MODFLOW-6: the next generation of groundwater modeling

MODFLOW-6 is the newest core version of the groundwater flow code developed by the MODFLOW team at the US Geological Survey (USGS) and collaborating industry leaders.

About MODFLOW

On March 29, 2023 we released Visual MODFLOW Flex 9.0, the latest version of the popular graphical user interface that brings together industry-standard codes for groundwater flow and contaminant transport, essential analysis and calibration tools, and stunning 3D visualization capabilities in a single environment. The conceptual model approach which is unique to Visual MODFLOW Flex in groundwater modeling user interfaces allows you to:

Visual MODFLOW Flex Improvements

MODFLOW is developed and used by the USGS for evaluating water resources under its jurisdiction and occasionally with other partners.  It is widely considered to be most popular finite-difference groundwater flow modeling code in the world. Although the early versions of MODFLOW were limited to saturated groundwater flow and simplified conceptualizations of boundary conditions, its open-source code and modular design allowed users to extend the capabilities of MODFLOW such that many related versions haven been developed with additional features and capabilities to simulate specific conditions. A brief timeline of the various versions is shown below and is described in our discussion on the Versions of MODFLOW.

The six core versions of MODFLOW are presented above the timeline, while specialized variants and partner software are presented below the timeline. Each of these codes (or engines) are described below.

What are the driving forces behind developing MODFLOW-6?

As shown in the timeline above, there have been many different versions of MODFLOW and related codes that have been developed over the years. Many of these disparate versions were designed to solve specific complexities that can arise in groundwater systems. This has led to a fracturing of the codes that is more difficult to maintain. More importantly, the associated features in each of these codes are also isolated such that the complexities that are incorporated across different versions of MODFLOW cannot be simulated simultaneously. This has prompted the USGS to develop a new core version of MODFLOW that modernizes the core functionality of MODFLOW, combines many of the most popular features that are available across the different versions of MODFLOW, and through its framework enables additional processes and features to be added in the future.

Major Features and benefits of using MODFLOW-6

MODFLOW-6 is a completely redesigned version of MODFLOW with an all new and more-readable input file structure that combines the major features from the most popular version/variants of MODFLOW that have been released over the last two decades:

• Structured grids, groundwater flow processes, and core boundary conditions (MODFLOW-2000/2005)
• Unstructured grids and ghost node-correction (MODFLOW-USG),
• Newton-Raphson formulation (MODFLOW-USG, MODLFOW-NWT),
• Dual-domain solute transport (MT3D-MS),
• Multiple connected/nested model grids (MODFLOW-LGR),
• Density-dependent flow and solute transport (SEAWAT)

as well as some new/unique features:

• full 3D anisotropy tensor support (via the XT3D formulation),
• separate time series input files,
• time-varying conductivities and storage parameter values,
• multiple stress package instances, and
• mover/exchange packages

Is the input/output similar to previous versions of MODFLOW?

Yes and No.  Although the overall modularity of input files has been retained in the new version and many of the packages will be familiar to long time users, the Input/Output formats for MODFLOW-6 are based on a new pseudo-object‑oriented paradigm that uses input blocks with keywords. Many of the packages have also been refactored so that some information has been moved to other, sometimes new, packages. Furthermore, in order to facilitate the integration of multiple models in a single solution, the models are controlled at a “simulation” level. The definition of the simulation includes the name and type of each model, how they are linked, and how they will be solved. Furthermore, the simulation also includes a single definition for time discretization – that is, information about temporal control, such as the number and duration of stress periods, is shared among all models in the simulation.

The upshot of these changes is that the files are generally easier to read and the order of the inputs is not as rigidly enforced as compared to previous versions due to the use of input blocks and keywords.

The major differences are described below and a more detailed discussion of the differences is described in the Visual MODFLOW Flex documentation for MODFLOW-6 translation settings here. Furthermore, it is strongly recommended that you take some time to read some of the references provided below.

MODFLOW-6 Input File Structure

MODFLOW-6 is a completely redesigned version of MODFLOW with an all new and more-readable input file structure that:

• is based on a Simulation Framework
• may include one or more groundwater flow model(s)
• may include one or more solute transport model(s)

Simulation Framework

The MODFLOW-6 input file structure is based on a new simulation framework. This allows you to compose a simulation consisting of one or more models of the same or different type under separate or combined solution groups. Currently, MODFLOW-6 includes support for groundwater flow (GWF) and transport (GWT) models; although the framework is extensible such that additional models may be added in the future (e.g. connected linear network flow and transport and/or surface water flow and transport). The simulation is composed in the main simulation file (MFSIM.NAM) which contains links to the model(s) and solution group(s). The simulation file also includes a link to the time discretization which handles by the time discretization (TDIS) package and is common to all models in the simulation.  This is somewhat different from the approach in previous versions of MODFLOW and MT3D-MS (and variants) where stress periods and transport steps were handled separately. The Models are solved as part of a solution group using the Iterative Model solution package (IMS), which is very similar to the solver used in MODFLOW-USG, the sparse matrix solver (SMS) package that includes both non-linear and linear solvers.  Models are linked through exchange packages. The following packages are required at the simulation level of MODFLOW-6:

Simulation-Level Packages

Groundwater Flow Model

The MODFLOW-6 groundwater flow (GWF) model includes most of the packages that are available in previous versions of MODFLOW; however, in some cases, certain input data have been moved to other or new packages. For example, the GWF model includes the node property flow (NPF) package which contains the information required to calculated intercell conductances, similar to the LPF package familiar to experienced MODFLOW users; however the transient storage terms are located in the optional storage (STO) package. For more details, please refer to the MODFLOW-6 input/output documentation that is included with the MODFLOW-6 distribution available at the USGS MODFLOW-6 webpage. The following packages are available in the GWF model of MODFLOW-6:

Groundwater Flow (GWF) Model Packages

Solute Transport Model

The MODFLOW-6 groundwater transport (GWF) model includes most of the packages that are available in MT3D-MS and related transport codes, including more advanced features from MT3D-USGS, SEAWAT, and USG-Transport; however, in some cases, certain input data have been moved to other or new packages. For example, the MT3D-MS included the RCT package which contains information on reaction(s), sorption, and mass transfer to an alternate (dual) domain; however, this information is separated in a GWT model between the mobile storage and transfer package (MST) and one or more (optional) Immobile Storage and Transfer (IST) packages. Another significant difference between MODFLOW-6 and its predecessors is that GWT models are limited to a single species – this limitation is overcome by the fact that you can include multiple GWT species in a single simulation. For more details, please refer to the MODFLOW-6 input/output documentation that is included with the MODFLOW-6 distribution available at the USGS MODFLOW-6 webpage. The following packages are available in the GWT model of MODFLOW-6:

Groundwater Transport (GWT) Model Packages

Support and Limitations of MODFLOW-6 in Visual MODFLOW Flex

Supported Features

Visual MODFLOW Flex includes support for many of the core features available from other engines:

The following MODFLOW packages are supported in the first release:

• Simulation Packages:
  • MFSIM.NAM – Simulation Name File
  • TDIS – Time Discretization
  • IMS – Iterative Model Solution
    
    
• Groundwater Flow Model (GWF): Simulate saturated groundwater flow using structured finite difference grids or unstructured control volume finite difference grids (Voronoi or Quadtree).
  • GWF.NAM – Groundwater Flow Model Name File
  • DIS – Discretization Package
  • DISV – Discretization by Vertices Package
  • IC – Initial Conditions (Heads)
  • OC – Output Control Package
  • NPF – Node Property Flow Package
  • STO – Storage Package
  • CHD* – Time Varying Constant Head Package
  • WEL* – Well Package
  • DRN* – Drain Package
  • RIV* – River Package
  • GHB* – General-Head Boundary Package
  • RCH – Recharge Package
  • EVT – Evapotranspiration Package
    
    
• Solute Transport Model (GWT): Simulate groundwater solute transport using structured finite difference grids or unstructured control volume finite difference grids (Voronoi or Quadtree), including a variety of sorption, reaction, and immobile domain models.
  • GWT.NAM – Groundwater Transport Model Name File
  • DIS – Discretization Package
  • DISV – Discretization by Vertices Package
  • IC – Initial Conditions (Concentrations)
  • OC – Output Control Package
  • ADV – Advection Package
  • DSP – Dispersion Package
  • MST – Mobile Storage Transfer Package
  • IST – Immobile Storage Transfer Package (up to three immobile domains)
  • SSM – Source Sink Mixing Package
  • CNC* – Time Varying Constant Concentration Package
  • SRC* – Mass Source Loading Package

* Multiple Instance Packages: MODFLOW-6 allows you to include multiple instances of certain packages (marked with * above) in a simulation and track budgets separately for each package.

Limitations

However, given the significant changes, it was not feasible to support all the functionality available in MODFLOW-6 in the first release (see tables above for specifics); however, our development team is continually working on improving Flex and we plan to add more support for MODFLOW-6 features based on your feedback. If there is a feature of MODFLOW-6 that is not yet supported that you’d like to see added in a future version of Visual MODFLOW Flex, please let us know at support@waterloohydrogeologic.com.

Learn more:

• Supported Packages and Engines in Visual MODFLOW Flex
• Supported Modeling Objectives (and engine cross-compatibilities):
  • Groundwater Flow
  • Groundwater Transport

Additional Resources

The following resources may be useful for learning more about MODFLOW-6:

• Official USGS MODFLOW-6 Website: https://www.usgs.gov/software/modflow-6-usgs-modular-hydrologic-model
• Documentation:
  • 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.

• 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.

• United States Geological Survey, 2022. MODFLOW-6 – Description of Input and Output (Version 6.4.1). United States Geological Survey, Water and Science Availability and Use Science Program, MODFLOW 6 Development Team. Dec 2022.