the e f f e c t i v e drainage area. The general topography o f the

watershed may be classed as r o l l i n g t o g e n t l y undulat ing

w i t h most o f the area under c u l t i v a t i o n of cereal g ra ins by

dry land farming. I t f a l l s i n a t r a n s i t i o n a l zone demarking

g l a c i a l and 1 acustr i ne reg ions and t h e r e f o r e includes two

p r i n c i p l e s o i l associat ions: H a v e r h i l l s i l t y c lay and c l a y

loams and Sceptre c lay .

Very b r i e f l y the procedures used i n developing and t e s t i n g

the i n f i l t r a t i o n a l g o r i t h m invo lve m o d i f i c a t i o n s to, o r replacement o f

those elements o f t h e land phase o f the WSRFS fol lowed by an evalua-

t i o n o f the improvement i n performance o f the model i n p r e d i c t i n g

streamflow rates, volumes and time elements o f the runoff hydrographs

from the Creighton t r i b u t a r y . I t i s s t ressed t h a t eva luat ion o f the

"improvement" i n model performance can n o t he based soley on the

agreement between t h e pred ic ted and measured flows. The reason f o r

emphasizing t h i s p o i n t i s t h a t the l n f i l t r a t l o n a lgor i thm uses as

i npu ts the outputs from the snowcover accumulat ion and a b l a t i o n sub-

r o u t i n e o f the NWSRFS which are based on values f o r d i f f e r e n t para-

meters; eg. area 1 dep le t ion , temperatures, me1 t fac to rs and o thers

recommended i n previous works (Cram, 1976; Wells, 1976 and D i v i s i o n o f

Hydrology, 1977). A c lose f i t o f "synthesized" and "measured" hydro-

graphs may be una t ta inab le because the a b l a t i o n process (or o thers)

has not been synthesized c o r r e c t l y by the system. I t i s suggested

t h a t b e t t e r measures o f the improvements i n streamflow synthesis a r e

def ined by the agreement i n runo f f volumes and i n the t ime elements o f

the hydrographs. Wi th respect t o the l a t t e r i t should be noted t h a t

the t ime per iod o f 6 h used by the model i s l i k e l y too l a rge f o r a

watershed the s i z e o f the Creighton T r i b u t a r y , whose basin lag i s

probably o f the same order o f magnitude.

4.2 WATER BALANCE CALCULATlONS

A simple, d i r e c t t e s t o f eva lua t ing the performance o f t he

i n f i l t r a t i o n model i n c a l c u l a t i n g streamflow from snowmelt i s t o

compare the t o t a l volume o f runof f , c a l c u l a t e d as the d i f f e rence

between snowcover water equivalent and i n f i l t r a t i o n , w i t h the amount

obtained from the streamflow hydrograph. Reasonable agreement i n

these values would be expected under the assumption tha t losses t o

depressional storage, evaporat ion and o the r f a c t o r s are small compared

t o the volume of d i r e c t runof f . As po in ted ou t above, the e f f e c t s o f

s torage on r u n o f f volumes i n the Creighton watershed are small.

These c a l c u l a t i o n s were completed f o r the Creighton water-

shed using data f o r two win ters , 1973/74 and 1974/75, which had con-

t r a s t i n g snowcover and premelt s o i l mo is ture condi t ions. The w i n t e r

o f 1973-74 was a year o f near record snowfa l l ; the average depth of

snowcover on the watershed was 55.6 cm haylng a snowcover water equiv-

a l e n t o f approximately 143 mm. It was preceeded hy a warm, d ry f a l l

i n which the average mois ture content o f the surface layer o f s o i l was

reduced t o the w i l t i n g p o i n t o r below, e s p e c i a l l y i n those s o i l s having

vege ta t i ve cover. The ex ten t o f s o i l c rack ing was not recorded,

however i t can be reasonably assumed t h a t i t was extensive because the

phenomenon has been observed i n subsequent years i n f i e l d s o f the same

s o i l type (Sceptre c l a y ) a t h igher mois ture l e v e l s than those measured

i n the f a l l o f 1973. I n add i t i on , f i e l d observat ions dur ing the

snowmel t runo f f per iod i n the sp r ing o f 1974 showed runof f from s tubb le

land t o be h i g h l y v a r i a b l e . On some f i e l d s small , but measurable,

sur face f lows were observed, on o thers the volumes were i n s i g n i f i c a n t

compared t o the amount o f snowcover water equ iva lent . The r e s u l t s o f

snowrnel t runo f f s tud ies conducted on sma 1 1 a reas (mi cro-watersheds)

located i n the Bad Lake watershed c lose t o the Creighton T r ibu ta ry

reported by Erickson e t a1 . (1978) showed a lower amount o f sur face

runof f from stubble i n 1974 compared t o the q u a n t i t i e s generated i n

o the r years f o r the same energy index of mel t . I n contrast , condi-

t i o n s dur ing the w in te r of 1974-75 would be l i kened more c l o s e l y t o

