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New To MODFLOW? Start Here!
MODFLOW is the U.S. Geological Survey's three-dimensional (3D) finite-difference groundwater model first published in 1984. Although originally conceived solely as a groundwater-flow simulation code, MODFLOW's modular structure has provided a robust framework for integration of additional simulation capabilities that build on and enhance its original scope. The family of MODFLOW-related programs now includes capabilities to simulate coupled groundwater/surface-water systems, solute transport, variable-density flow (including saltwater), aquifer-system compaction and land subsidence, parameter estimation, and groundwater management. The MODFLOW program is free, open-source software. Details on user rights for MODFLOW and other water-resource software developed by the USGS can be found in the Software User Rights Notice.
The USGS releases multiple versions of MODFLOW. There is a core MODFLOW version, which is developed and maintained by the USGS Office of Groundwater, and there are advanced versions. The core MODFLOW version is the one that is most widely used and most thoroughly tested. MODFLOW-2005 is presently the core MODFLOW version distributed by the USGS. Advanced MODFLOW versions include specialized MODFLOW variants and versions of MODFLOW that use newer formulations. These advanced MODFLOW versions are also available from the USGS.
There have been four major releases of the core MODFLOW version: MODFLOW-88, MODFLOW-96, MODFLOW-2000, and MODFLOW-2005. The first two versions of the code, MODFLOW-88 and MODFLOW-96, were based on the initial conceptualization of the program as a groundwater-flow model only. Specific aspects of a groundwater-flow system are simulated by these codes by use of independent, modular-programming components called "Packages," such as the Well Package and River Package. MODFLOW's modular design was further expanded with the release of MODFLOW-2000 by the addition of "Processes," which are defined as parts of the code that solve a major equation or set of related equations and that consist of sets of the underlying packages. The part of the code that solves the groundwater-flow equation became the Groundwater Flow (GWF) Process. Although additional processes have been developed for MODFLOW, the GWF Process remains the core process on which other MODFLOW simulation capabilities are built. The primary change in MODFLOW-2005 from MODFLOW-2000 is the approach used by MODFLOW-2005 for managing internal data.
Advanced versions of MODFLOW that use new formulations include:
The following is a list of specialized MODFLOW variants:
It is recommended that new MODFLOW users begin with MODFLOW-2005 and transition to more advanced versions once a fundamental understanding of groundwater modeling with MODFLOW has been developed.
MODFLOW is a command line executable program written in FORTRAN that reads ASCII text and binary files and writes ASCII text and binary output files. An example of an input file for MODFLOW is shown below (fig. 1). Although experienced MODFLOW users may be able to create MODFLOW input files by hand, most MODFLOW users rely on a graphical user interface (GUI) to prepare the input files and post-process the output files. The MODFLOW program itself does not generate contour plots or any other type of graphical output. These plots must be generated from MODFLOW results using other software programs. The USGS distributes several free pre- and post-processors for MODFLOW. Commercial GUIs are also available for sale by private vendors.
Related MODFLOW Programs
The USGS also has developed several programs that can be used to assist with MODFLOW simulations. These include programs for model calibration and sensitivity analysis (such as the parameter estimation PEST++ and UCODE_2005 programs), several graphical user interfaces, and pre- and post-processors to facilitate input-file preparation and presentation of simulation results, a particle-tracking post-processing package that computes three-dimensional flow paths throughout a simulation domain (MODPATH), and a program for computing detailed subregional water budgets (ZONEBUDGET). All of these programs can be accessed from the MODFLOW and Related Programs website.
Online MODFLOW Guide
Most MODFLOW versions are periodically updated. Some of these updates include changes to the format of the input and output files. These changes are described with release notes that are distributed with each program. The USGS also provides online guides for several MODFLOW versions. These online guides contain up-to-date input instructions and other helpful information. The MODFLOW-2005 guide can be found online.
Where to go from here
Successful use of MODFLOW typically requires a college-level class or professional training on groundwater modeling. In some situations, the USGS can provide training to governmental agencies with a cooperative agreement with the USGS; agencies can contact their cooperating USGS office for additional information. MODFLOW courses are also offered by several private companies. Those with groundwater modeling experience can download MODFLOW. For instructions on MODFLOW installation, please refer to the online help guide.
While training is recommended, experienced groundwater modelers interested in self study are encouraged to begin by:
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