This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
REM: A Model for Riparian Ecosystem Management
in Agricultural Watersheds1
Richard Lowrance and Adel Shirmoharnmadi
2
Abstract.--A model for Riparian Ecosystem Management
(REM) is presented. The model is driven by daily hydrologic
inputs and is designed to predict water quality changes in
riparian ecosystems of agricultural watersheds. Using the
principle of the Conservation of Mass, the model predicts
water and nutrient inputs from uplands to the riparian ecosystem. Predictions of changes in nutrient levels due to
interactions of water with soil, leaf litter, and vegetation
are made in four submodels. The next step in this modeling
effort is to develop a computer program and to verify the
model with available data sets from other watersheds.
Nutrient filtering by riparian ecosystems
is dependent on nutrient dynamics within
streamside forests and on the timing and amount
of nutrient transport from agricultural areas.
Hydrologic and nutrient transport models of
entire watersheds such as the Pesticide Runoff
Transport (PRT) model (Crawford and Donigian
1973), the Agricultural Runoff Model (ARM,
Donigian and Crawford 1976), and the
Agricultural Chemical Transport Model (ACTMO,
Frere et al. 1975) are available, but none
include explicit models of riparian areas. The
model presented here does not treat upland
nutrient dynamics and therefore requires that
nutrient concentrations in upland runoff be
specified as part of the input.
INTRODUCTION
Riparian forest ecosystems in the coastal
plain are effective filters of nutrients moving
in surface and subsurface flow from agricultural
fields (Jacobs and Gilliam 1983, Lowrance et al.
1984, Peterjohn and Correll 1984). A model of
nutrient filtering in riparian ecosystems is
needed in order to properly manage streamside
forests as functional parts of agricultural
watersheds. In this paper we will present
conceptual and mathematical models of riparian
ecosystems which might eventually be used to make
management decisions for nonpoint source pollution control. The specific example used in this
paper is a nitrogen model, but REM is designed to
model any of the essential plant nutrients.
Nutrient dynamics in the riparian ecosystem
are controlled by a combination of hydrologic,
physical, chemical, and biological processes.
The degree of nonpoint pollution control depends
on the interaction between the chemical load
transported from the uplands and the hydrologic
and nutrient cycling processes in the streamside
zone.
This discussion and the model presented here
assume a general land use of agricultural lands
on better drained upland soils and a forest of
mostly native tree and shrub species on poorly
drained bottomland soils. The model also assumes
that a shallow aquifer is part of the riparian
ecosystem and is available for evapotranspiration
from the vegetation. This situation exists or
has existed in parts of the lower coastal plain,
most oJ the upper coastal plain, and many areas
of the southern piedmont.
Water and nutrients enter the riparian
ecosystem from upland surface runoff, upland
subsurface runoff, and bulk precipitation.
Water and nutrients leave the riparian zone via
surface runoff and subsurface flow which together constitute streamflow. The important
processes affecting surface runoff inputs are
infiltration, sediment deposition, and exchange
of dissolved nutrients. All of these processes
take place at the interface between overland
flow and the soil/litter surface. Subsurface
flow into the riparian zone generally goes into
storage in a shallow alluvial aquifer. Water
and nutrients move from the alluvial aquifer
1
Paper presented at the First North American
Riparian Conference, Tucson, Arizona, April
16-18, 1985.
2Richard Lowrance is an Ecologist with the
Southeast Watershed Research Laboratory, USDAARS, and Adel Shirmoharnmadi is an Assistant Research Agricultural Engineer with the Department
of Agricultural Engineering, University of
Georgia, Tifton, Georgia, USA.
237
into streamflow or move into the root zone to be
available for uptake by vegetation.
LITTER and SOIL. Most of these parameters are
based on empirical data derived from coastal
plain riparian ecosystem studies. The
HYDROLOGY, NUTRIENT, VEGETATION, LITTER, and
SOIL components of the model are described in
detail below.
