This is the development repository for the USGS MODFLOW 6 Hydrologic Model. The official USGS distribution is available at USGS Release Page.

This repository contains branches of ongoing MODFLOW 6 development. The two main branches in this repository are:

• master -- the state of the MODFLOW 6 repository corresponding to the last official USGS release
• develop -- the current development version of the MODFLOW 6 program

The develop branch is under active and frequent updates by the MODFLOW development team and other interested contributors. We follow a fork and pull request workflow and require that pull requests pass our test suite before they are considered a possible candidate to merge into develop.

This repository may contain other branches with various levels of development code; however, these branches may be merged into develop or deleted without notice.

The develop branch often contains bug fixes and new features that are not yet part of the official USGS release. The updated user guide (mf6io.pdf) and binary executables for several operating systems of the develop branch are built and posted each night to the nightly-build repository.

Software distributions for the current and previous official USGS releases are available here.

This repository contains an ./autotest folder with python scripts for building and testing the MODFLOW 6 program and other workflow tasks. The testing workflow relies heavily on several related repositories including:

• modflowpy/pymake
• modflowpy/flopy
• MODFLOW-USGS/modflow6-testmodels
• MODFLOW-USGS/modflow6-largetestmodels
• MODFLOW-USGS/executables
• Deltares/xmipy

All new MODFLOW 6 functionality in the develop branch can be used directly with flopy; however, the installed version of flopy may need to be updated in order to become aware of new MODFLOW 6 changes. All MODFLOW 6 input is described using "definition files," which are located in the MODFLOW 6 repository under ./doc/mf6io/mf6ivar/dfn. These definition files may be updated on the develop branch as input changes to accommodate new options. The python script ./autotest/update_flopy.py can be run to automatically update the installed version of flopy to work with the develop version of MODFLOW 6. As a note of caution, be aware that running the update_flopy.py script will change the installed version of flopy, which could adversely affect existing MODFLOW 6 flopy scripts.

Instructions for building definition files for new packages are summarized in doc/mf6io/mf6ivar/readme.md.

MODFLOW is a popular open-source groundwater flow model distributed by the U.S. Geological Survey. For 30 years, the MODFLOW program has been widely used by academic researchers, private consultants, and government scientists to accurately, reliably, and efficiently simulate groundwater flow. With time, growing interest in surface and groundwater interactions, local refinement with nested and unstructured grids, karst groundwater flow, solute transport, and saltwater intrusion, has led to the development of numerous MODFLOW versions. Although these MODFLOW versions are often based on the core version (presently MODFLOW-2005), there are often incompatibilities that restrict their use with one another. In many cases, development of these alternative versions has been challenging due to the underlying MODFLOW structure, which was designed for the simulation with a single groundwater flow model using a rectilinear grid.

MODFLOW 6 is the latest core version of MODFLOW. It synthesizes many of the capabilities available in MODFLOW-2005, MODFLOW-NWT, and MODFLOW-LGR. MODFLOW 6 was built on a new object-oriented framework that allows new packages and models to be added, and allows any number of models to be tightly coupled at the matrix level. The Groundwater Flow (GWF) Model is the first model to be released in MODFLOW 6. It supports regular MODFLOW grids consisting of layers, rows, and columns, but it also supports more flexible grids that may conform to irregular boundaries or have increased resolution in areas of interest. The GWF Model consists of the original MODFLOW stress packages (CHD, WEL, DRN, RIV, GHB, RCH, and EVT) and four advanced stress packages (MAW, SFR, LAK, and UZF), which have been distilled from their predecessors to contain the most commonly used capabilities. MODFLOW 6 contains a new Water Mover (MVR) Package that can transfer water from provider packages to receiver packages. Providers can be many of the stress and advanced stress packages; receivers can be any of the advanced stress packages. This new capability makes it possible to route water between lakes and streams, route rejected infiltration into a nearby stream, or augment lakes using groundwater pumped from wells, for example. To modernize user interaction with the program, the MODFLOW 6 input structure was redesigned. Within package input files, information is divided into blocks, and informative keywords are used to label numeric data and activate options. This new input structure was designed to make it easier for users to adjust simulation options in an intuitive manner, reduce user input errors, and allow new capabilities to be added without causing problems with backward compatibility.

Hughes, J.D., Langevin, C.D., and Banta, E.R., 2017, Documentation for the MODFLOW 6 framework: U.S. Geological Survey Techniques and Methods, book 6, chap. A57, 40 p., https://doi.org/10.3133/tm6A57.

Langevin, C.D., Hughes, J.D., Banta, E.R., Niswonger, R.G., Panday, Sorab, and Provost, A.M., 2017, Documentation for the MODFLOW 6 Groundwater Flow Model: U.S. Geological Survey Techniques and Methods, book 6, chap. A55, 197 p., https://doi.org/10.3133/tm6A55.

Provost, A.M., Langevin, C.D., and Hughes, J.D., 2017, Documentation for the "XT3D" option in the Node Property Flow (NPF) Package of MODFLOW 6: U.S. Geological Survey Techniques and Methods, book 6, chap. A56, 40 p., https://doi.org/10.3133/tm6A56.

Langevin, C.D., Panday, S, and Provost, A.M., 2020, Hydraulic-head formulation for density-dependent flow and transport: Groundwater, v. 58, no. 3, p. 349–362.

Langevin, C.D., Hughes, J.D., Banta, E.R., Provost, A.M., Niswonger, R.G., and Panday, Sorab, 2020, MODFLOW 6 Modular Hydrologic Model version 6.2.0: U.S. Geological Survey Software Release, 22 October 2020, https://doi.org/10.5066/F76Q1VQV

This software has been approved for release by the U.S. Geological Survey (USGS). Although the software has been subjected to rigorous review, the USGS reserves the right to update the software as needed pursuant to further analysis and review. No warranty, expressed or implied, is made by the USGS or the U.S. Government as to the functionality of the software and related material nor shall the fact of release constitute any such warranty. Furthermore, the software is released on condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from its authorized or unauthorized use.