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

Abstract

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.

Quantifying the impact of lagged hydrological responses on the effectiveness of groundwater conservation

“…However, the relative contributions of vertical and lateral flows to net inflow are poorly understood (Butler et al, 2016(Butler et al, , 2020b.While recent work has found that reductions in aquifer net inflow can decrease the effectiveness of groundwater conservation programs over time (Butler et al, 2020b), it is highly uncertain how the mechanisms, timescales, and magnitudes of lagged responses from different water balance components vary. For example, estimates of the magnitude and transit time for groundwater recharge vary dramatically over the HPA due to thick unsaturated zones (Gurdak et al, 2008;Katz et al, 2016;McMahon et al, 2006;Zell & Sanford, 2020). As a result, we do not know which lagged responses may impact overall groundwater sustainability, nor the timescales and controlling processes.…”

Use case development for earth monitoring, analysis, and prediction (EarthMAP)—A road map for future integrated predictive science at the U.S. Geological Survey

“…Groundwater/surface-water modeling: The Center will use a cutout of the National Groundwater Model (NGWM; Zell and Sanford, 2020) to simulate transient groundwater flow, groundwater/surface-water interactions, and nitrogen loading from groundwater discharge to streams. The NGWM is an innovative, MODFLOW 6 based model of the shallow groundwater system for the contiguous United States.…”

GMD Perspective: the quest to improve the evaluation of groundwater representation in continental to global scale models

“…Given the strengths of regional models, a potential alternative to development of large-scale groundwater models would be combining or aggregating multiple regional models in a patchwork approach (as in Zell and Sanford, 2020) to provide global coverage. This would have the advantage of better respecting regional differences but potentially create additional challenges because the regional https://doi.org/10.5194/gmd-2021-97 Preprint.…”