MODPATH-LGR is a particle tracking post-processing program for computing
three-dimensional flow paths for steady-state and transient groundwater
flow in models with local-grid refinement (sometimes called embedded
models). Locally refined grids consist of a larger regional-scale parent
model and one or more smaller embedded local-scale child models. This
program uses output produced by MODFLOW-LGR, in which child grids in parent
grids are refined by using the Shared-Node Local Grid Refinement (LGR)
package of the three-dimensional finite-difference groundwater-flow model
MODFLOW published by the U.S. Geological Survey (USGS). The particle
tracking scheme is based on MODPATH, which uses an analytical expression
for particle movement within each finite-difference cell. Particles are
tracked among cells until discharged through model boundaries, weak sinks,
or other designated discharge zones. MODPATH-LGR tracks particles between
the parent and child models by using flows calculated at the interface
boundaries. Flows calculated by MODFLOW-LGR are modified to account for
fractional finite-difference cells along the shared-node interface between
parent and child models. Required program input includes standard MODPATH
input files for the parent and child models and a MODPATH-LGR control file.
This documentation describes how particles are transferred between parent
and child models and demonstrates program operation with two hypothetical
steady-state xsimulations.