New capabilities in MT3D-USGS for simulating unsaturated-zone heat transport

Changes in climate and land use will alter groundwater heat transport dynamics in the future. These changes will in turn affect watershed processes (e.g., nutrient cycling) as well as watershed characteristics (e.g., distribution and persistence of cold-water habitat). Thus, groundwater flow and heat transport models at watershed scales that can characterize and quantify thermal impacts of surface temperature change on groundwater system temperatures are needed to forecast changes to groundwater-linked ecosystems in riparian zones, streams, and lakes. Including unsaturated zone processes has previously been shown to be important for properly determining the timing and magnitude of groundwater recharge (Hunt et al. 2008). Similarly, heat transport dynamics in the saturated-zone, as well as connected surface-water systems, can be appreciably influenced by unsaturated-zone processes; in this way the unsaturated zone forms an inextricable link between land surface where change occurs and the groundwater system that transmit that change. This paper presents new capabilities for the existing MT3D-USGS transport simulator by adding functionality for simulating heat transport through the unsaturated zone. New simulation capabilities are verified through comparison of simulation results with those of the variably-saturated heat transport simulator VS2DH under steady and transient conditions for both water and heat flow. The new capabilities are assessed using a number of conceptualizations and include evaluations of convective and conductive heat flow. These additional capabilities increase the utility for applied watershed-scale simulations, which in turn should facilitate more realistic characterizations of temperature change on thermally sensitive ecosystems, such as stream habitat.