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MODFLOW - the Modular Three-Dimensional Finite-Difference Ground-Water Flow Model
Entry ID: modflow-99
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Abstract: The modular finite-difference ground-water flow model (MODFLOW) developed by
the U.S. Geological Survey (USGS) is a computer program for simulating common
features in ground-water systems. MODFLOW can be applied as a one-dimensional,
two-dimensional, or quasi-or full three-dimensional model. The modular program
design of MODFLOW allows for new simulation features to be added.
MODFLOW is ... designed to simulate aquifer systems in which (1) saturated-flow
conditions exist, (2) Darcy's Law applies, (3) the density of ground water is
constant, and (4) the principal directions of horizontal hydraulic conductivity
or transmissivity do not vary within the system. In systems where these 4
conditions are met, MODFLOW can simulate a wide variety of hydrologic features:
steady-state and transient flow can be simulated in unconfined aquifers,
confined aquifers, and confining units.
MODFLOW uses specified input to construct and solve equations of ground-water
flow in the aquifer system. The solution consists of head (ground-water level)
at every cell in the aquifer system at intervals called &time steps.& The head
can be printed and (or) saved on a computer storage device for any time step.
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
post-processing programs for detailed analysis of the simulated ground-water
The ground-water flow equation is solved using the finite- difference
approximation. The flow region is subdivided into blocks in which the medium
properties are assumed to be uniform. In plan view the blocks are made from a
grid of mutually perpendicular lines that may be variably spaced. Model layers
can have varying thickness. A flow equation is written for each block, called
a cell. Several solvers are provided for solving the resulting matrix problem;
the user can choose the best solver for the particular problem. Flow-rate and
cumulative-volume balances from each type of inflow and outflow are computed
for each time step.
ISO Topic Category
Quality Modflow is best suited for higher education students, scientists and
professionals seeking a utility to model ground water flow.
Use Constraints In order to use MODFLOW, initial conditions, hydraulic properties, and
stresses must be specified for every model cell in the finite-difference grid.
MODFLOW-96 is written in Fortran 77 with the following extension: use of
variable names longer than 6 characters. By default, the software is
dimensioned for use with models having up to 90,000 cells. ... MODFLOW-96 requires
that file names needed for the simulation be defined prior to execution. A
Name File is used for this purpose. Each record of the Name File specifies a
file type, unit number, and file name for each file used in the MODFLOW
simulation. The format of this file is described in the documentation.
Generally, the program is easily installed on most computer systems. The code
has been used on UNIX-based computer and DOS-based 386 or greater computers
having a math coprocessor and 4 mb of memory.
Belitz, K., and Phillips, S.P., 1993, Numerical simulation of
ground-water flow in the central part of the western San Joaquin
Valley, California: U.S. Geological Survey Water-Supply Paper 2396, 69
Prince, K.R., Franke, O.L., and Reilly, T.E., 1988, Quantitative
assessment of the shallow ground-water flow system associated with
Connetquot Brook, Long Island, New York: U.S. Geological Survey
Water-Supply Paper 2309, 28 p.
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