ARS Small Watershed Model
DeCoursey, D.G.
IIASA Collaborative Paper
December 1982
DeCoursey, D.G. (1982) ARS Small Watershed Model. IIASA Collaborative Paper. IIASA, Laxenburg, Austria, CP82-089 Copyright © December 1982 by the author(s). http://pure.iiasa.ac.at/2029/ All rights reserved.
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ARS SMALL WATERSHED MODEL
D. G . DeCoursey
December 1982
CP-82-89
C o Z Z a b o r a t i v e Papers r e p o r t work which h a s n o t been
performed s o l e l y a t t h e I n t e r n a t i o n a l I n s t i t u t e f o r
A p p l i e d S y s t e m s A n a l y s i s a n d which h a s r e c e i v e d o n l y
l i m i t e d review.
V i e w s or opinions expressed herein
do n o t n e c e s s a r i l y r e p r e s e n t t h o s e o f t h e I n s t i t u t e ,
i t s N a t i o n a l Member O r g a n i z a t i o n s , o r o t h e r o r g a n i z a t i o n s s u p p o r t i n g t h e work.
INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS
A-2361 L a x e n b u r g , A u s t r i a
AUTHOR
D r . D.G. DeCoursey i s a r e s e a r c h h y d r a u l i c e n g i n e e r a t t h e USDA-ARS N o r t h e a s t
Watershed Research C e n t e r , S e d i m e n t a t i o n L a b o r a t o r y , F o r t C o l l i n s , Colorado,
USA.
PREFACE
The global population growth and the increasing demand for agricultural
products led to the extension of agricultural land and the intensification
of land use. Therefore, optimal land use is a very important practical
problem of the interaction of agricultural management and the environment,
and the mathematical models are useful tools for analyzing this interaction.
During 1978-1981 a task "Environmental Problems of Agriculture" within the
Resources and Environment Area of IIASA used a complex field level model
(CREAMS) for analysis of the problems of soil erosion, nitrogen leaching,
phosphorus and pesticide losses. This model (CREAMS) which was developed by
U.S. Department of Agriculture was used in Czechoslovakia, England, FRG,
Finland, Poland, Sweden and USSR. The results of this use will be published
at IIASA as a CPS (forthcoming).
One of the special questions of the task "Environmental Problems of
Agriculture" was the problem of transition of a field level model to a
regional one. The SWAM model which is described in this paper is one of
the attempts to describe the management of land use within the catchment
area. The SWAM (Small Watershed Agricultural Model) uses the dynamic version
of the improved CREAMS model (CREAMS 11) and may be useful to investigate the
effect of agricultural management and evaluate the hydrologic sediments and
chemicals from a small watershed.
V. Svetlosanov
Task Leader
Land and Landcover Resources
ACKNOWLEDGEMENTS
The development of SWAM i s an ARS team e f f o r t t o which a l a r g e number
of people a r e c o n t r i b u t i n g . D r . Roger Smith i s developing much of t h e dynamic
v e r s i o n of t h e CREAMS I1 model and i s r e s p o n s i b l e f o r much of t h e m a t e r i a l
p r e s e n t e d i n t h a t s e c t i o n o f t h i s r e p o r t . He has r e c e i v e d a l o t of s u p p o r t
from o t h e r members of t h e CREAMS team. They a r e Drs. Walter K n i s e l , Chairman;
Ralph Leonard, George F o s t e r , A r l i n Nicks, Jimmy Williams and V i r g i n i a F e r r e i r a .
D r . Keith Saxton h a s p a r t i c i p a t e d a c t i v e l y i n d i s c u s s i o n s of p l a n t growth
modeling and w i l l be a i d i n g i n e v a l u a t i o n . D r . George F o s t e r c o n t r i b u t e d much
t o t h e development of sediment r o u t i n g on s o u r c e a r e a s through numerous d i s c u s s i o n s of o v e r l a n d e r o s i o n p r o c e s s e s . D r . C a r l o s Alonso i s r e s p o n s i b l e
f o r development of t h e channel r o u t i n g of b o t h w a t e r and sediments. He w i l l
a l s o b e p l a y i n g a major r o l e i n assembly and t e s t i n g of t h e e n t i r e SWAM model.
D r . Harry Pionke has been r e s p o n s i b l e f o r c o o r d i n a t i n g development of t h e
chemistry components' b e h a v i o r and t r a n s p o r t through t h e channel system.
D r s . B i l l Gburek and Yui Liong a r e r e s p o n s i b l e f o r development of t h e groundw a t e r component. Drs. Frank Schiebe and Ron Menzel a r e r e s p o n s i b l e f o r t h e
r e s e r v o i r model. D r . Myron Molnau c o n t r i b u t e d t o t h e development of t h e
snow accumulation and m e l t r o u t i n e s . D r . Wade N u t t e r was r e s p o n s i b l e f o r
a d o p t i n g , t h e model t o timbered a r e a s . D r . Clarence Richardson developed
t h e models being used t o g e n e r a t e t h e s y n t h e t i c c l i m a t o l o g i c a l i n p u t s .
M r . J . B . Burford h a s assembled t h e many d a t a s e t s needed f o r t e s t i n g . Others
such a s Drs. Harry Kunishi, Leonard Lane, Ken Renard, Ronald Schnabel and
John S c h r e i b e r have a l s o a i d e d i n i t s development. When t h e f i n a l document
i s completed, t h e s e and many o t h e r s w i l l have c o n t r i b u t e d .
ABSTRACT
I n t h i s paper I have attempted t o d e s c r i b e t h e SWAM development by ARS.
It began s e v e r a l y e a r s ago w i t h t h e work on CREAMS. The Small Watershed Model
u s e s t h e dynamic v e r s i o n of CREAMS I1 a s i t s core. The o u t p u t s from a l l s o u r c e
a r e a s w i t h i n a small watershed a r e r o u t e d through t h e channel and impoundment
systems t o t h e downstream p o i n t . The dynamic v e r s i o n of CREAMS I1 p r o v i d e s
a continuous r e c o r d of w a t e r , sediment and chemicals from each f i e l d i n t h e
watershed. A dynamic channel r o u t i n g scheme r o u t e s t h e water and sediments,
by p a r t i c l e s i z e f r a c t i o n , c a l c u l a t i n g b o t h a g g r a d a t i o n and d e g r a d a t i o n ; bed
armoring i s a l s o included. The r e s e r v o i r model c a l c u l a t e s p r o f i l e s of temper a t u r e and sediment c o n c e n t r a t i o n a s well a s t h e e f f e c t s of b i o l o g i c a l a c t i v i t y
on n u t r i e n t l e v e l s . Most of t h e s i g n i f i c a n t chemical b a l a n c e s and changes
a r e aonsidered a s t h e flow moves from r e a c h t o reach. However, much remains
t o be done t o make t h e combined program u s e f u l t o a wider range of a p p l i c a t i o n s .
T h i s c o n s i s t s of s e n s i t i v i t y a n a l y s e s t o reduce t h e model complexity i n insens i t i v e r e g i o n s and t o f i n d e f f i c i e n t ways t o a g g r e g a t e a r e a s such t h a t a b a s i n
s c a l e model can be developed.
TABLE OF CONTENTS
INTRODUCTION
INITIAL CONCEPTS
WATERSHED DELINEATION AND CLIMATIC INPUTS
SOURCE AREA RESPONSE
Precipitation
Intercept ion
Infiltration
Moisture Flow i n t h e Root Zone
Surface Detention
Surf a c e Runoff
Erosion
Evapotranspiration
P l a n t Growth and Decay
Nitrogen Cycle
Phosphorus Cycle
P e s t i c i d e Processes
Timbered Areas
CHANNEL, RESERVOIR AND GROUNDWATER PROCESSES
Routing Water and Sediments i n t h e Channel System
The Groundwater Component
Reservoir Processes
Movement of Chemicals through t h e Channel System
FUTURE EFFORTS
REFERENCES
ARS SMALL WATERSHED MODEL
D.G. DeCoursey
INTRODUCTION
A group of Agricultural Research Service CARS) scientists met in
Arlington, Texas, in 1980 to discuss the ARS response to expressed needs
for information and analytical tools that planners could use in responding to questions on nonpoint source pollution (Section 208 of Public Law
92-500, The Clean Water Act).
