The aqueduct capacity in certain areas of the Southern San Joaquin Valley is nearly reduced by half due to subsidence.
In California, 80% of the usable fresh water is located far from where it’s actually used, so water conveyance infrastructure is vital to the state’s existence. Spanning 444 miles, the California Aqueduct is a vital component of the State Water Project, serving as a crucial source of water for 27 million Californians, providing water for agriculture and municipalities. Unfortunately, land subsidence in the San Joaquin Valley has drastically diminished its capacity to deliver water, with reductions reaching up to 46% in some places. Addressing this problem will require a substantial investment of billions over the next 20 years.
Given the importance of water supply reliability to the state, the California Aqueduct Subsidence Program is a priority for the State Water Project. At the October meeting of the California Water Commission, the Commission received a briefing on the California Aqueduct Subsidence Program by Program Manager Jesse Dillon.
Subsidence in the Southern San Joaquin Valley nearly halves the aqueduct capacity in some areas
The area south of the Dos Amigos pumping plant to the Buena Vista pumping plant in the south is the most impacted. This 164-mile stretch, represented by the yellow and purple horizontal lines, relies entirely on gravity and a continuous downstream hydraulic slope to move the water.
The San Luis Canal is part of the joint-use facilities of the Central Valley Project; it was built by the Bureau of Reclamation and is operated and maintained by the State Water Project. The San Luis Canal begins at the San Luis Reservoir, which is located in Merced County, California. It extends southwest through the San Joaquin Valley and ultimately ends near the city of Kettleman City. The San Luis canal provides irrigation water supply to Central Valley Project customers for almost 1 million acres of some of the most productive agricultural lands in the San Joaquin Valley.
South of Kettleman City, the California Aqueduct, built and maintained by State Water Project, continues on from the San Luis Canal, delivering water to nearly 700,000 acres of farmland in Kings, Kern, and eventually, Ventura and San Diego counties. The California Aqueduct also supplies more than 20 million people from the Central Coast to San Diego with water for their municipal and industrial needs.
A December 2023 report documented subsidence in the San Joaquin Valley and how it has reduced the conveyance capacity of the San Luis Canal by 46% and the California Aqueduct by 44%. These capacity losses lead to decreased water deliveries, higher delivery costs, and increased operational risks. The reduced efficiency and rising maintenance costs are direct consequences on the system and prevent it from functioning as originally designed.
Measuring the subsidence
The graph below shows the measured subsidence on a yearly basis. The plot for the San Luis Canal is on top; the plot for the California Aqueduct is on the bottom. The lines would be flat if subsidence had not occurred. Instead, the differential subsidence creates sags or choke points in the conveyance system along the alignment. These sags make operating the system as originally designed challenging, causing constrictions in delivery capacity. As the subsidence continues, these reductions will worsen, and the shrinking freeboard height between the water surface and the top of the concrete liner will continue to increase operational risks, which must be balanced against downstream water demands to prevent further impacts.
SGMA requires groundwater basins to be sustainably managed and brought into balance by 2040 and plays a significant role in addressing subsidence. “It’s crucial that groundwater sustainability agencies along the San Luis Canal and California Aqueduct make decisive management decisions under the Sustainable Groundwater Management Act and strictly adhere to the groundwater sustainability plans they’ve developed for themselves,” said Mr. Dillon.
One of the major projects has been installing equipment to fill in data gaps along the San Luis Canal and the California Aqueduct facilities. This equipment facilitates the ability to more accurately and more frequently measure ground surface elevations, monitor groundwater levels at a multitude of different depths, and track subsurface soil compaction. The collected data helps to evaluate and inform the relationship between these factors and is made publicly available through DWR SGMA Data Viewer.
“These instruments, through the data they collect, will enhance our understanding of subsidence in San Joaquin Valley, move us closer to establishing causality, and guide recommended management decisions for groundwater sustainability agencies, DWR Sustainable Groundwater Management Office, and the water board to prevent critical infrastructure from future harm,” said Mr. Dillon.
To date, 9 of the 23 planned monitoring installations have been completed. Each installation includes equipment to measure ground surface elevations, monitor groundwater levels at different depths, and track subsurface soil compaction. These installations are already providing data that enhances the ability to monitor and respond to subsidence impacts more effectively.
Addressing the problem: A long-term, multi-billion dollar fix
Over the next 20 years, a multi-billion dollar investment will be needed to fully rehabilitate the San Luis Canal, which extends about 85 miles, and the California Aqueduct, which extends about 121 miles. The CASP is studying the options for long-term rehabilitation projects that will restore the original capacities and meet the original design standards.
The chart below illustrates the various phases, milestones, and key deliverables in the planning process for long-term infrastructure rehabilitation. Mr. Dillon pointed out that they don’t anticipate completing the planning effort until 2029, with plans being reviewed and approved until 2030.
The CASP has developed two models to help navigate the uncertainties of future conditions and focus on risk-based planning objectives: the probabilistic subsidence forecast model and the hydraulic conveyance capacity model. When used together, these models greatly enhance DWR’s ability to assess current conditions and forecast the upcoming challenges that need to be addressed in planning efforts.
The hydraulic conveyance capacity model output for the San Luis Canal and the California Aqueduct is shown below. The black stair step line illustrates the original design capacity of the system; as conveyance moves south, the capacity diminishes, which reflects the turnouts for deliveries along the alignment. The blue stair-step line represents the 2023 conveyance capacity of the system. The gap between the blue and black lines visually highlights the 45% reduction in capacity due to subsidence in the San Joaquin Valley.
“Understanding this difference is crucial in recognizing the impacts of subsidence on our infrastructure and planning and prioritizing future improvements,” said Mr. Dillon.
