Application of MODFLOW
MODFLOW simulates ground-water
flow in aquifer systems using the finitedifference
method. In this method, an
aquifer system is divided into rectangular
blocks by a grid (fig. 2). The grid of
blocks is organized by rows, columns,
and layers, and each block is commonly
called a “cell.”
Model Input
For each cell within the volume of the
aquifer system, the user must specify
aquifer properties. Also, the user specifies
information relating to wells, rivers, and
other inflow and outflow features for cells
corresponding to the location of the
features. For example, if the interaction
between a river and an aquifer system is
simulated, then for each cell traversed by
the river, input information includes layer,
row, and column indices; river stage; and
hydraulic properties of the river bed.
Model Output
MODFLOW uses the input to
construct and solve equations of groundwater
flow in the aquifer system. The
solution consists of head (ground-water
level) at every cell in the aquifer system
(except for cells where head was specified
as known in the input data sets) at
intervals called “time steps.” The head
can be printed and (or) saved on a
computer storage device for any time
step. Hydrologists commonly use water
levels from a model layer to construct
contour maps for comparison with similar
maps drawn from field data. They also
compare computed water levels at individual cells with measured water
levels from wells at corresponding
locations to determine model error (fig. 3).
The process of adjusting the model input
values to reduce the model error is
referred to as model calibration.
In addition to water levels,
MODFLOW prints a water budget for the
entire aquifer system. The budget lists
inflow to and outflow from the aquifer
system for all hydrologic features that
add or remove water.
Other program output consists of flow
rates for each model cell. MODFLOW
can write the flow rates onto a computer
storage device for any hydrologic feature
in a simulation. These cell-by-cell flow
rates commonly are read by postprocessing
programs for detailed analysis
of the simulated ground-water system.