We felt that the ARS could respond to
these needs of action agencies and planners because we had been doing
research in this area for many years.
At the meeting three levels of involvement were identified. The
first of these to receive attention was the development of a model to
simulate runoff, erosion and chemical movement from an agricultural
field. A state-of-the-art mathematical model of Chemical, Runoff and
Erosion from ~~ricultural
Management Systems known as CREAMS (Knisel,
1980) was published by USDA as Conservation Research Report No. 26, in
May, 1980. At this time, the model is being updated and improved.
The
dynamic version of this improvement is the core of the second model, that
of a small watershed (SWAM).
The purpose of this model development is to
simulate the response of a small watershed (2000 hectares) to alternative
land use or management changes within its catchment area.
It is limited
in size because responses from all the unit areas within the watershed
are routed downstream through the channel and any impoundments; and only
a few such a r e a s can be handled e f f a c t i v e l y .
The t h i r d model, i d e n t i f i e d
a t t h e meeting, was one designed t o s i m u l a t e t h e response from a b a s i n
s c a l e a r e a ( s e v e r a l hundred square k i l o m e t e r s ) .
I t w i l l be designed t o
a g g r e g a t e a r e a s such a s t h o s e a d d r e s s e d by CREAMS and i d e n t i f i e d i n d i v i d u a l l y i n SWAM.
The s m a l l watershed model w i l l be used i n a s i m u l a t i o n
mode t o h e l p i d e n t i f y t h e b e s t way t o a g g r e g a t e t h e s e a r e a s .
T h i s paper, t h e f i r s t of a s e r i e s of p a p e r s d e s c r i b i n g SWAM, desc r i b e s t h e v a r i o u s s u b r o u t i n e s t h a t a r e being combined i n t o t h e f i r s t
v e r s i o n of t h e model.
F u t u r e papers w i l l d e s c r i b e t h e s e n s i t i v i t y ana-
l y s e s , more r e f i n e d v e r s i o n s of t h e model and sample a p p l i c a t i o n s .
A
USDA p u b l i c a t i o n is being planned t h a t d e s c r i b e s t h e model i n d e t a i l ,
p r o v i d e s a u s e r s manual and t e c h n i c a l p a p e r s s u p p o r t i n g v a r i o u s components.
INITIAL CONCEPTS
The s m a l l watershed model is b e i n g designed t o show t h e e f f e c t of
changes i n l a n d u s e o r management on t h e h y d r o l o g i c , sediment, and
chemical response of a s m a l l watershed.
S i n c e i t u s e s CREAMS a s a b a s e ,
it is a p p l i c a b l e t o c o n s e r v a t i o n t i l l a g e , crop r o t a t i o n , double cropping
c o n t o u r i n g , s t r i p cropping, t e r r a c e s and g r a s s e d waterways.
of t h e model is e v a l u a t i o n of management a l t e r n a t i v e s ,
The emphasis
t h u s t h e response
should be r e a s o n a b l y a c c u r a t e f o r b o t h a b s o l u t e and r e l a t i v e e s t i m a t e s of
a specific practice a t a site.
The model i s b e i n g designed t o u s e on
engaged a r e a s , t h u s i t is i n t e n d e d t o be used w i t h o u t c a l i b r a t i o n .
For
most purposes c h a r t s , t a b l e s , graphs and maps w i l l p r o v i d e parameter
v a l u e guidance f o r t h o s e s i t u a t i o n s where o n - s i t e e s t i m a t e s o r p r e v i o u s
experience a r e not a v a i l a b l e t o a i d i n t h e i r s e l e c t i o n .
Because of t h e
wide v a r i e t y of s i t u a t i o n s t o which i t i s l i k e l y t o be a p p l i e d , t h r e e
t y p i c a l s i t e c o n d i t i o n s were c o n s i d e r e d i n i t s development,i) s m a l l
watersheds i n which t h e flow i s dominated by s u r f a c e flow i n t h e channel
system, i i ) s m a l l watersheds i n which t h e flow i s dominated by s u r f a c e
w a t e r impoundments, and i i i ) s m a l l watersheds i n which t h e flow i s domina t e d by s u b s u r f a c e flow.
By c o n s i d e r i n g a l l t h r e e s i t u a t i o n s i n t h e
development of t h e model we should b e a b l e t o a d d r e s s most s i t e problems
t h a t a r e combinations of t h e s e t h r e e .
I n i t s i n i t i a l form, t h e model w i l l b e v e r y comprehensive and w i l l
probably be more of a r e s e a r c h than a management t o o l .
.
I t was developed
t h i s way because we f e l t t h a t such a model could b e used i n more s i t u a t i o n s
than simpler models.
I t i s a l s o much e a s i e r , through s e n s i t i v i t y
a n a l y s e s , t o s i m p l i f y a comprehensive model where i t i s l e a s t s e n s i t i v e ,
than t o t a k e a s i m p l e model and make i t more comprehensive.
Because i t
i s comprehensive, a t l e a s t i n i t s i n i t i a l c o n f i g u r a t i o n , i t i s n o t
p r a c t i c a l f o r l o n g term (20 y e a r s o r more) s i m u l a t i o n s .
I t w i l l be a
continuous s i m u l a t i o n model of watershed response t o i n d i v i d u a l e v e n t s
f o r a period of s e v e r a l y e a r s .
A s i t i s r e f i n e d , through s e n s i t i v i t y
a n a l y s e s t o both s i m p l i f y i t and develop methods t o a g g r e g a t e a r e a s , i t
w i l l become more u s e f u l f o r l o n g term s i m u l a t i o n .
Because of t h e many p h y s i c a l p r o c e s s e s t h a t a r e b e i n g s i m u l a t e d , i t
i s d i f f i c u l t t o g r a s p a p i c t u r e of t h e whole model and a l l t h e i n t e r -
-
a c t i o n t h a t takes place.
F i g u r e 1 i s a schematic of t h e e n t i r e model
showing a l l of t h e p r o c e s s e s t h a t a r e t o be i n c l u d e d .
d i s c u s s e d i n more d e t a i l i n l a t e r p a r t s of t h i s paper.
model a s comprehensive a s t h i s a r e e x t e n s i v e .
Each of t h e s e i s
Inputs f o r a
F i g u r e 2 i s an expansion
of t h e major i n p u t groups a t t h e t o p of F i g u r e 1.
I n t h e n e x t s e c t i o n of
t h e r e p o r t I w i l l d e s c r i b e t h e dynamic v e r s i o n of CREAMS I1 t h a t s e r v e s
a s t h e s o u r c e a r e a and i s t h e c o r e of SWAM.
E s s e n t i a l l y i t is the c e n t e r
p a r t of F i g u r e 1, n o t e d a s s o u r c e a r e a p r o c e s s e s .
Subroutines have been developed t o provide sequences of d a i l y
r a i n f a l l , temperature and s o l a r r a d i a t i o n f o r u s e a t s i t e s where t h e s e
d a t a a r e n o t r e a d i l y a v a i l a b l e (Richardson, 1981).
a r e being developed f o r wind and pan e v a p o r a t i o n .
Similar subroutines
Techniques a r e a l s o
being i n v e s t i g a t e d t o d i s a g g r e g a t e t h e s e t o provide t h e h o u r l y o r breakp o i n t t y p e i n p u t s r e q u i r e d of t h e more comprehensive model.
sets a s p o s s i b l e a r e being assembled t o t e s t t h e model.
A s many d a t a
However, no one
d a t a s e t has been found t h a t is complete enough t o cover a l l a s p e c t s of
t h e model.
WATERSHED DELINEATION AND CLIMATIC INPUTS
I n o r d e r t o u s e t h e model, t h e e n t i r e watershed must be d i v i d e d i n t o
source areas.
sible.
These a r e a s a r e a s uniform i n s o i l s and l a n d u s e a s pos-
Response from each of t h e s e a r e a s i s s i m u l a t e d by d e f i n i n g t h e
source a r e a s a s a system of p l a n e s and V-shaped channels ( a minimum
number a r e u s e d ) .
A t t h i s p o i n t , t h e r e i s no a g g r e g a t i o n of a r e a s ; t h e
response of each a r e a i s c a l c u l a t e d i n d i v i d u a l l y .
