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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.

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Description: Several versions of MODFLOW have been released. The original version is
referred to as MODFLOW-88. MODFLOW-96 contains a variety of enhancements. The
most visible enhancements involve data input. MODFLOW-2000 is a major update
that fully integrates parameter estimation.

Description: MODFLOW-2000 simulates steady and nonsteady flow in an irregularly shaped flow
system in which aquifer layers can be confined, unconfined, or a combination of
confined and unconfined. Flow from external stresses, such as flow to wells,
areal recharge, evapotranspiration, flow to drains, and flow through river
beds, can be simulated. Hydraulic conductivities or transmissivities for any
layer may differ spatially and be anisotropic (restricted to having the
principal directions aligned with the grid axes), and the storage coefficient
may be heterogeneous. Specified head and specified flux boundaries can be
simulated as can a head dependent flux across the model's outer boundary that
allows water to be supplied to a boundary block in the modeled area at a rate
proportional to the current head difference between a "source" of water outside
the modeled area and the boundary block. MODFLOW is currently the most used
numerical model in the U.S. Geological Survey for ground- water flow problems.

In addition to simulating ground-water flow, the scope of MODFLOW-2000 has been
expanded to incorporate related capabilities such as solute transport and
parameter estimation.

Description: MODFLOW is a three-dimensional finite-difference ground- water flow model. It
has a modular structure that allows it to be easily modified to adapt the code
for a particular application. Many new capabilities have been added to the
original model. OFR 96-485 (complete reference below) documents a general
update to MODFLOW, which is called MODFLOW-96 in order to distinguish it from
earlier versions.

Description: MODFLOW is a three-dimensional finite-difference ground-water flow model. It
has a modular structure that allows it to be easily modified to adapt the code
for a particular application. Many new capabilities have been added to the
original model. This version includes all the major capabilities that were
documented as of September 1996.
Service Citation
Originators: McDonald, M.G., and Harbaugh, A.W.
Provider: U.S. Geological Survey
ISO Topic Category
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.
Service Provider
Ground Water, Software, Water Resources Division, U.S. Geological Survey, U.S. Department of the Interior

Service Provider Personnel
Phone: 703-648-4000;1-800-426-9000
Email: h2oteam at
Contact Address:
USGS National Center
Water Resources Division
12201 Sunrise Valley Drive
City: Reston
Province or State: VA
Postal Code: 20192-0002
Country: USA
Distribution Media
Distribution_Media: electronic or ftp
Distribution_Size: 590K to 853K depending on platform
Distribution_Format: self extracting DOS executable or UNIX tar file
Phone: (301) 614-6898
Fax: 301-614-5268
Email: Tyler.B.Stevens at
Contact Address:
NASA Goddard Space Flight Center
Global Change Master Directory
City: Greenbelt
Province or State: MD
Postal Code: 20771
Country: USA
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.
Creation and Review Dates
SERF Creation Date: 1999-10-13
SERF Last Revision Date: 2008-05-29
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