In Reply Refer To: April 17, 1992 Mail Stop 411 OFFICE OF GROUND WATER TECHNICAL MEMORANDUM NO. 92.08 Subject: PUBLICATIONS--Release of Open-File Report 91-494, "A Generalized Finite-Difference Formulation for the U.S. Geological Survey Modular Three-Dimensional Finite-Difference Ground-Water Flow Model" by Arlen W. Harbaugh The subject report describes the Generalized Finite-Difference (GFD) Package, which is a new option for the U.S. Geological Survey ground-water flow model, MODFLOW. The original model assumes that model nodes are in the center of cells and that transmissivity is constant within a cell. Based on these assumptions, the model calculates coefficients, called conductance, that are multiplied by head difference to determine flow between cells. Although these are common assumptions in finite-difference models, other assumptions are possible. The GFD Package reads conductance as input data, which allows the user to calculate conductance using any desired assumptions. There are a number of possible applications, such as radial flow simulation. The GFD program can be retrieved from the QVARSA Prime computer. On any Water Resources Division Prime computer, use the following commands to retrieve the source code and calling statements that must be added to the main program. FTR MODFLOW>GFD1.F77 FTS_DEPOT>== -SS QVARSA FTR MODFLOW>GFD1.CALL FTS_DEPOT>== -SS QVARSA Additionally, a complete version of MODFLOW for use on the Data General workstations is available as described in the MODELING Continuum (entry 106). This version of the model contains the GFD Package as well as several other options that have been published since the original model. We recommend that this report be brought to the attention of technical and management personnel concerned with ground-water modeling studies. Additional copies of the report are available from the Office of Ground Water. Joseph S. Rosenshein Acting Chief, Office of Ground Water Attachment Distribution: A and B (memo only) S, FO, PO (memo and report)PUBLICATIONS--Release of Open-File Report 91-494, "A Generalized Finite-Difference Formulation for the U.S. Geological Survey Modular Three-Dimensional Finite-Difference Ground-Water Flow Model"