Evaluation of Groundwater Resources in the Adelaida Area of San Luis Obispo County, California

Science Center Objects

Stakeholders in San Luis Obispo County are concerned that the increased demand for water use has, and will continue to, affect groundwater levels and availability in the Adelaida area. To address stakeholder concerns, the County of San Luis Obispo Board of Supervisors has asked the USGS to conduct a comprehensive evaluation of groundwater resources of the Adelaida area. 

Interactive Map  |  Water Supply  |  Land Use  |  Groundwater Flow  |  Outreach


Groundwater is the primary source of water in the Adelaida area and local stakeholders use private domestic and commercial wells to pump groundwater from the aquifer system. There is concern among stakeholders that the increasing demand for water is negatively affecting groundwater levels and groundwater availability. In response to these concerns, the County of San Luis Obispo Board of Supervisors (County Board) seeks to evaluate groundwater management solutions in the Adelaida area and has asked the United States Geological Survey (USGS), in cooperation with the San Luis Obispo County Flood Control and Water Conservation District (SLOFC&WCD), to provide a comprehensive evaluation of historic and current groundwater resources in the area. 

aerial view of Paso Robles, CA

View of Paso Robles and the Adelaida area near the central California coast as taken from an airplane flying overhead. (Credit: Geoff Cromwell, USGS. Public domain.)

The USGS will gain an increased scientific understanding of the hydrologic system in the Adelaida area by compiling and collecting hydrogeologic and hydraulic data in the defined study area. The California Department of Water Resources (DWR) has not defined the highlands of the Adelaida area as a groundwater basin, therefore the area is not subject to California Sustainable Groundwater Management Act (SGMA) regulations (CADWR, 2016). The eastern border of the Adelaida area is outside of of the Salinas Valley-Paso Robles Area (Paso Robles sub-basin) and Salinas Valley-Atascadero Area (Atascadero sub-basin) groundwater basins (CADWR, 2016; fig. 1). Publicly available hydrogeologic and hydraulic data (such as aquifer yield, water quality, or water availability) are limited throughout the Adelaida area, and to date, no investigation of the groundwater resources or water-bearing units of the aquifer system of the Adelaida area has been completed. The Adelaida Study (Study) will extend to the eastern area of the Paso Robles sub-basin and Atascadero sub-basin to better understand the groundwater connectivity between the sub-basin boundaries and the Adelaida areas.

This Study will benefit water managers, stakeholders and potential future interested entities and studies by providing: 

  • an improved understanding of the aquifer system, including identifying the water-bearing units 
  • evaluating the historic and current impacts of groundwater use in the Adelaida area 
  • quantifying the groundwater resources in an undefined local aquifer system 
  • advancing monitoring networks in the County for groundwater and water-quality data 
  • providing a more thorough understanding of geological controls on water availability 

Interactive Map

In order to understand the hydrologic system, many different types of data are being gathered from the surface water and groundwater system. Learn about these data types and explore them with the interactive map.


Water Supply 

Water districts for the communities of Paso Robles, Templeton, and Atascardero supply water sourced from a combination of groundwater from the Paso Robles and Atascadero sub-basins, underflow from the Salinas River, recycled water, and surface water from Nacimiento Reservoir (Carollo, 2012b). These communities primarily supply water to residents in the more populated urban and suburban zones in the eastern border of the Adelaida area and vicinity. In the northern part of the Adelaida area, the unincorporated community of Heritage Ranch sources its water downstream of Nacimiento Reservoir through its own water district; the service area of this water district is limited in its extent in the Adelaida area. Stakeholders outside these four communities likely rely exclusively on groundwater and utilize private domestic and commercial wells to meet their water demand. 

Back to top >>


Land Use 

Stakeholders in San Luis Obispo County have a specific need to evaluate and understand historic and current hydrologic conditions in the Adelaida area to consider the current and future population and agricultural water needs. In the highlands of the Adelaida area, groundwater is pumped for rural use and for agriculture, primarily vineyards.   


