Land subsidence along the California Aqueduct is threatening the ability of the State Water Project to deliver water, an important piece of infrastructure for millions of Californians. The recently released addendum to the 2023 SWP Delivery Capability Report from the Department of Water Resources reveal significant reductions in flow capacity as sinking land constricts the aqueduct’s channels, particularly in subsidence-prone areas of the San Joaquin Valley. With repairs estimated to cost billions, officials are exploring engineering fixes, collaboration with local agencies, and funding solutions to safeguard one of California’s most vital water delivery systems.
At June’s Metropolitan Imported Water Subcommittee meeting, Sarah Bartlett, interim manager for the State Water Project Supply and Programs Team, explained the challenges and outlined DWR’s efforts to address them.
HOW SUBSIDENCE IMPACTS THE CALIFORNIA AQUEDUCT
Land subsidence along the California Aqueduct occurs when the land sinks due to the compaction of underground layers, often as a result of overpumping groundwater. However, oil extraction can also contribute to this sinking, as the removal of oil can lead to the compaction of the surrounding rock layers.
Subsidence has been a well-known issue in the San Joaquin Valley for years. When the aqueduct was built in the 1960s, engineers designed it to remain functional even if the surrounding land continued to sink. At the time, there was hope that bringing in water through the aqueduct would reduce the need for excessive groundwater pumping and help address the problem at its source.
The graphic below shows how subsidence impacts flows in the California Aqueduct. The left side is flowing at the design grade; the right shows how subsidence creates a choke point in the aqueduct, restricting the flow.
This map shows the location of the bowls of subsidence located along the California Aqueduct in the San Joaquin Valley.
The most significant increase in subsidence in recent years has been in the two bowls located along the San Luis Canal section of the California Aqueduct. The San Luis canal is a joint-use facility that conveys water for both DWR for the State Water Project and for the US Bureau of Reclamation for the Central Valley Project. There has been a measurable uptick in subsidence in this area since 2014.
ADDRESSING THE PROBLEM
DWR established the California Aqueduct Subsidence Program (CASP) in 2017. In 2019, the CASP published a comprehensive study and supplement that quantified the problem and identified the need for action to maintain aqueduct operations and to quantify the future impacts of subsidence.
CASP developed two models: a hydraulic capacity model to quantify the maximum flow in a subsided aqueduct, and a probabilistic model that projects the magnitude of future land sinking under various levels of groundwater overdraft. Then, DWR modified the Cal Sim model to utilize the output from those two models to forecast the State Water Project’s delivery capability under subsided conditions. Last month, DWR published an addendum to the 2023 Delivery Capability Report detailing the results.
“The bottom line is that the analysis shows that current subsidence reduces the total Table A delivery capability of the State Water Project by 3% compared to the baseline 2023 report,” said Ms. Bartett. “We don’t really feel the impacts of these reductions, because DWR has modified aqueduct operating criteria to keep the water flowing, albeit at an increased power cost. If no action is taken, the probabilistic subsidence model projects significant reductions in aqueduct capacity in 2043, under continued overdraft conditions. The modeling shows potential reductions in table A delivery of 6 to 84% due to subsidence alone.”
To address the subsidence, DWR is planning both near-term and long-term fixes. In the near term, DWR is working to raise the liner along 27 miles of the San Luis Canal and remove a subsided check structure. This work was to begin this year, but has been delayed until 2028 to give the US Bureau of Reclamation time to secure its share of funding. The estimated cost for this work is $32 million, with the cost subject to a joint cost share between the state and the Bureau on a 55/45% basis, as outlined in the joint use agreement. In the long term, DWR is evaluating engineering alternatives to restore the flow in the California Aqueduct to original design parameters. This includes evaluating a fix-in-place alternative, which would reconstruct the aqueduct.
“If we were to follow normal cost reimbursement provisions, the current planning estimate for the cost of this work to the State Water Project contractors is approximately $3 billion, and about half of that would be Metropolitan’s responsibility,” said Ms. Bartlett.
DWR is coordinating with the Bureau and various stakeholders on evaluating various alternatives. The Bureau held a value planning workshop in May to assess alternatives in a big-picture, thematic way. Some of the themes addressed were reconstruction, bypassing the subsided areas, permanently changing operating criteria, and adding in-line pumping plants to bypass the problem. DWR is working with state and federal partners to secure additional funding to address subsidence.
