DELTA SCIENCE SPOTLIGHT: Fall X2 Action, Delta collaborative modeling project funded

At the August meeting of the Delta Stewardship Council, Delta Lead Scientist Dr. Laurel Larsen discussed a recent study on the Fall X2 action.  She also announced funding for a Delta collaborative modeling project.

Article spotlight: Flow Augmentations Modify an Estuarine Prey Field

The article for the August spotlight was recently published in the San Francisco Estuary and Watershed Science.  The article, Flow Augmentations Modify an Estuarine Prey Field, addresses how endangered fish populations respond to Delta inflow, an important issue for many processes, such as voluntary agreements and water project operations in the Delta.

Currently, one of the requirements is for the Central Valley Project and the State Water Project to augment flow through enhanced reservoir releases during the late summer and early fall in wet and above-normal years, known as the Fall X2 action.  The thinking behind this requirement is that higher flows push the range of favorable salinity for the Delta smelt to the marshes of Suisun, where the Delta smelt can take advantage of the abundant food and refuge that the marsh provides.  The results detailing how successful these costly flow actions might be for enhancing Delta smelt populations are mixed, and the requirement has long been controversial.

In the study, the authors, comprised of a team from ICF Consulting, the Department of Fish and Wildlife, the Department of Water Resources, and the US Fish and Wildlife Service, looked at how the availability and composition of zooplankton (an important food source for Delta smelt) compared across years with and without the flow augmentation (or Fall X2 Action).  They used data from two years with above-normal conditions and flow augmentation, 2018 and 2020, and two years with below-normal conditions and no flow augmentation, 2017 and 2019.

“They found that in years with flow augmentation, there were more of the zooplankton species preferred by Delta smelt, whereas species of zooplankton that are associated with higher salinity and are less favorable to Delta smelt were lower in abundance,” said Dr. Laurel Larsen.  “Studies like this do bolster confidence in the conclusion that high flows in late summer and early fall are good for Delta smelt by enhancing their food.  But this is also tricky because 2018 and 2020 were wetter than average anyway.  So it’s hard to attribute whether the improved zooplankton population observed in the sampling was actually due to that fall X2 action or the prevailing wetter conditions.”

Dr. Larsen noted that it’s a persistent challenge in many studies to determine the efficacy of management actions from the conditions that essentially triggered those management actions.  Also, because Delta smelt are so low in abundance in the wild, it’s hard to sample in reasonable numbers that are statistically robust, so it was not possible to document the effect of the improved zooplankton population on actual Delta smelt abundance.

“As the ISB has pointed out previously, if the voluntary agreements do get incorporated into the Bay-Delta Plan update, it will present new opportunities to better understand and try to get some more statistical certainty around that elusive flow fish relationship in the Delta,” said Dr. Larsen.

Flow Augmentations Modify an Estuarine Prey Field

By: Lee, Calvin Y.; Smith, April G.; Hassrick, Jason L.; Kalmbach, Andrew J.; Sabal, Megan C.; Cox, Daniel M.; Grimaldo, Lenny F.; Schultz, Andrew

Abstract: Zooplankton density and community composition in estuaries can be affected by variation in freshwater inputs, with important implications for higher trophic levels. In the San Francisco Estuary, management agencies have initiated autumn flow augmentations in the form of changes to reservoir releases or to exported water from the South Delta to increase and improve available habitat for endangered Delta Smelt, Hypomesus transpacificus, during the season when their body condition most influences fecundity. Autumn flow augmentation only occurs in years with higher precipitation, effectively moving the Low-Salinity Zone (LSZ) downstream to key foraging habitats for Delta Smelt in Suisun Bay and Suisun Marsh. To assess whether augmented flow enhanced prey resources for Delta Smelt, we compared autumn zooplankton abundance, biomass, spatial distribution, and community composition in years when flow was augmented (2017, 2019) with reference years when flow was not augmented (2018, 2020). In augmented years, we detected higher total zooplankton abundance and altered community composition in Suisun Bay and Suisun Marsh. Increased freshwater in these regions was associated with higher abundance of Pseudodiaptomus forbesi, a preferred prey of Delta Smelt, while species associated with higher salinities—Acartiella sinensis and Tortanus dextrilobatus—were less abundant. Thus, autumn flow augmentations can influence foraging habitat and prey availability for Delta Smelt, underscoring the complex responses of estuarine zooplankton communities to changes in response to flow and salinity regimes. This study is management- relevant because it shows that important Delta Smelt prey items increase in downstream regions when X2 is lower. Whether that results in a response in Delta Smelt abundance remains to be seen.

