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.