Model Structure
REM is based on dividing the riparian
ecosystem into three nutrient pools - a biological pool (VEGETATION) consisting of above and
belowground vegetation; a litter pool (LITTER)
consisting of both woody and non-woody litter on
the forest floor; and a soil pool (SOIL) consisting of organic and inorganic nutrients in the
unsaturated soil and in the saturated soil which
makes up the shallow alluvial aquifer (Figure 1).
These pools of nutrients a.re initialized based on
published data and/or field data and are altered
due to inputs and outputs each day. The inputs
and outputs are calculated in the nutrient load
component of the model (NUTRIENT) which is in
turn driven by the hydrology component (HYDROLOGY).
HYDROLOGY
The HYDROLOGY submodel is used to solve the
water balance equation for the riparian ecosystem and to calculate daily surface runoff
(Q
) and daily subsurface flow (Q
b)
usur
usu
inputs from the uplands. The general equation
for these calculations is:
The inputs to the riparian ecosystem are
surface runoff from uplands, subsurface runoff
from uplands, bulk precipitation, and nitrogen
fixation (for REM- nitrogen only). Outputs from
the riparian ecosystem are surface and subsurface
flow components of total streamflow, harvest of
biomass, and denitrification (for REM-nitrogen
only). Inputs are modified by the processes
taking place in VEGETATION, LITTER, and SOIL to
produce outputs.
f.. AS
AET
TF
For a rainday, assuming AET
Qusur infiltrates:
REM is driven by hydrologic inputs to the
riparian ecosystem and by climate as expressed
through evapotranspiration. Inputs to the model
include the hydrologic parameters needed to
calculate surface and subsurface inputs to the
riparian ecosystem and the various parameters
necessary to simulate fluxes in VEGETATION,
RIPARIAN PRECIPITATION
PR
surface runoff contribution to
total streamflow at the watershed
outlet.
subsurface runoff contribution to
total streamflow at the watershed
outlet.
change in alluvial aquifer storage.
actual evapotranspiration.
throughfall to riparian area.
where, Qosur
-
=
0 and that none of
(TF-TFI)
Qusur
=
Qosur
Qusub
=
Qosub + liAS-TF I
(2)
and
where, TFI
=
throughfall which infiltrates.
EVAPOTRANSPIRATION
ET
UPLAND PRECIPITATION
Pu
TOTAL
THROUGHFALL, TF
INFILTRATED
THROUGHFALL, TF1
STORAGE, 8.AS
Figure 1.--Flow diagram for the movement of water and
associated nutrients in REM.
238
(3)
For a non-rainday where Q
= 0, the upland
usur
subsurface flow can be determined as follows:
Qusub = Qosub + ~S+AET.
respectively. Surface runoff moving through
riparian forests loses significant quantities of
particulate N and P at the water/soil interface
(Peterjohn and Correll 1984). The runoff
enrichment factors,S andy, assume that exchange
of dissolved nutrients takes place with the
litter, and the exchange of sediment-associated
nutrients takes place with the soil.
(4)
The input data for HYDROLOGY are daily values of
Qosur' Qosub' ~s. bulk precipitation, and mean
daily temperature. The surface and subsurface
outputs (Q
and Q
) are calculated using
osur
osub
an approximate method for partitioning daily
streamflow data (Shirmohammadi et al. 1984a), and
the change in alluvial storage (6AS) is
calculated from groundwater elevation data using
the procedure presented by Shirmohammadi et al.
(1984b). Separately measured surface and
subsurface components of total streamflow can
also be used as input into HYDROLOGY for watersheds where these kinds of data are available.
