ASSESSING REGIONAL GROUNDWATER AVAILABILITY IN CALIFORNIA’S COASTAL BASINS

California coastal basins (CCBs), classified by the USGS as a principal aquifer system, is among the Nation’s largest producers of potable groundwater, specifically groundwater used for public supply. The CCBs consist of 131 groundwater basins distributed along the coastal regions of California. In the year 2000, fresh groundwater withdrawals from the CCBs accounted for an estimated 4% of total withdrawals from all aquifers across the U.S. The USGS is developing an updated estimate of groundwater availability in the CCBs. As part of this estimate, a unifying framework is being developed for analyzing and displaying water budget components for each of the 131 CCBs, including many that did not previously have groundwater flow models. This study is particularly timely and relevant to California’s Sustainable Groundwater Management Act of 2014 (SGMA). The wide hydrogeologic diversity among the CCBs presents a unique challenge for a system-wide modeling approach; therefore, a variety of methods are being employed in this study. To assist decision-makers in understanding the status and trends of groundwater availability in this disconnected system, the water budget components are being framed as consistently as possible. The framework is designed to highlight historic, current and potential future conditions for groundwater use and availability among the CCBs. The framework consists of a three-pronged modeling approach: (1) existing models are being updated, analyzed, and summarized; (2) new models are being built in areas without existing models; and (3) metamodels are being developed from existing models. For new model construction, numerical groundwater flow models are generated from multiple phases of hydrogeologic conceptual models with increasing complexity. The effect of increased complexity is evident in groundwater flow model estimates of water budget components. For metamodels, existing models are condensed to a statistical regression of model outputs on model inputs. These metamodels can be used to predict the effect of changes in model inputs, such as additional pumping, on various water budget components. Overall, a synthesis of various model datasets is being integrated into a web-based tool to view the water budget status in a variety of basins.