Runoff Modeling of a Coastal Basin to Assess Variations in Response to Shifting Climate and Land Use: Implications for Managed Recharge

We quantified the distribution of hillslope runoff under different climate and land-use conditions in a coastal, mixed land-use basin, the Pajaro Valley Drainage Basin (PVDB), California, USA, in order to evaluate opportunities to improve groundwater supply. We developed dry, normal, and wet climate scenarios using high-resolution historic data and compared contemporary land use to pre-development land use under the different climate scenarios. Relative to pre-development conditions, urban and agricultural development resulted in more than twice as much simulated runoff generation, greater spatial variability in runoff, and less water available for recharge; these differences were most pronounced during the dry climate scenario. Runoff results were considered in terms of potential to support distributed stormwater collection linked to managed aquifer recharge (DSC-MAR), which routes excess hillslope runoff to sites where it can infiltrate and enhance groundwater recharge. In the PVDB, 10% of the annual groundwater deficit could be addressed by recharging 4.3% of basin-wide hillslope runoff generated during the normal scenario, and 10.0% and 1.5% of runoff during the dry and wet scenarios, respectively. Runoff simulation results were combined with an independent recharge suitability mapping analysis, showing that DSC-MAR could be effective in many parts of the PVDB under a range of climate conditions. These results highlight the importance of strategically locating DSC-MAR projects at the confluence of reliable supply and favorable subsurface hydrologic properties.