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"

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)