"normal"; the average depth and water equ iva lent o f the snowcover were

29.9 cm and 71.8 mrn respec t i ve l y and the average premelt s o i l mo is ture

content 27.4% by volume.

The r e s u l t s o f the water budget c a l c u l a t i o n s are g iven i n

Table 2. It should be noted t h a t the values o f the snow water equiva-

l e n t (SUE) and the premel t soi 1 moisture content (0 ) were der ived P

from data c o l l e c t e d from comprehensive snow surveys conducted on the

watershed and from measurements made w i t h a neutron gauge o f the f a l l

so i 1 moisture regime on a dense network (23 gauges) i n f i e l d s located

immediately adjacent t o the watershed having the same s o i l type and

cropping pat terns. From the data i n Table 2, i t can be seen t h a t the

area l ly-weighted volumes o f runo f f ca l cu la ted as the d i f f e rence (snow

water equivalent (sWE) - snowmel t i n f i 1 t r a t i o n INF)) are i n c lose

agreement w i t h those volumes obta ined from the discharge hydrographs.

The r a t i o of "calculated" t o "measured" volume was 1.02 f o r 1973-74

and 1.16 f o r 1974-75. The res idua l q u a n t i t i e s (SWE - INF - measured

re .- O L E c m o 3 P I L Ere

In- 0)m PL E

* - n m

. . 5 UI

S Z 2 : E P

Y- 0)

O E S L U L - U C U - 0

m > C Y- o w - o

c o 0) 0 - C a 0 3 C v )PL 3 m 0 n ore U

c o g C a- o w m c

- - w 3 In -- C

0) - c 3 $ r 0 3 w

n o m - 0 ) - 0 )

n - * m *- h L E m o m .- - re

L > . re c m - 0

- - 0 ) C s C O 2 In

In c u m o a l o - *- 3 0) U U O L

0-X 3 = u r n In u c m Q O I n - 0 0 ) E -- U E a r I .- L 0) - 3 3 K C O W 0 3 I n 0 ) *- = . - I n U

0 m @ E r e

z r e w w - 0 - .- c .- S O 3 LC . - InL C .- - c u -'-c I m m re u U s

0 ) w C O o w - a).- 0 - 0 ) - w

3 E m m U In > L 0 ) a l Y - - 0

5 ' 0 3 " • v m

I n - I n a l o , > I n - C - al m m o n 1

%.-a a u 0) w 3 - C

- > m o m m a ~ > m 3 nu^ e, 3 0 ) - 3 m + ' > I n o o m In-n K L

0) 0 m u , 0 C E u u .- w - 0 - II In

L - al -0 0 a*-- 0 c % . - r e m w m 0 ) w 3 - -0 U nn .- In

- m a I n0 -VV 0 ) - a ez

m n

0 a- a 0

0

a 0 CO a- 0

-j. cr\ tn m

0

m o a m m - h *h l h l h - . . . 0 0 0

* m a hl h l a m * O \ D h - - - 0 . . . - 0 0 0

- m * m

0

- l n m a * * m h l m a m * . . . . . . 0 0 0 0 0 0

D 0)

U U U '0 -0 -0 0) a- aJ a l 0 ) 0 ) U E U w w w .- .- .- .- .- .- E - E .- c .- E E E .- .- .- A 3 4 4 2 4

Ln hl 0 CO

0

C O O * \ D m 0 3 - * a \ O h m . . . 0 0 0

ha03 C O * - a o a m - a . . . . . . a m - m a -

- - m m U w 0 0 k +

Q) 0 )

