California Groundwater Basin Boundaries Change

The California Department of Water Resources (DWR) released final 2016 modifications to California’s groundwater basin boundaries, completing a critical step in the implementation of the state’s Sustainable Groundwater Management Act (SGMA).   DWR presented the final basin boundaries to the California Water Commission, which approved them. Included in the approved boundary modifications were changes proposed by the Orange County Water District to DWR's Basin 8-1, the Coastal Plain of Orange County Groundwater Basin.

When SGMA was established in 2014, it initially established the basin boundaries as defined in Bulletin 118, a comprehensive report on California groundwater resources that DWR periodically updates. SGMA also established a process for local agencies to request that DWR modify basin boundaries to assist local agencies in forming the required Groundwater Sustainability Agencies (GSAs). Of the 54 requests for changes to basin boundaries, DWR approved 39, denied 12, and three were deemed incomplete. Most of the modifications were made to basins in the Central Valley and included refinements reflecting waterways, county lines and geologic information.

After the passage of SGMA, the Orange County Water District reviewed the boundaries of Basin 8-1, which were published in the first Bulletin 118 in 1975 and in subsequent updates in 1980 and 2003. Having carefully reviewed the boundaries established for Basin 8-1 and comparing these with more recent and accurately-digitized geologic maps, OCWD staff identified inaccuracies in some areas. These inaccuracies could have been due to distortions caused by digitizing paper maps created in the 1970s and then projecting them onto current and more accurate geographic information system (GIS) base maps. OCWD staff considered the distortions to be significant enough from a scientific perspective to warrant modification to Basin 8-1 boundaries to improve accuracy and consistency with the originally-intended DWR geologic boundaries.

To improve the accuracy of the Basin 8-1 boundary, OCWD staff re-digitized the Basin 8-1 boundary to more accurately follow the geologic boundaries using properly georeferenced geologic base maps. OCWD shared the proposed Basin 8-1 boundary with other agencies within the basin and made further modifications based on feedback from the cities of La Habra and Corona. This new boundary modification was approved by DWR.

The new Basin 8-1 boundary will assist in achieving sustainable groundwater management by correctly defining the groundwater basin boundaries. OCWD has been collaborating with other agencies within Basin 8-1, including the City of La Habra and Irvine Ranch Water District to present a report to DWR to show that Basin 8-1 has been sustainability managed for the past decade. This report will be presented to DWR by the end of this year.