Addition of Process-Based Stream Temperature Modeling Capabilities to MODFLOW 6

MODFLOW is a well-known and widely used groundwater flow simulator.  Characteristics that have historically defined MODFLOW remain in place: it is open-source, freely available, well-documented, and intuitive.  A complete rewrite of MODFLOW in 2017 has facilitated the adoption of several new model types embedded directly into the MODFLOW framework.  In addition to simulating groundwater flow, MODFLOW 6 now also includes solute transport, particle tracking, and a new heat-transport model called the Groundwater Energy (GWE) transport model.  Many other enhancements are actively being developed.  As with all model types available within the MODFLOW 6 hydrologic simulator, the GWE model leverages the design concept commonly referred to as packages – modules that represent specific features of the hydrologic system being modeled.  For example, the Streamflow Routing (SFR) package can be activated to simulate flow in streams.  If desired, users also can simulate heat transport within a stream network by activating the Streamflow Energy (SFE) transport package.  The SFE package simulates advective heat transport within the stream network while also accounting for advective and conductive heat exchange with the underlying groundwater system.  Although the initial release of GWE offered basic heat transport functionality in stream networks through the SFE package, detailed representation of heat exchange between stream reaches and the atmosphere was not included.  However, recent SFE development efforts are focused on adding functionality to represent heat exchange with the atmosphere.  New processes by which heat may be exchanged with the atmosphere are short- and long-wave radiation and sensible and latent heat fluxes.  When finished, the new process-based stream temperature modeling capabilities will work with the other MODFLOW features, including the application programming interface (API), parallel simulation, the input data model (IDM), and support within the popular FloPy Python library.