Modification to the USGS Modular Flow Model


             UNITED STATES DEPARTMENT OF THE INTERIOR
                        GEOLOGICAL SURVEY
                        RESTON, VA. 22092

In Reply Refer To:                            June 24, 1985
WGS-Mail Stop 411


GROUND WATER BRANCH TECHNICAL MEMORANDUM NO. 85.04

Subject:  PUBLICATIONS--Modification to the USGS Modular Flow
                        Model

After a year of experience with the U.S. Geological Survey Modular
Ground Water Flow Model we have identified a number of
modifications which should be made to the program.  The
modifications are listed on attachment A.  The modification to
module BCF1FM corrects a problem that could cause significant
errors when simulating a multi-layer system in which the top layer
and at least one other layer are convertible from confined to
unconfined (LAYCON=2 or 3).  The problem could cause incorrect
results to be calculated without warning messages.  It will,
however, cause an imbalance in the volumetric budget.

The other modifications correct problems that were encountered on
a few computers.  Most compilers are robust enough to circumvent
the problems.  Some compilers, however, detect the problems and
generate compiler diagnostics.  Probably none of these problems
will have caused erroneous results to be calculated.

The Open-File report 83-875 was intended as an interim report.  We
intend to prepare the final report within the next few months.  If
you have found errors in the Open-File report we would like to
hear about them.  Please write to:

          Michael G. McDonald
          U. S. Geological Survey Mail Stop 433
          12201 Sunrise Valley Drive
          Reston, Virginia 22092

If you gave a copy of the model to other people please forward a
copy of this memo and the attachments to them too.



                                   (s) Eugene P. Patten
                                   Chief, Ground Water Branch

WRD Distribution:  A, B, S, FO, PO