Documentation of computer programs to compute and display pathlines using results from the U.S. Geological Survey modular three-dimensional finite-difference ground-water flow model
A particle tracking post-processing package was developed to compute three-dimensional path lines based on output from steady-state simulations obtained with the U.S. Geological Survey modular 3-dimensional finite difference groundwater flow model. The package consists of two FORTRAN 77 computer programs: (1) MODPATH, which calculates pathlines, and (2) MODPATH-PLOT, which presents results graphically. MODPATH uses a semi-analytical particle tracking scheme. The method is based on the assumption that each directional velocity component varies linearly within a grid cell in its own coordinate direction. This assumption allows an analytical expression to be obtained describing the flow path within a grid cell. Given the initial position of a particle anywhere in a cell, the coordinates of any other point along its path line within the cell, and the time of travel between them, can be computed directly. Data is input to MODPATH and MODPATH-PLOT through a combination of files and interactive dialogue. Examples...