How Capturing Floodwaters Can Reduce Flooding and Combat Drought

Farmers toil at the mercy of nature’s whims, which can prove particularly vexing in California. 

Even before climate change, bouncing between drought and deluge was routine in the Central Valley, the state’s richest farming region. Humans have amplified these natural cycles by pumping greenhouse gases into the atmosphere, studies show, creating a future filled with what scientists recently dubbed “whiplash events.”

California got a taste of whiplash four years ago, when one of its wettest winters immediately followed one of its deepest, longest droughts. Heavy runoff from rivers in the Sierra Nevada damaged the main spillway of the Oroville Dam, the largest in the nation, forcing more than 180,000 people to evacuate.

Such dramatic swings will create even more headaches for California farmers and water managers, who have more than their share in a good year. 

Now, as California farmers grapple with reduced federal water allocations amid an intensifying drought, a recent study challenges policymakers to think about floods. 

The increased frequency of extreme drought and flooding under climate change in the nation’s largest economy will intensify the already fierce competition for water, said Xiaogang He, a hydrologist who led the research while a postdoctoral scholar at Stanford University. 

But the potential for more storms also points to an opportunity. If climate change is likely to unleash more floods on California, reasoned He, why not try to capture that water and send it to where it’s needed most.

In the study, published in Science Advances, He and his colleagues provided the first statewide analysis of the floodwater potentially available to restore depleted groundwater basins under future climate change scenarios. The increase in floodwater available to replenish over-drafted aquifers over the next 30 years, they found, would be enough to fill 192,000 Olympic swimming pools each year under an intermediate-emissions scenario, and 232,000 pools under a high-emissions scenario. 

Potential additional floodwaters increased even more toward the end of the century, with enough available to fill up to 552,000 Olympic swimming pools under the high-emissions scenario. But the extent to which this additional water could be used to replenish basins would be limited by constraints on the ability to capture and divert it, they found.

Their analysis focused on the highest levels of streamflow likely to accompany future spikes in big storms. Harvesting those high-water flows, they concluded, could help mitigate flood risk while boosting the state’s dwindling groundwater supplies.

Those supplies are critically low in the backbone of the state’s $50 billion agricultural industry, the San Joaquin Valley. Farmers there have sucked so much water from the Tulare Basin to grow crops in an arid landscape that parts of the San Joaquin Valley, including one entire town, have sunk several feet.

The state’s groundwater sustainability law gives water managers roughly 20 years to replenish, or recharge, aquifers by balancing pumping and resupply levels. 

Studies show that farmers may need to retire hundreds of thousands of acres worth billions of dollars to reduce groundwater demand and comply with the requirements of the sustainability law, said He, now assistant professor of civil and environmental engineering at the National University of Singapore. “But a lot of farmers may not be willing to retire their crops, especially if they’re perennial high-profit crops like grapes or almonds.”

He undertook the ambitious task of estimating the potential for capturing runoff throughout the state from whatever nature may have in store to help farmers adapt to the new pumping restrictions and future droughts.

Toward that end, the team designed a computer model to simulate streamflow and climate conditions under different emissions scenarios, and used statistical methods to account for human actions like irrigation and reservoir operation. They prioritized regions where investing in structures to divert floodwaters to the increasingly parched south would make the most sense. 

“What’s really nice about this paper is that they look at a lot of different scenarios about what we might be seeing in terms of future runoff,” said Ellen Hanak, director of the Public Policy Institute of California’s Water Policy Center, who was not involved in the research. “And that’s really key to thinking about how much extra water might be available for recharge.”

The study also predicted, in line with previous research, that the wetter parts of the state will get wetter and the drier parts drier. 

That will further complicate what’s arguably the biggest challenge for California water managers: Three-quarters of the rain, hail and snow that feeds reservoirs falls in the north of the state, while 80 percent of demand for that water comes from the southern two-thirds. To manage the mismatch between water availability and need, state and federal agencies over the last century built an extensive network of dams, reservoirs and canals to shuttle water from north to south. 

For decades, however, cities and farms supplemented the water from reservoirs with groundwater. A dramatic increase in groundwater pumping during the last drought, which lingered from 2012 to 2016, prompted the state to enact the 2014 Sustainable Groundwater Management Act. 

By then, though, decades of unregulated pumping had extracted far more water from underground aquifers than was replenished. Eleven of the state’s 21 “critically overdrafted” groundwater basins are in the fertile San Joaquin Valley.

The dire condition of the San Joaquin Valley’s groundwater inspired the state to explore innovative ways to safeguard its most precious resource.

“Groundwater recharge has been a tool for a long time,” said the Public Policy Institute of California’s Hanak. But the sustainability law, she said, “has really kicked interest to a higher level.”

Embracing Nature’s Whims

California’s water system is built around three main storage systems: reservoirs, groundwater and snowpack. “As the climate warms, you’re getting less precipitation as snow and it’s melting sooner,” said Hanak. “You don’t want to lose that water, and groundwater basins are a great place to store it.”

