MODPATH

Criterion	Explanation
General Description	Software coupled with MODFLOW to generate groundwater flow path.
Model Domain	Subsurface saturated zone
Developer	U.S. Geological Survey (USGS)
Hardware computing requirements	None specified. The program is packaged for use on computers with Windows (versions 7, 8, 10) and may be modified for use on other operating systems.
Code language	Fortran – 2003 standard
Original application	MODPATH is a particle-tracking post-processing program designed to work with the USGS finite-difference groundwater flow model, MODFLOW. Particles may be tracked forward or backward in time through either steady-state or transient-flow fields. The model is often used to compute recharge capture areas for wells and other hydrologic features.
Public/proprietary and cost	Public; No cost
Physically or empirically based	Physically based
Mathematical methods used	MODPATH is based on a three-dimensional network of rectangular grid cells (structured grid). Each structured grid cell has six faces that connect it to either a neighboring cell or a boundary of the system. The model calculates inflow and outflow across cell faces, and flow rates are saved as output in MODFLOW budget file (used by MODPATH to construct the groundwater velocity distribution that forms the basis for particle-tracking calculations). MODPATH uses linear interpolation to compute velocity components within the cell from the values at the cell faces. The velocity component time derivatives are integrated and solved taking the exponential, yielding the solutions for the x, y, and z coordinate of a particle as a function of time. The time at which a particle will exit through the face of a cell is calculated, and then this time is plugged into the particle location equations to determine the exit location. This process is repeated until the particle reaches a cell with no possible exit face or meets some other specified termination location. MODPATH repeatedly applies this algorithm to build particle paths. In cases where multiple neighboring grid cells are connected to a single cell face, MODPATH divides cells into subcells so that the simple linear velocity interpolation method required for the analytical solution will work.
Input data requirements	The model input files are a combination of MODFLOW input and output files plus MODPATH-specific input files.
Outputs	Particle coordinate output files are simple text files with spatial and temporal coordinates defining particle paths. From these, standardized graphics files can be created for displaying in various two-and three-dimensional viewers. The capture area for a well can be displayed by plotting the starting locations of all the particles that discharge to the cell containing the well.
Pre-processing and post-processing tools	Visual MODFLOW (Waterloo Hydrogeologic), Groundwater Vistas (Rockware)
Representation of uncertainty	The model does not have built-in uncertainty representation. This can be assessed for each specific case using uncertainty analysis.
Prevalence	MODPATH is widely applied to MODFLOW-based groundwater flow simulation studies, according the USGS. It is also widely used to delineate sources of water to discharge sites and aquifers in systems. Publications include: Anne M. Stewart 2018, Scientific Investigations Report 2017-5155. Hydrologic assessment and numerical simulation of groundwater flow, San Juan Mine, San Juan County, New Mexico, 2010–13. Paul Misut 2014, Scientific Investigations Report 2014-5036. Simulation of zones of contribution to wells at site GM–38, Naval Weapons Industrial Reserve Plant, Bethpage, New York. Daniel T. Feinstein, Leon J. Kauffman, Megan J. Haserodt, Brian R. Clark, Paul F. Juckem 2018, Scientific Investigations Report 2018-5038. Extraction and development of inset models in support of groundwater age calculations for glacial aquifers. Fio, J.L. and Deverel, S.J., 1991. Groundwater flow and solute movement to drain laterals, western San Joaquin Valley, California, II. Quantitative hydrologic assessment, Water Resources Research, 27, 2247 - 2257.
Ease of use for public entities	Code is readily available for electronic retrieval on the internet at http://water.usgs.gov/software/modpath.html Or by contacting the USGS for distribution: U.S. Geological Survey Hydrologic Analysis Software Support Program 437 National Center Reston, VA 20192 h2osoft@usgs.gov
Ease of obtaining information and availability of technical support	Limited support is provided for correcting bugs and clarification on how MODPATH is intended to work. Email modflow@usgs.gov Or mail requests to: Earth Systems Modeling Branch Integrated Modeling and Prediction Division Water Mission Area, U.S. Geological Survey 411 National Center 12201 Sunrise Valley Drive Reston, VA 20192 Training in the use of MODPATH is included in the USGS MODFLOW training courses Modeling of Ground-Water Flow Using Finite-Difference Methods (GW2096TC) and Advanced Finite-Difference Modeling of Ground-Water Flow (GW3099TC), offered annually at the USGS National Training Center.
Source code availability	MODPATH is available online from the USGS at the following address: http://water.usgs.gov/ogw/modpath/ A software distribution package also contains supporting documentation, such as input instructions, output file descriptions, and example problems.
Status of model development	Current release of MODPATH version is 7 (7.2.001) is the fourth major release since its original publication. Improvements include unstructured grid support and input/output format modifications that minimize the difference between MODPATH simulations based on the different versions of MODFLOW.
Challenges for integration	For integration with surface water modeling, temporal and spatial scales and calculations associated with MODFLOW modeling are different that those used for surface water models.

References

See https://water.usgs.gov/cgi-bin/man_wrdapp?modpath for a summary of MODPATH.

Pollock, D.W., 2016, User guide for MODPATH Version 7—A particle-tracking model for MODFLOW: U.S. Geological Survey Open-File Report 2016–1086, 35 p., http://dx.doi.org/10.3133/ofr20161086.

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