The graph below is from the probabilistic subsidence forecast model, which relies on an empirical relationship between historical subsidence and annual water deliveries from the Central Valley Project and the State Water Project to users in the San Joaquin Valley. It shows the results of a probability distribution based on various input conditions and responses, such as the likelihood and duration of low water deliveries.
The model also factors in expected future reductions in subsidence rates due to the implementation of the Sustainable Groundwater Management Act, which requires groundwater basins to be sustainably managed and brought into balance by 2040. However, the timeline for actual SGMA implementation is uncertain, so these factors are treated as variables in the forecast model.
“The model is an improvement over earlier trend extrapolations because it better captures the uncertainties surrounding subsidence forecasts and how they may impact future conditions,” said Mr. Dillon. “The key takeaway here is the projected future subsidence represented by the jagged lines in the figure will continue to dig deeper and deeper below the originally intended water surface profile, as shown by the blue line.”
“The relatively near-term 2043 top-of-liner projections significantly affect the water deliveries before we can implement long-term solutions,” he pointed out. “This urgency highlights the need for near-term action projects to address these constrictions in both the San Luis canal and California Aqueduct.”
So, while DWR is working hard on these long-term solutions, the reality is that the effects of subsidence are happening right now, affecting the ability to deliver water effectively and potentially leading to reduced availability for agricultural users and municipal needs.
Near-term actions needed while long term solutions sought
To shore up deliveries in the meantime, five projects at a cost of $60 million will be implemented over the next four years to ensure continued and reliable water deliveries. The “Interim Action” Projects include approximately 42 miles of liner raises in Pools 17 & 18, Pools 20 & 21, Pool 24, Pool 31, and the removal of the gates at Check 17. The primary goal of these projects is to improve flexibility in maintaining the operating water surface in the near term despite ongoing subsidence.
The graphic on the slide shows how they plan to address the subsidence bowls. The upper schematic shows how the water surface is constrained by the standard operating order requirement to keep the water surface one foot below the subsided top of the original concrete liner. This limitation can hinder the ability to optimize water delivery. The lower schematic shows how by adding additional concrete liner in dark gray, the water surface can be maintained at a higher elevation.
“This flexibility is crucial to managing water deliveries effectively, especially as we continue to navigate the challenges posed by ongoing subsidence,” said Mr. Dillon. “By implementing these interim projects, which occur entirely within the existing right of way of the facilities, we are taking proactive steps to ensure reliable water delivery while we work on the longer-term solutions.”
In closing, Mr. Dillon noted the significant progress the program has made over the past three years. “We recognize that groundwater management is a complex and often charged issue that will require strong collaboration across the San Joaquin Valley. However, it’s essential to remember that the impacts to our water conveyance infrastructure affect 20 million Californians who depend on this system for their water supply. The State Water Project fully acknowledges this subsidence remediation and the restoration of our conveyance capacity as a top priority. We are committed to working together with all those affected to ensure we can maintain and eventually restore the capacity and reliability of our water delivery systems.”
Commissioner Kimberly Gallagher asked for clarification on slide 14, noting that the black line and blue line meet at Edmonton.
“The overall capacity of the system is not as affected at the downstream and once we get past Buena Vista pumping plants,” said Mr. Dillon. “There’s the long 146-mile stretch where it’s all gravity-fed from Dos Amigos down to Buena Vista. That’s where those sags really start to affect things because we’re relying on gravity to move the water. Beyond Buena Vista, it’s pretty much flat lines based on that upstream choke point at Pool 24. So once we get past that point, the capacity of the system isn’t really affected right now because of the augmentations that we’ve made to our operating order. The original freeboard criteria that were originally established were much higher, and in order to balance deliveries and try and adjust the system as we need to in order to work with subsidence, we’ve reduced that freeboard criteria to one foot, and that has allowed us to buy back some of that loss that we would have had. Similar to the interim action we’re proposing, we will raise that water surface by adding liner. Similarly, we’ve been able to mitigate some of that loss by raising the water surface.”
So what is the long-term solution?
Commissioner Sandi Matsumoto asked, what is the long-term solution?
“Inelastic subsidence has occurred, and we will not be able to pump the balloon back up, essentially,” said Mr. Dillon. “There are a number of different alternatives that we’re going to explore … Raising embankments was shown in one of the figures early on, where we had essentially raised the embankment and the liner to get back to that original operating water surface. It makes our canal deeper; it’s impactful, but it’s easy for us to quantify because it’s on our existing alignment; we know the features that will be impacted. So that’s a pretty straightforward alternative, and we have begun looking at that in terms of quantifying total costs, but we’re not there yet.
Other alternatives include going off alignment. I know that was done on the Friant Canal – a parallel bypass. Other alternatives include installing a new pumping plant along the existing alignment. That 164-mile stretch is gravity-fed; if we stuck a low lift pumping plant in the middle on the downstream end of one of those bowls, we could do a low lift, get it back up, and work with it that way.”
John Yarbrough, Deputy Director of the California State Water Project, noted that the Department is interested in slowing down and stopping the subsidence, so we’re not in the same situation 15 years down the road. “So that’s driven a lot of our more active engagement through the SGMA process,” he said. “There’s a lot of petroleum extraction in that area adjacent to the canal, and so understanding its role in subsidence versus groundwater pumping is motivated by how to stop subsidence in the long term by making groundwater and petroleum extraction more sustainable. That’s important as we do our long-term planning.”
- California Aqueduct Subsidence Program, webpage at the Department of Water Resources
- Land Subsidence Along the California Aqueduct, webpage at the USGS