Environmental Engineering Reference
In-Depth Information
Box 22.4
ISISMOD - A coupled groundwater-surface water (GW-SW) model
ISISMOD is a fully coupledGWand SWmodel based on combining existing iSIS FlowandMODFLOW
models. Coupling between the programs is based on the following concept.
MODFLOW solves the groundwater flow equation and provides the groundwater heads that iSIS
uses to calculate the corresponding leakage, based on the water level of the surface water body. The
leakage is exported back toMODFLOWand the groundwater heads are recomputed. Iteration between
the programs ceases when the difference between two successive leakage volume calculations is less
than a specified tolerance. The issue of the different time steps, normally required for the simulation of
both systems independently, was addressed by calculating (in iSIS) an average leakage term during a
MODFLOW time step, using groundwater heads linearly interpolated every iSIS time step.
The governing equations for aquifer and surface water interaction are described below. The general
expression for the leakage between a surface water body and an aquifer can be expressed as follows:
Ql
ks
t
¼
L
area
ð
hs
HGW
Þ
where:
Ql
¼
leakage flow that passes between a water body and an aquifer (L
3
T
1
);
k
s
¼
vertical hydraulic conductivity of the streambed of the water body (LT
1
).
t
¼
thickness of the streambed (L);
L
area
¼
cross-sectional area normal to the leakage flow direction (L
2
);
h
s
¼
water elevation of the surface water body (L);
HGW
¼
groundwater head on the aquifer in correspondence with the surface water body (L).
The resulting continuity equation for an open channel, including a leakage contribution from an
aquifer, can be expressed as:
d
d
x
þ
d
Q
h
d
t
þ
ql
¼
0
where:
kv
L
area
ð
hs
HGW
Þ
¼
ql
dx
¼
leakage per unit length of channel (L
3
T
1
L
1
).
iSISMOD was developed specifically to allow a wide range of interaction cases, including a large
floodplain area interacting with multiple flow cells, as depicted below:
ql