Click here to access the article.

Delta collaborative modeling project funded

The Delta Science Program has been engaged with others to develop collaborative modeling resources for the science community.  More recently, the vision for a so-called ‘co-laboratory’ was furthered in the Delta Science Program’s Integrated Modeling Framework Workshop in February.

In late 2022, the Legislature allocated almost $100 million to the UC Office of the President to stimulate research directly applicable to California’s climate resilience goals.  The funds are intended to develop new agency, academic, and community partnerships and develop actionable tools and resources with long-term benefits to the state that would directly enable it to reach more of its climate resilience goals.

Recognizing that opportunity, Dr. Larsen began brainstorming ideas with colleagues at agencies and the academic community, ultimately partnering with Ted Grantham at UC Berkeley and Eric Danner at NOAA in scoping the co-laboratory with a focus on evaluating how different strategies for water operations would impact water allocations within the Sacramento San Joaquin River system under different climate change projections, and how those altered water allocations would in turn impact ecosystems and human communities.

Dr. Larsen announced that the UC Office of the President announced they would fund the $8.2 million project.  The project will work with diverse community members and agency representatives to develop a library of plausible future policies regarding water allocations in the Sacramento San Joaquin basin, as well as projected climate futures.  Using the Cal Sim model, the team will create a library of over 100 different Cal Sim scenarios with clear visualizations and explanations and drill down into the multifaceted trade-offs associated with those water allocations.  The project will bring together partners from UC campuses, DWR, the State Water Resources Control Board, Metropolitan Water District, Tribes, Restore the Delta, other NGOs, and the Delta Stewardship Council.

”I truly believe that this project could be a game changer for how we use science to help address needs for decision-making in this rapidly changing world,” she said.  “This truly is a win for Delta science.  And I think we can expect great things from this team.”

Other news

The search is on for a new Delta Lead Scientist:  At the meeting, Dr. Larsen announced she would be stepping down on November 30 to take a leadership position at a university outside the United States.  The search for a new lead scientist is underway.

Vacancy on the Delta Independent Science Board:  The Delta Independent Science Board is comprised of ten prominent scientists who provide oversight of the science supporting adaptive management of the Delta.  Each member can only serve two four-year terms.  Dr. Stephen Brandt on the Science Board will conclude his second term in January 2024.  The Delta Stewardship Council has been recruiting scientists with expertise in fish ecology and has three applicants for the position.  Interviews will begin in early September.   They are hoping to have a recommendation at the October Council meeting.

First part of the independent peer review of the State Water Project Delivery Capacity Report (DCR) complete:  The DCR is a biannual report that provides essential information about the current and projected future water supply capacity of the State Water Project.  DWR requested a review of the data and methods used in that report to model conditions that have changed and will continue to change due to climate change.  The first phase focused on whether the historical hydrologic data used in the model should be adjusted to account for non-stationarity due to climate change and is now complete.  View documents here.  Dr. Larsen said that the general conclusion from the reviewers is that the new method to account for this change is an improvement over the previous method.  Reviewers suggested that the analyses could be presented more clearly to make a stronger case for those methods.  And they offered several suggestions for further improvements for future editions of the DCR.  The second part, now underway, will focus on risk-informed climate change scenarios to model the future performance of the State Water Project system.

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