The United States Geological Survey (USGS) has just released the latest MODFLOW version: MODFLOW 6. 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. For example, a regional-scale groundwater model may be coupled with multiple local-scale groundwater models.

The first release of MODFLOW 6 contains one type of hydrologic model, the Groundwater Flow (GWF) 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.

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:

Internal calculations of groundwater flow
Stress packages
Advanced stress packages

Information for current MODFLOW users

MODFLOW 6 has most of the functionalities of MODFLOW-2005, MODFLOW-NWT, MODFLOW-USG and MODFLOW-LGR. However, current users will find some differences. Some packages have been divided, renamed, removed and others are no longer supported.

MODFLOW 6 input files have a different format, nonetheless, it includes a conversion program to translate previously existing MODFLOW 2005, MODFLOW NWT or MODFLOW LGR files to the new format.

Also, many tests were performed to make sure that the MODFLOW 6 simulation results were identical to the ones from previous versions.

Changes between the current and previous versions

Here we present some major differences between the GWF model (MODFLOW 6) and previous versions:

The GWF Model in MODFLOW 6 is based on a generic unstructured design, patterned after the MODFLOW-USG design. This allows a GWF Model cell to be connected at the matrix level to any number of adjacent cells. A GWF Model cell can also be connected to a cell in another GWF Model or to other types of hydrologic features.
The GWF Model in MODFLOW 6 supports three alternative input packages for specifying the grid used to discretize the groundwater system:
- Discretization (DIS) Package (traditional MODFLOW grid).
- Discretization by Vertices (DISV) Package
- unstructured Discretization (DISU) Package
The BAS input file no longer exists. Initial heads are specified with Initial Conditions
(IC) Package, and constant heads are specified using the Time Varying Specified Head (CHD) Package.
Formulations implemented in MODFLOW-NWT and MODFLOW-USG to manage dry or nearly dry cells have been synthetyzed into one.
Temporal discretizacion information is specified at the simulation level instead rather than for an individual model (Timing Module).
Aquifer properties to calculate hydraulic conductance are specified in the Node Property Flow (NPF) package. This replaces the Layer Property Flow (LPF) package, Block-Centered Flow (BCF) package and Upstream Weighting (UPW) package.
Storage aquifer properties are specified in the Storage (STO) package, If this package does not exist, steady-state conditions are considered.
Only one evapotranspiration package is considered in MODFLOW 6. It contains the functionalities of MODFLOW-2005 EVT Package, the Segmented Evapotranspiration (ETS) Package and the Riparian Evapotranspiration (RIP-ET) Package.
The Multi-Aquifer Well (MAW) Package replaces Multi-Node Well (MNW1 y MNW2) Package.

There are many more new MODFLOW 6 characteristics that have not been mentioned here. Find more information in the documentation: https://water.usgs.gov/ogw/modflow/6beta.html

After reviewing the documentation, we can say that this new MODFLOW version is bringing many changes focused on simplifying the use of the different packages and the creation of input files. It is also important to mention that models created with previous MODFLOW versions can be converted to MODFLOW 6, so there will be no problem with backward compatibility.