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How California is using recent floods to prepare for future drought

Diverting and storing excess floodwaters in aquifers could help replenish depleted groundwater supplies

During a break in the rain, farm workers drain lettuce fields as a storm slams Salinas, Calif., on Friday. (Melina Mara/The Washington Post)
5 min

As more heavy rain pummels California, the torrential downpours that have flooded large swaths of the state could help address another climate-change-related problem: drought.

This month, state water regulators approved a plan to divert floodwaters from the San Joaquin River to replenish critical groundwater stores that have been depleted after drought fueled by rising global temperatures.

It’s one way the state, which experts say has not been prepared for periods of excess water, is attempting to capitalize on historic amounts of rain and snow after experiencing three of its driest years on record.

“California’s new climate reality is it requires us to better manage extreme wet conditions and extreme dry conditions,” said Jule Rizzardo, assistant deputy director for the division of water rights at the California State Water Resources Control Board. “This is one of the really important tools in our toolbox: to be able to capture those high flows and recharge our groundwater supplies.”

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What’s happening in California?

The State Water Board recently approved a federal petition to take up to more than 600,000 acre-feet of floodwaters from the San Joaquin River — the longest river in central California — and divert large amounts of that water to places where it can soak into the ground and replenish an aquifer under the San Joaquin Valley. Gov. Gavin Newsom (D) followed that action by signing an executive order on Friday temporarily lifting certain restrictions and regulations to make it easier to capture floodwaters for groundwater recharge without harming water quality or natural habitats.

“Many of our groundwater basins are in serious states of decline and this is an opportunity that we have now — because of the situation we find ourselves in with the improved precipitation — to use some of that water,” said Eric Oppenheimer, chief deputy director at the water board. The plan, he noted, also has a “secondary benefit” of helping to mitigate potential flood impacts.

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How does groundwater recharge work?

The technical term for the process of diverting or taking surface water and storing it underground is “managed aquifer recharge,” said Helen Dahlke, a professor and groundwater hydrologist at the University of California at Davis.

“We do it in a controlled way,” Dahlke said. “We are not talking about unintentional or natural recharge that’s happening just as a result of precipitation, rainfall falling onto soils and then infiltrating.”

Instead, she said, the approach typically involves taking water from one location, directing it onto a certain parcel of land and letting it percolate down into an aquifer.

Spreading water over designated areas is one of the most common recharging methods, Dahlke said. Pipelines, canals or ditches route water from the source, such as a river or aqueduct, to the recharge location. As it seeps through the ground, natural filtration often occurs, ridding the water of certain contaminants and pollutants before it reaches the groundwater table, she said.

“Instead of just letting rivers go wild with wild flooding everywhere, we’re basically managing where are we taking water out, where are we putting it and doing it in a controlled way so that there is no harm to infrastructure or life,” she said.

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What are the potential downsides?

Diverting floodwaters and storing it for later use could have environmental and legal implications.

There are potential water quality concerns, said Jonathan Yoder, director of the State of Washington Water Research Center and a professor at Washington State University.

“Introducing water of different characteristics to an aquifer can also introduce problems,” Yoder said. “Even if the water that’s being introduced doesn’t have a particular toxicity, it can change the character of groundwater.”

Additionally, capturing floodwaters for recharge could create legal issues due to complex water rights and environmental rules.

For instance, senior water rights holders downstream might claim that diversions of floodwaters upstream could negatively impact them, said Sharon Megdal, director of the University of Arizona Water Resources Research Center.

“You have to look at this in the context of your whole system: Your hydrologic system, your legal system, and then how the users are using the water,” Megdal said.

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That includes protected wildlife. In California, some environmental groups have opposed the recent actions, arguing that taking water from the San Joaquin River could lead to lower flows than required and might potentially harm Chinook salmon, the Los Angeles Times reported.

State water regulators say the recent orders are not likely to pose much risk to both senior water rights holders and fisheries, noting that the planned diversions will happen during flood conditions when there are high flows and excess water. The approved petition notes that the San Joaquin River plan would “capture high flows that would otherwise go unused.”

Is recharge an effective solution?

Some major challenges include having enough land for water to be spread out and the necessary infrastructure to move the water, Dahlke said. It’s also a process that typically needs to be done repeatedly, she said.

Still, Yoder said the strategy should be considered in areas where surface water storage opportunities are in short supply and it can yield a “double dividend”: both flood and drought mitigation.

Dahlke added that any groundwater dependent regions may likely be good candidates for managed aquifer recharge. In California, groundwater accounts for 41 percent of the state’s total water supply on an annual average basis — a figure that jumps to as much as 58 percent during critically dry years.

“I know it’s hard to imagine now because we’re in some pretty severe flooding situations in parts of our state and it’s hard to think about the next dry period,” Rizzardo said. “But we know it’s coming.”