Recharging California’s Diminishing Aquifers

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California Ground Water

With increasing attention on methods of recharging depleted aquifers, groundwater moves to the frontlines of drought mitigation in California

California Ground Water

Images show drought conditions and changing elevation, or land subsidence, between 1965 and 2013. Image Credit: USGS California Water Science Center

For the better part of a decade California has experienced a drought that has significantly depleted water reserves throughout the state. Surface water has long been the focus of California’s water supply planning, but a new awareness of groundwater is changing that. With the dawning recognition that groundwater is a finite and diminishing resource, new projects for recharging groundwater reserves are changing the nature of California’s approach to water management.

Groundwater constitutes roughly half of California’s freshwater supply, with the other half supplied by surface water sources such as reservoirs. Until 2014, there was no statewide system for groundwater monitoring or regulation. Property owners could use as much groundwater as they wanted without keeping tabs on how much they were using.

Under pressure from the drought, in the fall of 2014 California passed the first statewide legislation to address the problem. The Sustainable Groundwater Management Act (SGMA) provides standards and assistance to local water agencies, and mandates that local systems establish Groundwater Sustainability Agencies to oversee improved monitoring and sustainable management of groundwater stores.

To explain the relationship between surface water and groundwater, Graham Fogg, a professor of hydrogeology at University of California, Davis, used the metaphor of bank accounts.

“Imagine you had all of your money in two bank accounts that are linked,” Fogg said. “For one of them you know the balance and the inputs and outputs, but for the other one you don’t. It would be really hard to manage your money sustainably. When one account gets low you have automatic withdrawal from the other. That’s how we manage our water.”

In addition to his post at UC Davis, Fogg is a director at UC Water, an organization formed after the passage of SGMA to unite the expertise of UC system researchers and help the state move forward with future planning.

We have to be much more aggressive about not only groundwater management but recharging the groundwater in ways that we haven’t in the past,” Fogg said.

Recharge is the naturally occurring process by which water seeps into the ground and below river and stream beds after rain, filling up aquifers and the space between rocks and soil. Irrigation also is responsible for groundwater recharge. During drought conditions, not enough water is returning to the ground to replace the amount that users are pumping out of the ground. In California, depleted groundwater supplies are causing multiple problems such as subsidence and saltwater intrusion.

On this topic, Andrew Fisher is ahead of the game. A professor of hydrogeology at University of California Santa Cruz, Fisher began studying groundwater 15 years ago when he sought to focus his research efforts on a topic that would have an impact in the public realm. Fisher had “an epiphany” that groundwater recharge was that topic.

Groundwater’s relationship to surface water. Image credit: USGS

Recharge is “an area that’s a hydrologic frontier, an area where there’s a lot that’s not known and where as a scientist I could make a contribution that would make a difference,” Fisher said.

To confront these issues, Fisher founded the Recharge Initiative to involve students in collaboration with government agencies, farmers, policymakers, and other stakeholders to study and implement recharge projects statewide. Through the Initiative, Fisher directs his students in research into local and regional conditions that support or deplete groundwater stores, including mapping where recharge occurs and computer modeling to test their results and make predictions to optimize groundwater recharge projects.

Fisher advocates managed recharge, an umbrella term for using various techniques to actively introduce excess surface water, including stormwater, treated wastewater, and agricultural runoff, into aquifers. Certain areas of California have been using some of these methods for decades. For instance, the city of Roseville manages injection wells that force water into the ground, and the Coachella Valley diverts excess flows from the Colorado River into spreading basins to collect water and allow it to seep into the ground. But until recently, these methods have not been widely used.

Part of the problem is a lack of data about what methods work best in different regions. Because groundwater resources, surface water availability, geology, climate, and financing are so different from place to place, what works in one place won’t necessarily work in another.

Fisher is also a director at UC Water, which is currently supporting several new recharge projects, including floodplain recharge and agricultural field flooding. Before the 19th century, the rivers in the Central Valley would regularly flood, filling up the floodplain with water that would eventually percolate down to underlying aquifers. This natural recharge disappeared 150 years ago when levees were built to control the flooding. Recently, The Nature Conservancy (TNC) has removed the levees on a study site along the Cosumnes River. Because the area is not urbanized and the floods are not a danger to homes or businesses, TNC is working with UC Water and other partners to let the floodplain flood, and measure the recharge benefit.

A San Joaquin Valley almond orchard is flooded to recharge the aquifer underneath. Image Credit: The Almond Board of California

Another promising recharge project is the winter irrigation of fallow farm fields. The project involves re-routing excess water from wet winter conditions onto farms.

“Irrigation has probably increased the groundwater recharge perhaps tenfold,” Fogg said.

Helen Dahlke, a researcher at UC Davis, is currently conducting a number of demonstration and field test studies on the benefits and drawbacks of winter irrigation, including its impacts on crops, groundwater levels, and groundwater contamination.

“We’ve pumped roughly 100 million acre-feet out of the groundwater aquifers in the Central Valley,” Dahlke said. “So that is a void that can be filled again.”

For instance, in an almond orchard in Modesto, Dahlke and partners are applying six vertical inches of water from city storm drains to the orchard. Over the course of the winter, they will flood the field with two feet of water that would otherwise end up in the Tuolumne River. They will monitor the tree response to the additional water to make sure it doesn’t negatively impact the trees or the almond yield the following summer.

These projects seem promising, but with climate change advancing, where will the water come from? Many climate models forecast that the state will receive the same amount of precipitation that it has historically, but that it will come in different ways, such as bigger floods alternating with more frequent and severe droughts. This shift in climate patterns will require new strategies by water managers to divert higher flood flows, when they happen, into groundwater basins.

Fisher and his colleagues conducted a study in 2012 that looked at extreme precipitation for the last 120 years, and found that the average amount of rain each year hasn’t changed, but more of the rain is falling during a smaller number of more intense storms.

“Everything else being equal, shorter more intense rain storms tend to soak into the ground less,” Fisher said. “You tend to get more runoff, and that leads to flooding, and even when it doesn’t lead to flooding, it adds to less recharge.”

The effect is amplified under changes in land use such as increasing urbanization and agricultural development, which makes preparation for climate change a challenge. In addition, Fisher noted that climate models have a difficult time making precipitation predictions.

Fogg adds that over a geologic time scale, California is prone to long periods of drought. Whether the changes are due to climate change or not, he said, “We anticipate that the drought could continue for a long time …We have to try and prepare for long periods of scarcity or at least intermittent periods of scarcity which means not predominantly relying on storage of water in our surface reservoirs to weather those droughts.”

Using a combination of data from ground measurements and satellites like NASA’s Gravity Recovery and Climate Experiment (GRACE) enabled researchers to create this model of the U.S. groundwater stored in aquifers. NASA researchers say that it will take 11 trillion gallons of water to recover from ongoing drought conditions in California. Image Credit: NASA

The variety and diversity of local conditions, coupled with the changes wrought by a shifting climate, underscores the need for so many different approaches to recharge. In studying recharge, hydrogeologists have learned that it does not occur in most places.

You might look across an area of a thousand acres, and if you were to throw water equally on that acreage, you might get 95 percent of the recharge from only 5 or 10 of those acres,” Fisher said.

Currently, there is no statewide mapping system to collect and monitor conditions to determine the most effective locations for managed recharge projects.

But with the new legislation taking effect, the impetus and financing are finally there for water systems across the state to start considering the viability of groundwater projects, Fisher said.

“Right now there are dozens of groundwater basins and regions around the state that are scrambling to get organized,” he said. “There’s still just a tremendous amount of work to be done to help figure out what the best way is to manage resources.”