Introduction

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Last modified on Wednesday, January 31, 1996 - 6:01:24 PM

The groundwater flow program MODFLOW is a program which takes one set of numbers, performs a variety of arithmetic operations on them, and reports a second set of numbers. People may chose to interpret the second set of numbers as a set of head and/or drawdown values together with a set of fluxes. However, this interpretation may not always be correct. The results may be

1. physically impossible for an actual flow system,
2. theoretically possible but with no true counterpart in the real world, or
3. an approximation of an actual flow system within a certain range of input parameters.

This document is intended to increase your skill in using MODFLOW. It is a supplement to the original documentation of MODFLOW rather than a replacement for it. It explains how to format the input files for MODFLOW and how to avoid some common errors. It does not describe the theoretical basis for or the inner workings of MODFLOW.

MODFLOW is a widely used groundwater modeling program. It can be used to simulate groundwater flow in one, two, or three, dimensions and can be used for both steady state and transient models. It does not simulate advective transport. However, another widely used program, MT3D, can use the output from MODFLOW for that purpose.

One of the strengths of MODFLOW is that it has a series of packages which each simulate a separate hydrologic process. This makes it possible to use MODFLOW to simulate a wide variety of hydrologic situations. It is relatively easy to update the program by writing a new package. One major difficulty in using MODFLOW is that it can be difficult to create correctly formatted input files for the program. This document is meant to aid anyone from a beginner to an experienced MODFLOW user in the preparation of correctly prepared MODFLOW input files.

Finally, If you wish to write a new package for MODFLOW, I strongly recommend that you document the package as completely as did McDonald and Harbaugh, 1988. The documentation should include

1. A discussion of new theoretical developments (if any)
2. A description of the mathematical basis for the package
3. A comparison of the model results with analytical solutions if such exist
4. Detailed input instructions
5. Examples of input and output files
6. A narrative for each module including
   1. an introductory statement and
   2. a series of numbered statements describing individual processes within the module keyed to numbered comments within the source code and numbered boxes in the flow chart
7. A flow chart including
   1. a list on the side briefly describing the purpose of submodules and flags and
   2. numbered boxes keyed to numbered comments in the source code and numbered descriptions in the narrative
8. Source code including numbered comments describing the function of individual blocks of code and keyed to numbered descriptions in the narrative numbered boxes in the flow chart
9. A complete list of variables for each module and submodule.

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