Hydrology at the Continental Scale: Evaluating Groundwater Surface Water Interactions Across the U.S. with an Integrated Hydrologic Model

Abstract:

We use a fully integrated hydrologic model to simulate physically based dynamic interactions from the groundwater through the land surface at 1 km² spatial resolution across more than 6,000,000 km² of continental US. This is accomplished with ParFlow-CLM which incorporates 3D variably saturated groundwater flow with overland flow and a fully coupled water energy balance at the land surface. While 1 km² resolution may be considered course relative to many critical zone processes, it is orders of magnitude finer than standard global and continental scale approaches. Furthermore, existing global land surface and water balance models do not resolve lateral groundwater flow. Incorporating physically based groundwater-surface water interactions into continental scale simulations can help bridge the gap between these approaches and provide novel insights into scaling and spatial heterogeneity. Model outputs are used to characterize groundwater surface water exchanges across a wide range of hydroclimatic settings and spatial scales not feasible with other approaches. 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 moderating temporal variability in many settings. Within the Budyko framework for surface water partitioning, we demonstrate that groundwater surface water exchanges can systematically bias relationships between runoff and evapotranspiration in predictable ways for watersheds ranging in sized form in watersheds ranging from 100 to 1,000,000 km². Furthermore, model results were used to evaluate the partitioning between evaporation and transpiration. Results demonstrate a novel connection between lateral groundwater flow and terrestrial water budgets and reconcile systematic differences between global observations and global land surface models.