A BRIEF DESCRIPTION OF THE U.S.G.S. MODFLOW PROGRAM AND AUXILIARY PROGRAMS

The U.S.G.S. MODFLOW program is a widely used finite-difference three-dimensional ground water flow model. The latest version of the source code is available from the U.S. Geological Survey. Executable versions of the original MODFLOW program are available from several different vendors, as well as some modified versions of the program, auxiliary programs that work directly with MODFLOW, programs for particle tracking, contaminant transport, parameter estimation and optimization, and pre- and postprocessing software.

USGS MODFLOW Package (as documented in McDonald and Harbaugh, 1988)

MODFLOW is a finite-difference ground-water model to simulate two-dimensional areal or cross sectional, and quasi or fully-three-dimensional, transient flow in anisotropic, heterogeneous, layered aquifer systems. The model is based on a block-centered finite-difference approach, using variable grid spacing in x-,y-, and z-direction. Layers may be simulated as (semi-) confined, unconfined, or convertible between the two conditions. The model can also handle layers that pinch out (representing aquifers, aquitards, or layers within an aquifer). The model allows for analysis of external influences such as constant and time-varying wells, areal recharge, drains, evapotranspiration, and streams. Furthermore, MODFLOW has a full implementation of 1st, 2nd, and 3rd type boundary conditions, both constant and varying in time.

The original (or standard) MODFLOW program consists of a main program and a series of packages: the Basic Package, the Block Centered Flow package, the well package, the recharge package, the drain package, the evapotranspiration package, the river package, and two solvers, the Strongly Implicit Procedure solver (SIP) and the Slice-Successive Over-Relaxation solver (SSOR). The following section provides a short description of these packages:

BAS -- Basic Package -- handles those tasks that are part of the model as a whole. Among those tasks are specification of boundaries, determination of time-step length, establishment of initial conditions, and printing of results;

BCF -- Block-Centered Flow Package -- calculates terms of finite difference equations which represents flow within porous medium; specifically flow from cell to cell and flow into storage;

WEL -- Well Package -- adds terms representing flow to wells to the finite difference equations;

RCH -- Recharge Package -- adds terms representing areally distributed recharge to the finite difference equations;

RIV -- River Package -- adds terms representing flow to rivers to the finite difference equations;

DRN -- Drain Package -- adds terms representing flow to drains to the finite difference equations;

EVT -- Evapotranspiration Package -- adds terms representing evapotranspiration to the finite difference equations;

GHB -- General-Head Boundary Package -- adds terms representing general-head boundaries to the finite difference equations;

SIP -- Strongly Implicit Procedure Package -- iteratively solves the system of finite difference equations using the Strongly Implicit Procedure; and

SOR -- Slice-Successive Over-Relaxation Package -- iteratively solves the system of finite difference equations using the Slice-Successive Over-Relaxation technique (SSOR).

The original MODFLOW package from the U.S.G.S. includes only source code files and does not come with a preprocessor to prepare input data files, or any postprocessing software to prepare graphics of simulated heads, drawdown, etc.

Since MODFLOW's publication in 1988, the U.S. Geological Survey has released several add-on packages. To use these packages, they have to be integrated into the MODFLOW runtime versions, requiring source code modification and recompilation. Additional packages often included in commercially distributed versions of MODFLOW include:

PCG2 -- Preconditioned Conjugate-Gradient 2 Solution Procedure Package (Hill, 1990) iteratively solves the finite difference equations using a variant of the Preconditioned Conjugate-Gradient technique.

STR1 -- Streamflow Routing Package (Prudic, 1989) facilitates interaction between ground water and surface water while accounting for the amount of flow (runoff) in streams using the Manning formula.

BCF2 -- Block-Centered Flow 2 Package allows converting no-flow cells to variable-head cells (McDonald et al., 1991);

BCF3 -- Block-Centered Flow 3 Package includes alternative interblock transmissivity formulations (Goode and Appel, 1992). BCF3 an update of the original BCF package and requires the BCF2 package installed.

HFB1 -- Horizontal Flow Barrier Package (Hsieh and Freckleton, 1993) for simulation of thin, vertically oriented, low permeability geologic features that impede the horizontal flow of ground water.

TLK1 -- Transient Leakage Package (Leake et al., 1994) for simulating transient leakage and ground-water storage changes in compressible confining units using efficient integro-differential equations.

GFD1 -- General Finite Difference Flow Package (Harbaugh, 1992) is a generalized finite-difference formulation to facilitate user input of conductance terms in stead of MODFLOW calculating them; if used, the package replaces the BCF package (only one internal flow package can be in use at a time).

IBS1 -- Interbed Storage Package (Leake and Prudic, 1991) simulates compaction of compressible fine-grained beds within or adjacent to aquifers due to aquifer pumping.

CHD1 -- Time-Variant Specified Head Boundaries Package (Leake and Prudic, 1991) allows specification of time varying boundary conditions as part of the initial input..

Other MODFLOW Packages

Other MODFLOW versions

Various extended DOS versions of MODFLOW are available from commercial vendors, including Geraghty & Miller, International Ground Water Modeling Center (IGWMC), and Scientific Software. Other versions of MODFLOW include:

Stand-alone preprocessors/postprocessor/shells for MODFLOW

Links between GIS systems and MODFLOW

Particle Tracking programs that link with MODFLOW

Contaminant Transport programs that link with MODFLOW

Parameter Estimation

Optimization

Fore more information contact:

International Ground Water Modeling Center
Colorado School of Mines, Golden, CO 80401
phone: 303/273-3103, fax: 303/384-2037
email: igwmc@mines.edu

Prepared by Paul K.M. van der Heijde, July 1996