Methods of a g g r e g a t i o n
by s i m i l a r crops, s o i l s o r s m a l l mixed l a n d u s e watersheds w i l l be
s t u d i e d by using t h e model i n a s i m u l a t i o n mode, l o o k i n g a t t y p i c a l
responses from a v a r i e t y of d i f f e r e n t watershed c o n f i g u r a t i o n s .
paper w i l l r e p o r t on t h e s e e v a l u a t i o n s .
A future
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Output from t h e s o u r c e a r e a watersheds ( w a t e r , sediments and chemic a l s ) w i l l be r o u t e d through t h e channel and impoundment systems t o t h e
o u t l e t of t h e watershed.
Thus t h e c h a n n e l s must be d e s c r i b e d by c r o s s
s e c t i o n , bank and bed m a t e r i a l s and v e g e t a t i v e cover a t r e p r e s e n t a t i v e
n o d a l p o i n t s ; such a s a t channel confluences and major change i n c r o s s
s e c t i o n , s l o p e , r a d i u s of c u r v a t u r e o r bed and bank c o n d i t i o n s .
depth-area-capacity
The
curve and o u t l e t s t r u c t u r e of a l l impoundments a r e
a l s o needed o r must be e s t i m a t e d .
P o i n t s of groundwater r e t u r n t o t h e
channel o r l o s s e s from t h e channel t o groundwater must be i d e n t i f i e d .
C l i m a t i c i n p u t t o t h e model c o n s i s t s of b r e a k p o i n t r a i n f a l l , maximum
and minimum d a i l y a i r t e m p e r a t u r e s and s o l a r r a d i a t i o n .
computations u s e pan e v a p o r a t i o n and wind speed.
A l t e r n a t i v e ET
A t t h e p r e s e n t t i m e we
a r e c o n c e n t r a t i n g on development of t h e model u s i n g b r e a k p o i n t r a i n f a l l
because w e have much more confidence i n t h e h y d r o l o g i c r e s p o n s e of t h e
model w i t h t h i s i n p u t r a t h e r t h a n d a i l y v a l u e s .
When w e s t a r t work on
t h e v e r s i o n designed f o r l o n g term s i m u l a t i o n we w i l l probably use d a i l y
inputs.
SOURCE AREA RESPONSE
The dynamic v e r s i o n of CREAMS 11, which forms t h e n u c l e u s of t h e
SWAM model, i s a s i g n i f i c a n t r e v i s i o n of t h e CREAMS model.
The t h r e e
independent components of CREAMS (hydrology, e r o s i o n and chemicals) have
been combined t o e n a b l e t h e i n t e r a c t i o n s t h a t e x i s t t o o p e r a t e p r o p e r l y .
Many of t h e p r o c e s s e s have been updated and new ones added e . g . p r e c i p i t a t i o n i n t e r c e p t i o n by t h e p l a n t canopy, t h e e f f e c t s of t i l l a g e on i n f i l t r a t i o n and a dynamic s o i l e r o s i o n component.
Figure 3 is a conceptuali-
z a t i o n of t h e movement of w a t e r i n t h e model.
The f o l l o w i n g d i s c u s s i o n
r e f e r s t o t h e p r o c e s s e s shown i n F i g u r e 3.
Precipitation
P r e c i p i t a t i o n i n p u t i s d i v i d e d i n t o snow o r r a i n f a l l a s a f u n c t i o n
of t h e maximum and minimum d a i l y a i r temperature.
p e r a t u r e is l e s s than o r e q u a l t o 0
assumed t o be snow.
equivalent.
0
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I f weighted a i r tem-
F) t h e p r e c i p i t a t i o n i s
Snow accumulation i s i n t h e form of mm. of w a t e r
Snowmelt (Khanjani and Molnau, 1 9 8 2 ) i s a f u n c t i o n of incom-
i n g s o l a r r a d i a t i o n , albedo, shading and maximum and minimum a i r temperat u r e , r a i n f a l l , a s p e c t and s l o p e of t h e s i t e , and h e a t g a i n from t h e s o i l
surface.
Snow e v a p o r a t i o n i s a f u n c t i o n of t h e maximum and minimum a i r
temperature, s o l a r r a d i a t i o n and albedo, which changes w i t h age of t h e
snow and is a f u n c t i o n of accumulated degree hours.
Figure 3.
Water movement on source areas
Interception
Before r a i n f a l l r e a c h e s t h e s o i l s u r f a c e , some of i t is i n t e r c e p t e d
by t h e p l a n t canopy.
s t o r y , is p o s s i b l e .
A m u l t i - s t o r y canopy, such a s t r e e s over an underL i t t e r on t h e ground under a row c r o p o r o t h e r
canopy is a l s o c o n s i d e r e d t o i n t e r c e p t r a i n f a l l .
I n t e r c e p t P o n is c a l -
c u l a t e d a s a l i n e a r f u n c t i o n of t h e l e a f a r e a index.
Mulch is d e s c r i b e d
a s p e r c e n t cover and i t s i n t e r c e p t i o n t r e a t e d t h e same a s l e a f a r e a .
Evaporation i s assumed t o come f i r s t from i n t e r c e p t i o n s t o r a g e b e f o r e
t h a t from t h e s o i l s u r f a c e .
Any d e f i c i t i n i n t e r c e p t i o n s t o r a g e is
f i l l e d by r a i n f a l l i n t h e n e x t p r e c i p i t a t i o n e v e n t .
Infiltration
P r e c i p i t a t i o n t h a t p a s s e s through t h e canopy and l i t t e r i n f i l t r a t e s
into the soil.
Two a l t e r n a t i v e methods a r e a v a i l a b l e t o e s t i m a t e i n f i l -
t r a t i o n ; t h e Smith and P a r l a n g e (1978) and t h e Green and Ampt (Moore,
1981) e q u a t i o n s .
Brooks-Cory
Parameters of t h e e q u a t i o n s a r e e s t i m a t e d by u s i n g t h e
(Corey e t a l . ,
the s o i l properties.
1965 and L a l i b e r t e e t a l . , 1966) r e l a t i o n s and
S i n c e t i l l a g e a f f e c t s t h e macro p o r e s , i t is con-
sidered i n calculating i n f i l t r a t i o n rate.
The s i z e of t h e macro p o r e s
a r e a f u n c t i o n of r e l a t i v e s u r f a c e roughness and t h e s o i l t y p e .
depth of t i l l a g e depends upon t h e implement and i t s use.
The
Both t h e s u r -
f a c e roughness and macro pore s i z e decay w i t h p r e c i p i t a t i o n f o l l o w i n g a
t i l l a g e operation.
Moisture Flow i n t h e Root Zone
The s o i l i n t h e r o o t zone can b e d e s c r i b e d by a s many a s 10 h o r i z o n s
o r layers.
Each of t h e s e l a y e r s i s c h a r a c t e r i z e d by i t s p o r o s i t y , pore
s i z e d i s t i b u t i o n i n d e x , 15-bar w a t e r c o n t e n t , 0.3-bar
e f f e c t i v e saturated hydraulic conductivity.
w a t e r c o n t e n t and
Each of t h e l a y e r s is f u r -
t h e r d i v i d e d by t h e computer s u b r o u t i n e i n t o a s many a s 15 l a y e r s f o r
computational purposes.
Routing of water between t h e l a y e r s is accom-
p l i s h e d by a l i n e a r i z e d step-wise s o l u t i o n of t h e Richards e q u a t i o n i n
which t h e g r a v i t y and d i f f u s i o n terms a r e t r e a t e d s e p a r a t e l y t o i n s u r e
s t a b i l i t y and l e n g t h e n b o t h t h e a l l o w a b l e time and t h e d i s t a n c e i n c r e ments.
Roots e x t r a c t w a t e r from t h e s o i l l a y e r s i n p r o p o r t i o n t o t h e
s o i l m o i s t u r e p o t e n t i a l i n each l a y e r ( s e e s e c t i o n on p l a n t growth).
Surface Detention
S u r f a c e d e t e n t i o n i s a f u n c t i o n of roughness, s o i l t y p e , t i l l a g e and
amount of p r e c i p i t a t i o n s i n c e t h e l a s t t i l l a g e e v e n t .
