Critically Overdrafted Basins
The Sustainable Groundwater Management Act (SGMA) directs the Department of Water Resources (DWR) to identify groundwater basins and subbasins in conditions of critical overdraft. As defined by SGMA, “A basin is subject to critical overdraft when continuation of present water management practices would probably result in significant adverse overdraft-related environmental, social, or economic impacts.”
Overdraft occurs where the average annual amount of groundwater extraction exceeds the long-term average annual supply of water to the basin. Effects of overdraft result can include seawater intrusion, land subsidence, groundwater depletion, and/or chronic lowering of groundwater levels.
In 2015, DWR evaluated California’s groundwater basins for conditions of critical overdraft. DWR consulted with the State Climatologist and identified a base period for this evaluation of water years 1989 to 2009, which includes wet and dry periods and has the same mean precipitation as the long-term mean. DWR used available data, reports, and other information to identify basins with obvious and reported adverse impacts.
In August 2015, DWR developed a draft list of 21 basins subject to conditions of critical overdraft and presented this list to the California Water Commission, on the DWR website, and to the public through a webcast meeting. DWR sought public comments for 60 days and these comments were considered. A description of the evaluation process and the final list is presented in Bulletin 118 – Interim Update 2016.
As required in the SGMA, all Bulletin 118 basins designated as medium or high priority and critically overdrafted shall be managed under a Groundwater Sustainability Plan (GSP) by January 31, 2020. All other medium and high priority basins must be managed under a GSP by January 31, 2022.
Maps and Resources
Bulletin 118 Interim Update 2016
The 2016 Interim Update of Bulletin 118 includes a detailed description of the process for evaluating critical conditions of overdraft.
Public Meeting Presentation
Presentation covers background of Critically Overdrafted Basins, process for determining Critical Overdraft status, as well as a review of the draft results and next steps.
- Recorded Webcast
Senior Engineering Geologist
901 P Street
P.O. Box 942836
Sacramento, CA 94236