B z I n B z I n - a m - n I n - 3 m - 3 m m - L m w L L V ) O LL u, 0

* Ln h h L 1 m * h h tn m - -

hl I A W h 0 - h l W m a m C O a h l m - . . . - 0 0 0

runo f f ) were 0.0073 x lo6rn3 i n 1973-74 and Q.0640 x l o 6 m 3 i n 1974-75, which represent an average depth o f water on the watershed of 0.60 mm

and 5.6 mrn respec t i ve l y . Such c lose agreement I n "calculated" and

"measured" volumes was unexpected consider ing the l e y e l s of accuracy

o f measurement o f the d i f f e r e n t hydro log ica l components and the

methods used i n eva luat ing the d i f f e r e n t terms. I t could be argued

t h a t the assumption o f c l a s s i f y i n g the infiltration p o t e n t i a l o f the

t o t a l area i n s tubb le i n 1973-74 as "unl imi ted" i s no t v a l i d . Note:

i f the s tubb le area was classed as " l im i ted " t h i s would lead t o an

e r r o r equal t o approximately 30% o f the SWE or , the measured volume o f

runo f f . However, as pointed ou t above, i t i s known from f i e l d exper-

ience o f the i n t e r a c t i o n o f s o i l moisture and s o i l c rack ing o f the

l a c u s t r i n e c l a y o f the area and f i e l d observat ions of snowmelt runo f f

dur ing the m e l t sequence i n 1974 t h a t i t would be i n c o r r e c t i n des-

c r i b i n g the i n f i l t r a t i o n p o t e n t i a l o f the watershed i n 1973-74 as

" l imi ted" . One can on ly pos tu la te the exact d i v i s i o n of the t o t a l

area o f the watershed t o "unl i m i ted" and "1 i m i ted" classes. Neverthe-

less, i t i s considered tha t the r e s u l t s obta ined i n the two years do

support the p r o p o s i t i o n tha t the model w i l l prov ide est imates o f

i n f i l t r a t i o n acceptable f o r p r a c t i c a l , opera t iona l app l i ca t i ons .

4 . 3 EVALUATION OF NWSRFS APPLIED TO CREIGHTON WATERSHED

4.3.1 I n t r o d u c t i o n

A s h o r t desc r ip t i on o f the s o i l moisture accounting rou t ine

(LAND) o f t h e U. S. Nat ional Weather Service Sacramento model (NWSRFS)

i s provided t o f a m i l i a r i z e the reader w i t h the major p a r t s o f the

system. F igu re 12 i s a schematic f lowchar t o f t h i s subrout ine. As

shown i n the f i g u r e the rou t ine u t i l i z e s two subsurface moisture

zones: an upper zone conta in ing a tension and a f r e e water component

and a lower zone having one tension and two f r e e water storage compon-

ents. Input t o the rou t ine i s i n the form o f p r e c i p i t a t i o n o r snow-

me1 t runo f f and evapotranspirat ion (ET) can occur from a1 1 storages

and the stream channel. I n f i l t r a t i o n occurs f i r s t t o the upper zone

tension water and then t o f r e e water ( i f there i s any l e f t over) .

When the upper zone capacity i s s a t i s f i e d then p r e c i p i t a t i o n and snow

PRECIPITATION MPUT 1 PERVIOUS AREA IMPERVIOUS *DIW

DIRECT ANOR

Figure 12. Schematic Diagram o f NWSRFS LAND subroutine (Peck, 1976).

UPPER] ZONE - EXCESS

f , ,/. * SURFACE RUNOFF

TENSION WATER ,, - UZR *

, FREE WATER 4' UZFW

/ 0,

E T

- 4 UZTW ,,//

INTERFLOW - E T

I 1 I

PERCOLATION i - ST REAM FLOW - E l 4

ZRRC. REIP

SARVA

A -

TOTAL C H W L I N F l W

u -

SSGIT : FREE ',WATER L Z S R SUPREMWTAL

DISTRBUIK)N FUNCTION -

7ENSK)N WATER i p i s 4 BASE FLW -

+

b LzTw i ..... LZFP .................... i LIFS '7 TOTAL BASE

E T

FLOW SLASURFKC DIX)u#iZ

mel t water go d i r e c t l y t o r u n o f f . Water moves from the upper zone t o

the lower zone by pe rco la t i on . Baseflow from the two lower zone f ree

water storages can be i n the form o f channel (surface) f l o w o r non-

channel f low. Output o f t h i s r o u t i n e i s processed by a channel rou t -

ing a lgor i thm.

