To replenish California’s chronically depleted aquifers, the state’s Department of Water Resources is taking a hard look at a new line of attack: Pairing more sophisticated reservoir operations with groundwater recharge. Water managers are aiming to make greater use of the increased floodwater that’s expected to come with flashier, more intense storms and earlier snowmelt.

The new approach is known as FIRO-MAR, which stands for Forecast-Informed Reservoir Operations-Managed Aquifer Recharge. In December, DWR released a study focused on the five main tributary watersheds of California’s second-largest river, the San Joaquin, that provides the most comprehensive assessment of the concept’s potential yet. The study suggests that by weaving together better weather forecasts, more-flexible reservoir operations and strategically targeted recharge locations, “FIRO-MAR has the potential to increase the volume of recharge more than fourfold, and it can greatly diminish the size and frequency of flood flows.”

The approach — a new twist on the well-established idea of using floodwater to recharge aquifers, known as Flood-MAR — could allow for an average annual recharge of an additional 410,000 acre-feet of water across the five watersheds, which run from the crest of the Sierra down to an area roughly from Stockton to Fresno. DWR’s study found that peak annual recharge could potentially be as high as 2.6 million acre-feet — more than the capacity of New Melones Lake, California’s fourth-largest reservoir. (An acre-foot equals 325,851 gallons, enough to supply the average annual needs of roughly 3 households, depending on their location.)  

A newly minted law, Senate Bill 72, could give further impetus to putting FIRO-MAR into action. It requires DWR to identify 9 million acre-feet of additional water supply by 2040 to offset losses anticipated as the climate continues to warm.

Still, FIRO-MAR can only deliver on its full promise if state water rights permitting regulations are relaxed, and if proponents can prove that diverting floodwater won’t harm other users’ water rights or salmon and other endangered species.

Stretching Water Supplies with More-Flexible Management

Climate change is making California’s weather more volatile and is already bringing larger and more frequent bursts of flooding as well as more-intense droughts. Warmer temperatures are also causing more wintertime precipitation to fall as rain instead of snow, and they’re leading to the earlier, faster melting of what snowpack does accumulate — as has dramatically happened this winter.

“You can use existing infrastructure and optimize how it’s operated to come up with additional water supply that currently is not available to water managers or the environment.” - Joel Metzger, DWR’s deputy director of statewide water resources planning

FIRO-MAR, like Flood-MAR more generally, relies on aquifers for storing water as a substitute for the state’s shrinking snowpack, which normally holds on to winter precipitation until it melts and runs off in spring. But by incorporating better reservoir operations, FIRO-MAR potentially allows for even more water to be captured from more severe storms and channeled into aquifers — without having to build any major new infrastructure.

“In the watershed studies, we’re not looking at any new dam construction,” says Joel Metzger, DWR’s deputy director of statewide water resources planning. “You can use existing infrastructure and optimize how it’s operated to come up with additional water supply that currently is not available to water managers or the environment.”

Making it work would require modernizing the way dams and reservoirs are operated. Many of California’s large dams are bound by strict operating constraints to ensure that they maintain adequate flood-control capacity. But those “rule curves” are relatively inflexible and often ill-suited to the highly variable conditions that are a hallmark of California weather. Most are decades old and haven’t kept pace with rapid advances in weather-forecasting accuracy — particularly the ability to predict the path and intensity of atmospheric rivers. Those powerful winter storms arrive from far out in the tropical Pacific and, on average, can carry the equivalent of 25 times the flow of the Mississippi River.

“FIRO,” says DWR supervising engineer Jim Wieking, “allows you to flex that rule curve based on the forecast.”

To figure out where to put that water, DWR and local water agencies have been using the Groundwater Recharge Assessment Tool (GRAT), a type of modeling software developed by the nonprofit organizations Sustainable Conservation and Earth Genome, to identify areas where water can be recharged most effectively.

Traditionally, dedicated recharge facilities allow water to either percolate or be pumped directly into the underlying aquifer. But DWR’s FIRO-MAR study evaluated not just those types of facilities but more than 350,000 acres of “on-farm recharge,” in which water is applied directly to orchards or vineyards — or to fields lying idle between planting cycles — and infiltrates into the aquifer from there. In fact, in each of the five San Joaquin River tributary watersheds evaluated in the study, on-farm recharge accounted for an average of 70 percent of the total amount of water that could potentially be recharged.

Increased recharge could help water agencies in the San Joaquin Valley meet another looming challenge: The Sustainable Groundwater Management Act, which requires that they stabilize their groundwater supplies by the early 2040s.

DWR’s study also found that, if used in a more directed manner, FIRO-MAR could improve drinking water reliability, particularly for disadvantaged communities; help combat land subsidence caused by excessive groundwater pumping; potentially boost water availability for groundwater-dependent ecosystems and struggling species like salmon and steelhead; and create new wetland habitat for migratory shorebirds.

