A drone view of Fresno Irrigation District's Lambrecht Basin in Fresno, California, which provides groundwater recharge and groundwater banking, California Department of Water Resources (DWR) is securing resources to assist with expanding the groundwater recharge efforts. Photo taken April 27, 2023. Ken James / California Department of Water Resources

DELTA LEAD SCIENTIST: A tool for optimizing groundwater recharge; update on nutria eradication efforts

At the July meeting of the Delta Stewardship Council, Delta Lead Scientist Dr. Laurel Larsen spotlighted an article that developed a model to optimize groundwater recharge, gave an update on nutria eradication efforts in the Delta, and highlighted the activities of the Delta Science Program.

Dr. Laurel Larsen began her report by noting that a local TV station has been running a week-long series on how California stores its water, with the premise that California’s climate is changing rapidly, creating challenges for water management.  More precipitation is falling as rain and snow, and variability is also increasing, underscoring the need for storage that balances flood protection with water supply.

The previous night’s segment looked at the potential for groundwater storage, which could store needed water and replenish depleted aquifers.  Groundwater depletion leads to dry wells, land subsidence, and mounting expenses for continued groundwater extraction, among other impacts.

Groundwater storage is increasingly being recognized as a priority statewide and will likely loom large in future conversations around the state with implications for the Delta.  Dr. Larsen noted that plans to actively use aquifers from the Central Valley as an underground storage reservoir may require exporting more water from the Delta during wet years, reducing Delta outflow.  It may also result in reduced inflows from the San Joaquin River system.  However, she pointed out that the later use of that stored water within the Central Valley during dry periods may reduce demand for exports during those times.

While the benefits of added water supply reliability might be clear, potential negative impacts include the cost of conveying and storing water underground.   Reducing flushing flows through the Delta during very wet water years could offset any large improvements to water quality those flows might have made or affect the cues that fish use to migrate.  Dr. Larsen also noted that some concern has been expressed that policymakers may favor exports for underground storage over meeting water quality standards in the Delta, which could have a suite of social, economic, and ecological costs.

There are several ways to recharge a groundwater aquifer, but it often involves flooding large swaths of land, which might require compensating the landowner for using it.  Infrastructure might need to be upgraded to accommodate the delivery of large amounts of excess water, and new points of diversion may be required.  And if the water is sourced from the Delta or other distant locations from the place of recharge, there might be pumping and conveyance costs to consider.

The main challenge is that there are many possible points of diversion and locations for the recharge to occur when you consider the Sacramento-San Joaquin system and all of its tributaries.  Those different sites are variable in terms of their associated costs, their feasibility with respect to the landowner’s willingness to participate, and the efficiency of storage.   Dr. Larsen explained that efficiency in the groundwater recharge context refers to the fact that groundwater and surface water are quite well connected, and in some locations, quite a bit of water might be lost to streamflow relatively soon after recharge, meaning that it would be unavailable when needed during the drought years.

Dr. Larsen noted that, fortunately, there are ways that science can contribute to resolving the challenges and uncertainties and potentially accelerate the development of new policies related to subsurface storage.  This month’s article in the spotlight, Optimizing managed aquifer recharge locations in California’s Central Valley using an evolutionary multiobjective genetic algorithm coupled with a hydrological simulation model, came from UC Davis and was funded through the US Department of Agriculture and the National Science Foundation.  The research team created a tool to help resolve some of the planning uncertainty around costs and storage efficiency.

“What they did is pretty neat and creative,” said Dr. Larsen.  “First of all, they developed several different decision scenarios, in which they looked at different maximum and minimum thresholds for diverting water to aquifer recharge; an example of a combination of possible thresholds is shown on the slide on the upper left.  The scenarios also involve different decisions about whether the water would be sourced locally and diverted to nearby fields or whether it might come from places like the Delta.  Then they took the historical streamflow record, also shown in the upper left, and determined how much water would be available for recharge based on the decision criteria in the different scenarios.”

“So once they know how much water is available, as shown in light blue on the graph on the upper left, there is still an almost intractable combination of ways that the water could be diverted and directed into aquifers.  And this is where their creativity really came in.  Using a groundwater model and an economic model, they explored possible solutions using what’s known as a genetic algorithm, which is inspired by how actual genetics and the concept of survival of the fittest works.”

“In their modeling strategy, solutions for where the water is diverted from and where it’s diverted to are randomly selected; those solutions are run through the groundwater model to determine storage efficiency and their economic model to determine costs.  The solutions that result in the best combination of storage and low cost are selected from all these random possibilities.  They are then mated with each other, meaning that some of the diversion points and recharge locations associated with one best solution will be mixed with the diversion points and recharge locations from the other best solution.  And the offspring will be evaluated for how well they produce high storage, low-cost solutions.”