I n the o r i g i n a l (unrevised) model upper and lower zone s o i l

moisture parameters a r e assigned values based on the recession charac-

t e r i s t i c s o f the hydrograph. Assignment o f r e a l i s t i c est imates o f

these p roper t i es i s near an impossible task f o r i n t e r m i t t e n t streams.

The LAND r o u t i n e can be c a l i b r a t e d and used w i t h reasonable

confidence f o r the summer months, however i t was never designed t o -- operate under w i n t e r and sp r ing condi t ions, e s p e c i a l l y i n f rozen

s o i l s . Th is r o u t i n e a l l ows p e r c o l a t i o n and baseflow t o occur a l l

w in te r long so t h a t a t the end o f w in te r the f r e e water storages a r e

almost empty. The r e s u l t i s t h a t most o f the snowmelt i n f i l t r a t e s and

no runof f i s generated. The opera tor can compensate f o r t h i s problem

by choosing input parameters based on numerous c a l i b r a t i o n runs but

t h i s procedure o f t e n degenerates i n t o an excercise o f empi r ica l curve

f i t t i n g and lacks completely i n physical i n t e g r i t y .

4.3.2 Revised Program o f NWSRFS

A schematic f l ow c h a r t o f the revised land phase subrout ine,

based on the concepts o f i n f i l t r a t i o n t o frozen s o i l s presented above

i s shown i n Fig. 13. The subrout ine, designated WLAND i s c a l l e d d a i l y

from the LAND r o u t i n e as long as there i s snow on the ground. When

WLAND i s c a l l e d a l l p a r t s o f LAND deal ing w i t h i n f i l t r a t i o n , evapotran-

sp i ra t i on , p e r c o l a t i o n and basef low a re bypassed f o r t h a t day.

W i th in the WLAND r o u t i n e the watershed i s t rea ted as th ree

separate areas and c l a s s i f i e d as t o i n f i l t r a t i o n p o t e n t i a l ; Rest r ic ted ,

L imi ted and Unl i m i ted (see sect i on 2.2). A t the s t a r t o f a run each

s o i l type i s given the same values f o r the var ious mois ture storage

terms. Over the course o f t he run these values vary independently o f

one another.

Experimental evidence has shown tha t the normal zone o f

i n f i l t r a t i o n f o r snowrnelt water i n frozen P r a i r i e s o i l s i s 30 cm.

Figure 13. Schematic Diagram of Subroutine WLAND.

Identify and Classify areas of a Watershed according to their

Infiltration Potential

1

1 v 1 Unl imi ted Limited

i Restricted

/ \

-

1 Calculate Premelt Moisture,

8 .. Eqs. 3a or 3b P

i

1 -

Calculate Total Inf i 1 tration, INF from Snowcover Water Equivalent, SUE

a n d 8 ... Eq.2 P

\

C

I r

4

RUNOFF, DRO = 0

+

2% -

Compare INF and DSRO

4

I NF = INF - DSRO v Y

DRO = DSRO - INF

I

Calculated Daily Snowcover Runoff or Snowmelt Amounts from Snow Ablation Routine

DSRO

- DRO = DSRO

. ' .- DSRO < IkF DSRO = 0 DSRO 7 INF

I i- 1 f

Therefore, i n the "revised" model, the upper zone i s the so i l l aye r ,

0-30 cm and t h e lower zone extends from 30 cm t o 200 cm. From an

opera t iona l p o i n t o f view i t becomes very easy t o determine the maxi-

mum ava i l a b l e storage parameters when the depths a re def ined from the

bu lk dens i t y o f s o i l o f the respect ive zone and the s p e c i f i c g r a v i t y

(assumed -2650 kg/m3 f o r mineral so i 1 s) . The WLAND subrout ine does no t a l l o w p e r c o l a t i o n from the

upper t o the lower zone. This I s i n keeping w i t h experimental e v i -

dence t h a t m e l t water en te r ing the zone o f i n f i l t r a t i o n i s frozen i n

place. Baseflow from the lower zone i s a l s o prevented because ' t h i s

area would be f rozen o r the amounts woul-d be i n s i g n i f i c a n t . Note:

Gray e t a l . (19831 r e p o r t upward m i g r a t i o n i n the lower zone due t o

f reez ing. For the "Unl imited" case, i n f i l t r a t i o n can occur t o both

upper and lower zones, f o r the "Limited" case, i n f i l t r a t i o n occurs

on ly t o the upper zone. No i n f i l t r a t i o n occurs i n the "Restr ic ted"

case.