WORKING WITH LOCAL GSAs
Another focus of DWR and the State Water Contractors is to collaborate with the GSAs in the area. Under SGMA, the various groundwater sustainability agencies (GSAs) are required to manage their respective groundwater basins without causing undesirable results by 2040, including significant and unreasonable subsidence that substantially interferes with surface land uses, such as the California Aqueduct. DWR, the State Water Contractors, and Metropolitan are submitting comments on groundwater sustainability plans and exploring potential programs to provide surface water to subsidence-prone areas.
The map shows the various agencies and groundwater sustainability plans located along the California Aqueduct. The aqueduct itself is color-coded to show areas of sinking since the 1967 survey.
“The most subsidence has occurred in the purple shaded zone, which shows the West Side subbasin, which is managed by the Westlands Water District GSA,” said Ms. Bartlett. “Metropolitan, DWR, and the State Water Contractors recently weighed in on their amended plan with comments that it was not sufficiently protective of the aqueduct. We believe allowing subsidence to continue through 2040 will be detrimental to the California Aqueduct operations and the proposed near-term fixes, which are in violation of SGMA.”
The Kern subbasin is currently under review for probation by the State Water Resources Control Board, in part due to insufficiently addressing subsidence issues affecting the aqueduct. However, in the Kern subbasin, shown in yellow south of Westlands, the subsidence isn’t as severe. The Westside District Water Authority GSA has recently adopted a groundwater extraction moratorium within a two-and-a-half-mile buffer along the aqueduct to eliminate potential impacts. The moratorium may also help provide data to show that oil recovery may be a contributing factor in this area.
The recently published addendum is a call to action, rather than a long-term planning document, according to DWR. They recommend continuing to use data from the 2023 Delivery Capability Report, published last year, for long-term planning in urban water management plans, as DWR has plans to address capacity constraints through both near-term and long-term actions.
WHO WILL PAY FOR THE MULTI-BILLION FIX?
During the discussion portion, subcommittee members balked at the $3 billion price tag, of which Metropolitan’s portion would be $1.5 billion. “Since MWD didn’t cause the problem, why would we be on the hook for paying such a substantial portion of that?” asked Chair Mark Gold.
“The contractors are definitely not interested in paying for something that was perhaps caused by others,” said Ms. Bartlett. “All options are on the table right now. DWR is really focused on identifying the best solution to fixing the aqueduct and working collaboratively on value engineering. Then there will be future discussions on who pays and how that payment happens.”
General Manager Deven Upadhyay stated that the 2014 Sustainable Groundwater Management Act is now beginning to yield results. “We’re weighing in as Metropolitan and also through the State Water Contractors when we’re seeing things develop with the SGMA actions in the valley that we don’t think are adequate to protect our long-term interests, such as addressing the California Aqueduct. I think we’re having an effect, and the state is also very interested in follow-through there.”
“We are encouraging DWR to follow through with their plan to rehabilitate the aqueduct capacity as that’s the easiest climate adaptation project that DWR has in their hands right now,” he said, noting that the recently passed Proposition 4 might be a source of funds. “DWR needs to address where the funding comes from, not by first going to the State Water Contractors, but by using the tool that was passed by the voters with the climate bond and then arguing for the federal funding to come in on the joint use piece which we advocated for in Washington DC.”
“As you can gather from my background, I’m very much used to the Polluter Pays principle, so that the folks who are responsible and liable for the harm that was caused should have basically a really big share of paying for that,” said Chair Gold. “Do folks ever talk about the water agencies responsible for allowing that harm to occur actually paying their fair share to create some resilience into the system?”
“As somebody who knows the Central Valley pretty well, there’s been a legacy of abandonment,” said Chair Adan Ortega. “In the past, there was a vested interest in the land owners to sustain the health of the valley. However, when overdrafting accelerated, especially after the 1980s and 1990s, and agriculture became globalized, many of the big players in the valley found profit in other places. There was the overextraction, as well as, to some degree, the abandonment of the local economy, including the suppliers, chemical vendors, and irrigation companies. The subsidence is a big physical manifestation of the transformation to globalized agriculture. And it’s hard to find who … many of the people that are there today were not the people that started it.”
“The question you pose has to be asked, but on the other hand, I think we need an expedited solution to this so that we can get the maximum deliveries possible.”