S u r f a c e Runoff
S u r f a c e r u n o f f o c c u r s o n l y a f t e r ponding h a s o c c u r r e d and s u r f a c e
detention is f i l l e d .
Using r a i n f a l l b r e a k p o i n t s , s u r f a c e w a t e r i s
r o u t e d , w i t h a k i n e m a t i c approximation, of f l o w o v e r a p l a n e / c h a n n e l
cascade.
I t c a n p r o v i d e f o r convergent o r d i v e r g e n t flow, furrow geome-
t r y , and r e c e s s i o n i n f i l t r a t i o n .
The roughness parameter i s a f u n c t i o n
of t i l l a g e , t h e e f f e c t s of p r e c i p i t a t i o n energy s i n c e t h e l a s t t i l l a g e
o p e r a t i o n and l i t t e r o r p l a n t d e n s i t y i n t h e c a s e of r a n g e o r p a s t u r e
land conditions.
Erosion
Sediment eroded from t h e s o i l s u r f a c e i s c a l c u l a t e d u s i n g t i m e
i n t e r v a l s determined by t h e r a i n f a l l and i n f i l t r a t i o n r a t e s .
of t h e p r o c e s s e s i n v o l v e d i s shown i n F i g u r e 4.
A schematic
The t r a n s p o r t sub-
r o u t i n e s have been a d a p t e d from Kineros (Smith, 1976a and b and Smith,
1981 a and b ) .
Sediment p a r t i c l e s a r e assumed t o b e detached by b o t h
r a i n f a l l and flowing w a t e r and t r a n s p o r t e d by t h e w a t e r .
Interrill
detachment of s o i l p a r t i c l e s by r a i n f a l l o c c u r s whether o r n o t t h e r e i s
s u r f a c e runoff.
However, s i n c e t h e y cannot b e t r a n s p o r t e d , r a i n f a l l
detachment i s n o t c o n s i d e r e d u n t i l r u n o f f b e g i n s .
I t i s a f u n c t i o n of
r a i n f a l l i n t e n s i t y and a s o i l e r o d i b i l i t y f a c t o r r e l a t e d t o s o i l t y p e .
Detachment i n r i l l s by f l o w i n g water i s a f u n c t i o n of t h e e x c e s s t r a n s port capacity.
A bookkeeping scheme e n a b l e s us t o c a l c u l a t e a l l of t h e above f o r a
range of p a r t i c l e s i z e s and d e n s i t i e s .
S i n c e b o t h r i l l and i n t e r i l l
e r o s i o n a r e c o n s i d e r e d , t h e c o n c e n t r a t i o n s of b o t h s u r f a c e a p p l i e d and
i n c o r p o r a t e d c h e m i c a l s , t h a t a r e a t t a c h e d t o s o i l p a r t i c l e s , can b e
i d e n t i f i e d and c a l c u l a t e d .
The topography i s c o n s i d e r e d t o b e made up of
c a s c a d e s of p l a n e s and c h a n n e l s , a l l of t h e many c o n s e r v a t i o n p r a c t i c e s
such a s s t r i p c r o p p i n g , t e r r a c e s , g r a s s waterways, convex, concave,
complex s l o p e s and s m a l l impoundments c a n b e s i m u l a t e d and b o t h aggradat i o n and d e g r a d a t i o n r a t e s determined.
Output c o n s i s t s of t h e q u a n t i t i e s
of e a c h of s e v e r a l c l a s s e s of sediment p a r t i c l e s i z e s i n t r a n s p o r t d u r i n g
each t i m e i n t e r v a l .
a l s o defined
.
Both o r g a n i c m a t t e r and any a t t a c h e d chemicals a r e
.El
INPUT
n
ADSORBED
CHEMICALS
SURFACE
AGGRADATION
DEGRADATION
PARTICLES
DETACHED
1
SEDIMENT
a ORGANIC
MATTER
TRANSPORTED
Figure 4 .
ORGANIC
MATTER
DETACHED
Sediment eroded on source a r e a s .
Incoming energy d i s s i p a t i o n i s s p l i t between s o i l s u r f a c e evaporat i o n and e v a p o t r a n s p i r a t i o n .
Evaporation from t h e exposed s o i l s u r f a c e
i s i n f l u e n c e d by any mulch t h a t may b e p r e s e n t and i s a modified form of
t h e Penman-Montieth method ( R i t c h i e , 1972).
E v a p o t r a n s p i r a t i o n from t h e
p l a n t s u r f a c e d u r i n g i n t e r s t o r m p e r i o d s i s assumed t o occur a t p o t e n t i a l
r a t e u n t i l i t cannot b e provided by w a t e r from t h e r o o t zone.
Limiting
c o n d i t i o n s i n any l a y e r b e g i n s a t a m o i s t u r e c o n d i t i o n 20 p e r c e n t above
t h e 15 b a r t e n s i o n v a l u e .
No w a t e r i s a v a i l a b l e from s o i l w i t h a moisI f n e t s o i l l a y e r water is l e s s than
t u r e t e n s i o n of 15 b a r s o r g r e a t e r .
t h e 20 p e r c e n t v a l u e , t h e amount provided i s a f u n c t i o n of t h e remaining
w a t e r above t h e 15 b a r t e n s i o n v a l u e .
P l a n t growth i s l i m i t e d by r e g i o n s
of s o i l t h a t a r e completely s a t u r a t e d o r have a m o i s t u r e t e n s i o n g r e a t e r
than 15 b a r s .
P o t e n t i a l e v a p o t r a n s p i r a t i o n r a t e can be c a l c u l a t e d by
R i t c h i e ' s method ( R i t c h i e , 1972) o r u s i n g pan e v a p o r a t i o n w i t h a c o e f f i cient.
I f t h e i n t e r s t o r m p e r i o d i s l e s s t h a n a day, p o t e n t i a l evapo-
t r a n s p i r a t i o n i s d i s t r i b u t e d between dawn and dusk i n a s i n u s o i d a l
pattern.
Dawn and dusk t i m e s a r e c a l c u l a t e d by an a l g o r i t h m from Mohler
and G i f f o r d (Khanjani and Molnau, 1982).
P l a n t Growth and Decay
The e x t e n t of p r o t e c t i o n provided by t h e canopy i s c a l c u l a t e d by a
p l a n t growth model ( s e e F i g u r e 5 ) .
Actual p l a n t growth i s a m o d i f i c a t i o n
of p o t e n t i a l growth; w i t h p o t e n t i a l growth being a f u n c t i o n of t h r e e
i n p u t v a r i a b l e s d e f i n e d f o r each crop.
They a r e i ) t h e energy e f f i c i e n c y
of biomass p r o d u c t i o n , i i ) maximum p o t e n t i a l d r y m a s s p r o d u c t i o n and i i i )
average degree-days t o m a t u r i t y .
modify t h e p o t e n t i a l growth curve.
Water, temperature and n i t r o g e n s t r e s s
T h i s form of growth model can v e r y
e a s i l y show regrowth r e s p o n s e t o h a r v e s t i n g , such a s f o r a l f a l f a o r o t h e r
g r a s s crops.
However, y i e l d of t h e crop i s n o t a primary o b j e c t i v e of
t h e model a t p r e s e n t , t h u s i t is assumed t o be a f u n c t i o n of t h e d r y
m a t t e r produced.
D i e back a f t e r r e a c h i n g m a t u r i t y c o n v e r t s t h e l e a f a r e a
t o s t a n d i n g d r y m a t t e r which t h e n b e g i n s t o f a l l and becomes s u r f a c e
residue.
P a p e r s d e s c r i b i n g t h e s e p r o c e s s e s w i l l be p r e p a r e d by t h e team
working on r e v i s i o n of t h e CREAMS model i n t h e n e x t few months.
INPUT
CANOPY
CROP
YIELD
AREA
WATER
LAYER
NUTRIENT
UPTAKE
BIOMASS
ACCUMU
LATION
ADSORBED
NEtP
TRANSWRTED
-
LAYER
I
-
RELEASED
SOLUBLE
Nap
-
EXTRACTED
Figure 5.