Thinking Bigger

Currently, FIRO-MAR is only being used in a relatively limited scope at Prado Dam on the Santa Ana River in Southern California. But several water agencies in the San Joaquin Valley have been eyeing the possibility of using the approach in their own jurisdictions.

One of those is the Turlock Irrigation District (TID), which operates the largest locally owned reservoir in the state, the two-million-acre-foot Don Pedro Reservoir on the Tuolumne River. The district supplies irrigation water to some 150,000 acres of farmland, as well as wholesale water to the cities of Turlock and Ceres to treat for drinking water.

“Snowpack is essentially a second reservoir on the Tuolumne, and it has historically run off to line up very well with our irrigation demands,” says Josh Weimer, TID’s director of external affairs. “What we’re seeing the last couple of years, specifically, is that that peak runoff is happening earlier. And so we need to be creative and look at ways to put that water to use when it’s available.”

“… peak runoff is happening earlier. And so we need to be creative and look at ways to put that water to use when it’s available.” - Josh Weimer, TID’s director of external affairs

For several years, TID has been testing out its own small-scale pilot recharge projects within the district. But DWR’s study suggests that if FIRO-MAR is employed more broadly in the Tuolumne River watershed alone, it could allow for a fivefold increase in the amount available for recharge, yielding an average of 163,000 acre-feet per year, with a maximum of 803,000 acre-feet.

“A lot of the stuff in the watershed study reinforced what we have been trying to do,” Weimer says. “So we’ve been having many conversations with (DWR) to see how we can all potentially work together and move something forward.”

Challenges ahead

Still, making FIRO-MAR work at a wider scale will require overcoming several significant hurdles.

First, FIRO would have to be integrated into the rule curve for each dam. The U.S. Army Corps of Engineers, which issues the rule curves, has grown increasingly receptive to wider use of FIRO, but the update process still typically takes several years.

A bigger challenge is the state water rights-permitting process, which is overseen by the State Water Resources Control Board (State Water Board). DWR based its study on the total volumes of flood flows that hydrologic models suggest will come with future storms — considerably more water than would be legally available under the state’s current permitting guidelines

Over the past decade, the State Water Board has gradually streamlined and introduced more flexibility into its permitting guidelines for “high flow events.” But some water agencies contend that more could be done.

“In 2022, we actually filed for flood water rights on the Tuolumne, but there hasn’t been any movement on that application,” says TID’s Weimer, noting that water agencies can’t plan or obtain project financing without water rights in hand. “We want to get certainty that when that water’s available, we’d be able to utilize it. Until you have that, it’s going to be difficult.”

But Amanda Montgomery, the permitting manager for State Water Board’s Division of Water Rights, says regulators are faced with a delicate balancing act. Because many rivers in California are overallocated — meaning that more water rights have been issued than is typically available — even flood flows are often already earmarked for other water users.

“Those opportunities that are left — the pieces of pie that are not spoken for — are usually pretty small and pretty focused,” she says.

Using FIRO-MAR to divert floodwater in the San Joaquin tributaries could also create conflicts with water users who depend on water exported from the Sacramento-San Joaquin Delta downstream, primarily via the State Water Project and Central Valley Project pumps there. DWR’s study found that, on average, for each two acre-feet of water diverted upstream, flows downstream near the Delta would be reduced by one acre-foot.

There could also be significant negative effects on habitat for salmon and steelhead, which are protected under the state and federal endangered species acts. The study found “a consistent impact of 25 to 62% reduction in potential off-channel salmonid habitat,” which serves as a nursery for young fish before they head to the Pacific Ocean.

DWR acknowledges these potential impacts and challenges in the study.

“What we wanted to do was show the realm of the possible,” Metzger says, essentially to reveal what he calls “the top end of what is achievable realistically.”

Working out what FIRO-MAR might look like in practice on a larger-scale would require working with regulatory agencies and interested parties to carefully evaluate the operational and environmental tradeoffs.

“What we wanted to do was show the realm of the possible.” - Joel Metzger, DWR

“The only way to achieve the goals of (FIRO-MAR) would be to take a look at some of those regulations, working with the State Water Board,” Metzger says, as well as “taking a look at the impacts to Delta exporters and understanding the tradeoffs there for the State Water Project and the Central Valley Project contractors.”

For now, at least, moving the concept forward on the ground seems to largely be in the hands of local water agencies. Metzger says that Proposition 4, a 2024 statewide bond measure to enhance water and climate resilience, could provide a source of money for such agencies to pilot the concept in their own watersheds. According to a recent analysis by the state Legislative Analyst’s Office, there’s currently about $1.8 billion of Proposition 4 funding still available for safe drinking water, drought, flood and water resilience projects.

Turlock Irrigation District’s Weimer says that the path to implementing FIRO-MAR on a broader scale will likely be paved with local efforts.

“This isn’t one-size-fits-all, and every watershed is different,” he says. “And so I think looking at this watershed by watershed is the right way to do it.”

Western Water, our flagship publication by Foundation journalists, is available online. Check out our latest articles. Reach writer Matt Jenkins at mjenkins@watereducation.org.

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