UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY RESTON, VA. 22092 In Reply Refer To: November 18, 1982 Mail Stop 411 GROUND WATER BRANCH TECHNICAL MEMORANDUM NO. 83.02 Subject: PUBLICATIONS--"Modifications and corrections to the finite-difference model for simulation of three-dimensional ground-water flow" by Lynn J. Torak The report accompanying this memorandum describes modifications that have been incorporated into the finite-difference model for simulation of three-dimensional ground-water flow by Trescott (1975) and Trescott and Larson (1976). These changes extend the application of this model to problems involving head-dependent sources and sinks (i.e., rivers, evapotranspiration, and springs or drains). Changes were also made which may enhance convergence of an iterative solution by the Strongly Implicit Procedure (SIP). This report contains other corrections to the model which eliminate errors in (a) formulating coefficients for water-table simulations using the Basic 3-D approach (Equation 3); (b) formulating coefficients in the vertical direction when the Quasi 3-D approach is taken (Equation 4, Trescott, 1975, with TK values read in); (c) the computation of vertical flow; and (d) punched output format of the PHI matrix. The report first describes the nature of the modifications that Lynn made to the 3-D model, his personal experiences with their implementation, and additional data-input instructions. This is followed by a description of errors inherent to the 3-D model and corrections that were made. This report describes two versions of the 3-D model. One version contains the corrections and modifications described in this report; the other version contains only the corrections to the standard 3-D model as it is described in Trescott (1975) and Trescott and Larson (1976). Both versions reside on the ADMAHL computer in Reston. The modified and corrected version of the 3-D model may be accessed as DSN=VG4E91L.HEDDDEP.FORTGI.FORT/,UNIT=3330- 1,VOL=SER=SYS312,DISP=SHR The corrected version of the 3-D model may be accessed as DSN=VG4E91L.STANDTD.FORTGI.FORT/,UNIT=3330- 1,VOL=SER=SYS010,DISP=SHR All program statements of modifications and corrections are identified in these versions by an asterisk (*) or a digit (0-9) in column 80. The original program sequence numbers appearing in columns 73-79 were preserved whenever possible so that a comparison of the modified and/or corrected version with the original model can easily be made. Additional copies of this report are available from the Ground Water Branch. (s) Eugene P. Patten Acting Chief, Ground Water Branch Attachment WRD Distribution: A (w/o attachment) B (w/o attachment) S (w/o attachment) PO (w/o attachment) FO (with attachment)"Modifications and corrections to the finite-difference model for simulation of three-dimensional ground-water flow"