SOIL
The SOIL submodel is used to calculate
daily changes in the riparian forest soil
nutrient pool. The soil nutrient pool includes
the nutrients in alluvial storage. The major
processes in the SOIL submodel, decomposition
and uptake, are driven by the relationship
between daily actual evapotranspiration and
the maximum daily AET for the forest. The SOIL
submodel solves the equation:
NUTRIENT
6SOIL =
Fn+Fm+Ft)
MAXDEN · (- -- 3
+(1-Y)Qusur.Csed + 6AS·Cusub
The NUTRIENT submodel is used to calculate
the daily hydrologic flux of nutrients entering
and leaving the riparian ecosystem. The basic
equation for outputs is:
LOAD - [ Q
· C
] + [Q
• C
]
osur
osur
osub
osub
where,
( 5)
MAXAET
MAXDEN
LOAD= Q · [(Y-1)·c
+(S-1)·C
]+(1-a)·TF·Ctf+
usur
sed
so 1
6
Qusub.Cusub - AS·Cusub - AET"Css
( 6)
c
c
sol
usub
c
ctf
ss
a
s
y
MAXDEC
MAXUPTK
Expressing equation 5 in terms of upland
inputs and riparian hydrologic fluxes, one can
define LOAD as follows:
where, LOAD
c
sed
(MAXDEC-MAXUPTK)·(~ET)-
F , F , F
n
m
t
total streamflow load of nutrient
concentration of sediment
associated nutrient in upland
surface runoff,
concentration of dissolved
nutrient in upland surface
runoff,
concentration in upland subsurface runoff,
concentration in throughfall,
concentration in unsaturated soil
solution,
fraction of TF which infiltrates,
factor for enrichment or
depl~tion of dissolved concentration by litter,
factor for enrichment or
depletion of sediment associated
concentration by soil.
( 7)
maximum daily litter
decomposition rate,
maximum daily uptake rate
by vegetation,
maximum daily AET for
riparian ecosystem,
maximum daily denitrification rate.
factors rel;ted to soil
N0 -N, soil moisture, and
3
soil temperature, respectively. These vary from 0
to 1.
For nutrients other than nitrogen, the output
due to denitrification is omitted.
The maximum rates of decomposition, uptake,
and denitrification are determined from literature values or field data. The maximum daily
AET rate is calculated from long-term (10 year)
water balance computations in order to find the
maximum actual evapotranspiration from a site.
The factors used to control maximum
denitrification are based on empirical data of
the relationships between denitrification rates
and soil moisture, soil nitrate, and soil
temperature. The denitrification factors assume
that the maximum activity takes place in
0
saturated soil at a soil temperature of 35 C.
Growing plants and denitrifying bacteria compete
for the available N0 -N in the soil. Based on
3
an extensive study of environmental factors
affecting denitrification in bottomland soils
(Hendrickson 1981), F and F are maximum in
n
m
May. The nitrogen factor is minimal in July,
and the moisture factor is minimal in October.
The temperature factor peaks during August.
The daily hydrologic data needed in NUTRIENT
are passed from HYDROLOGY. The concentration
data are given as input to the submodel based on
literature values, field data, or other models.
Values of the factors a, S, and y are input to
NUTRIENT based on infiltration characteristics of
soils and literature values of enrichment or
depletion of surface runoff by litter or soil,
239
At this point in its development, the model
is based on a body of empirical evidence about
the function of coastal plain riparian ecosystems. REM incorporates hydrologic and nutrient
cycling realism through a large quantity of empirical factors requiring estimation by the
user. Despite this need for many empirical
relationships, REM represents an attempt at a
simple usable management model which avoids the
difficulties of simulating the effects of
individual storm events on riparian ecosystem
processes.
LITTER
The LITTER submodel calculates daily changes
in the riparian ecosystem litter nutrient pool.
The LITTER submodel solves the equation:
6LITTER = LITFALL + [(1-S) ·Q
usur
·C
usur
] -
AET
MAXDEC·(MAXAET)
(8)
where, LIT.t!'ALL = litterfall based on Julian date.
L~tterfall includes both woody and nonwoody
litter and is based on literature values or field
data. Other parameters in equation 8 are defined
in previous sections.