OVER
LAND
FLOW
-
SOLUBLE
N 8 P
RANSPORTED
Biomass growth and decay
ADSORBED
NEtP
Decomposition of surface residue is a function of contact area with
the soil.
If the field is plowed, the surface biomass is incorporated
and decomposes at a rate determined by mineralization rate in the nitrogen cycle.
If the field is not plowed, only the material in contact
decomposes by mineralization. As it decomposes additional material comes
in contact with the soil until eventually d l is decomposed. At-the
present time, the nutrients considered in the decomposition are phosphorus and nitrogen.
Both the nutrients and carbon accumulations are
based on the carbon-nitrogen ratios of the plant and average nutrient
content.
The concentration or density of plant roots in any one soil layer is
assumed to be great enough to provide that fraction of the water that the
hydrostatic pressure gradient indicates should come from that layer.
Thus, only the penetration depth of the roots is needed. It is assumed
to be a nonlinear function of plant growth, i.e. root penetration is
faster early in the season. Nutrient uptake by the roots is assumed to
be sufficient to meet the plant's needs.
Nutrient stress occurs only
when the nutrients in the soil profile are insufficient.
Heat Flux:
Since many of the nutrient processes as well as snowmelt
are dependent upon soil temperature, a heat flux model is provided.
It
solves the second order differential equation for heat flow using the
same soil layers as those used for water movement, but with the nodes
assumed to be at the layer boundaries.
The temperature, several cm.
below at the base of the root zone, is assumed constant. Only daily
average temperatures are used for the soil surface boundary--no daily
fluctuations are considered.
Nitrogen Cycle
The nitrogen cycle in the soil is very complex.
Figure 6 shows
those components considered. Inputs of nitrogen are assumed to be fertilizer, manure, plant residues, nitrogen fixation and rainfall.
Micro-
organisms in the soil convert the organic forms of nitrogen to ammonia
and nitrate; the rate of conversion is a function of the soil moisture
and temperature conditions. The nitrification of ammonia to nitrate is
not considered as a separate process; it is incorporated in mineralization. Nitrate in soil solution is taken up by the plants, redistributed
with the soil water, leached below the root zone into ground water,
(
FIXATION
1
FERTILIZER
MANURE
-=C
ADSORBED
EROSION
ADSORBED
LAYER
Figure 6 .
EXTRACTED
Nitrogen c y c l e on source a r e a s
e x t r a c t e d from s u r f a c e s o i l l a y e r s by s u r f a c e r u n o f f , immobilized and
d e n i t r i f i e d . Adsorption-desorption isotherms r e l a t e t h e s o l u b l e ammonia
t o t h a t adsorbed on s o i l p a r t i c l e s .
The adsorbed ammonia p l u s o t h e r
sediment a s s o c i a t e d n i t r o g e n i s s u b j e c t t o e r o s i o n w i t h t h e s o i l .
Expres-
s i o n s f o r t h e n i t r o g e n c y c l i n g p r o c e s s e s a r e based on t h e Phoenix and
EPIC models (McGill e t a l . ,
1981 and Williams, 1982).
Phosphorus Cycle
The phosphorus p r o c e s s e s a r e shown i n F i g u r e 7 .
I n p u t s c o n s i s t of
p l a n t r e s i d u e s and f e r t i l i z e r o r manure i n t h e form of s o l u b l e phosphates.
Within t h e s o i l , m i n e r a l i z a t i o n o f o r g a n i c forms t o t h e i n o r -
g a n i c o r immobilization of i n o r g a n i c t o o r g a n i c a r e n o t c o n s i d e r e d .
S o l u b l e PO
i s s t r o n g l y adsorbed on s o i l p a r t i c l e s t h u s t h e s o i l can a c t
4
a s a scavenger.
Phosphorus can be leached from t h e s o i l s u r f a c e , b u t is
not considered t o move through t h e s o i l p r o f i l e e x c e p t i n c e r t a i n circumstances.
The amount e x t r a c t e d from t h e s u r f a c e by runoff i s a f u n c t i o n
of an e x t r a c t i o n c o e f f i c i e n t , t h e phosphorus c o n c e n t r a t i o n i n t h e s o i l
s o l u t i o n and flow r a t e .
Adsorbed PO
removes t h e s o i l p a r t i c l e s .
is subject t o l o s s a s erosion
4
P l a n t uptake i s a f u n c t i o n of demand.
P e s t i c i d e Processes
The movement of p e s t i c i d e s on s o u r c e areas is shown i n F i g u r e 8 .
The method of a p p l i c a t i o n i s c o n s i d e r e d ; i t can be e i t h e r s u r f a c e a p p l i e d
o r incorporated i n the s o i l .
The amount of a s u r f a c e a p p l i e d p e s t i c i d e
r e a c h i n g t h e s o i l s u r f a c e i s a f u n c t i o n of t h e canopy cover and, a f t e r
r e a c h i n g t h e s o i l , i s t r e a t e d much t h e same way a s i n c o r p o r a t e d p e s t i cides.
Decay r a t e s , a p p r o p r i a t e f o r t h e s p e c i f i c p e s t i c i d e s , a r e a p p l i e d
t o c a l c u l a t e , from day t o day, t h e amount of t h e p e s t i c i d e remaining i n
o r on t h e s o i l s u r f a c e .
Appropriate adsorption/desorption isotherms a r e
provided t o show what p a r t of t h e p e s t i c i d e i s adsorbed and what p a r t is
i n solution.
The s o l u b l e forms a r e r o u t e d w i t h t h e w a t e r through t h e
s o i l zones o r e x t r a c t e d i n s u r f a c e r u n o f f .
E x t r a c t i o n i s a f u n c t i o n of
t h e c o n c e n t r a t i o n s i n an assumed mixing d e p t h , t h e flow r a t e and an
extraction coefficient.
with the s o i l particles.
The adsorbed p e s t i c i d e s a r e s u b j e c t t o e r o s i o n
r
INPUT
SOLUBLE
P
EXTRACTED
-
FORMS
OF
P
EROSION
SOLUBLE
ADSORBED
RANSPORTED
Figure 7 .
Phosphorus c y c l e on source a r e a s
P
ADSORBED
PESTICIDES
INCORPORATION
AMOUNT
AMOUNT
PLANT
AOSORBED
FORMS
n
SOLUBLE
MATERIALS
RANSPO
Figure 8 .
I
Pesticides on source areas
ADSORBED
MATERIALS
1
The amount of p e s t i c i d e t h a t r e a c h e s t h e p l a n t s u r f a c e i s s u b j e c t t o
v o l i t a l i z a t i o n , d e g r a d a t i o n and wash-off.
The amounts t h a t a r e v o l i t a -
l i z e d o r degraded a r e a f u n c t i o n of t h e s p e c i f i c p e s t i c i d e .
The amount
t h a t i s washed o f f i s a f u n c t i o n of t h e c r o p t o which i t i s a p p l i e d and
i t s a d s o r p t i o n i n t o t h e w a x on t h e p l a n t s u r f a c e .
A mass b a l a n c e p l u s a
decay f a c t o r i s used t o d e t e r m i n e t h e amount washed o f f .
The f a t e of
t h a t which washes o f f i s dependent upon t h e s i z e of t h e e v e n t and whether
o r not t h e r e is s u r f a c e runoff.
See t h e p r e v i o u s d i s c u s s i o n of p e s t i -
c i d e s on t h e s o i l s u r f a c e .
Timbered Areas
The r a t e of movement of w a t e r , sediment and chemicals from timbered
a r e a s w i l l b e handled by m o d i f i c a t i o n s i n parameter v a l u e s o f t h e equat i o n s o f flow, e r o s i o n and sediment t r a n s p o r t ; and changes i n t h e evapotranspiration subroutine.
Changes i n t h e ET s u b r o u t i n e a l l o w t h e rela-
t i v e c o n t r i b u t i o n s of s o i l s u r f a c e e v a p o r a t i o n and p l a n t t r a n s p i r a t i o n t o
f o l l o w a n o n - l i n e a r rate a s a f u n c t i o n of l e a f - a r e a - i n d e x
t i o n of a m a x i m u m LA1 of t h r e e .
without r e s t r i c -
The i n f l u e n c e of t h e f o r e s t f l o o r a s a
d i f f u s i o n b a r r i e r t o s o i l e v a p o r a t i o n i s accounted f o r by m o d i f i c a t i o n of
t h e s o i l - w a t e r t r a n s m i s s i o n and c r o p r e s i d u e - s o i l
cover parameters.