The procedure is iterated over several “generations” until the solutions produce a smooth curve, as shown on the graph on the right, known as a “Pareto Front.”  The curve represents designs that maximize storage while minimizing costs.  The graph shows that policymakers would have several combinations of cost and storage to choose from, depending on how they balance the two factors.   The authors made Pareto Fronts for each of their policy and engineering scenarios available as a web tool.

The graph on the right shows a rather conservative scenario in which just the top 5% highest flows are taken for aquifer recharge, the recharge occurs near the point of diversion, and there’s a relatively conservative cap on the maximum amount of flows that can be conveyed by the infrastructure per month.

There are several things to be learned from this analysis, Dr. Larsen said.  “For one, as storage increases, so does cost (not surprisingly) because you progressively use the lowest cost sites first and eventually might need to do things like pump water uphill.   Policies that allow more water to be used, for example, the highest 10% of flows rather than 5%, produce substantial cost savings; that’s not shown on this graph, but I provide some numbers in my report.   As you expand the allowable water sources and include the Delta, you do produce solutions that achieve more storage for less cost than when the water sources are more restricted.  But because we’re talking about a lot of storage, the total cost ends up being fairly high.”

However, Dr. Larsen noted that the storage volumes through an approach like this are fairly massive, potentially exceeding the volume of Shasta Lake in the conservative scenario depicted on the graph.  Yet, the price tag is over an order of magnitude less than the cost of constructing additional surface water reservoirs.

“I just put a comparison with the proposed Sites reservoir, which has a capacity of 1.8 million acre-feet at a cost of $4.4 billion, whereas the costs that we’re looking at for these groundwater storage solutions on the graph are on the order of hundreds of millions of dollars.”

Dr. Larsen noted that there is more work to be done at a policy level to pave the way towards managed aquifer recharge, but it is a priority of the Newsom administration; it’s also relevant to the recommendations of the ISB in their recent water supply reliability review.

“Overall, this paper that I’m highlighting is a great example of how a couple of models and a scenario-based approach could be used to directly inform policy decisions and public conversations about the trade-offs associated with those decisions,” she said.  “This is very well aligned with major ongoing initiatives within the Delta science program.”

Optimizing Managed Aquifer Recharge Locations in California’s Central Valley Using an Evolutionary Multi-Objective Genetic Algorithm Coupled With a Hydrological Simulation Model

By Georgios KourakosGiuseppe BrunettiDaniel P. BigelowSteven WallanderHelen E. Dahlke

Managed aquifer recharge (MAR) can provide long-term storage of excess surface water for later use. While decades of research have focused on the physical processes of MAR and identifying suitable MAR locations, very little research has been done on how to consider competing factors and tradeoffs in siting MAR facilities. This study proposes the use of a simulation-optimization (SO) framework to map out a cost-effectiveness frontier for MAR by combining an evolutionary algorithm with two objective functions that seek to maximize groundwater storage gains while minimizing MAR cost. We present the theoretical framework along with a real-world application to California’s Central Valley. The result of the SO framework is a Pareto front that allows identifying suitable MAR locations for different levels of groundwater storage gain and associated MAR project costs, so stakeholders can evaluate different choices based on cost, benefits, and tradeoffs of MAR sites. Application of the SO framework to the Central Valley shows groundwater can be recharged from high-magnitude (95th percentile) flows at a marginal cost of $57 to $110 million per km3. If the 10 percent largest flows are recharged the total groundwater storage gain would double and the marginal costs would drop to between $30 and $50 million per km3. If recharge water is sourced from outside local basins (e.g., the Sacramento-San Joaquin Delta), groundwater storage gain is approximately 25%–80% greater than can be achieved by recharging local flows, but the total cost is about 10%–15% higher because of additional lift cost.

Click here to read this article.

Update on nutria eradication efforts

As part of the Delta Science Program’s core function of coordinating science related to major management challenges, Council staff are actively involved in the Delta Interagency Invasive Species Coordination Team and other teams focused on managing individual invasive species.

Nutria are large semi-aquatic rodents native to South America with distinctive orange teeth.  Beyond their native habitat, nutria can be incredibly damaging to wetlands, agriculture, and infrastructure.  They feed on plant material and crops and can burrow up to 50 meters into banks and levees, compromising their stability.  In places like Louisiana, nutria have caused massive destruction of coastal marsh areas.

Since they were discovered in California in 2017, just over 3500 nutria have been removed by the California Department of Fish and Wildlife, the state agency leading nutria eradication efforts.

The first reproducing population in the Delta was discovered in September 2018 south of Lathrop, and since then, over 100 nutria have been removed from that specific location, with the last nutria taken in March 2021.  Also, at the time, nutria were detected near Rough and Ready Island and French Camp Slough between May 2019 to February 2021.

