It is often necessary to import a MODFLOW simulation that was not generated by GMS. Since GMS uses the standard MODFLOW file format, this is not a problem in most cases. However, there are a few steps and precautions that should be taken.
GMS 3.0 uses the standard MODFLOW file formats described in the MODFLOW documentation (McDonald & Harbaugh, 1988). These formats use a fixed field approach where each number must be properly aligned in the correct column. The files you are importing must match this format exactly. If the files were generated for a version of MODFLOW that has been modified to accept free-field formatted files, the files may need to be edited before they can be successfully imported to GMS 3.0.
GMS 3.1 interfaces with MODFLOW96. The MODFLOW96 used with GMS requires all input files to be in free format. GMS 3.1 is able to import MODFLOW88 and MODFLOW96 files.
GMS 4.0 and later versions interface with MODFLOW2000. All input files are saved and used in free format. GMS 4.0 and 5.0 also are able to import MODFLOW88 and MODFLOW96 files.
GMS uses a MODFLOW super file to organize the files used by a MODFLOW simulation. This file is not a standard MODFLOW file. One approach to importing an externally defined simulation is to create a MODFLOW super file containing the name of the MODFLOW name file using a text editor. A MODFLOW96 and MODFLOW2000 can be imported by simply opening the namefile. A simpler approach to importing the simulation is available if the MODFLOW simulation is MODFLOW88. This approach involves opening the Basic package file first. Then if necessary, the user will be prompted for the location of the other MODFLOW88 files. The Basic file is opened by selecting the Open command from the File menu.
MODFLOW96 and MODFLOW2000 both use a name file. GMS 4.0 and later versions can read this namefile if the extension is *.mfn or *.nam .
MODFLOW allows multiple sources/sinks (rivers, wells, drains, general head) to be assigned to a single cell. With a transient simulation, the sources/sinks are listed in the package files on a stress period by stress period basis.
In the case of rivers, drains, and general head, the source/sink typically remains active during the entire duration of the simulation. In the case of wells, however, it is not uncommon for a well to be active (turned on) during part of the simulation and inactive (turned off) during part of the simulation. A well which is inactive at a given stress period can be specified in the input file one of two ways: the pumping rate can be specified as zero or the well can simply be omitted for the stress period. When GMS imports a well file, it generates a time series of the pumping rate (for ease of editing) for each well.
If a cell contains multiple wells and some of the wells are missing for some of the stress periods, it may be impossible for GMS to generate the time series. For example, if a cell contains three wells during one stress period but only one during the next stress period, it is impossible to determine which of the three original wells the remaining well corresponds to.
When GMS exports a Well package file, this problem is avoided by always including all wells for each stress period and using a zero pumping rate to indicate inactive wells.
Related Links:
Reading a MODFLOW Simulation
MODFLOW Interface
Packages Supported in GMS