4.3.2.1 Input parameters. Three new parameters were added t o the

program inpu t l i s t . They a re the f r a c t i o n s o f the bas in i n each i n f i l -

t r a t i o n c lass , which a r e usua l ly q u i t e easy t o est imate. For example,

i f the f a l l was very d r y and there was no midwinter mel t , i t would be

reasonable t o se t the "Unl imited" area equal t o the area i n crops

(cereal g r a i n s o r grass) , the "Limited" area-equal t o the area i n

f a l l o w and the "Restr ic ted" area se t t o zero. I n the event o f a

midwinter me l t , some o f the watershed area would be s h i f t e d t o the

"Restr ic ted" c lass . I n the event o f a very wet f a l l , summerfallow

would become "Restr icted", s tubb le would be "Limited" and no area o f

the basin would be considered "Unlimited".

4.3.2.2 Program ou tpu t . Test runs were performed f o r the snowrnelt

runof f i n 1974 and 1975 on Creighton Watershed. S o i l moisture data

f o r the f a l l o f 1973 ind i ca ted a very d ry cond i t i on and v i sua l obser-

v a t i o n showed a l a rge amount o f cracking i n the s tubb le areas. There-

fo re , a1 1 s tubb le was put i n t o the "Unl i m i ted" c lass and the remainder

(grass and fa1 low) was put i n t o the "Limited" class. Two runs were

performed, one w i t h each model ( o r i g i n a l and rev ised NwSRFS), us ing

the same inpu t data.

Table 3 compares the major ou tpu ts from the o r i g i n a l and

rev ised NWSRFS systems appl ied t o snowrnelt on the Creighton watershed

i n 1974. The r a i n p l u s snow mel t i npu t column i s the same f o r bo th

models. The "melt" p o r t i o n o f the t o t a l , up t o A p r i l 18, i s the

output o f the snowpack ab la t i on rou t ine , the remainder i s r a i n f a l l on

the watershed. Note, w i t h both models a l a rge amount o f i n f i l t r a t i o n

occurs e a r l y i n the me l t period. The rev ised model showed streamflow

r u n o f f beginning on A p r i l 14th; no s i g n i f i c a n t r u n o f f was produced by

the o r i g i n a l model throughout the me l t sequence. The t o t a l volume o f

runof f s imulated by the revised model i s on l y 6% h igher than the

measured f low. F igure 14 shows the "simulated" and measured hydro-

graphs. I t i s i n t e r e s t i n g t o note the c lose assoc ia t ion i n the time-

o f - r u n o f f and the depth o f runof f from the rev ised system and the

measured hydrograph dur ing the e a r l y p a r t s of the runof f per iod (2-

3d). Note: i n the s imula t ion no at tempt was made t o sequence the

i n f i 1 t r a t i o n amounts (see sec t ion 3.5) as a1 1 snowcover runof f was

assumed t o i n f i l t r a t e u n t i l the capac i t y o f the s o i l was s a t i s f i e d .

The l a r g e d i f f e r e n c e i n shape o f the "simulated" and measured hydro-

graphs i s n o t considered t o i n v a l i d a t e the i n f i l t r a t i o n a lgo r i t hm

r a t h e r i t r e f l e c t s t h a t the channel lag and r o u t i n g r o u t i n e of t he

system requ i res major modi f i ca t ions (e.9. v e r i f i c a t i o n o f parameters)

when app l i ed t o the watershed. The problem o f us ing the system on

small watersheds because o f i t s r e l a t i v e l y l a rge t ime i n t e r v a l (6 h)

was discussed prev ious ly .

The r e s u l t s obtained f o r t he 1975 snowmelt event (see Table

4; F ig. 15) show s i m i l a r c h a r a c t e r i s t i c s t o those reported above f o r

1974; e.g. the " o r i g i n a l " NWSRFS g r o s s l y underestimated both the

streamflow r a t e s and volumes. For 1975, a year when the e n t i r e water-

shed was considered t o have a l i m i t e d i n f i l t r a t i o n p o t e n t i a l , the

s imulated volume o f streamflow by the rev ised system exceeded the

measured amount by about 23 percent.

Table 3. Comparison of outputs from o r i g i n a l and rev ised NWSRFS model app l i ed t o the Crelghton watershed i n 1974 using the same inputs f o r each system. Un i ts are m i l l i m e t e r s .

