Basin Boundaries

Historically, there were four groundwater subbasins within Yolo County as defined by DWR’s Bulletin 118: Yolo (5-21.67), Colusa (5-21.52), Solano (5-21.66), and Capay Valley (5.21-68) subbasins (see the figure below; 2003).


The Yolo County Flood Control and Water Conservation District (District), on behalf of the Water Resources Association of Yolo County (WRA) and the Yolo County Farm Bureau (Farm Bureau) submitted a formal basin boundary modification request to DWR on March 31, 2016.  The proposed basin boundary modification was a jurisdictional modification involving a consolidation of the Capay Valley (5-21.68) and portions of the Colusa (5-21.52), Yolo (5-21.67), and Solano (5-21.66) subbasins that lie within Yolo County and a boundary adjustment along the Yolo County line.  The modification created a new subbasin called the Yolo subbasin, or Yolo Basin.

The basin boundary modification was approved by DWR on October 18, 2016 as shown below.  The new basin boundaries were included in the interim update of Bulletin 118, due January 1, 2017 — known as Bulletin 118 (2016).


The Yolo Subbasin Groundwater Agency is currently working with Reclamation Districts 150 and 999 to submit a Basin Boundary Modification request to modify the Yolo Subbasin boundaries.  Incorporating RD 150 and RD 999 into the Yolo Subbasin facilitates holistic, effective management of groundwater in Yolo County, and allows for greater economies of scale in complying with SGMA.

Draft Yolo Subbsin BBMRS_20171013


From the 2016 basin boundary request:

The leadership of the WRA and Farm Bureau believe that the most functional and efficient basin boundary for the subbasins overlapping Yolo County would be to simply use the existing County geopolitical borders.  The modification reduces the potential complexities associated with keeping the original Bulletin 118-defined subbasins as is; effectively streamlines the processes involved in forming a Groundwater Sustainability Agency (GSA) and developing a Groundwater Sustainability Plan (GSP) since only one basin needs to be considered.  The  Yolo subbasin will promote sustainable groundwater management by building on the existing structure of the WRA and by improving efficiency between land use planning and water resource management in Yolo County.  Additionally, the Yolo subbasin provides the following advantages:

  • Utilizes the existing WRA-based water resource planning governance structure, and builds on the existing local agency technical and programmatic relationships in Yolo County;
  • Preserves the existing rights and authorities of the WRA’s member agencies, including land use authorities of the County and Cities;
  • Ensures the interests of private well owners are considered;
  • Serves as the California Statewide Groundwater Elevation Monitoring (CASGEM) Monitoring Entity;
  • Incorporates the Yolo County Water Resources Information Database (WRID) monitoring network;
  • Utilizes the Integrated Groundwater and Surface water Model (IGSM), Integrated Water Flow Model (IWFM), and Water Evaluation And Planning system (WEAP) surface and groundwater models; and
  • Advances the subsidence monitoring network that has served Yolo County for the past 15 years.


Please direct questions or comments to Kristin Sicke at

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