There were no subsequent detections until October 2022, when a family group of nutria was detected on lower Sherman Island.  There was concern that high flows and wet conditions during the 2022 to 2023 water year would aid in spreading invasive species like nutria.  So in April of this year, the CDFW Nutria Eradication Program, or NEP, made a temporary operational shift in response to these conditions, increasing their staff presence and expanding further northward into the Delta.  Now nearly all NEP field staff are part of an early detection rapid response effort to address any nutria that could have potentially been washed into the Delta from flooding and high flow events in the San Joaquin River system where the nutria had formerly been trapped actively.

In May 2023, the NEP conducted the first nutria detector dog field trials on multiple currently and previously infected sites within the Delta.  During the field trial, the dogs located an additional area of nutria presence on Sherman Island.  Following that detection, eight additional nutria were trapped and removed from the site, with the last taken on June 1.

The dogs did consistently detect nutria in sites that were already known or suspected to be infested, and they did not detect any nutrient present at the Delta sites with previous detections or infestations, indicating that those previous local eradication efforts were successful.  The nutria detector dogs program was funded by a Delta Conservancy Prop One grant, and they will be an increasingly valuable tool as nutria densities decrease and the remaining animals become increasingly difficult to detect.

As a result of the expanded operations of the NEP into the Delta, one nutria was captured on camera on May 22, 2023, on Hammond Island, which is to the west of Sherman Island, and trapping operations and intensified survey efforts commenced immediately to characterize the extent of nutria presence and rapidly remove the infestation.  And then since that time since June 1, the department has captured 34 nutria on Hammond and Southern Grizzly Island, including four pregnant females.

One thing that has been unclear is whether the ramp-up and program activity, including the use of detector dogs, might have driven the dispersal of nutria from Sherman Island further outward.  In the past few years, the NEP has observed increased movement among remaining individuals when a local population is depleted due to management actions.  So as such, the NEP sees spikes and detections across the landscape despite decreasing overall population size.

Finally, despite the concerns associated with recent nutria detections in the western Delta, in the Suisun marsh area, the NEP continues to see steady declines in the annual number of sites with nutria taken, the number of nutria taken, and the density taken by site.  Dr. Larsen noted that this might be an artifact of the drought that occurred throughout the years leading up to this one.

“With increased water in the system comes increased habitat and additional battles for the NEP,” said Dr. Larsen.  “In the face of these challenges, the NEP continues to expand both its Delta-centered staffing levels and its toolbox, such as the use of detector dogs and Judas nutria, which are tagged nutria designed to betray the location of other populations of nutrias.  And so these early detection and rapid response efforts continue to locate and remove the nutria from the Delta as quickly as possible.”


Activities of the Delta Science Program

Adaptive Management Forum:  The second part of the Adaptive Management Forum took place on June 27.  Part two built on the theme of adaptive governance explored within the first session.  Participants worked through a visioning process to collaboratively identify aspirational goals for governance and adaptive management for the Delta by the year 2050.  They brainstormed strategies that might be needed to achieve those goals and the governance components to ensure that those strategies could be implemented and maintained.

“A few examples of the goals identified in different groups were to create more inclusive governance systems in which all perspectives have a seat at the table and establish an equitable water rights system reflective of changing water availability and historical injustices,” said Dr. Larsen.  “This is a very controversial topic statewide right now, but this reflects what some breakout groups came up with.  Also, to develop an equitable, evidence-based and risk-informed governance system where information flows easily.”

Integrated Modeling Framework Workshop:  A comprehensive summary report is now available, which captures the full set of topics covered during the two-day workshop and provides a short summary of every workshop session with a link to the video.  The summary report is intended to serve as a foundation for developing a white paper highlighting key recommendations and near-term actions critical to advancing the vision of collaborative and integrative modeling for the Delta.  A white paper is expected to be released in the fall.

State of Bay-Delta Science Executive Summary:  An Executive Summary for the State of Bay-Delta Science will be released soon.  It is a high-level summary of the full report released in January 2023, which focuses on some of the management recommendations and key findings from each chapter.

NCEAS partnership to focus on the human dimensions of resource management in the Delta:  The Delta Science Program just began a collaborative working group in partnership with the National Center for Ecological Analysis and Synthesis (or NCEAS).   This provides training for scientists spanning 16 different agencies and academic institutions and is designed to address gaps and understanding of the human dimensions of the Delta, as identified in the Delta social science community of practice’s Advancing Interdisciplinary research, training, and workshop event, which took place in October of last year.  The working group’s theme involves multi-benefit approaches to managing the Delta as a social-ecological system and integrating human dimension data into research and management decision-making.

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