A new look at regional groundwater flow using a high-resolution integrated hydrologic model of the Continental U.S.

While regional groundwater flow has been a topic of research for more than a century, open questions still remain regarding the relative importance of topography, geology and climate in determining groundwater configuration. Previous research on this topic has utilized analytical solutions, observations and regional hydrologic models. However, to date no study has considered groundwater flows at the continental scale using an integrated groundwater surface water model. Here we present the results of a high-resolution (1km) simulation of the majority of the Continental US generated using ParFlow, a physically based integrated hydrologic model. The model is forced with spatially distributed recharge derived from historical precipitation and evaporation and the subsurface is composed of laterally heterogeneous geologic units. Results illustrate clear multi scale behavior and regional shifts in the relative control of topography, geology and climate on water table depth and groundwater flux. Previous analysis at the continental scale, has used non-dimensional water table ratios to classify water table configurations as topographically dominated or recharge dominated, while groundwater exchanges with the surface have been classified using the ratio of runoff to total recharge. In agreement with previous studies, model results highlight relatively greater topographic control and more significant groundwater exchanges with streams in the arid west than in the humid east. However, by using an integrated modeling platform that solves the surface and subsurface systems simultaneously we are also able to demonstrate the limitations of metrics that rely on simplifications. While we observe regional patterns in groundwater that are consistent with large-scale gradients in aridity and geographic features; at the local scale the interactions with subsurface heterogeneity are complex. In some cases subsurface heterogeneity is clearly manifest in groundwater fluxes while in others topographic gradients control the flow. Results demonstrate the complexity of the natural system and the difficulty in classifying groundwater behavior using simplified approaches.