1.   Purpose and objectives

     The MODFLOW model simulates groundwater flow in a porous medium in three dimensions. It is designed for use by experienced groundwater hydrologists. A modular structure was used for the program and documentation in order to make the model easy to understand and easy to  modify when necessary.

2.   Description
      
     The MODFLOW model uses the finite-difference approximation to simulate transient, or time-varying, flow in three dimensions in a heterogeneous and anisotropic porous medium. Steady-state conditions can also be simulated. The principal axes of hydraulic conductivity must be aligned with the 
coordinate directions, and water of constant density is assumed. The time  derivative is evaluated using the backward difference approximation. Wells, rivers, drains, evapotranspiration, and recharge can be simulated and are represented as head dependent source or sink terms in which the head outside the model is user specified.  Layered aquifers can be represented in the so-called quasi-three-dimensional approximation. Nodes in this approximation can change from being confined to unconfined, and vice versa, as the computation progresses.

3.   Input

     Aquifer hydraulic parameters, boundary conditions, initial conditions, and stresses are required model input. The input is from text files with the data laid out in a prescribed order and format. The input data must correspond to the specified grid structure. Many MODFLOW users make use of separate programs to help discretize the data and lay them out in the required form. However, no such support programs are included as part of MODFLOW itself.

4.   Output

     The primary model output is the head at each model node. In addition a water budget is calculated, and the flow through each model cell can be stored in a disk file. As is the case with input, support programs are often used to analyze and display the output, though again these are not included in MODFLOW.

5.   Operational requirements and restrictions

     MODFLOW is written in the FORTRAN 77 language. It can run on a wide range of computers from personal computers to super computers. A math coprocessor is strongly recommended for' performance reasons. It should be noted that some FORTRAN compilers require a math coprocessor. The computational time and memory requirements depend on the size of the problem being simulated.

6.   Form of presentation

     The source code in FORTRAN 77 is supplied on a 3 1/2" floppy diskette or via the Internet, together with the MODFLOW documentation report in English (see reference).

7.   Operational experience

     MODFLOW, which is probably the most widely used groundwater model in the world, today, has been used extensively inside and outside of the U.S. Geological Survey since 1983.

8.   Originator and technical support

     U.S. Geological Survey, Office of Ground Water.

9.   Availability

     From the HOMS National Reference Centre for USA for 3 1/2" floppy or printed documentation; or electronically on the Internet at
http://water.usgs.gov/software/ or http://www.geogr.uni-jena.de/software/

10.  Conditions on use

     Program and electronic version of the documentation are available free of charge; there is a charge for printed documentation.

Reference:

McDonald, M. G. and Harbaugh, A. W., 1988, A modular three-dimensional finite-difference groundwater flow model: U.S. Geological Survey, Techniques of Water Resources Investigations, Book 6, Chapter Al, 586 pp.