Technical Studies

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What is Known About the Paso Robles Groundwater Basin?


The Paso Robles Groundwater Basin is presently the primary water source for the North County. The basin is large - approximately 505,000 acres (790 square miles). Communities from Garden Farms to San Miguel and Templeton to Shandon and Creston rely on the basin's groundwater. Rural residences, urban development, vineyards and other agricultural uses, and wineries all pump water from the underground basin to use for drinking, landscape and agricultural irrigation, and to meet the day-to-day requirements of living in an arid environment.

Groundwater levels in many parts of the basin have declined over the last 12 years.

Sub-Basin vs. Sub-Area

A sub-basin is defined as a portion of a basin that can be subdivided for hydrologic study purposes. Hydraulically, a sub-basin is interdependent on the basin as a whole, but is locally independent of pumping depressions and recharge effects.

A sub-area is defined as a portion of a basin that is subdivided for water planning purposes.


The County Board of Supervisors has spent several years studying the basin hydrogeology and the demand and supply of the basin's groundwater (these studies are available under the "Technical Studies" section). In February 2011, the Board of Supervisors approved the Paso Robles Groundwater Basin Resource Capacity Study (RCS). It links the state of the basin to land use policy, basin monitoring, and water conservation. The RCS concludes that the groundwater basin is approaching or has reached its perennial yield.

The RCS established a Level of Severity for the groundwater basin (there are three severity levels for resources, from level I to level III). The Board established a Level of Severity III for the main basin and a Level of Severity I for the Atascadero Sub-Basin.

California law does not allow the County to limit how much water a property owner pumps from the ground. The County must therefore use only the authority it has to address this issue. The RCS recommended groundwater monitoring, water conservation, and land use measures to address groundwater demand. The Board of Supervisors adopted the following recommended actions from the RCS:

Monitoring and Conservation Actions

  1. Initiate the development of a groundwater monitoring program.

  2. Continue studies of the groundwater basin and stakeholder coordination efforts.

  3. Develop and implement, in collaboration with other water purveyors within the Paso Robles Groundwater Basin and the Atascadero Sub-basin, a water conservation outreach and education program for the rural area.

  4. Continue to conduct biannual groundwater measurements.

  5. Amend Title 8 in accordance with the recommendations in the Resource Management System Annual Summary Report.

  6. Require that the new wells be a part of the District groundwater level measuring program if needed.

Land Use Measures Actions

  1. Require new discretionary development that does not use supplemental water to use the California Urban Water Conservation Councils (CUWCC) best management practices for water conservation and offset 100 percent of its new water use with non-agricultural water.

  2. In rural areas of the basin, do not approve General Plan amendments that result in a net increase in the non-agricultural use of groundwater. Prohibit new land divisions in the rural areas of the basin. All discretionary development shall offset its water use with non- agricultural water, except that proposed Agricultural Processing uses (as defined in the Land Use Ordinance), including outdoor and other appurtenant water use, shall be subject to project- specific land use and/or water conservation mitigation measures required by the review authority based on environmental review.

  3. New wineries shall use best management practices consistent with the Best Management Practices.

  4. Revise the Growth Management Ordinance and the Resource Management System to substantially limit yearly non-agricultural development in the rural areas of the basin.

  5. Develop a landscape ordinance that will limit the amount of turf and other high-water use features on all parcels within the Paso Robles Groundwater Basin, Including the Atascadero Sub-basin.

  6. Work with stakeholders to develop best management practices for prevalent land uses in the basin similar to the winery BMP.