A Geographic Information System (GIS) was used to expedite assembly of input‐data sets for a model of ground‐water flow in the Middle Patuxent River basin in Howard County, Maryland. The model grid was developed with GIS and used to select attributes from GIS data coverages. These attributes were then output from GIS into the input‐data sets, which included model boundaries, regolith thickness, bedrock‐surface altitude, stream locations, stream length, stream‐bottom altitude, and trans‐missivity. The ability to change large sets of spatial data quickly and accurately with GIS enhances the model‐calibration process. (KEY TERMS: GIS; ground water; Piedmont; Maryland; MODFLOW.)