SACRAMENTO — Now that researchers know they can make significant aquifer improvements with groundwater recharge projects, the next frontier will be determining where and when it can best be done.
Studies by the University of California and the Almond Board of California have found success in improving the health of water tables by flooding alfalfa fields and almond orchards during the winter.
But what’s still to be determined is how late in the season the intentional flooding can occur without hurting crops and yields, and where all the most optimum places are for undertaking such projects, researchers say.
For instance, orchards with sandy, porous soils have done the best at absorbing water when it’s applied. But even some of those have harder rock layers underneath that cause “a lot of lateral movement of water,” said Peter Nico, a soil and environmental biogeochemist at the Lawrence Berkeley National Laboratory.
“We know about soils up and down the (Central) Valley,” Nico said during a recent almond industry conference in Sacramento. “What we know less about is the layers underneath.”
For instance, the less porous layers are often not continuous; they fade in and face out, Nico said. Their location is important in determining where water goes, he said.
Finding areas in an orchard where water seeps through the soil more quickly could facilitate more targeted recharge projects, noted Gabriele Ludwig, the Almond Board’s director for sustainability and environmental affairs.
“Maybe you don’t have to put flooding across a whole field,” Ludwig said. “Could you put it in portions or locations that have a speedier connection to lower-down depths where you’re more rapidly able to get water in and you don’t have to use as much area?
“I know one of the things that would really make a difference ... is if you could just target certain areas that you could completely set aside and maybe take out of production,” she said. “It gives us an opportunity that may be less risky to growers.”
While California’s groundwater reserves have been systematically depleted since the 1920s, the notion of recharge really took hold with the passage of the 2014 Sustainable Groundwater Management Act, which includes intentional recharge of farm fields as an available option, said Helen Dahlke, a UC-Davis hydrologist who has led some of the research.
UC scientists have been working with growers throughout the valley to find fields with soils conducive to recharge and set up pilot projects, as have groups such as the Almond Board.
State-funded aquifer recharge projects already underway put at least 306,727 acre-feet of water per year back into the ground, according to Stanford University estimates.
Planners are discussing recharge as a means to not only replace groundwater that was lost and restore troubled aquifers but also provide an option to store water without needing to put it behind a dam, Ludwig said.
Other states are also getting into the act. For instance, the Idaho Water Resource Board is on pace to recharge a record amount of water into the Eastern Snake Plain Aquifer during the 2017-2018 season, having already put 317,000 acre-feet of water into the ground.
In California, the Almond Board and the environmental group Sustainable Conservation have teamed in recent years to fund orchard-flooding research on test plots. UC-Davis researchers have been trying to determine whether groundwater recharge could take place without negatively impacting orchard health and crop yields.
The test plots in Modesto, Delhi and Orland — all almond orchards — have different soils, Dahlke said. The site in Modesto has 20-year-old Nonpareil and Monterey trees on “moderately good” soil, the Delhi site is “like dune sand” and the Orland orchard has an older stand in find, sandy soil but with confining layers underneath, she said.
While it took 48 hours for water to pass the root zone in the Modesto orchards, it took about six hours in the other two, Dahlke said.
“At the Orland site, it was so wet that it only took 5 feet of water then started to run off,” she said.
The researchers found that recharge projects during almond trees’ winter dormancy period had little impact on the following season’s yields. In fact, the Delhi site saw an increase in yields, perhaps because the wet winter flushed the excess salt from the soil, Dahlke said.
“Based on our field studies, we don’t see any obvious warnings that winter recharge has a negative impact on tree health,” Dahlke said.
Researchers are still gathering data on actual root impact from the projects, Ludwig said. Another caveat is that additional information is needed on whether the projects worsen nitrate leaching into groundwater, or whether putting additional water on may have a diluting effect, she said.
Also to be determined is how or whether recharge projects could be done when trees are active. Waterlogged trees don’t respond well, so such efforts might only work in the most porous of soils, and even then a grower might have to add nitrogen, Ludwig said.
The trees’ displeasure at being waterlogged was evident after heavy rains caused flooding in orchards in the winter and spring of 2017, when “we did definitely see orchards that were unhappy standing in water too long,” Ludwig said.
The project is one of numerous efforts in various crop fields throughout crop fields being done with the help of UC Cooperative Extension advisers. In the Scott Valley in far northern California, for example, rancher Jim Morris obtained permission to take winter stormwater from a local water district’s irrigation canal and apply different amounts of water to different sections of a field to test the tolerance of his alfalfa to the practice.
Dahlke said she doesn’t think groundwater recharge could reverse subsidence in the southern Central Valley, where collapses that have occurred in the hard clay subsurface are irreversible.
In general, though, there would be enough water in the state to address overdraft if it weren’t for such limiting factors as a lack of infrastructure, legal barriers and environmental needs, Dahlke said.
“With recharge we can definitely address some of the ongoing groundwater overdraft and at least bring groundwater basins into balance, meaning stabilize the water tables over longer term instead of seeing them decline at a steady rate,” she said in an email.
Fully restoring groundwater to historic conditions is probably unlikely, though, since climate change and an increase in water demand from population growth are expected to put continued pressure on aquifers, she said.