Transactions on Ecology and the Environment vol 10, © 1996 WIT Press, www.witpress.com, ISSN 1743-3541 A comparison of 2-D and 3-D pre- and post- processing techniques in groundwater flow modeling S.A. Sorby, A.S. Mayer, C.G. Tallman, J.G. Johnson Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan, USA Abstract In the past, the use of 3-D groundwater models has been hindered by the lack of adequate pre- and post-processors for entering data and interpreting results. In order to overcome these difficulties, an interface between I-DEAS solid modeling software and MODFLOW has been developed. The pre-processing capabilities of the software interface allow the user to build a geologic model from field data, discretize the model into a finite-difference grid and define boundary conditions and material properties on the grid. Simulation is accomplished by the execution of MODFLOW within the framework. The results of the simulation can then be visualized with 3-D color images. This paper describes the development and application of the software interface with full 3-D capabilities. The software interface is demonstrated with an example problem. The same problem is analyzed by "traditional" 2-D methods using popular 2-D pre- and post-processors. A comparison is made between these two types of analysis. 1 Introduction Groundwater modeling is an increasingly important area in environmental engineering, in response to the deterioration of groundwater quality. For most problems encountered in groundwater modeling, numerical methods have become widely recognized and accepted means of solution. One of the disadvantages of groundwater modeling methods is the vast amount of data required to solve and interpret problems. One of the most widely accepted Transactions on Ecology and the Environment vol 10, © 1996 WIT Press, www.witpress.com, ISSN 1743-3541 226 Computer Techniques in Environmental Studies finite-difference codes for groundwater modeling is MODFLOW (McDonald and Harbaugh, 1988), which was developed by the USGS. While this model is a powerful analysis tool, use of this and other public domain codes is limited by a lack of an adequate, graphical pre- and post-processor. This deficiency makes the groundwater modeling task cumbersome, and results are often difficult to interpret. The lack of an adequate pre- and post-processor for three-dimensional groundwater flow analysis has serious implications, because several simulations often are required for an effective characterization of a groundwater flow problem. Multiple simulations are needed because it is often the case that the input data is uncertain and/or incomplete. Thus, there is often a need to perform a series of simulations to test the sensitivity of the results as a function of the possible range of data values. An additional limiting factor is the ability to visualize the output results, that is, the patterns of groundwater flow as functions of space and time. A typical model simulation results in the output of hundreds to tens of thousands of numerical values. The objective of this project, is to develop a software interface between a groundwater modeling code (MODFLOW) and a fully interactive, graphical pre- and post-processor in order to enhance the groundwater modeling visualization capabilities and increase the productivity of current groundwater modeling efforts. The most widely-used groundwater flow model in the U.S. is MODFLOW (McDonald and Harbaugh, 1988). The MODFLOW model is based on a three- dimensional, finite difference solution to the groundwater flow equation. The model can be used to simulate flow in unconfined and confined aquifers and provides for flows associated with external stresses such as wells, areal recharge, evapotranspiration, drains and streams. The model source code is written in FORTRAN. The MODFLOW model was originally written in 1984, but has been updated as progress has been made in the field of groundwater flow modeling. Application of the MODFLOW model involves the superposition of a finite difference grid over the groundwater aquifer of interest. The aquifer is thus subdivided into a series of rectangular or cubic cells, defined by nodes at the center of the cell or at each corner. The unknown of interest, i.e., groundwater pressure head, is solved for at each nodal location. For groundwater flow solutions, the following parameters must be defined at each node or cell: hydraulic conductivities for each coordinate dimension, aquifer thickness, aquifer storage coefficients and withdrawal or injection rates. In addition, boundary and initial conditions must be defined as needed. I-DEAS (Integrated Design Engineering Analysis Software) was developed Transactions on Ecology and the Environment vol 10, © 1996 WIT Press, www.witpress.com, ISSN 1743-3541 