MODFLOW 6: USGS Modular Hydrologic Model

For over 30 years, the MODFLOW program has been widely used by academics, 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 MODFLOW version (previously MODFLOW-2005), there are often incompatibilities that restrict their use with other MODFLOW versions. In many cases, development of these alternative MODFLOW versions has been challenging due to the underlying program structure, which was designed for the simulation of a single groundwater flow model using a regular MODFLOW grid consisting of layers, rows, and columns.

Overview of MODFLOW 6

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MODFLOW 6 is an object-oriented program and framework developed to provide a platform for supporting multiple models and multiple types of models within the same simulation. This version of MODFLOW is labeled with a "6" because it is the sixth core version of MODFLOW to be released by the USGS (previous core versions were released in 1984, 1988, 1996, 2000, and 2005). In the new design, any number of models can be included in a simulation. These models can be independent of one another with no interaction, they can exchange information with one another, or they can be tightly coupled at the matrix level by adding them to the same numerical solution. Transfer of information between models is isolated to exchange objects, which allow models to be developed and used independently of one another. Within this new framework, a regional-scale groundwater model may be coupled with multiple local-scale groundwater models. Or, a surface-water flow model could be coupled to multiple groundwater flow models. The framework naturally allows for future extensions to include the simulation of solute transport.

Groundwater Flow (GWF) and Groundwater Transport (GWT) Models

MODFLOW 6 presently contains two types of hydrologic models, the Groundwater Flow (GWF) Model and the Groundwater Transport (GWT) Model. The GWF Model for MODFLOW 6 is based on a generalized control-volume finite-difference (CVFD) approach in which a cell can be hydraulically connected to any number of surrounding cells. Users can define the model grid using

1. a regular MODFLOW grid consisting of layers, rows, and columns,
2. a layered grid defined by (x, y) vertex pairs, or
3. a general unstructured grid based on concepts developed for MODFLOW-USG.

For complex problems involving water-table conditions, an optional Newton-Raphson formulation, based on the formulations in MODFLOW-NWT and MODFLOW-USG, can be activated. The GWF Model is divided into "packages," as was done in previous MODFLOW versions. A package is the part of the model that deals with a single aspect of simulation. Packages included with the GWF Model include

• those related to internal calculations of groundwater flow (discretization, initial conditions, hydraulic conductance, and storage),
• stress packages (constant heads, wells, recharge, rivers, general head boundaries, drains, and evapotranspiration), and
• advanced stress packages (streamflow routing, lakes, multi-aquifer wells, and unsaturated zone flow).

An additional package is also available for moving water available in one package into the individual features of the advanced stress packages. The GWF Model also has packages for obtaining and controlling output from the model.

The GWT model for MODFLOW 6 simulates three-dimensional transport of a single solute species in flowing groundwater. The GWT Model solves the solute transport equation using numerical methods and a generalized CVFD approach, which can be used with regular MODFLOW grids or with unstructured grids. The GWT Model is designed to work with most of the new capabilities released with the GWF Model, including the Newton flow formulation, unstructured grids, advanced packages, and the movement of water between packages. The GWF and GWT Models operate simultaneously during a MODFLOW 6 simulation to represent coupled groundwater flow and solute transport. The GWT Model can also run separately from a GWF Model by reading the heads and flows saved by a previously run GWF Model. The GWT model is also capable of working with the flows from another groundwater flow model, as long as the flows from that model can be written in the correct form to flow and head files. 

XT3D Flow Formulation

A new groundwater flow formulation was developed specifically for the GWF Model in MODFLOW 6 (and to represent dispersion in the GWT Model). This new flow formulation is called the XT3D option. The XT3D option extends the capabilities of MODFLOW by enabling simulation of fully three-dimensional anisotropy on regular or irregular grids in a way that properly takes into account the full, three-dimensional hydraulic conductivity tensor. It can also improve the accuracy of groundwater flow simulations in cases in which the model grid violates certain geometric requirements. Thus, the XT3D option is an alternative to the Ghost-Node Correction (GNC) Package, which was developed for MODFLOW-USG.

Input Structure

In addition to the many new or redesigned capabilities, the MODFLOW 6 input structure has also been 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.

Capabilities in the Current Release

This current MODFLOW 6 release with the GWF and GWT Models synthesizes many of the capabilities in existing MODFLOW variants:

• MODFLOW-2005 — the GWF Model contains revisions of the commonly used flow packages, stress packages, and advanced stress packages
• MODFLOW-NWT — the GWF Model supports an optional Newton-Raphson approach for water table aquifers
• MODFLOW-USG — GWF Models can be developed using regular MODFLOW grids or unstructured grids
• MODFLOW-LGR — any number of GWF Models can be specified for a single simulation; these GWF Models are tightly coupled at the matrix level
• MODFLOW-CDSS — multiple stress and advanced stress packages of the same type can be specified for a single GWF Model
• MT3D/MT3D-USGS — the GWT Model simulates solute transport for a single chemical species
• SEAWAT — the GWF Model contains a Buoyancy Package for simulating variable-density groundwater flow.  Fluid density can be calculated from concentrations in a coupled GWT Model

For MODFLOW users with existing models, the MODFLOW 6 distribution includes a conversion program that will translate a MODFLOW-2005, MODFLOW-NWT, or MODFLOW-LGR (Version 2) model into the MODFLOW 6 format.

