U.S. Geological Survey modpath(1) NAME modpath - A particle-tracking postprocessor model for MODFLOW ABSTRACT MODPATH is a particle-tracking postprocessing package that was developed to compute three-dimensional flow paths using output from steady-state or transient ground-water flow simulations by MODFLOW, the U.S. Geological Survey finite- difference ground-water flow model. The particle tracking package consists of two Fortran computer codes: (1) MODPATH, which calculates particle paths, and (2) MODPATH-PLOT, which displays results graphically. MODPATH is useful as a visualization tool to help understand flow patterns in simulated ground-water flow systems. It is also useful for delineating sources of water to discharge sites and aquifers in systems simulated with MODFLOW. METHOD MODPATH uses a semi-analytical particle tracking scheme that allows an analytical expression of the particle's flow path to be obtained within each finite-difference grid cell. Particle paths are computed by tracking particles from one cell to the next until the particle reaches a boundary, an internal sink/source, or satisfies some other termination criterion. Both steady-state and transient ground-water flow systems can be analyzed with MODPATH. HISTORY Version 5.0 2008/11/5 - Allows binary files larger than 2 gigabytes. Can automatically detect binary head and budget files that are single or double precision. An option was added to track particles outside of the grid as required for performing parameter estimation based on travel time. Version 4.3 2003/07/03 - Fixed problem that prevented MODPATH from working with mixed steady-state and transient stress periods. Fixed problem in which recharge and ET were sometimes applied to the wrong cells when COMPACT BUDGET was used. Fixed problem with displaying contours. Added a check for negative cell thickness. Version 4.2 2001/04/18 - Modified to improve use on personal computers. The code was modified to make it possible to use non-standard compiler options for opening unformatted head and budget files. This can make it possible for MODPATH and MODPATH-PLOT to read head and budget files generated by MODFLOW runfiles that were created by a different compiler than the one used to compile MODPATH and MODPATH-PLOT. Also, a new version of Interacter has been loaded with the PC version of MODPATH-PLOT, which supports more graphics devices. Version 4.1 2000/07/06 - Modified to work with MODFLOW-2000. MODPATH version 4 has been modified to read the cell elevations from the MODFLOW-2000 discretization file. The discretization file also defines the number and length of stress periods, so this information is no longer read from MODPATH's main data file. Another change involves the way MODPATH determines information about stresses. MODPATH versions 3.2 and earlier read the MODFLOW input files for stresses. These were read in order to identify the locations of stresses and the faces to which a stress is applied (variable IFACE). For the WEL and RCH Packages, the stress input files were also read to define the stress flow rate rather than using data in the budget file. MODPATH version 4 no longer reads any stress files. Instead, the stress information is obtained entirely from the budget file. MODFLOW-2000 has been modified to save AUXILIARY variables in the budget file, and MODPATH uses AUXILIARY variable IFACE to define the IFACE variable that was previously read from the MODFLOW input files for stresses. Version 3.2 1999/06/14 - Dynamic memory allocation added to allow for very large problems and minimize the need for source-code recompilation. Additional graphical output options provided in precompiled MODPATH-PLOT. Fixed problem causing MODPATH to fail when trying to read the MODFLOW head file for cases where the MODFLOW-96 XSECTION option was used to save the heads as a single 2D cross section. Fixed problem causing end-of-file error when reading the cell-by-cell budget file when list-oriented stress packages (e.g. rivers, drains, etc.) had zero entries for one or more stress periods. Fixed bug causing the automatic particle generation option to fail for transient simulations where the number of columns was larger than the number of rows. 1994 - version 3 released (no version 2 was released) 1989 - version 1 released DATA REQUIREMENTS Data input for MODPATH and MODPATH-PLOT is a combination of data files and interactive keyboard input. Keyboard input also can be redirected to a data file. Both programs are designed to work with MODFLOW. The number of new data files required by MODPATH is minimized by making use of MODFLOW data files whenever possible. OUTPUT OPTIONS Output from MODPATH is in the form of a variety of numerical data files containing spatial coordinates of particles and traveltime. Output from MODPATH-PLOT is primarily graphical. The graphical output is dependent on the capabilities of the GKS graphics system installed on the computer system. SYSTEM REQUIREMENTS MODPATH and MODPATH-PLOT are written in standard Fortran 90, and they can be compiled with any standard Fortran 90 compiler. In addition to a Fortran compiler, MODPATH-PLOT requires a graphics subroutine library known as GKS (Graphical Kernel System). GKS is a standardized set of graphics routines that are available commercially for most computer systems. The code has been used on UNIX-based computers and on Windows-based computers. DOCUMENTATION Pollock, D.W., 1994, User's Guide for MODPATH/MODPATH-PLOT, Version 3: A particle tracking post-processing package for MODFLOW, the U.S. Geological Survey finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 94-464, 234 p. Pollock, D.W., 1994, Source Code and Ancillary Data Files for the MODPATH Particle Tracking Package of the Ground- Water Flow Model MODFLOW -- Version 3, Release 1: U.S. Geological Survey Open-File Report 94-463, 2 p., two 3.5-inch diskettes. CONTACTS Operation: U.S. Geological Survey Office of Ground Water David Pollock 411 National Center Reston, VA 20192 dwpolloc@usgs.gov