I n t e r c e p t i o n a l g o r i t h m s i d e n t i f y f o u r d i f f e r e n t v e g e t a t i v e t y p e s ; longl e a f e d c o n i f e r s , s h o r t - l e a f e d c o n i f e r s , mature hardwoods and mixed hardwoodpine.
A d i f f e r e n t i n t e r c e p t i o n p a t t e r n i s d e f i n e d f o r each.
CHANNEL, FESERVOIR AND GROUNDWATER PROCESSES
The p r e v i o u s s e c t i o n s a r e a d e s c r i p t i o n of t h e s o u r c e a r e a r e s p o n s e ,
t h a t i s t h e c o r e of SWAM.
The b a l a n c e of t h i s p r e s e n t a t i o n is a d e s c r i p -
t i o n of t h e r o u t i n g of w a t e r , sediment and chemicals through t h e channel
system; t h e groundwater flow; and t h e movement of w a t e r sediment and
chemicals through r e s e r v o i r s .
Routing Water and Sediments i n t h e Channel System
Water:
C h a r a c t e r i s t i c s of t h e channel t h a t a r e needed f o r r o u t i n g
w a t e r through t h e channel system were d e s c r i b e d b r i e f l y i n t h e s e c t i o n of
t h i s paper t h a t d e s c r i b e d t h e Watershed D e l i n e a t i o n and C l i m a t i c I n p u t s .
Using t h e s e d e s c r i p t i o n s of t h e channel system and t h e hydrographs of a l l
i n p u t s t o a g i v e n p o i n t (node) i n t h e channel system (upstream c h a n n e l ,
l a t e r a l i n f l o w s , r e s e r v o i r outflow, and groundwater), r o u t i n g t o t h e n e x t
node i s shown i n F i g u r e 9.
Within t h e channel, w a t e r can move e i t h e r
i n t o o r o u t of t h e channel banks, channel l o s s e s a r e c a l c u l a t e d a s a
f u n c t i o n of t h e channel p e r m e a b i l i t y .
Out-of-bank
flow i s s u b j e c t t o
I n f i l t r a t e d water moves back i n t o t h e
e v a p o r a t i o n and i n f i l t r a t i o n .
channel, a f t e r t h e storm, a s groundwater o r i t goes i n t o deep s t o r a g e .
I f t h e reach i s upstream from a r e s e r v o i r o r i n a backwater s i t u a t i o n ,
then s o l u t i o n of t h e flow e q u a t i o n s is based on t h e d i f f u s i v e wave approximation.
I f t h e flow i s u n o s b t r u c t e d , s o l u t i o n i s based on t h e k i n e m a t i c
wave approximation.
Output from t h e r e a c h i s i n p u t t o t h e n e x t reach
downstream.
Sediment: The composition of sediment t r a n s p o r t e d through t h e
channel r e a c h is dependent upon t h e c h a r a c t e r i s t i c s of t h e bed m a t e r i a l ,
t h e t r a n s p o r t e d l o a d , flow c o n d i t i o n s and channel c o n f i g u r a t i o n .
P r o c e s s e s considered i n r o u t i n g sediments a r e shown i n Figure 10; they
a r e based on t h e USDA Sedimentation Laboratory Model (Alonso e t a l , 1981).
Residual t r a n s p o r t c a p a c i t y , which determines whether t h e channel
reach w i l l agrade, degrade o r remain i n e q u i l i b r i u m is based on t h e
c a r r y i n g c a p a c i t y of t h e flowing w a t e r , t h e sediment l o a d t h a t i t
r e c e i v e s and t h e p a r t i c l e s i z e d i s t r i b u t i o n of t h e sediment l o a d .
I f the
sediment l o a d i s i n balance w i t h t h e c a r r y i n g c a p a c i t y , t h e l o a d p a s s e s
through t h e r e a c h unchanged.
t i o n w i l l occur.
I f i t is g r e a t e r t h a n t h e c a p a c i t y , deposi-
P a r t i c l e s a r e deposited, s t a r t i n g with the l a r g e s t s i z e
f r a c t i o n , u n t i l t r a n s p o r t c a p a c i t y i s reached.
Composition of t h e bed i s
then determined and a new bed e l e v a t i o n e s t a b l i s h e d .
I f t h e sediment
l o a d is l e s s than t r a n s p o r t c a p a c i t y , e r o s i o n of t h e bed o c c u r s .
The
s m a l l e s t s i z e f r a c t i o n s a v a i l a b l e i n t h e bed a r e t h e n removed u n t i l
t r a n s p o r t c a p a c i t y is achieved o r t h e remaining s u r f a c e l a y e r i s composed
of m a t e r i a l too l a r g e t o b e t r a n s p o r t e d .
T h i s t h e n becomes t h e armor
l a y e r and t h e bed composition and e l e v a t i o n a r e determined.
The sediment
l o a d and i t s composition is t h e n passed on t o t h e n e x t reach.
The Groundwater Component
Groundwater movement i n t o t h e channel system is based on t h e flow
l i n e o r s t r e a m t u b e concept.
Low flow c o n d i t i o n s i n t h e watershed a r e
used t o i d e n t i f y t h o s e reaches where groundwater e n t e r s t h e channel
system.
Groundwater w e l l s a r e used t o e s t a b l i s h t h e groundwater d i v i d e
then r e p r e s e n t a t i v e flow p a t h s a r e drawn.
DeCoursey, 1982 and I j o n g e t a l .
1981).
See F i g u r e 11 (Liong and
The flow p a t h s a r e t h e n grouped
.
GROUND
WATER
MAIN
CHANNEL
LATERAL
INFLOWS
i
T
1
CHANNEL FLOW
CHANNEL
-
/
/
/
/
/
RESERVOIR
OUTFLOW
CHANNEL
CHARACTERISTICS
OVERBANK F L O W
STORAGE
,I
GROUNDWATER
STCRAGE
INPUT TO
NEXT REACH
F i g u r e 9.
-
GROUNDWATER
DEEP
STORAGE
Stream flow r o u t i n g w i t h i n a channel r e a c h
/
CHANNEL
FLOW
RANSPORTED
SEDIMENT
LOAD
/
RESIDUAL
:TRANSPORT
CAPACITY
BED
-
CHANNEL
SEDIMENT
SOURCES
0
a LOAD
COMPOSITION
COMPOSITION
SEDIMEM
TRANSPORTED
F i g u r e 10.
Sediment t r a n s p o r t w i t h i n a c h a n n e l r e a c h
/GROUND
WATER
TYPICAL FLOW
-------
PATHS OF
GROUND WATER
TO A CHANNEL
------
SYSTEM
INTERCEPTING
CROSS SECTION ALONG A FLOW PATH
CONDUCTIVITY)
-----
Figure 11.
Groundwater processes
into representative lengths for each of the channel reaches and the input
calculated on a daily basis.
Flow into the channel is a function of the
saturated hydraulic conductivity, the porosity, the length of the flow
path, the hydraulic head of the water surface elevation at the divide
over water elevation in the channel, and the thickness of the aquifer at
the channel. Both convergent and non-convergent flow are considered. A
kinematic routing scheme moves water that passes below the root zone,
through the unsaturated zone above the water table, to the water table.
This raises the elevation of the groundwater, thus changing the flow rate
and rate of groundwater flow recession.
If a flow path crosses several
fields or response areas, total flow, through the root zone from all
fields is averaged across the entire length of the path to get a single
weighted average increase in the water table level.
Any chemicals, such
as nitrate, that the percolating water may carry into the groundwater are
assumed to be uniformly mixed with the groundwater below the field.
These concentrations are mixed with inflowing water from gradient and
down-gradient in the same way.
This simplified approach to estimating
the quantity and chemical concentrations of groundwater provides reach
inputs that can be added to surface runoff and routed through the channel
system.
Reservoir Processes
Reservoirs and small impoundments such as farm ponds probably have
more impact on the quality of water in a channel system than any other
structural or land use conservation practice.
