MODFLOW (USGS Modular Groundwater Flow Model)

Criterion	Explanation
General Description	MODFLOW simulates groundwater flow using a finite difference equation solution to the groundwater flow equation.
Model Domain	The physical domain of the model includes the unsaturated and saturated zones.
Developer	US Geological Survey
Hardware computing requirements	The code has been used on UNIX-based computers and personal computers running various forms of the Microsoft Windows operating system.
Code language	FORTRAN. The most recent version, MODFLOW 6 was programmed in FORTRAN using a modern programming style. Many of the simulation components were programmed using an object-oriented design described in Adams and others (2009) provides a comprehensive description of the details for implementing object-oriented programming concepts using FORTRAN. The object-oriented design is different from the procedural program design that was used for previous MODFLOW versions.
Original application	The original MODFLOW model was developed in 1984 to simulate groundwater flow using a finite difference equation solution to the groundwater flow equation. It used a modular framework in which independent subroutines (modules) which simulate different features of the hydrologic system.
Public/proprietary and cost	Public, no cost
Physically or empirically based	Physically based
Mathematical methods used	MODFLOW solves the groundwater flow equation using a finite-difference numerical solution.
Input data requirements	Hydraulic conductivity, storativity, boundary fluxes (recharge, lateral fluxes), boundary conditions (constant head, head-dependent boundary, etc.)
Outputs	Outputs include spatial distribution of heads, water budgets, and boundary flows.
Pre-processing and post-processing tools	Multiple pre-and post-processing programs can be used with MODFLOW. These include Visual MODFLOW (Waterloo Hydrogeologic), Groundwater Vistas (Rockware), USGS Model Muse and Model Viewer.
Representation of uncertainty	Uncertainty is represented through the results of sensitivity analysis which can be used to provide ranges of model outputs.
Prevalence	Of all the groundwater flow models widely available, MODFLOW, is regarded by many as the most widely used by government agencies and consultant firms (Loudyi et al. 2014).
Ease of use for public entities	There is a high level of use ease. The code is readily available for free. Training and support are readily available. Multiple graphic interfaces are available.
Ease of obtaining information and availability of technical support	There is substantial online information available through the USGS. The USGS continues to support MODFLOW with upgraded models and additional features. User groups exist. Support is provided at the USGS for correcting bugs and clarification of how MODFLOW 6 is intended to work. Only limited assistance can be provided for applying MODFLOW 6 to specific problems by emailing modflow@usgs.gov or by contacting Chris Langevin, Joe Hughes, or Ned Banta at: 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
Source code availability	The source code is available for modification if needed at https://water.usgs.gov/ogw/modflow/MODFLOW.html#related
Status of model development	MODFLOW has evolved since its initial release. Modules have been added. Examples include integration with surface-water and watershed models, addition of the unstructured grid package, explicit modeling of unsaturated-zone flow and development of the farm processes package.
Uses in Delta	MODFLOW has been used in the Delta to simulate groundwater hydrologic conditions for the entire Delta (CVHM-D) to simulate groundwater resources impacts analysis for the BDCP EIR/EIS, and individual islands (e.g. Deverel et al. 2017).
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. The water must have a constant density, dynamic viscosity (and consequently temperature) throughout the modelling domain (SEAWAT is a modified version of MODFLOW which is designed for density-dependent groundwater flow and transport). The principal components of anisotropy of the hydraulic conductivity tensor does not allow non-orthogonal anisotropies, as could be expected from flow in fractures. Challenges for use in the Delta include correctly simulating island geometry using finite differences; simulating ET and drains can be also challenging. MODFLOW unstructured grids may be better suited.

References

Adams, J.C., Brainerd, W.S., Hendrickson, R.A., Maine, R.E., Martin, J.T., and Smith, B.T., 2009, The Fortran 2003 handbook—The complete syntax, features and procedures, Springer Science + Business Media, 713 p., Accessed June 27, 2017, at https://doi.org/10.1007/978-1-84628-746-6.

Deverel, S. J, Leighton, D. A, Lucero, C., & Ingrum, T., 2017, Simulation of Subsidence Mitigation Effects on Island Drain Flow, Seepage, and Organic Carbon Loads on Subsided Islands Sacramento–San Joaquin Delta. San Francisco Estuary and Watershed Science, 15(4). Retrieved from https://escholarship.org/uc/item/4q340190.

Loudyi, Dalila; Falconer, Roger; and Lin, Binliang, 2014, "MODFLOW: An Insight Into Thirty Years Development Of A Standard Numerical Code For Groundwater Simulations", CUNY Academic Works, http://academicworks.cuny.edu/cc_conf_hic/168.

See https://en.wikipedia.org/wiki/MODFLOW for summary and history.

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