EVALUATING GROUNDWATER SURFACE WATER INTERACTIONS ACROSS THE CONTINENTAL U.S. WITH AN INTEGRATED HYDROLOGIC MODEL (Invited Presentation)

We evaluate groundwater surface water interactions using a fully integrated, simulation of the continental US that incorporates dynamic interactions from the groundwater to the land surface. Results are based on a one-year transient simulation spanning water year 1985 using ParFlow-CLM to simulate 3D variably saturated groundwater flow with a fully coupled water energy balance at the land surface. The model provides high-resolution (1 km²) outputs over a large spatial extent (~6.3 million km²) that are used to characterize groundwater surface water exchanges across a wide range of hydroclimatic settings and spatial scales not feasible with other approaches. Model outputs are validated against the more than 1.2 million groundwater and surface water observation available over the simulation period. We compare spatial patterns in groundwater depth, land energy partitioning and basin productivity to identify areas of strong interaction between the surface and subsurface. Results illustrate the importance of lateral groundwater flow in supporting surface water availability and variability in many settings. We also use the Budyko framework to evaluate the impact of recharge on evapotranspiration and runoff partitioning in watersheds ranging from 100 to 1,000,000 square kilometers. We find that recharge can significantly impact the tradeoffs between runoff and evapotranspiration, but that the Budkyo relationship is still applicable when recharge is appropriately corrected for. Overall, this work demonstrates the potential for integrated hydrologic models to improve our understanding of groundwater interactions in large heterogeneous systems.