Therefore, this component
of the model has received considerable attention.
very simplified form in Figure 12.
It is presented in a
Given the following physical charact-
eristics of the reservoir; initial temperature, suspended sediment,
dissolved solids profiles; chemical structure; hydrologic and meteorologic data; morphometric data; inflow data; and outflow relationships--a
series of subroutines calculates changes that take place in the temperature, suspended sediment and dissolved solids profiles.
These subrou-
tines take into consideration density currents that develop as inflow
enters the reservoir (Dhamothran and Stefan, 1980). After changes in the profiles
are calculated, chemical and biological process changes are simulated.
Phosphorus processes include macrophyte and plankton uptake and decay and
sediment sorption-desorption isotherms in both the epilimnion and hypolimnion.
The sedimentation of detritus and effects of rooted macrophytes
are also considered in the phosphorus structure.
PHYSICAL
CHPRPCTERISTICS
INITIAL TEMP.. SUSP. SEDIMENT.
DISSOLVED SOLIDS PROFILE
CHEMICAL AND BIOLOGICAL STRVCTURE
PROCESSES
RESERVOIR
WATER QUALITY
Figure 12.
Reservoir Processes
A t t h e p r e s e n t time, t h e n i t r o g e n c y c l e i s handled i n a v e r y simple
way t a k i n g i n t o c o n s i d e r a t i o n t h e o r g a n i c m a t t e r l e v e l and n i t r a t e concent r a t i o n i n t h e r e s e r v o i r a s compared t o t h e i n f l o w i n g w a t e r , t h e r e s i dence t i m e and temperature.
The model w i l l be r e f i n e d , comparable t o
t h a t of phosphorus, i n t h e n e a r f u t u r e .
The p e s t i c i d e p r o c e s s e s simulated i n c l u d e a s o r p t i o n - d e s o r p t i o n
i s o t h e r m b a l a n c e between t h e s o l u b l e f r a c t i o n and t h a t adsorbed on t h e
suspended sediments i n both t h e e p i l i m n i o n and hypolimnion.
Outflow from
t h e r e s e r v o i r a t t h e end of each day ( i f t h e r e i s any) i n c l u d e s t h e
suspended sediments and a s s o c i a t e d w a t e r q u a l i t y c o n s t i t u a n t s .
v a l u e s a r e i n p u t t o t h e channel system downstream.
These
S u b r o u t i n e s provide
a mechanism f o r c a l c u l a t i n g t h e t r a y e f f i c i e n c y of t h e sediments, n u t r i e n t s and p e s t i c i d e components of flow ( s e e F i g u r e 1 3 ) .
Movement of Chemicals through t h e Channel System
Nitrogen:
The n i t r o g e n compounds of most concern i n channel flow
a r e n i t r a t e s and sediment a s s o c i a t e d N ( s e e F i g u r e 1 4 ) .
For p r a c t i c a l
purposes, n i t r a t e s a r e n o t adsorbed, b u t move o n l y i n s o l u t i o n .
The
sediment a s s o c i a t e d N i s c o n s i d e r e d t o move p r i m a r i l y w i t h t h e o r g a n i c
f r a c t i o n of t h e sediment.
The NH4, which i s computed a s p a r t of t h e
sediment a s s o c i a t e d N , i s m o s t l y adsorbed.
An a l o g r i t h m f o r NH4 t r a n s -
p o r t , determined by a p a r t i t i o n i n g c o e f f i c i e n t based on i o n exchange
e q u i l i b r i u m i s b e i n g developed f o r t h o s e s p e c i a l s i t u a t i o n s where NH
important.
is
4
Because flow through t h e channel system is r e l a t i v e l y r a p i d ,
changes due t o N i n c o r p o r a t i o n , n i t r i f i c a t i o n o r d e n i t r i f i c a t i o n a r e
assumed t o be i n s i g n i f i c a n t .
I f . t r a v e l time through some channel systems
i s s u f f i c i e n t l y long s o t h a t t h e s e p r o c e s s e s should b e c o n s i d e r e d , t h e
r e s e r v o i r N c y c l i n g model could be i n c o r p o r a t e d .
Outputs of t h e n i t r o g e n
r o u t i n g p r o c e s s i n c l u d e t h e c o n c e n t r a t i o n s and masses of n i t r a t e i n
s o l u t i o n and t h e sediment a s s o c i a t e d N .
Phosphorus:
The phosphorus o u t p u t from t h e s o u r c e a r e a s a r e s o l u b l e
i n o r g a n i c PO4 and adsorbed PO4 ( s e e F i g u r e 1 5 ) .
The o t h e r P f r a c t i o n s
such a s s o l u b l e o r g a n i c , o r g a n i c m a t t e r o r m i n e r a l P forms a r e n o t
modeled.
E q u i l i b r i u m i s assumed between t h e s o l u b l e and adsorbed PO
4'
These two f r a c t i o n s comprise roughly 20-40 p e r c e n t of t h e t o t a l P and a r e
t h e most b i o l o g i c a l l y a c t i v e P f r a c t i o n s .
t h e t e c h n i q u e used i s a mass
b a l a n c e based on t h e E q u i l i b r i u m Phosphorus Concentration (EPC) p u b l i s h e d
by Kunishi and T a y l o r , 1977; and Taylor and Kunishi, 1971.
Merging flow
.
SOLUBLE
PEST. IN
FLOW
SOLUBLE
PEST. IN
LATERAL
FLOW
SOLUBLE
PEST. IN
CHANNEL
PEST.
ON
CHANNEL
SEDIMENTS
LATERAL
SEDIMENTS
SOLUBLE
PEST. IN
RESERVOIR
OUTFLOW
PEST. ON
SEDl MENTS
FROM
RESERVOIRS
LOSS IN
FLOW TO
PEST.
DEPOSITED
SOLUTION
SEDIMENTS
a
PEST.
TRANSPORT
F i g u r e 13.
P e s t i c i d e s movement w i t h i n a channel reach
SEDIMENTS
NO, IN
LATERAL
NO, IN
CHANNEL
FLOW
Nl(r IN
CHANNEL
FLOW
NO, IN
G .w.
FLOW
N 8 NH4 I N
CHANNEL
SEDIMENTS
N 8 NH4 ON
RESERVOIR
SEDIMENTS
N 8 NH4 IN
SEDIMENTS
NO, IN
RESERVOIR
OUTFLOW
N 8 NH4
NO,
1N
SOLUTION
DEPOSITED
I
/
'
Figure 1 4 .
TRANSPORTED
Nitrogen movement w i t h i n a channel reach
PO, IN
LATERAL
PO, IN
CHANNEL
FLOW
PO, ON
LATERAL
SEDlMENTS
PO, ON
CHANNEL
SEDIMENTS
PO4 IN
RESERWIR
OUTFLOW
--
PO, ON
RESERVOIR
SEDIMENTS
PO4 IN
DEPOSITED
SOLUTION
FLOW
SEDIMENTS
TOTAL
TRANSPORTED
Figure 15.
Phosphorus movement within a channel reach
SEO IMENTS
o r sediments a r e each c h a r a c t e r i z e d by a l i n e a r b u f f e r curve which i s
t h e n recomputed f o r t h e combined mass, minus t h a t PO
transformed t o
4
u n a v a i l a b l e P forms ( f i x a t i o n ) . The d i r e c t l o s s of s o l u b l e PO i s com4
puted on t h e b a s i s of w a t e r l o s s t o groundwater r e c h a r g e . S o l u b l e PO4
s o u r c e s i n c l u d e f i e l d r u n o f f , groundwater and l a t e r i n f l o w .
It can move
i n t o t h e groundwater o r b e adsorbed on sediments d e p o s i t e d i n t h e channel.
Adsorbed PO
s o u r c e s a r e channel and l a t e r a l i n f l o w s from f i e l d s
4
and r e s i d u a l sediments i n t h e channel. PO f i x a t i o n i s computer a s a PO
4
4
loss.
Carbon:
The primary s o u r c e of carbon i s s o i l o r g a n i c m a t t e r and
eroded o r g a n i c m a t t e r t r a n s p o r t e d i n t h e sediment phase.
I t i s computed
a s a mass b a l a n c e w i t h f i e l d i n f l o w s being t h e primary s o u r c e .