MODFLOW 6 Publications

Five USGS publications documenting MODFLOW 6 are included in the software distribution:

• 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., Provost, A.M., Panday, Sorab, and Hughes, J.D., 2022, Documentation for the MODFLOW 6 Groundwater Transport Model: U.S. Geological Survey Techniques and Methods, book 6, chap. A61, 56 p., https://doi.org/10.3133/tm6A61.
• Hughes, J.D., Leake, S.A., Galloway, D.L., and White, J.T., 2022, Documentation for the Skeletal Storage, Compaction, and Subsidence (CSUB) Package of MODFLOW 6: U.S. Geological Survey Techniques and Methods, book 6, chap. A62, 57 p., https://doi.org/10.3133/tm6A62.

Download Current Version of MODFLOW 6

The current version of MODFLOW 6 is version 6.4.4, released February 13, 2024.

• Release Notes [2.2MB PDF]
• MODFLOW 6 v.6.4.4 for Microsoft Windows; distribution includes documentation, binary executables, source code, test problems, and utility programs [91.8MB ZIP]
• MODFLOW 6 Description of Input/Output [3.1MB PDF]
• Distributions are available for the MacOS and Linux operating systems in our version-controlled public MODFLOW 6 Repository.

Programs Related to MODFLOW 6

The USGS also has developed several programs that can be used to assist with MODFLOW 6 simulations.

1. ZONEBUDGET for MODFLOW 6
   A program for computing detailed subregional water budgets for GWF models and advanced stress packages. This program is based on ZONEBUDGET for MODFLOW-2000 and MODFLOW-2005. ZONEBUDGET for MODFLOW 6 is included in the MODFLOW 6 release.
2. Mf5to6 Converter 
   A program for converting MODFLOW-2005, MODFLOW-NWT, and MODFLOW-LGR model datasets to MODFLOW 6 datasets. Mf5to6 is included in the MODFLOW 6 release.
3. MODPATH version 7
   MODPATH is a particle-tracking post-processing model that computes flow paths using output from MODFLOW.
4. GRIDGEN
   GRIDGEN is a program for generating layered quadtree grids for MODFLOW-USG.
5. FloPy
   FloPy is a Python package for creating, running, and post-processing MODFLOW-based models.
6. ModelMuse
   ModelMuse is a graphical user interface for creating, running, and post-processing MODFLOW-based models as well as several other types of model.

Previous MODFLOW Core Versions

Supported Versions

The following previous Core Versions of MODFLOW are still actively supported by the USGS.

• MODFLOW-2005

Superseded Versions

The following software is not actively supported by the USGS. The software has been superseded by MODFLOW 6. The software versions below are provided online for historical reference only, and the pages may contain outdated information or broken links.

• MODFLOW-2000
• MODFLOW-96
• MODFLOW-88

Find MODFLOW-Related Software

Visit the MODFLOW and Related Programs page for a list of MODFLOW-related software.

How to Cite MODFLOW 6

This USGS software has two citations associated with it.

1. The report citation is for the original report or article documenting the underlying theory, methods, instructions, and (or) applications at the time the initial version of the software was released. This digital object identifier (DOI) is for the report.
2. The software release citation is for the software/code itself (now referred to by USGS as a "Software Release") and references a specific version of the code and associated release date. This DOI links to the code.

In instances where an author is citing use of this software, it would be appropriate to cite both the report documenting the code and the specific software release version that was used.

Report Citation for MODFLOW 6

Langevin, C.D., Hughes, J.D., Banta, E.R., Provost, A.M., Niswonger, R.G., and Panday, Sorab, 2017, MODFLOW 6 Modular Hydrologic Model: U.S. Geological Survey Software, https://doi.org/10.5066/F76Q1VQV

Software/Code Citation for the Current Version

Langevin, C.D., Hughes, J.D., Provost, A.M., Russcher, M.J., Niswonger, R.G., Panday, Sorab, Merrick, Damian, Morway, E.D., Reno, M.J., Bonelli, W.P., Boyce, S.E., and Banta, E.R., 2024, MODFLOW 6 Modular Hydrologic Model version 6.4.4: U.S. Geological Survey Software Release, 7 February 2024, https://doi.org/10.5066/P9FL1JCC