Regional Managed Aquifer Recharge and Runoff Analyses in Santa Cruz and northern Monterey Counties, California

This study used spatial analysis and modeling to develop practical screening tools to find suitable locations in Santa Cruz and northern Monterey Counties (adjacent to Monterey Bay) where there may be good opportunities to improve groundwater resources using distributed stormwater collection linked to managed aquifer recharge (DSC-MAR). New data sets and methods developed through this project are now available and can be applied by water managers, environmental stewards, and other public and private stakeholders. The study is the first comprehensive, regional effort to: (1) quantify and map suitability for DSC-MAR and (2) evaluate the supply of stormwater runoff in support of MAR. The first part of this project focused on quantifying and mapping spatial variations in suitability for infiltration and storage of excess surface water, using a geographic information system (GIS). Numerous datasets were acquired, processed, patched and combined to identify locations where there is alignment of properties that are most favorable for DSC-MAR. The assessment included surface and subsurface GIS coverages. Surface coverages (available for the full study region) provided an initial screening for MAR suitability and included parameters such as elevation, soil properties linked to infiltration, and bedrock geology (determining “presence or absence” of an underlying aquifer). Subsurface coverages (available only for certain areas within the project region) allowed for a more detailed assessment of opportunities to infiltrate and store stormwater runoff, and included hydrogeologic parameters such as the geometry (lateral extent, thickness) of aquifer and confining layers, transmissive and storage properties, the thickness of soil and rock layers above the shallowest aquifer, and recent changes in groundwater levels. The second part of this project focused on modeling basin runoff response, including spatial variations under a range of climate scenarios (dry, normal, wet), with models driven by historical precipitation data. We used an open source, process-based numerical model (Precipitation Runoff Modeling System – PRMS) to assess potential opportunities for stormwater collection, based on soil properties, vegetation cover, and other hydrologic properties within and among small hydrologic units (25-250 acres) throughout two drainage basins. Once the climate scenarios were developed and simulations were completed, we analyzed model-generated outputs with an emphasis on relations between precipitation and hillslope runoff, before this runoff reaches "blue line" streams. There are many opportunities in the study region locate and develop managed aquifer recharge projects that are supplied with stormwater runoff. More water can be collected during wet years, of course, but that simply emphasizes the importance of developing and running these kinds of projects so that benefits can be achieved under favorable hydrologic conditions. The products of this work (maps, datasets) should be used mainly for screening purposes, complemented by direct assessments on a site-by-site basis. Maps and datasets specific to each basin are available for public access (download) through the Resource Conservation District of Santa Cruz County’s website (http://www.rcdsantacruz.org/managed-aquifer-recharge). Results of this work have direct implications for this region, where communities must make do with limited local resources, but also may serve as a template for other parts of the state, where planning and implementation of new projects to improve the security and sustainability of groundwater resources is expected to be increasingly common