Erosion
and d e p o s i t i o n of carbon a s s o c i a t e d w i t h s t r e a m bottom sediments i s
estimated (see Figure 16).
Carbon i s r e q u i r e d a s i n p u t t o t h e r e s e r v o i r
model and p e s t i c i d e a d s o r p t i o n c a l c u l a t i o n s .
FUTURE EFFORTS
Most of t h e s u b r o u t i n e s d e s c r i b e d i n t h e d i s c u s s i o n have
been developed and t e s t e d and some of them have been combined.
I n the
n e a r f u t u r e a l l of them w i l l be assembled i n t o one l a r g e program and
s e n s i t i v i t y a n a l y s e s and t e s t i n g begun.
However, t h e r e a r e s e v e r a l
a g r i c u l t u r a l p r a c t i c e s , a p p l i c a t i o n s , o r consequences t h a t have n o t been
addressed.
model.
A s t i m e p e r m i t s , w e w i l l a t t e m p t t o i n c o r p o r a t e them i n t o t h e
These i n c l u d e t i l e d r a i n a g e , i r r i g a t i o n , groundwater i n f l o w t o
ponds o r r e s e r v o i r s , e r o s i o n from c o n c e n t r a t e d s o u r c e s such a s g u l l i e s ,
b a c t e r i a l and b i o l o g i c a l a c t i v i t y , p o i n t s o u r c e s of p o l l u t i o n , a complete
t r e a t m e n t of temperature, adequate coverage of o r g a n i c m a t t e r decomposit i o n , n u t r i e n t l e a c h i n g and s u r f a c e e x t r a c t i o n of chemicals.
After
s e n s i t i v i t y a n a l y s e s , methods of a g g r e g a t i n g a r e a s w i l l b e deveioped and
e f f o r t s t o develop a b a s i n s c a l e model begun.
SOLUBLE
CARBON
SOLUBLE
CARBON
CARBON ON
CHANNEL
SEDIMENTS
LATERAL
CARBON ON
LATERAL
SEDIMENTS
U
SOLUBLE
CARBON
RESERVOIR
OUT FLOW
I
CARBON
IN
CARBON
ON
SOLUTION
SEDIMENTS
u
TRANSPORTED
Figure 16.
Carbon movement w i t h i n a channel r e a c h
CARBON
DEPOSrrED
SEDIMENTS
REFERENCES
Alonso, C.V., D.K. Borah
graded sediments in
Stability, a report
the U.S. Army Corps
and S.N. Prasad. 1981. Numerical model for routing
alluvial channels. Appendix J of Stream Channel
prepared by the USDA Sedimentation Laboratory for
of Engineers, Vicksburg District, 89 pp.
Borah, D.K., C.V. Alonso and S.N. Prasad. 1981. Single event numerical model
for routing water and sediment on small catchments. Appendix I of Stream
Channel Stability, a report prepared by the USDA Sedimentation Laboratory
for the U.S. Army Corps of Engineers, Vicksburg District, 112 pp.
Corey, G.L., A.T. Corey and R.H. Brooks. 1965. Similitude for non-steady
drainage of partially saturated soils. Colorado State University
Hydrology Papers No. 9. 38 pp.
Dhamothran, S., and H. Stefan. 1980. Mathematical model for temperature
and turbidity stratification dynamics in shallow reservoirs.
Amer. Soc. of Civil Engrs. Proc. of Symp. on Surface Water Impoundments, Minneapolis, Minnesota.
Khanjani, M., and M. Molnau. 1982. Snowmelt runoff computations for
CREAMS. ASAE Annual Summer Meeting, Madison, Wisconsin. Paper 82-2051.
Knisel, Walter G. (editor) 1980. CREAMS: A Field Scale Model for Chemicals,
Runoff, and Erosion from Agricultural Management Systems. U.S. Department
of Agriculture, Conservation Research Report No. 26, 640 pp.
Kunishi, H.M., and A.W. Taylor. 1977. Predicting pollution potential of
phosphorus at heavy application rates. Proceedings of the International
Seminar on Soil Environment and Fertility Management at Intensive
Agriculture, Tokyo, Japan. pp. 349-356.
Laliberte, G.E., A.T. Corey and R.H. Brooks. 1966. Properties of unsaturated
porous media. Colorado State University Hydrology Papers No. 17. 40 pp.
Liong, S. Y . , D. G. DeCoursey and E. H. Seely. 1981. Response f u n c t i o n s of
s u b s u r f a c e flow. Proceedings I n t . Symp. on Rainfall-Runoff Modeling,
Mississippi S t a t e University, Mississippi State, Mississippi.
Liong, S.Y. and D.G. DeCoursey. 1982. Development of p r e d i c t i o n e q u a t i o n s
f o r a p h r e a t i c a q u i f e r i n r e s p o n s e t o i n f i l t r a t i o n : dimensional a n a l y s i s .
Water Resources B u l l e t i n , Amer. Water Resources Assoc. Vol. 1 8 , No. 2,
pp. 307-310.
McGill, W.B., e t a l . 1981. Phoenix, a model of t h e dynamics of c a r b o n and
n i t r o g e n i n g r a s s l a n d s o i l s . T e r r e s t r i a l N i t r o g e n C y c l e s , C l a r k E.
Rosswell, ed. Ecol. Bul. 33, pp. 49-115.
Moore, I. D. 1981. I n f i l t r a t i o n e q u a t i o n s modified f o r s u r f a c e e f f e c t s .
ASCE J o u r n a l of I r r i g a t i o n and Drainage, Vol. 107, No. I R I , pp. 71-86.
Richardson, C.W. 1981. S t o c h a s t i c s i m u l a t i o n of d a i l y p r e c i p i t a t i o n ,
t e m p e r a t u r e . a n d s o l a r r a d i a t i o n . Water Resources Research, Vol. 17,
No. 1, pp. 182-190.
R i t c h i e , J.T. 1972. Model f o r p r e d i c t i n g e v a p o r a t i o n from a row c r o p
w i t h incomplete c o v e r . Water Resources Research, Vol. 8 , No. 5 ,
pp. 1204-1213.
Smith, R.E. 1976a. S i m u l a t i n g e r o s i o n dynamics w i t h a d e t e r m i n i s t i c
d i s t r i b u t e d w a t e r s h e d model. Proc, of t h e T h i r d I n t e r Agency
1-173.
S e d i m e n t a t i o n Conference, pp. 1-163
-
Smith, R.E. 1976b. F i e l d t e s t of a d i s t r i b u t e d watershed e r o s i o n / s e d i m e n t a t i o n
model. S o i l Erosion: P r e d i c t i o n and C o n t r o l , S o i l Cons. Soc. of Amer.
pp. 201-209.
Smith, R.E., and J . Y . P a r l a n g e . 1978. A p a r a m e t e r - e f f i c i e n t h y d r o l o g i c
i n f i l t r a t i o n model. Water Resources Research, Vol. 1 4 , No. 3,
pp. 533-538.
Smith, R.E. 1981a. Mathematical s i m u l a t i o n of w a t e r and sediment flow
p r o c e s s e s on t h e catchment s u r f a c e . The I n s t i t u t i o n of E n g i n e e r s ,
A u s t r a l i a . C i v i l E n g i n e e r i n g T r a n s a c t i o n s , 5 pp.
Smith, R.E. 1981b. A k i n e m a t i c model f o r s u r f a c e mined sediment y i e l d .
Trans. of t h e Amer. Soc. of Agr. Engrs. Vol. 24, No. 6, pp. 1508-1514.
T a y l o r , A.W. and H.M. Kunishi. 1971. Phosphate e q u i l i b r i a on s t r e a m sediment
and s o i l i n a watershed d r a i n i n g a n a g r i c u l t u r a l r e g i o n . A g r i c u l t u r a l
and Food Chemistry, Vol. 1 9 , pp. 827-831.
Williams, J . R . 1982. EPIC: A Model f o r A s s e s s i n g t h e E f f e c t s of E r o s i o n o n
S o i l P r o d u c t i v i t y . T h i r d I n t . Conf. on State-of-the-Art i n E c o l o g i c a l
Modeling. Colorado S t a t e U n i v e r s i t y , May 24-28.