Environmental Engineering Reference
In-Depth Information
A uniform value of diffuse recharge can be
assigned to the entire model domain, or values
can be assigned on a cell-by-cell basis. The simu-
lated domain is sometimes divided into a num-
ber of regions in which recharge is assumed
uniform (Cooley, 1979 ); these regions are some-
what analogous to the HRUs used in watershed
models. Historically, many groundwater-flow
models invoked a steady-state assumption,
wherein recharge was assumed constant in
time, because there seldom were sufficient data
to calibrate a transient model. Recent advances
in linking watershed and groundwater-flow
models ( Section 3.6 ) allow recharge to vary over
time, even to the point of having daily values
of recharge.
Focused recharge can be simulated by using
the River Package in MODFLOW-2005. For each
finite-difference cell, the user must specify
stream stage and a streambed conductance
term that accounts for streambed thickness
and hydraulic conductivity and stream area
(length times width) within the cell (Harbaugh,
2005 ). Exchange of water between the stream
and aquifer is calculated according to the Darcy
equation. If stream stage for a model cell is
greater than calculated head for that cell, water
flows from the stream to the aquifer at a rate
dictated by model input parameters. If calcu-
lated head exceeds stream stage for a model
cell, the stream acts as a sink for groundwater
discharge.
Recharge to confined aquifers occurs largely
in areas where the aquifers are unconfined, but
many aquifers receive substantial amounts
of replenishment in the form of vertical flow
through confining beds. Flow through overly-
ing confining beds was the largest source of
water for the Ozark aquifer under predevelop-
ment conditions (Czarnecki et al ., 2009 ). As con-
fined aquifers are developed by humans, the
importance of flow through confining beds as
a source of aquifer replenishment is enhanced
because declining water levels in the aquifers
induce additional flow into the system through
these beds.
Overlying confining layers may or may not
be included as active model layers when simu-
lating flow in a confined aquifer. If the confining
(a)
Node
Element
Aquifer
boundary
(b)
Node
Cell
Figure 3.11 Example of areal finite element grid (a)
and finite difference grid (b) overlain on an aquifer.
Approximations to Equation ( 3.8 ) are solved for values of
head at each node. Multiple grid layers are required for
simulating three-dimensional groundwater flow.
recharge fluxes from these boundaries are
determined by the model on the basis of cal-
culated heads and therefore are not known
a priori. Focused recharge can also be rep-
resented as a point source with a specified
volumetric flow (e.g. by using the MODFLOW
Well Package) for sources such as intermittent
streams.
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