- Author: Mike Hsu
Light irrigation before flooding stimulates microbes to remove nitrates from soil
With California enduring record-breaking rain and snow and Gov. Gavin Newsom recently easing restrictions on groundwater recharge, interest in “managed aquifer recharge” has never been higher. This process – by which floodwater is routed to sites such as farm fields so that it percolates into the aquifer – holds great promise as a tool to replenish depleted groundwater stores across the state.
But one concern, in the agricultural context, is how recharge might push nitrates from fertilizer into the groundwater supply. Consumption of well water contaminated with nitrates has been linked to increased risk of cancers, birth defects and other health impacts.
“Many growers want to provide farmland to help recharge groundwater, but they don't want to contribute to nitrate contamination of the groundwater, and they need to know how on-farm recharge practices might affect their crops,” said Matthew Fidelibus, a University of California Cooperative Extension specialist in the UC Davis Department of Viticulture and Enology.
A recently published study by UC scientists sheds new light on how nitrates move through an agricultural recharge site and how growers might reduce potential leaching. Researchers analyzed data from two grapevine vineyards at Kearney Agricultural Research and Extension Center in Fresno County – one flooded for two weeks, and other for four.
Understanding initial nitrate levels crucial
A key factor in mitigating contamination is understanding how much nitrate is in the soil at the outset, said study author Helen Dahlke, a UC Davis hydrologist and leader of UC Agriculture and Natural Resources' strategic initiative on water. In areas with little precipitation and cropping systems that require greater amounts of synthetic fertilizer, the accumulation of residual nitrate – resulting from nitrogen in the fertilizer not taken up by the plants – can be quite high.
“The percentage of nitrates in some soils can really increase over the years, particularly if you have many dry years in a row where you don't have access to irrigation water or natural precipitation flushing some of those nitrates out of the soil,” Dahlke said.
While intense rains in recent weeks have helped dilute nitrate concentrations naturally, farmers looking to participate in recharge during the dry years ahead should consider flooding their fields with greater volumes of water.
“If you're doing this for the first time – on-farm recharge in the winter – check your residual soil nitrate levels because if they're very high, you should apply a lot of water in order to make sure that the residual nitrate is diluted down,” said Dahlke, who also added that growers should check their soil properties for suitability of recharge projects.
She recommended using, as a “good first approximation,” the online Soil Agricultural Groundwater Banking Index map, a project led by Toby O'Geen, a UC Cooperative Extension soil resource specialist.
Researchers looking at other ways to reduce nitrates
Even before flooding the fields for recharge, there are several practices that can lower initial nitrate levels and risk of leaching. Cover crops such as alfalfa and triticale, for example, can help take up residual nitrates that accumulate from fertilizing a main crop over time.
Dahlke and Fidelibus – a co-author of the San Joaquin Valley vineyard study – both pointed to pre-flooding irrigation that encourages denitrification, a process in which soil microbes transform nitrates into gaseous forms of nitrogen.
“Those denitrifying microbes need to be stimulated to do the work,” said Dahlke. “What we have found is that if you do a little bit of irrigation before you start the flooding, increasing the soil moisture can get those microbes started and they can take out more nitrate from the soil.”
The timing and quantity of fertilizer applications are also major factors in reducing leaching. Although more growers are following high-frequency, low-concentration practices to maximize uptake by crops, Dahlke said there needs to be more emphasis on incorporating nitrogen transformation processes – such as denitrification – in the nutrient management guidelines that farmers follow.
“Implementing thoughtful nutrient management plans will play a particularly important role in participating farms,” Fidelibus added.
A more holistic view of groundwater recharge
In short, choices made during the growing season can affect those in the winter recharge season – and vice versa. For example, applying compost or other organic amendments to soil can give microbes the “fuel” they need for sustained denitrification.
“What we have found is that our denitrifying bacteria often run out of steam because they don't have enough carbon to do the work,” Dahlke said. “Like us, microbes need energy to do the work, and for microbes this energy comes from soil carbon.”
Then, adding moisture via recharge to that field with high organic content can stimulate mineralization and nitrification, processes in which microbes transform the organic nitrogen into ammonium – and subsequently nitrates – that the plants can then take up. Those naturally occurring nitrates would thus reduce the need for the grower to apply synthetic fertilizer.
“The winter on-farm recharge experiments have shown that altering the moisture regime in the winter has consequences for the nitrogen budget in the summer growing season,” Dahlke explained. “Theoretically, what we need to be doing is better integrating both seasons by keeping an eye on the soil-nitrogen balance across the whole year so that we can ensure, at the end of the growing season, the residual nitrate in the soil is minimized.”
The study, published in the journal Science of The Total Environment, was part of the post-doctoral work of former UC Davis researcher Elad Levintal. In addition to Fidelibus and Dahlke, other authors are Laibin Huang, Cristina Prieto García, Adolfo Coyotl, William Horwath and Jorge Rodrigues, all in the Department of Land, Air and Water Resources at UC Davis.
