Hydrological analysis of the California Basin Model

California’s water resources are declining and recent droughts have only exaggerated the dire conditions. As rivers and surface water reserves run dry, water managers are forced to drill deeper wells, mining their precious groundwater resources. To better understand California’s water, we developed a surface and subsurface model of the California Basin using HydroGeoSphere (HGS), a 3D finite element control-volume surface and variably-saturated subsurface flow model with evapotranspiration processes. Once the integrated HGS model reached steady-state conditions, groundwater and surface water extractions were turned on, and the HGS model was run to 2011. We then implemented the two-way coupled HydroGeoSphere-Weather Research and Forecasting (WRF) model over a 200 day period. This paper investigates feedbacks between groundwater and evapotranspiration and finds an average decrease of 0.029 mm/day in evapotranspiration per meter of water table depth over the period January 1 st to July 20 th 2011. The simulation results showed that a minimum net precipitation rate of 0.178 m/year (R 2 = 0.998) is required every year to sustainably maintain current water use and groundwater extractions. The simulated results replicated similar drawdown behavior (49 mm/year simulated) as the GRACE remote sensing satellites (43 mm/year measured).