A National Groundwater Model with Improved Resolution of Deep Groundwater Dynamics

Large scale estimates of groundwater dynamics are limited by the model structure, data availability, and aquifer heterogeneity. Previous continental-scale groundwater studies have used near-surface soil or bedrock geology to infer shallow aquifer properties, and homogenized or omitted the deep aquifers which are most heavily relied upon for water supply. To estimate groundwater availability and change in shallow and deep aquifer systems, we used historical USGS aquifer water level data, and estimates of storativity, layer thickness, and permeability, from calibrated regional groundwater models. Groundwater recharge was estimated in two ways: (1) using a land-surface hydrologic model to estimate net recharge, and (2) using statistical parameterization estimating recharge as a function of precipitation, aquifer lithology, and land use. Where available, we use recharge estimates from published studies to inform our modeling. The gridded model structure has a surface layer into which net recharge is added. Subsequent lower layers represent known major aquifers and confining units, or an equivalent representation of multiple layers. Saturated flow between layers and across grid cells is determined based on head and an effective hydraulic conductivity factor which is a function of layer thickness and contact area. Pumping withdrawals were estimated using agricultural crop water requirement models, municipal data, and USGS water use reports. The model was calibrated using hydrologic data between 1950 and 2010, and will be used to optimize land use and urban development within the country under future climate and economic scenarios.