/h3>/h3>/h3>/h3>- Author: Pamela Kan-Rice
Following an open call, Helen Dahlke has agreed to take on the role of leader for the Strategic Initiative for Water Quality, Quantity and Security.
Dahlke is associate professor in integrated hydrologic sciences in the UC Davis Department of Land, Air and Water Resources. She brings a rich history of experience having completed her undergraduate and master's degrees in her native Germany before earning her Ph.D. at Cornell University. After her Ph.D., she did postdoctoral work at Stockholm University in Sweden before joining UC Davis in 2013.
Dahlke's current research interests include surface water – groundwater interaction, water resources management, vadose zone transport processes, and applications of DNA nanotechnology in hydrology. She comes with a broad appreciation of the multiple roles for addressing issues facing water across the state from the mountains to the sea. One of her main research efforts focuses on testing the feasibility of using agricultural fields as recharge sites for groundwater replenishment.
“We welcome and thank Helen for adding this new role to her ongoing activities,” said Mark Bell, vice provost for strategic initiatives and statewide programs. “The SI leaders are the champions for the broad umbrellas of work across the organization.”
The Strategic Initiatives help people connect while helping unify, communicate and advocate for UC ANR's work across the state. The SI leaders are part of Program Council, which provides input for programmatic policy and direction for the organization.
- Author: Pamela Kan-Rice
We are pleased to announce that - following an open call - Helen Dahlke is taking on the role of leader for the Water Strategic Initiative (officially known as the Water Quality, Quantity and Security SI).
Helen is associate professor in integrated hydrologic sciences in the UC Davis Department of Land, Air and Water Resources. Helen brings a rich history of experience having done her undergraduate and masters in her native Germany before doing her Ph.D. at Cornell University. After her Ph.D., she did postdoctoral work at Stockholm University in Sweden before joining UC Davis in 2013.
Helen's current research interests include surface water – groundwater interaction, water resources management, vadose zone transport processes, and applications of DNA nanotechnology in hydrology. She comes with a broad appreciation of the multiple roles for issues facing water across the state from the mountains to the sea. One of her main research efforts focuses on testing the feasibility of using agricultural fields as recharge sites for groundwater replenishment.
We welcome and thank Helen for adding this new role to her ongoing activities. The SI leaders are the champions for the broad umbrellas of work across the organization. The SIs help people connect while helping unify, communicate and advocate for the work across the state. The SI leaders are part of Program Council, which provides input for programmatic policy and direction for the organization.
Please welcome Helen in her new and expanded role.
Mark Bell
Vice Provost Strategic Initiatives & Statewide Programs
- Author: Madison Sankovitz, student intern
When California experiences drought due to a lack of rain and snow and the reservoirs don't fill up, people pump water out of the ground to meet their needs. But that practice has its limits, as groundwater aquifers -- underground layers of porous rock -- get depleted, similar to how water squeezes from a sponge.
Many of California's groundwater aquifers, especially in the San Joaquin Valley, are critically overdrafted. They are being depleted faster than they are being recharged by water from the surface percolating through the soil to groundwater. Overdrafting is a concern because California relies on groundwater aquifers as a water storage and supply resource. They must be protected to ensure water security in the future.
To direct surface water back into the aquifers, recharge basins are built, but they are limited in number and volume. Thinking of agricultural fields as potential recharge basins opens the possibility of increasing the number of locations where groundwater aquifers can be recharged.
Alfalfa is a crop that has a unique potential to support this practice because it is widely grown in the San Joaquin Valley and, in many cases, the plants are dormant or semi-dormant during the winter months. University of California researchers Helen Dahlke, Nick Clark and Khaled Bali are investigating how alfalfa copes with additional water in the winter and early spring, when snowmelt runoff occurs, for groundwater recharge.
“We hypothesized that water can be recharged in dormant and early regrowing established alfalfa fields successfully, and with little to no harm to the plants and the productivity as far as the farmer is concerned,” said Clark, a UC Cooperative Extension farm advisor.
The findings have shown great promise for recharging groundwater without negatively impacting alfalfa yield but may diminish quality. “You could do recharge in winter and then turn the water off completely and still get a cutting or two of alfalfa before the summer,” said Bali, a UCCE irrigation water management specialist.
Like Bali, Clark thinks alfalfa growers could use their fields for recharging groundwater while taking steps to limit loss of crop value.
“We did find that the practice of recharging groundwater in alfalfa fields can have a negative impact on the feed quality of the alfalfa when first harvested after the flooding,” Clark said.
“One big recommendation we have is that the alfalfa fields should be in their later years of production so if something disastrous happens, there is not so huge of investment return lost,” said Clark. “There is current research being conducted by UC Cooperative Extension specialists Dan Putnam and Khaled Bali examining practical solutions on the farm level that growers can implement to minimize the risk of damage to alfalfa when also flooding fields for groundwater recharge.”
