Environmental Engineering Reference
In-Depth Information
100
1.0
Figure 3.9
Rainfall and calculated
recharge for one computational
element, site L (Zhang
et al
.,
1999
).
Rainfall
Recharge
80
0.8
60
0.6
40
0.4
20
0.2
0
0
6/92
8/92 11/92 2/93
5/93
8/93 11/93 2/94
5/94
8/94 11/94
Date
two-dimensional horizontal flow is simulated,
or by multiple layers of cells for simulation of
three-dimensional flow. An approximation of
Equation (
3.8
) is developed for each cell, and
the set of equations for all cells is solved sim-
ultaneously to determine head values at each
node. Depending on the size of the domain,
t here may be t housa nds, even m il l ions, of com-
putational cells. Required input data include
hydraulic conductivity, recharge and other
sources or sinks, aquifer geometry, initial
head values, and boundary conditions. No-flow
boundaries are the usual default boundary
type; constant head boundaries and boundar-
ies where flux is constant or varies over time
or as a function of head can also be used, as
explained in documentation for MODFLOW-
2005 (Harbaugh,
2005
).
Groundwater-flow models use different
approaches for simulating diffuse and focused
recharge. Diffuse recharge is usually assigned
as a constant flux (i.e. a volumetric flow per
unit horizontal surface area, L/T) to a cell rep-
resenting the water table; within MODFLOW-
2005, diffuse recharge is simulated by using
the Recharge Package. Focused recharge from
a stream or lake or other surface-water body
occurs to model cells that underlie those sur-
face features and is typically simulated with
fixed head or head-dependent boundaries;
0.50
0.1 m
0.25
0
0.50
0.25
1 m
0
Date
Figure 3.10
Observed (dots) and simulated (lines) soil-
water contents at site P, depths of 0.1 and 1 m (Zhang
et al
.,
1999
).
has been used in many studies, including
Cooley (
1979
;
1983
), Boonstra and Bhutta (
1996
),
Tiedeman
et al
. (
1997
), Sanford
et al
. (
2004
), and
Dripps
et al
. (
2006
).
Most groundwater-flow models solve
Equation (
3.8
) numerically using either the
finite-difference or finite-element method.
The simulated domain is discretized into a
grid of computational cells and nodes (
Figure
3.11
). The groundwater system may be repre-
sented by a single layer of cells, in which case
