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.