Calibrated Simulation of the Long-Term Average Surficial Groundwater System and Derived Spatial Distributions of its Characteristics for the Contiguous United States

While the physical processes governing groundwater flow are well understood, and the computational resources now exist for solving the governing equations in three dimensions over continental-scale domains, there remains substantial uncertainty about the subsurface distribution of the properties that control groundwater flow and transport for much of the contiguous United States (CONUS). The transmissivity of the shallow subsurface is a key parameter for the simulation of water table position, shallow groundwater flow, and base-flow discharge, but is not well-characterized at large regional to continental scales. We used a process-based inversion of CONUS-extent groundwater information to generate national data sets of (a) the transmissivity of the shallow groundwater system, (b) the depth to the water table, (c) groundwater discharge as base-flow, and (d) long-term average water content in the unsaturated zone. CONUS-extent coverage was developed in the form of 75 subdomain models, with the spatial distribution of long-term average transmissivity for each subdomain model calibrated against water-levels derived from U.S. Geological Survey (USGS) observation wells, NHDPlusV2 first-order perennial streams, and National Wetlands Inventory (NWI) freshwater wetlands. Estimated transmissivities were lower in the western CONUS than the eastern CONUS, and across the CONUS both transmissivity and depth to water correlate with recharge, elevation, and topographic slope. These generated data sets provide spatially distributed, long-term average estimates of subsurface properties and hydrological states that we anticipate will complement other environmental modeling efforts as explanatory variables, boundary conditions, or transport pathways. Plain Language Summary We developed a simulation of the shallow groundwater system for the contiguous United States (CONUS) and compared it to large data sets of water table measurements in order to generate a map of surficial transmissivity, which is a property that controls the flow of water between its entrance into the groundwater system and its discharge to streams and rivers. Better maps of this property are one ingredient to improved understanding of how the landscape stores and releases water at large spatial scales. Key Points Calibrated simulations of the shallow groundwater system were used to estimate the CONUS-extent spatial distribution of transmissivity Correlations were explored between the estimated transmissivities and recharge, elevation, and topographic slope Simulated depth to water and groundwater discharge were used to derive unsaturated zone properties and long-term average base-flow on the NHDPlusV2 stream network