Rain Snowcover Runoff I n f i l t r a t i o n Simulated Flow Date and

Observed Rev i sed Or ig ina l Revised O r i g i n a l Revised Or ig ina l Flow

Me NWSRFS NWSRFS* NWSRFS NWSRFS NWSRFS NWSRFS

Apr. 8 9

10 1 1 12 13 14 15 I 6 17 18 19 20 2 1 2 2 2 3 2 4 2 5 26 2 7 28 2 9 30

To ta l s

- - - - R e v i e c a d M o d a l

- - - j! - - j ! - - - - j !

O b e a r v a d F l o w

- - - - - - O r i g i n a l M o d a l - - -

I 1 I- I A I 1- I I 1

A p r . 1 May 1 June 1

T i m e

Figure 14. Observed and simulated streamflow hydrographs from snowmelt f o r the Creighton Watershed, 1974. Revised model - i n f i l t r a t i o n model simulated i n NWSRFS; Or lg lna l model - land subroutine of NWSRFS unchanged.

Table 4. Comparison o f ou tpu ts from o r i g i n a l and rev ised NWSRFS model app l i ed t o the Creighton watershed i n 1975 us ing the same inputs f o r each system. U n i t s a re m i l l i m e t e r s .

Ra i n Snowcover Runoff l n f i 1 t r a t i o n Simulated Flow Observed

Date and Revised O r i g i n a l Revised O r i g i n a l Revised O r i g i n a l F 1 ow NWSRFS NWSRFS ' NWSRFS NWSRFS NWSRFS NWSRFS

Apr. 10 1 1 12 13 i 4 15 16 17 18 19 20 2 1 2 2 2 3 24 2 5 26 2 7 2 8 29 3 0

Tota 1 s

A p r . 1

2. 5

2. 0 A V) \

m E V

1.5

3 0

l-l

LL 1. 0

X rl -d

0 0

0. 0

M a y 1

T i m e

- - - - - - - - - -

- f l R " l s a d Mad" - - - - - -

I \ - - -

. 5 - - - Original Modal - -

I I I I I L I June 1

Figure 15. Observed and simulated streamflow hydrographs from snowmelt f o r the Creighton Watershed, 1975. Revised model - i n f i l t r a t i o n model simulated i n NWSRFS; O r i g i n a l model - land subroutine o f NWSRFS unchanged.

5. CONCLUS I ONS AND RECOMMENDATI.ONS

The s tudy repo r ted h e r e i n i s bes t -de f i ned as a "bas ic"

reasearch i n v e s t i g a t i o n i n t o model ing t he snowmelt i n f i l t r a t i o n phen-

omenon t o f r ozen P r a i r i e s o i l s . W i t h i n t h e r e p o r t t he concept and

e m p i r i c a l r e l a t i o n s h i p s d e s c r i b i n g an i n f i l t r a t i o n model f o r o p e r a t i o n a l

systems used t o s y n t h e s i z e o r s imu la te s t r eamf l ow from snowmelt a r e

presented. No s p e c i f i c conc lus ions can be p resen ted on t he e x a c t

"degree" t h a t t he i n f i l t r a t i o n model w i l l improve t h e hydrograph

syn thes is by an e x i s t i n g f o r e c a s t system. P r e l i m i n a r y r e s u l t s us i ng

t he Nat iona l Weather Se rv i ce R i ve r Fo recas t i ng System, whose l and

subrou t ine was m o d i f i e d t o r e f l e c t the e s s e n t i a l p r i n c i p l e s o f i n f i l -

t r a t i o n model, showed s i g n i f i c a n t improvement i n s imu la ted r u n o f f

volumes and r a t e s f r om those ob ta ined w i t h t h e o r i g i n a l system.

The main advantages o f t he model a re : i t i s s imple, i t i s

p h y s i c a l l y based and i t makes use o f r e l a t i o n s h i p s e s t a b l i s h e d f rom

f i e l d da ta c o l l e c t e d i n the P r a i r i e s . I t i s recommended t h a t t h e

model be w r i t t e n i n computer language and i t s performance t e s t e d i n an

e x i s t i n g o p e r a t i o n a l system (e.g. SSARR, NWSRFS o r o t h e r ) when appl i ed

t o syn thes iz ing s t reamf low f r om snowmelt on a l a r g e watershed.

6. REFERENCES CITED

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