The MODFLOW 6 Modular Hydrologic Model

MODFLOW 6 is the latest core release of the U.S. Geological Survey’s modular hydrologic model. MODFLOW 6 synthesizes many of the recent MODFLOW advances into a single program and provides an object-oriented framework for adding new types of models and packages. The first release of MODFLOW 6 contains the Groundwater Flow (GWF) Model, which combines most of the capabilities of MODFLOW-2005, MODFLOW-NWT, MODFLOW-USG, and MODFLOW-LGR. The GWF Model supports both structured and unstructured grids, with the capability to remove thin or inactive cells from the simulation. When thin cells are removed from the simulation, the model automatically connects overlying and underlying cells. The GWF Model includes a new formulation, called XT3D, for representing full, three-dimensional anisotropic groundwater flow and for improving the flow approximation for certain grid types. The GWF Model in MODFLOW 6 contains the six, traditional MODFLOW stress packages (Well, Drain, River, General-Head Boundary, Recharge, and Evapotranspiration) and four advanced hydrologic stress packages (Multi-Aquifer Well, Streamflow Routing, Lake, and Unsaturated Zone Flow). MODFLOW 6 also includes a new and generalized way to move water between these hydrologic stress packages. This new capability can be used to route water from drains into a stream, for example, or to send rejected infiltration into a nearby lake. The object-oriented framework also makes it possible to tightly couple any number of parent, child, and grandchild models, or to couple an unstructured GWF Model of a shallow aquifer with a deep aquifer GWF Model that uses a regular grid. Future plans for MODFLOW 6 include the addition of a new subsidence package, variable-density groundwater flow, and transport.