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), FOGROUND WATER - Corrections to computer code for modeling three-dimensional ground-water flow.