Groundwater accounts for about 90 percent of the world’s freshwater reserves, excluding the polar ice caps. California’s unique hydrology and succession of droughts have drained many of the state’s natural underground reservoirs, even as the prospect of more heavy rainstorms and earlier snowmelt threatens to increase flood risk.

The state recognized the urgent need to “rehabilitate and modernize” the state’s water and flood infrastructure three years ago, when it launched the Flood-Managed Aquifer Recharge (Flood-MAR) program.

Flood-MAR also seeks to integrate the disparate elements of California’s famously complex water system to better manage surface and groundwater, upgrade canals and aqueducts that move water and identify opportunities to harness floodwater to recharge aquifers.

The state’s interest in integrating flood and groundwater management inspired He to figure out how much future floodwater might be available to recharge aquifers and where high-water flows are likely to occur in the face of huge uncertainties.

“A very nice thing about doing a study in California is that the state cares so much about climate change,” said He. “So we can get very high quality climate change scenarios that reflect California’s climate.”

His team’s models showed that increases in high-volume flows as a result of climate change are likely to be concentrated in a shorter window during the wet season. Shorter periods of heavy rain in several regions, mostly in Northern California, will increase flood risks by straining dams, levees and facilities meant to contain rising waters.

But even climate models tailored to California carry a range of uncertainties about exactly when, where and how much floodwater might occur over the next several decades. And since reinforcing or building structures to catch, store and divert floodwaters can run into billions of dollars, policymakers need tools to determine whether the benefits justify the costs.

A model can’t capture the universe of uncertainties embedded in policymaking decisions, but it can simulate a range of possibilities for a specific factor. For example, the amount of water available to recharge basins is set by “high-magnitude flow” thresholds—flows that exceed water rights allocations and levels legally required to protect freshwater ecosystems and could be used to recharge groundwater.

But the definition for “high magnitude flow” varies, He said, depending on how much water different experts think should be in a river. Hydrologists, for example, use a lower threshold than salmon and fisheries biologists, who generally favor higher flows for ecological reasons.  

Streamflow volume typically rises fairly rapidly during a storm, hits a peak and then declines, said study coauthor David Freyberg, an associate professor of civil and environmental engineering at Stanford. The researchers looked at just the highest peaks.

It’s easier to remove water when flows are lower than when a river is raging, but that has “huge impacts” on salmon and downstream water supply, among other issues, Freyberg said. So they focused on the high flows, even though it’s more challenging to remove that water. “Whether or not we can actually do that, and how much of it we could do, I view it as still an open question,” he said.

He added that the next challenge would be to figure out how to capture floodwaters in such a way that the benefits are worth the costs.

Policymakers will also have to weigh the balance between reducing flood risks and the environmental benefits of letting floodplains flood. Another reason the researchers considered just high-magnitude flows, Freyberg said, “was to acknowledge that we don’t want to just stop all flooding, because flooding is really important for ecosystems.”

Still, knowing that a watershed faces a high risk of flooding down the road may give policymakers the incentive they need to build structures to divert extra water. That leaves the increasingly difficult problem of moving large volumes of water from Northern California to the southern reaches of the state. 

Planning for a Changing Climate

The prospect of a wetter north and an even drier south is certain to place greater strain on the Sacramento-San Joaquin Delta, the already overtaxed fulcrum of the state’s water system.

The massive network of reservoirs, dams and canals operated by federal and state water projects was designed to move water from north to south through the delta. “That’s a major bottleneck already for moving water, just about at any time,” Hanak said, “because it’s a complicated ecosystem and environmental constraints on pumping have increased over time.” 

That’s why the state is exploring alternatives for moving water, including tunnels, she said. “If you want to get water from north to south in the state, you’ve got to fix the delta.”

Accounting for all the political, environmental, technical and socioeconomic drivers of water policy around the delta, which Hanak called “a world unto itself,” was beyond the scope of He’s research. But by focusing on the physical availability of floodwater, He and his colleagues hoped to help policymakers decide where building canals and recharge basins would offer the biggest return on investment.

What this research and other studies have shown is that current systems aren’t robust enough to convey the water that’s already available, Hanak said. “Now the question comes, what’s cost effective, because you don’t want to build stuff if it’s mostly going to be empty. So what are the smart plays in terms of expanding infrastructure, given the probabilities of what you’re going to be able to do with it?”

For He, tackling California’s future water challenges will require finding ways to integrate management of droughts, floods, water allocations and groundwater depletion—all overseen now by a highly decentralized, fragmented network of water districts and agencies. 

Eighty percent of the land subject to California’s groundwater sustainability law is managed by multiple entities, He said, with as many as 24 separate authorities in a single groundwater basin. Decisions to build floodwater infrastructure in even a single basin would call for an extraordinary level of collaboration. 

In some places, it might make more sense to consider changing the way reservoirs operate, He said, which in any event should update decisions about how much water is released, and when, to account for climate projections.

“Water policy is really complicated,” Freyberg said. “As soon as you recommend one thing, there’s always a counterargument.”

Still, he added, any plans to boost groundwater sustainability must explicitly consider climate change, rather than designing a future based on historical or current conditions. 

“We have to be thinking about the changing climate,” Freyberg said. “You have to take changing climate seriously.”