Computer Techniques in Environmental Studies 227 by the Structural Dynamics Research Corporation (SDRC, 1992) in the early 1980s as a geometric modeler and mesh generator for finite-element analysis. The geometric database manager in I-DEAS enables a user to create a file translator for use with virtually any other external finite element or finite difference program. Data files for external solvers can be written from I-DEAS and converted to their required format for execution. If the source code is available, the external solver can also be modified to output a file which contains the results of the numerical analysis in standard I-DEAS format. In this way, the user can take full advantage of the meshing and post-processing capabilities of I-DEAS while still using the solver which is most suited to his or her needs. 2 Framework Development In this project, a framework has been developed to link the MODFLOW code with I-DEAS software. The framework has been developed in the following steps. First, programs were written using the I-DEAS programming language for automation of I-DEAS pre-processing capabilities including the creation of a geologic model from raw site data, application of a three-dimensional finite- difference grid to this model and designation of all boundary and initial conditions, external stresses, and material properties. Second, additional programs were developed to write data from I-DEAS into the format needed for input to MODFLOW. These programs are written in the C programming language and they can be executed directly from within I-DEAS. Third, the source code for MODFLOW was modified so that the results of the groundwater analysis can be output to I-DEAS readable files in addition to the standard MODFLOW output files. Finally, a program was created to run within I-DEAS to read in the results of the numerical analysis and automate some of the I-DEAS post-processing commands. The pre-processor allows the user to graphically build a three-dimensional geologic model and designate relevant hydrogeologic properties. The pre- processor was constructed using I-DEAS programming language and is menu- and query-driven. The programs written in this manner are invisible to the user. I-DEAS programming files are written as a combination of I-DEAS menu picks, menu and input commands (where users are queried for input data) and statements which are similar to typical FORTRAN statements. A portion of an I-DEAS program file is shown in Figure 1. Transactions on Ecology and the Environment vol 10, © 1996 WIT Press, www.witpress.com, ISSN 1743-3541 228 Computer Techniques in Environmental Studies K : #pmod: K : #menu "Parameter to Modify:" choice 0 4 , K : "A-AIl" "L-Stress Period Length" , K : "N-Number of Time Steps" "TSM-Time Step Multiplier" K : #if (Z_INP_STAT eq 0) then goto sp K : #if (ZJNP.STAT eq 1) then goto imod K : #if(Z_INP_STAT ne 3) then goto pmod K : #if (choice eq 1) then #modfl=l; #index=num; #goto len C : ** get a new value for the parameter K : #nv: K : #input "Enter new value:" newv K : #if (Z_INP_STAT eq 0) then goto imod K : #if (Z_INP_STAT eq 1) then goto pmod K : #if (Z_INP_STAT ne 3) then goto nv K : #if (choice eq 2) then #perlen=newv; #goto writ K : #if (choice eq 3) then #nstp=newv; #goto writ K : #tsmult=newv C:** write new data K : tfwrit: K : /MO LAB 1 K:DON K : XF perlcn K : YF nstp K : ZF tsmult Figure 1: Example I-DEAS Program File In this program file, lines beginning with C : are comments, and lines beginning with K : are command lines. Command lines which do not start with a # are strings of I-DEAS menu picks. Menu commands insert a menu on the screen which is identical to standard I-DEAS menus from which the user can graphically pick options. Input commands query users at the I-DEAS prompt line and assign a variable name to the user input. I-DEAS program files are written so that there is a main program and several sub-routines. The data file format required by the groundwater model was analyzed and a file translator program was created to be run from within I-DEAS. The file translator draws upon the grid, material property, and other data generated from pre-processing and writes out the data files suitable for direct input to MODFLOW. This activity is also invisible to the user. The user can then execute MODFLOW from within the framework, without exiting I-DEAS. A link between the output of the groundwater modeling code and I-DEAS was developed. The link involves the addition of subroutines to the source code of the model such that a universal file is generated as a part of its output. Universal files are ASCII files which contain geometric information in a format which is acceptable to I-DEAS. In this way, the results of the groundwater modeling can be read directly into I-DEAS for post-processing.