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The documentation is presented for a microcomputer code for solving the two dimension and quasi three dimension formulation of the general ground-water flow equation. PREFACE: GETTING STARTED WITH THE DISKETTE The source diskette contains both the 2 dimension and the 3 dimension programs. There are two directories on the diskette: XY and XYZ. The two programs are in the two directories: 2D in XY, and 3D in XYZ. Each directory contains the Fortran source program. There are also data sets in both directories which are set-up for test problems; the test problems are further explained in the appendices. In addition there is an executable program for MS-DOS computers designated MAIN; the test problem can be run by simply entering MAIN. The diskette contains all the data necessary to execute the codes. When the program executes it generates output: 1. The input data files are written to a file GWDATA where they are available following a run. 2. The head change along with the material balance following each designated time step is written to the screen and to a file named GWLEVEL; a final head is also written to this file. GWLEVEL is available after the simulation run. 3. The final head is also written to a file GWSURF. This final head can be used for input to a graphics package or it can be read as an initial head for a continued simulation. These files require additional disk space. The source diskette containing the two codes is almost entirely full and is write protected. To execute either code the contents of the directories XY or XYZ must be copied to the hard disk or another diskette. The input files must be changed to simulate different problems. The input files can be edited using the MS-DOS line editor EDLIN 1 , or one of any number of word processing programs. The input files are all in a structured format. Care must be taken to preserve the fixed format in editing the input files. One common error is inputing data that violate the read formats of the program; this usually results in a fatal error during execution. All the data are read by one subroutine LOAD: one can check the read formats in the listing of LOAD in the appendix. Should the input files become too scrambled the test program files can be read again from the source diskette. The codes are compiled and linked to run without a mathematics co-processor chip. The programs will run faster with a co-processor; however, the Fortran source code must be recompiled and relinked for use with a co-processor. The codes will accommodate large problems; I have run a 7-layer problem with 40 by 30 cells in each layer within the usual 640K memory limitation on a personal computer (PC). A million-year simulation for this problem took several hours to execute in a 286, AT type PC. 1. The use product names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. INTRODUCTION One of the necessary tools of the ground-water hydrologist is a means to solve the general equation of ground-water flow. With the advent of digital computer space, it is possible to solve the non-homogeneous equations in two or three space dimensions. A number of numerical methods are available for the solution of the general partial differential equation. Microcomputers have now become available to almost any serious hydrologist throughout the world. With this thought in mind I have set out to develop a relatively simple numerical code to solve the non-homogeneous ground-water flow equations in two and three spatial dimensions. In writing the code there were several objectives: 1. The code should run on a 16 bit computer in single precision; that is, no double precision variables. 2. The code should be written in elementary Fortran 77 so that it is readily transferable between various computers. 3. The code should be readily understandable to anyone knowledgeable with computer programming as well as the basic ground-water flow equations. 4. The code should be designed in such a way that modifications and changes can be readily introduced. With these objectives in mind, these codes have been modularized into subroutines that perform specific tasks; with the exception of LOAD, subroutines have been kept to between approximately 50 and 100 statements so that their functions are readily understood. An attempt has been made to relate the code names of variables to names that have meaning to hydrologists. Each of the spatially dimensioned variables is kept in separate, two and three dimensional arrays so that they are readily identifiable with the physical problem. Because the code is written in FORTRAN 77 it can be run on a variety of small systems, including IBM PC compatible, Macintosh, as well as UNIX based systems. The diskette included with this document contains the FORTRAN source code. It has a compiled, executable program for PC compatible systems. For other systems the source code must be compiled. Some of the file numbers may have to be changed to correspond to the requirements of other operating systems; otherwise there are no significant modifications necessary. In solving the numerical problem an iterative technique, SIP, which provides good results with all the variables in single precision, has been utilized. There are some rules-of-thumb to keep in mind in solving the numerical problem; some of them are indicated in the sections below. The general idea is to have a relatively simple basic code which one could grasp and utilize on almost any computer with perhaps an hour or so of familiarization.

Microcomputer codes for simulating transient ground-water flow: in two and three space dimensions