Runoff modeling of a coastal basin to assess variations in response to shifting climate and land use: Implications for managed recharge

We quantified hillslope runoff distribution in a coastal, mixed-use basin, the Pajaro Valley Drainage Basin (PVDB), under different climate and land use conditions, evaluating the potential for distributed stormwater collection coupled with managed aquifer recharge (DSC-MAR) to improve groundwater supply. We developed dry, normal, and wet climate scenarios using historic precipitation and temperature data, and we compared contemporary land use to pre-development land use by replacing developed areas in the model with plausible native vegetation types. Relative to pre-development land use, urban and agricultural development resulted in up to 2.3 times as much simulated runoff generation, greater spatial variability in runoff, and less basin-wide groundwater recharge; these differences were most pronounced during the dry climate scenario. These results provide quantitative motivation for DSC-MAR projects, which collect excess hillslope runoff and route it toward underlying aquifers to improve groundwater supply. Aquifers in the PVDB are in a state of chronic overdraft and we found that addressing 10% of the region's annual groundwater deficit would require recharging 4.3% of basin-wide runoff during the normal scenario, and 10.0% and 1.5% of runoff during the dry and wet scenarios, respectively. During the dry scenario, runoff generation was focused within a smaller area; the 10% of the basin that generated the most runoff accounted for 46% of basin-wide runoff during the dry scenario and just 31% in the wet scenario. An MAR suitability map combining soil, bedrock, and aquifer properties with runoff supply shows that DSC-MAR projects could be effective in many locations throughout the basin during all climate scenarios and highlights the importance of strategically locating DSC-MAR projects to maximize potential water supply benefit.