LITERATURE CITED
Crawford, N.H., and A. S. Donigian, Jr. 1973.
Pesticide transport and runoff model for
agricultural lands. EPA-660/274-013.
Office of Res. and Development, U.S.
Environmental Protection Agency,
Washington, D.C. 211 p.
Donigian, A. S., Jr., and N.H. Crawford. 1976.
Modeling pesticides and nutrients on
agricultural lands. Environmental
Protection Technology Series,
EPA-600/2-76-043. Office of Res. and
Development, U.S. Environmental Protection
Agency, Washington, D.C. 311 p.
Frere, M. H., C. A. Onstad, and H. N. Holtan.
1975. ACTMO, an Agricultural Chemical
Transport Model. U.S. Department of
Agriculture, Agricultural Research Service,
ARS-H-3. U.S. Govt. Printing Office,
Washington, D.C. 54 p.
Hendrickson, 0. Q., Jr., 1981. Flux of nitrogen
and carbon gases in bottomland soils of an
agricultural watershed. Athens, GA, University of Georgia, 210 p. Dissertation.
Jacobs, T. C. and J. W. Gilliam. 1983. Nitrate
loss from agricultural drainage waters:
implications for nonpoint source control.
Water Resources Research Institute, University of North Carolina, Raleigh, NC.
Report No. 209.
Knisel, W. J. (editor) 1980. CREAMS: A Model
of Chemicals, Runoff, and Erosion from
Agricultural Management Systems. U.S.
Dept. of Agric., Agric. Res. Serv.
Lowrance, R., R. Todd, J. Fail, Jr., 0.
Hendrickson, Jr., R. Leonard, and L.
Asmussen. 1984. Riparian forests as
nutrient filters in agricultural
watersheds. BioScience 34:374-377.
Peterjohn, W. T. and D. L. Correll. 1984.
Nutrient dynamics in an agricultural
watershed: observations on the role of a
riparian forest. Ecology 65:1466-1475.
Shirmohammadi, A., W. G. Knisel, and J. M.
Sheridan. 1984a. An approximate method
for partitioning daily streamflow data.
Journal of Hydrology 74(3/4):335-354.
Shirmohammadi, A., J. M. Sheridan, and L. E.
Asmussen. 1984b. Hydrology of alluvial
stream channels in southern coastal plain
watersheds. ASAE paper No. 84-2502,
presented at the 1984 Winter ASAE Meeting,
New Orleans, Louisiana, 23 p.
VEGETATION
The VEGETATION submodel calculates daily
changes in the living vegetation nutrient pool
using the equation:
6VEGETATION = MAXUPTK·(~E~
- LITFALL - HARVEST
(9)
where, HARVEST = daily removal of woody material.
Harvest data are based on field data and
allow the effects of forest clearing to be
simulated. Forest clearing affects the values
used for MAXUPTK since this is based on mass/area
and is decreased when fewer trees remain per unit
area. Tree harvest also adds a pulse of litterfall to the forest floor although future litterfall is decreased.
DAILY BALANCE
REM calculates the final riparian nutrient
pool each day from the previous day's level and
the changes calculated in SOIL, LITTER, and
VEGETATION submodels:
FRIPNUT = IRIPNUT + 6SOIL
+ 6LITTER +6VEGETATION
(10)_
where, FRIPNUT = nutrient pool for day (i)
IRIPNUT =nutrient pool for day (i-1).
Model output on a daily basis includes SOIL,
LITTER, VEGETATION, FRIPNUT, and LOAD.
POTENTIAL USES
The Riparian Ecosystem Management model, once
it is refined and functional, will be useful
either by itself or combined with existing models
of nutrient transport from agricultural fields
such as CREAMS (Knisel 1980). Coupled with a
field-scale model of nutrient transport, REM would
process upland inputs without relying on partitioning of total streamflow in order to calculate
Qusur and Qusub"
240