GROUND WATER - Corrections to computer code for modeling three-dimensional ground-water flow.



             UNITED STATES DEPARTMENT OF THE INTERIOR
                         GEOLOGICAL SURVEY
                      RESTON, VIRGINIA  22092

                                                       GW Branch

                                                    May 12, 1977
                                                  Code 4351 5016

GROUND WATER BRANCH TECHNICAL MEMORANDUM NO. 77.06

Subject:  GROUND WATER - Corrections to computer code for modeling
                         three-dimensional ground-water flow.

Documentation of the subject computer model was given in the
reports that accompanied Ground Water Branch Technical Memoranda
Nos. 76.05 and 77.02.  These reports are:

     Trescott, Peter C., 1975, Documentation of finite-
difference model for simulation of three-dimensional
ground-water flow:  U.S. Geol. Survey Open-File Rept.
75-438, 32 p.

     Trescott, P. C., and Larson, S. P., 1976, Supplement to
Open-File Report 75-438, Documentation of finite-
difference model for simulation of three-dimensional
ground-water flow:  U.S. Geol. Survey Open-File Rept.
76-591.

Peter C. Trescott and Steven P. Larson have determined that the
following changes need to made to the
computer code and documentation:

I.  For a problem where the modified 3-D finite-difference
equation is used (i.e., T values are used)
and TK(I,J,K) values are not read with data input but are
calculated in the COEF subroutine, the FACT(K,3) multiplication
factor (see p. III-5 in 75-438) should be input as
FACT(K,3)/[delta]Z(K).  As an example, consider a problem for
which,

     (1)  FACT(K,1) = FACT(K,2) = 2.0;
     (2)  vertical hydraulic conductivity is 10 times less
          than it is in the horizontal plane;
     (3)  [delta]Z(K) = 100 m;
     (4)  TK(I,J,K) is to be calculated in the code; then
          FACT(K,3) = 0.2/100. = 0.002

This correction does not apply if TK(I,J,K) values are read from
cards for a problem using the modified 3-D finite-difference
equation nor for any problem using the basic 3-D finite difference
equation.

II.  In the COEF subroutine, replace

          DATA N3/1/                    C0F 195
with
          N3 = 1                         C0F 195

Please bring this memorandum to the attention of all personnel
concerned with modeling ground-water systems.  Any
questions regarding these corrections should be directed to
Messrs. Trescott or Larson.



                         (s) Charles A. Appel
                         Charles A. Appel
                         (for) Chief, Ground Water Branch

WRD Distribution:  A, S, B (Limited), FO