The California Department of Water Resources (DWR) began conducting land-use surveys in 1950’s as a way to better understand current and future water needs. DWR land-use surveys focus mostly on agricultural land and include more than 70 different crop categories, however urban and native vegetation information is also collected. Surveys are compiled from aerial imagery as well in person field visits. (California Department of Water Resources, 2020).  


Farmland M​apping and Monitoring Program (FMMP) 

The California Department of Conservation’s Farmland Mapping and Monitoring Program (FMMP) was established in 1982 in response to a rising need for understanding the location, quality, and quantity of agricultural land and changes in this land over time. The FMMP produces hybrid resources quality and land use maps as well as statistical data. These data are used for analyzing impacts on California’s agricultural resources. Since DWR land-use studies are conducted less frequently than FMMP studies, the FMMP data provides more detailed insight into potential changes in land-use over time (California Department of Conservation, 2019). 


donut charts comparing irrigated land use in the Adelaida Area of San Luis Obisbo County, CA for 1984, 1996, 2004, 2014 and 2016

Irrigated land-use in the Adelaida Area of San Luis Obisbo County, CA for 1984, 1996, 2004, and 2014. Numbers indicate acres and percent of total acres. Source: California Department of  Conservation Farmland Mapping and Monitoring Program, https://www.conservation.ca.gov/dlrp/fmmp. (Public domain.)

Back to top >>


Groundwater Flow 

The direction of groundwater flow in the Adelaida area highlands is unknown at present, but likely flows from west to east, following the topographic gradient from the highlands towards the Paso Robles and Atascadero subbasins. Groundwater in the Paso Robles and Atascadero subbasins flows north and east towards the Salinas River (Todd Engineers, 2007). Folds, faults, and fractures in the basement and consolidated sedimentary rocks may provide conduits for groundwater flow, may compartmentalize the aquifer system, and (or) may impede groundwater flow.  

The study will be completed over a five-year period using staff from the USGS California Water Science Center, while drawing on expertise of local stakeholders, including the SLOFC&WCD. 

Back to top >>



The USGS participates in community outreach meetings hosted by the County of San Luis Obispo Public Works department.  The meetings offer an opportunity to provide updates on the study.  Recordings of the meetings are available online



California Department of Conservation [2019], Program Overview: California Department of Conservation web page, accessed November 3, 2020, at https://www.conservation.ca.gov/dlrp/fmmp/Pages/Program_Overview.aspx

California Department of Water Resources, 2000, 1996 San Luis Obispo County Land Use Survey, accessed January 3, 2020 at https://gis.water.ca.gov/app/CADWRLandUseViewer/.

California Department of Water Resources, 2016, Bulletin 118-Interim Update 2016, California's Groundwater, Working Toward Sustainability: California Department of Water Resources, 226 p., accessed November 10, 2018 at https://water.ca.gov/Programs/Groundwater-Management/Bulletin-118

California Department of Water Resources [2020], Land Use Surveys: California Department of Water Resources web page, accessed November 5, 2020, at https://water.ca.gov/Programs/Water-Use-And-Efficiency/Land-And-Water-Use/Land-Use-Surveys.

LandIQ, 2017, i15_Crop_Mapping_2014_Final, Geospatial Dataset prepared for California Department of Water Resources, accessed January 3, 2020 at https://gis.water.ca.gov/app/CADWRLandUseViewer/.

Todd Engineers, 2007, Update for the Paso Robles Groundwater Basin, San Luis Obispo, California, accessed January 6, 2020 at https://www.slocounty.ca.gov/Departments/Public-Works/Forms-Documents.aspx 

Carollo, 2012b, Master Water Report – Volume II of III: San Luis Obispo County Flood Control and Water Conservation District, San Luis Obispo, California, 347 p., accessed October 1, 2019 at https://www.slocounty.ca.gov/Departments/Public-Works/Forms-Documents/Water-Resources/Master-Water-Report.aspx