User's documentation for MODFLOW-96, an update to the U.S. Geological Survey modular finite-difference ground-water flow model

Abstract

A number of changes have been made to the U.S. Geological Survey modular finite-difference ground-water flow model, which is commonly known as MODFLOW. Existing MODFLOW input files will work with the revised model. Also, the basic structure and computational methods of the model have been maintained. Those familiar with the original model should have no difficulty adapting to the revised model. Some of the most significant changes are: 1. All files are opened with Fortran OPEN statements. The user must prepare an additional file that specifies the model files to be opened. 2. The array-reading subroutines have been enhanced to make it easier to prepare the array control records. When using the new control record, all the parameters are read using free format; however, the original fixed-format control records can still be used if desired. 3. For input not read with array-reading subroutines, an option to use free format has been added. This avoids the need to place data values in specified columns of input records, and thus will prevent the common error of entering data in incorrect columns. If this option is not used, data are read using the original fixed format fields. 4. The capability to produce an output listing that is 80 columns wide (rather than 132) has been added in order to facilitate printing on 8.5-inch wide paper and viewing on a video display screen. All the miscellaneous output has been reduced to 80 columns, and several new formats that are 80 columns wide have been added to the array-printing subroutines. 5. A new method for specifying output control has been added. This new method uses words rather than numeric codes for specifying the various output options. New options have also been added such as the ability to save calculated heads in formatted files and save budget data in more compact unformatted files. The old method of specifying output control can still be used. 6. Budget calculations have been changed to double precision, which improves the accuracy of budget calculations. Inaccuracy of budget calculations was sometimes noticeable in the original model when a grid of 100,000 or more cells was used or when a large conductance was used in a head-dependent stress term. 1U.S. Geological Survey, Reston, VA 2McDonald Morrissey Associates, Reston, VA