Environmental Engineering Reference
In-Depth Information
from those of drainage through the unsatu-
rated zone and groundwater flow (
Section 2.2
).
Time and space discretizations are often quite
different between the two types of models.
Watershed models invariably operate on a daily
time-step basis. Daily time steps can be used in
groundwater-flow models, but because ground-
water movement is relatively slow, monthly
or yearly time steps are more common; use of
daily time steps in a multiyear simulation may
require substantial computation time. Steady-
state groundwater-flow models that assume
no change in water levels over time have been
widely used in the past, but the steady-state
assumption defeats the purpose of a combined
model. Typically, a hydrologic response unit
(HRU) would overlie multiple computational
cells of a groundwater-flow model. So a proce-
dure is required to map and distribute fluxes
between an HRU and the appropriate compu-
tational cells.
One of the more difficult challenges in link-
ing a watershed and groundwater-flow model is
the simulation of water movement through the
unsaturated zone between the soil zone and the
water table. This issue is addressed in
Section 3.3
for unsaturated zone water-budget models, and
much of the discussion in that section is relevant
here as well. The three-dimensional nature of a
combined model introduces additional complex-
ity, as does the fact that recharge fluxes must be
calculated for thousands of computational cells
within the groundwater-flow model. Most com-
bined models assume one-dimensional vertical
flow through the unsaturated zone and use one
of the approaches described in
Section 3.3
, such
as a bucket model, a transfer-function model,
or an approach based on the Richards equa-
tion. More complex models, such as MIKE-SHE
(Graham
et al
.,
2006
) and InHM (VanderKwaak
and Loague,
2001
), allow for horizontal water
movement within the unsaturated zone by solv-
ing the Richards equation numerically in three
dimensions. This approach provides a detailed
representation of water movement within the
unsaturated zone and can be useful when unsat-
urated zone hydraulic properties are not uni-
form, but the approach can be quite demanding
in terms of computational resources.
3.6 Combined watershed/
groundwater-flow models
In recent years, both watershed modelers and
groundwater-flow modelers have come to rec-
ognize the benefits offered by a linked water-
shed and groundwater-flow model. A combined
model allows watershed modelers to realistic-
ally capture the dynamic nature of surface and
groundwater exchange as controlled by the
head gradient between a stream and an aqui-
fer. Measured groundwater levels provide add-
itional constraints for model calibration. For
groundwater-flow modelers, a combined model
provides physically based representations of
recharge and discharge processes. Both diffuse
and focused recharge can vary in space and time,
as influenced by climate, geology, topography,
vegetation, and other factors. Also, a combined
model is able to accommodate settings where
watershed boundaries do not exactly coincide
with aquifer boundaries. Flow can be simulated
in an aquifer, for example, that underlies only
a part of a watershed. At present, only a small
number of combined watershed/groundwater-
flow models are available: SWAT/MODFLOW
(Sophocleous and Perkins,
2000
), MIKE-SHE
(Graham
et al
.,
2006
), and GSFLOW (Markstrom
et al
.,
2008
), but new and improved models
should continue to appear in the future.
To envision a combined watershed/ground-
water-flow model, one need only replace the
groundwater reservoir shown in
Figure 3.6
with a spatially extensive aquifer within which
groundwater may move horizontally and verti-
cally. Simulated recharge, depending on model
design, can be influenced by water-table height.
Similarly, groundwater discharge to a stream
can be calculated on the basis of groundwater
levels and stream levels. Thus, a combined model
allows groundwater recharge and discharge to
be represented in a manner more physically
realistic than approaches used in stand-alone
watershed and groundwater-flow models.
A number of challenges arise in attempt-
ing to link a watershed model and a
groundwater-flow model. Precipitation, runoff,
and streamflow occur at time scales different