Clark emphasized that farmers face incredibly complex water-management issues today. The drivers that influence their decisions reach far beyond the farm, so he collaborates with UC Davis professor Helen Dahlke, who studies integrated hydrologic sciences.
“It requires an approach from multiple disciplines to address that complexity,” he said. “Helen brings hydrological expertise, while I can focus on agronomy. So the information we provide is more holistic and relevant on a larger scale, but still practical and applicable for farmers.”
Grower input on the research findings has been critical to their success. “With their feedback, we were able to reform some of the project methodologies to achieve results that were even more relevant to the local conditions,” said Clark.
With the prolonged drought, innovations in groundwater recharge are becoming more crucial. Future research will continue to focus on how growers can use their land in multifaceted ways to improve California's water sustainability far into the future.
- Author: Jeannette E. Warnert
When droughts strike California, people who rely on shallow domestic wells for their drinking, cooking and washing water are among the first to feel the pain. Aquifers have become depleted from decades of overuse. Drilling deeper is an option for farmers, but prohibitively expensive for low-income residents in disadvantaged communities in the San Joaquin Valley.
A UC scientist believes managed aquifer recharge on agricultural lands close to populations with parched wells is a hopeful solution.
Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater. If relatively clean mountain runoff is used, the water filtering down to the aquifer will address another major groundwater concern: nitrogen and pesticide contamination.
“The recharge has the potential to clean up groundwater,” she said.
Five years ago, UC Cooperative Extension specialist Toby O'Geen developed an interactive map that identifies 3.6 million acres of California farmland with the best potential for replenishing the aquifer based on soil type, land use, topography and other factors. Dahlke and her colleagues analyzed the map and identified nearly 3,000 locations where flooding suitable ag land will recharge water for 288 rural communities, half of which rely mainly on groundwater for drinking water. The research was published by Advancing Earth and Space Science in February 2021.
“If we have the choice to pick a location where recharge could happen, choose those upstream from these communities,” Dahlke said. “Recharge will create a groundwater mound which is like a bubble of water floating in the subsurface. It takes time to reach the groundwater table. That bubble floating higher above the groundwater table might just be enough to provide for a community's water needs.”
Filling reservoirs under the ground
Many climate models for California suggest long-term precipitation amounts will not change, however, the winter rainy season will be shorter and more intense.
“That puts us in a difficult spot,” Dahlke said. “Our reservoirs are built to buffer some rain storms but are mainly built to store the slowly melting snowpack in the spring. In the coming years, all the water will come down earlier, snowmelt likely in March and April and more water in winter from rainfall events.”
She is working with water districts and farmers to consider a change in managing water in reservoirs.
“We want to think about drawing reservoirs empty and putting the water underground during the fall and early winter. Then you have a lot of room to handle the enormous amounts of runoff we expect when we have a warm atmospheric river rain event on snow in the spring,” she said. “However, farmers are hesitant. They like to see water behind the dams.”
Interest in groundwater banking has been lifted with the implementation of the 2014 Sustainable Groundwater Management Act (SGMA). The law requires governments and water agencies to stop overdraft and bring groundwater basins into balanced levels of pumping and recharge by 2040. Before SGMA, there were no statewide laws governing groundwater pumping and groundwater was used widely to irrigate farms when surface supplies were cut due to drought.
“For some of the drought years, overdraft was estimated to be as high as 9 million acre-feet a year,” Dahlke said.
Dahlke believes wintertime flooding for groundwater recharge can help water districts meet SGMA rules. “We have to do anything we can to store any surplus water that becomes available to save it for drier times and our aquifers provide a huge storage for that,” she said.
Farming impacts
The Dahlke lab is collaborating with UC Agriculture and Natural Resources farm advisors and specialists and with scientists at other UC campuses to learn about agronomic impacts of flooding a variety of agricultural crops, including almonds, alfalfa and grapes.
In the San Joaquin Valley, UC Cooperative Extension irrigation specialist Khaled Bali led an intermittent groundwater recharge trial on alfalfa. The researchers applied up to 16 inches per week with no significant impact on alfalfa yield.
“You could do groundwater recharge in winter and then turn the water off completely and still get a cutting or two of alfalfa before summer,” he said.
This past winter, Dahlke was prepared to flood 1,000 acres of land with water from the Consumnes River. Even though winter 2020-21 was another drought year, the research will go on. Her team was able to flood a 400-acre vineyard and, in collaboration with scientists from UC Santa Cruz, deploy sensors in the field to measure infiltration rates to better understand whether sediment in the flood water could clog pores in the soil. Her team also collaborates with Ate Visser of Lawrence Livermore National Laboratory in using isotope and noble gas data to determine the groundwater age and flow.
The Dahlke Lab's groundwater banking project has planned more studies in groundwater basins across the state to close knowledge gaps on suitable locations, technical implementation and long-term operation. They also plan to address operational, economic and legal feasibility of groundwater banking on agricultural land.