Study Finds That Subsidence, Groundwater Over-Pumping Could Limit Future Water Deliveries if No Action is Taken

A recently released technical report concludes that the sinking of land in the Central Valley due to over-pumping of groundwater, referred to as subsidence, has restricted the amount of water the State Water Project (SWP) can deliver in a year by 3 percent. By 2043, if no action is taken, the current trajectory of subsidence, combined with climate change, could reduce deliveries by 87 percent. The SWP, one of California’s primary water storage and delivery systems, helps supply water to 27 million Californians and 750,000 acres of farmland and businesses throughout the state.

 

The technical report, an addendum that builds on the Delivery Capability Report (DCR) released in 2024, analyzed the capability of the SWP to deliver water under both current and potential future conditions in the year 2043. The new findings underscore the importance of eliminating groundwater overdraft in the Central Valley and repairing existing damage to the state’s main water-delivery arteries.

 

The Delivery Capability Report presented findings that if no new adaptation actions are taken, SWP delivery capability and reliability could be reduced by as much as 23 percent in 20 years due to changing flow patterns and extreme weather shifts. Such a decline is equivalent to the loss of nearly 500,000 acre-feet, enough to supply 1.7 million homes for a year.

 

The addendum analyzes additional potential changes to future deliveries due to damage to major SWP canals tied to groundwater pumping and oil and gas production. It also looks at the impacts under different combinations of hydrology and subsidence scenarios.

 

Looking ahead to 2043, the addendum finds that:

• The long-term average delivery capability of the SWP is reduced by 18 to 87 percent compared to the Delivery Capability Report baseline.
• Under a moderate climate change scenario coupled with a moderate level of subsidence, average annual SWP deliveries could decline 400,000 acre-feet to 1.8 million acre-feet.
• Under extreme climate change and subsidence conditions, the project’s average annual deliveries could decrease by 1.5 million acre-feet, to 295,000 acre-feet.
• Failure to halt subsidence and fix canal chokepoints will limit the state’s ability to move water into storage in high-precipitation years and cut into supplies the state needs to endure dry years. 

The addendum also explores different levels of compliance with the Sustainable Groundwater Management Act (SGMA) because of the uncertainties around how and when groundwater basins will be brought into sustainable management. Excessive pumping of groundwater as well as oil and gas production along some stretches of the canals have caused the overlying surface of the land to fall.

 

Sinking land has created dips or “bowls,” where water pools, restricting the movement of water. SWP operators have adjusted operations for years to overcome the effects of subsiding land, but they have reached the limits of those adjustments. The effect of 2023 levels of subsidence can be observed in the need to run water closer to the top of the concrete liner in subsided canals than standard operating procedures would dictate.

 

Strategies to Address Subsidence

In 2017, DWR released its first in-depth study of the magnitude, location, and effects of subsidence on the California Aqueduct, a critical part of the SWP. DWR then created the California Aqueduct Subsidence Program to work with the U.S. Bureau of Reclamation, the owner of the San Luis Canal, and local groundwater sustainability agencies on a range of solutions to prevent the effects of subsidence from impending operations.

 

DWR is protecting SWP water deliveries by implementing projects to restore SWP capacity, engaging with ground water sustainability agencies to stop additional detrimental subsidence, and working with its partners to secure additional funding sources for this work. DWR is committed to ensuring that the dire outcomes of a future with unchecked subsidence do not occur.  

To learn more about the impacts of subsidence on SWP facilities in the San Joaquin Valley, visit the DWR webpage.