First-of-its-Kind Watershed Study Highlights How Innovative Tools Help Build Climate Resilience in the San Joaquin Valley

Published: April 18, 2024

California's changing climate brings new challenges each year for water managers as they navigate extreme shifts from drought to flood while working to ensure safe, reliable water supplies for California's 39 million residents. Water managers address these challenges in their local watersheds, which are often at the forefront of the impacts of climate change.

The Department of Water Resources (DWR) is working with local and regional water agencies such as the Merced Irrigation District to conduct cutting-edge climate vulnerability assessments of watersheds in the San Joaquin Valley and evaluating how flood protection and groundwater recharge strategies can be used to adapt to climate vulnerabilities.

When water falls as rain or snow, it flows down into California's various creeks, streams, rivers, and reservoirs. These bodies of water and the land that surrounds them make up a watershed. California has a wide range of watersheds in various shapes and sizes, which help provide water supply, flood management, ecosystem support, hydropower, recreation, and other benefits to those within and connected to them.

Because these watersheds are interconnected and experience unique local challenges, it's more important than ever that California's water managers and agencies work together to proactively plan and develop innovative tools and approaches to adapt to these unprecedented climate extremes. DWR's recently released California Water Plan Update 2023highlights the need to develop water infrastructure and management practices at a watershed scale to do more with what we get from extreme weather, protect communities from flooding, and maximize groundwater recharge and storm water capture for future water supply needs when dry conditions return.

New Merced Watershed Study:

A recently completed first-of-its-kind study, the Merced River Watershed Flood-MAR Reconnaissance Study,looks at projected climate change impacts in the Merced watershed and how strategies like Flood-Managed Aquifer Rechargeand others can help reduce flood risk, improve water supply, replenish groundwater supplies, and support ecosystems in the Merced River watershed.

"The need to do these studies increases as temperatures rise and snowmelt comes earlier and earlier each year. This study looks at how to best utilize our built and natural infrastructure to capture these extreme river flows that are not controlled by levees, dams, and diversions to protect communities against flooding, support ecosystems, and create opportunities to boost critical groundwater supplies in a coordinated way," said DWR Director Karla Nemeth.

Flood-Managed Aquifer Recharge, or Flood-MAR, involves the collection of high flows from heavy precipitation events or snow melt, and then moving it to locations such as agricultural fields or other managed lands for groundwater recharge. This approach has proven feasible in the short term, helping address some recent flooding in California, and recharging nearly 400,000 acre-feet of flood water statewide to replenish groundwater basins during the 2023 storms. This study suggests that the state now has an opportunity to prepare more effective solutions during the next wet season and beyond.

This study also looked at strategies that combine recharge with Forecast-Informed Reservoir Operations(FIRO) and local infrastructure improvements to help manage California's changing climate, improve how water from heavy precipitation events is captured and moved to support future water supply needs, and support a more flexible water management system.

FIRO involves local, state, and federal water agencies working together, using the best available science and technology, to leverage improved forecasting to manage large storm events and runoff going into state reservoirs. This approach helps water agencies both coordinate flood releases while also maximizing opportunities to increase water supplies. The study assesses potential improvements that can be achieved by combining both FIRO and recharge.

Key Study Findings:

The three-year study, conducted in partnership with the Merced Irrigation District, tested the Merced watershed's hydrology - rainfall and runoff patterns - and water system across a wide range of climate scenarios that may occur by 2040 and 2070. The study has generated insightful information regarding climate change, using future climate scenarios and how they impact water management of the Merced watershed. Key findings emphasize that:

• Due to rising temperatures, there's an 80 percent chance that Merced River's 100-year peak outflow will surpass the downstream flooding threshold by 2040. This highly probable outcome is driven by greater runoff volume during winter storms, consequently increasing the vulnerability of downstream infrastructure and communities.
• In the absence of actions to achieve groundwater sustainability, increased reliance on groundwater from higher agricultural demands (driven by higher temperatures) and reduced surface water availability in dry years would result in a 98 percent probability of increased groundwater overdraft under projected 2040 climate conditions.
• Ecosystem species and habitats are also expected to be negatively impacted as a result of declining groundwater levels and decreasing instream flows that support salmonid habitat.

The good news? The study also identifies key strategies and important data and toolsets needed to support watershed-scale, multi-benefit planning and implementation that can be used to support a more climate resilient watershed.

The Merced study asserts that:

• Using the existing water system and following current streamlined water rights guidance to divert flood waters for groundwater recharge can improve water supply.
• Partnering with flood and ecosystem managers on projects that incorporate Forecast-Informed Reservoir Operations and recharge can significantly enhance flood protection, improve water supply, and ecosystem benefits.
• Strategic infrastructure improvements and directed recharge can increase the efficiency of groundwater recharge and provide important benefits and outcomes for subsidence mitigation, groundwater dependent ecosystems, shorebird and pollinator habitats, underserved communities, and groundwater basin retention.
• Flood-MAR strategies that include changes in reservoir operations, such as Forecast-Informed Reservoir Operations, are essential to achieving multiple benefits across multiple water management sectors.

Work Continues: Understanding Climate Change in Other Watersheds

The study and its findings will help inform water management decisions and identify strategies that can help improve conditions not only in the Merced watershed, but other watersheds in the San Joaquin Valley.

As outlined in California Water Plan Update 2023, DWR will use this study to inform the 2027 update of the Central Valley Flood Protection Plan and will work with other local partners and regional agencies, including the U.S. Bureau of Reclamation, U.S. Army Corps of Engineers, and Center for Western Weather and Water Extremes,to complete similar studies for additional San Joaquin watersheds including Calaveras, Stanislaus, Tuolumne, and Upper San Joaquin. DWR plans to complete these studies in 2025.

To learn more about Flood-MAR and to read the Merced River Watershed Flood-MAR Reconnaissance Study, visit DWR's webpage.