Groundwater Basin Prioritization

CASGEM Basin Prioritization Results - June 2014

Prioritization Map

In January 2014, DWR conducted 5 workshops to introduce draft Basin Prioritization results and solicit public comments. The Basin Prioritization results, finalized in June 2014, address the comments received and include the most recent available groundwater use data.

Basin Prioritization is being used to align resources in the implementation of the CASGEM Program. Due to limited resources, DWR will focus on the High and Medium priority groundwater basins first.

The list of High and Medium priority groundwater basins that are not monitored under the CASGEM program will be provided to State water grant programs. Although CASGEM is a voluntary program, not participating could result in ineligibility for a State water grant or loan. Grant eligibility will be determined by the respective grant program.

Final Basin Prioritization findings indicate that 127 of California's 515 groundwater basins and subbasins are High and Medium priority. These basins account for 96% of California's annual groundwater pumping and supply 88% of the population which resides over groundwater basins. The remaining 388 basins are Low and Very Low priority and comprise 75% of the groundwater basins in the State.

Final Statewide results are compiled into a summary table in PDF or Excel organized by priority and includes the basin name, number, total score, Hydrologic Region, and DWR Region.

Individual Basin Summary Sheets are available for each of the 515 basins. Each summary provides the ranking value for each of the eight components described above, the basin total ranking score, and resultant basin priority of high, medium, low, or very low.

Please visit the our Groundwater Information Center Interactive Map, a web-based application. With this application you can view individual Geographic Information Systems (GIS) layers containing geospatially-referenced groundwater-related information, including a layer depicting CASGEM Groundwater Basin Prioritization information.  

Additionally, please see the table below for Final Basin Prioritization results by Hydrologic Region with percent of groundwater use and overlying population for all alluvial groundwater basins.

Hydrologic Region (HR) CASGEM Groundwater Basin Ranking HR Basin Count Percent of Totals for High & Medium Ranked Basins
High Ranking Range
Medium Ranking Range
21.07 – 13.43
Low Ranking Range
13.42 – 5.75
Very Low Ranking Range
GW Use* Overlying Population*
North Coast 0 8 2 53 63 82% 62%
San Francisco Bay 0 7 0 26 33 90% 63%
Central Coast 9 15 0 36 60 97% 90%
South Coast 13 22 4 34 73 99% 94%
Sacramento River 5 18 4 61 88 96% 98%
San Joaquin River 7 2 0 2 11 100% 100%
Tulare Lake 7 1 1 10 19 99% 98%
North Lahontan 0 2 3 22 27 12% 55%
South Lahontan 2 4 4 67 77 84% 96%
Colorado River 0 5 9 50 64 82% 61%
Statewide: 43 84 27 361 515 96% 88%

*Note: Estimated percentages are based on total groundwater use and population overlying all alluvial groundwater basins in the hydrologic region.

Ranking detailed information and associated maps are available for viewing and downloading in either PDF or Microsoft Excel format. Below, you can access the maps and lists for the entire State or for DWR's four regional boundaries. Each dataset is offered in different sorting orders. The detailed dataset includes values used in determining the individual component ranking values but is only available by basin number.

Datasets Map By Basin Priority By Basin Number By Basin Name
Statewide PDF PDF/Excel PDF/Excel PDF/Excel
Northern Region PDF PDF/Excel PDF/Excel PDF/Excel
North Central Region PDF PDF/Excel PDF/Excel PDF/Excel
South Central Region PDF PDF/Excel PDF/Excel PDF/Excel
Southern Region PDF PDF/Excel PDF/Excel PDF/Excel
Detailed Dataset        
Statewide     PDF/Excel  

Warning: Files are large and may take some time to download.

CASGEM Basin Prioritization Process

There are 515 alluvial groundwater basins and subbasins in California as defined in DWR's Bulletin 118. These basins contribute close to 40 percent of the California's annual water supply in an average year and as much as 45 percent in dry years. During extensive dry or drought years, groundwater can provide close to 60 percent of the water supply. Statewide, approximately 30 million people, or 80 percent of Californians, live in areas overlying alluvial groundwater basins. Some communities are 100 percent reliant on groundwater.

The California Department of Water Resources (DWR) implemented the California Statewide Groundwater Elevation Monitoring (CASGEM) Program in response to legislation enacted in California's 2009 Comprehensive Water package. As part of the CASGEM Program and pursuant to the California Water Code (CWC §10933), DWR is required to prioritize California groundwater basins, so as to help identify, evaluate, and determine the need for additional groundwater level monitoring.

The CWC requires a statewide prioritization of California's groundwater basins using the following eight criteria:

  1. Overlying population;
  2. Projected growth of overlying population;
  3. Public supply wells;
  4. Total wells;
  5. Overlying irrigated acreage;
  6. Reliance on groundwater as the primary source of water;
  7. Impacts on the groundwater; including overdraft, subsidence, saline intrusion, and other water quality degradation; and
  8. Any other information determined to be relevant by the Department.

The CASGEM Groundwater Basin Prioritization (Basin Prioritization) is a statewide ranking of groundwater basin importance that incorporates groundwater reliance and focuses on basins producing greater than 90% of California's annual groundwater. Although the results are a statewide assessment; it is important to recognize the statewide findings are not intended to diminish the local importance of groundwater including in the smaller size or lower-use groundwater basins.

Prioritization Brochure

The (1) CASGEM Groundwater Basin Prioritization Brochure summarizes the process.

The detailed explanation of the Basin Prioritization Process is included in the Final (2) CASGEM Groundwater Basin Prioritization Process. Appendix A with the figures and tables is available here.

Updated FAQs

Prioritization Technical Paper

Workshop Materials from January 2014

In January 2014, DWR conducted 5 workshops to introduce - Basin Prioritization process and draft results as well assolicit public comments. The final Basin Prioritization results address the comments received and include the most recent available groundwater use data. Please see below for information from the January 2014 workshops.

Presentation from January workshops
Webcast from January 16, 2014 workshop held in Sacramento
Public Comments with DWR responses
CASGEM Overview and Next Steps Brochure (December 2013)


If you have additional questions please contact the appropriate Region Office.