Fallow fields could be used as percolation basins for groundwater recharge.

As farmers pump groundwater to keep their crops alive during the California drought, many of the state's aquifers are being drained rapidly. UC Agriculture and Natural Resources (UC ANR) researchers are working on a new approach to replenish these critical underground supplies once the rains return: using farm fields as recharge basins during winter months.

Already, a number of water agencies around the state deliberately recharge groundwater supplies by spreading water on open land and allowing it to percolate into aquifers. But dedicated sites for this type of recharge are scarce. So the UC ANR team decided to figure out if some of the California's millions of acres of farms and ranches could be brought into service.

"We wanted to see if we could support a more sustainable groundwater supply," said Anthony O'Geen, a UC ANR Cooperative Extension  specialist  (UCCE) in the Department of Land, Air and Water Resources at UC Davis and the lead author of a peer-reviewed article on the topic in the current issue of California Agriculture journal.

Integrating data on the physical properties of soil, salinity, topography and the tolerance of crops to standing water, O'Geen and his colleagues developed an index of the suitability for groundwater recharge of land in all agricultural regions in California. One finding: As much as 3.6 million acres of agricultural land statewide has good potential for groundwater recharge.

In dry years, groundwater can account for more than half of the irrigation water used in the state. But few groundwater basins are actively recharged. Instead, they are replenished naturally -- and at an uncertain rate -- by, for instance, percolation of rainwater into soil and seepage from rivers and lakes as well as water supply canals and irrigation ditches.

Deliberately recharging groundwater allows aquifers to be managed more like surface reservoirs, and has the potential to increase the state's water storage capacity by millions of acre-feet. During periods like the current drought, there's little or no extra water available for groundwater recharge. But in wet years, it may be possible to devote substantial volumes to replenishing aquifers.

Just how much extra water might be recharged, and on what kinds of fields, are two questions the UC ANR researchers are pursuing now.

Helen Dahlke, an assistant professor in the Department of Land, Air and Water Resources at UC Davis and a co-author on O'Geen's paper, has been conducting field experiments to evaluate two main variables: How much water can be recharged over a period of several weeks, and whether all that water hurts the crops planted in the field.

So far, the results are very promising.

Over a six-week period in February, March and April, Dahlke oversaw a test in Siskiyou County in which 140 acre-feet of water were applied to 10 acres of alfalfa. That's well over twice the amount of irrigation water the field typically gets in an entire year.

"It was just pouring into the ground," Dahlke said.

The water percolated readily into the earth and the groundwater table in the vicinity of the farm rose quickly. Just as important: by June the alfalfa field that had been watered so heavily was just as healthy as a control plot. Alfalfa is known to "drown" if watered very heavily in summer months, but it appears that the winter dormancy of alfalfa is helping the crop to tolerate saturated soils for some time. Field trials near the UC Davis campus have corroborated the Siskiyou County results, Dahlke said, though additional tests in more soil types and warmer climates (e.g. the southern Central Valley) are needed.

At least two similar tests in almond orchards in the Central Valley are planned for the coming year, as are additional alfalfa trials.

Groundwater recharge on farm fields is still several big steps away from becoming widespread, O'Geen and Dahlke said. In addition to the physical and biological parameters of the practice, UC ANR researchers are also investigating infrastructure, policy and institutional barriers. The first results from that work should be available later this year, Dahlke said.

To read the groundwater study in California Agriculture, visit http://californiaagriculture.ucanr.edu.

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California Agriculture is a peer-reviewed journal of research in agricultural, human and natural resources published by UC ANR. For a free subscription, visit californiaagriculture.ucanr.edu, or write to calag@ucanr.edu.

“A significant number of regions in California won't have groundwater available in another generation or two if we continue business as usual.” -- UC scientists Thomas Harter and Helen Dahlke

During the drought, California is pumping from wells. UC scientists make recommendations to manage groundwater.

In the special edition of California Agriculture released today (July 16), UC Cooperative Extension specialist and UC Davis professor Thomas Harter and UC Davis professor Helen Dahlke call attention to the stress being placed on California's aquifers as well as the catastrophic consequences of not having this hidden resource available in future droughts.

In the University of California's premiere journal for agricultural research, the groundwater experts make the following key recommendations:

• Groundwater is most effectively managed at the local or regional basin level, with support from the state.
• Local groundwater management entities must be given better tools, such as clear mandates to assess, measure, monitor and allocate their groundwater and control its extraction.
• The definition of groundwater sustainability can be set at the state level and translated into specific actionable thresholds that must be enforced locally, with a credible threat of state enforcement should the local efforts be unsuccessful.
• Much better data collection, analysis, reporting and data integration are needed to provide transparency, to support local management efforts and to properly inform the public. This requires much stronger planning and support within the DWR and SWB.

“Fundamentally, even more needs to be done," Harter and Dahlke write. "Local land-use decisions on urban and agricultural development, which have critical impacts on groundwater resources, must be consistent with groundwater management objectives. This will require significant communication between land-use and groundwater managers. Effective integration with water quality management and surface water management efforts, which are governed separately, is also required. And none of these efforts can occur without sustained funding through a mix of local and state sources.”

In their outlook article, Harter and Dahlke also explore one of the most promising ideas to protect our aquifers: groundwater banking.

The idea is that during storms or flood control releases, excess surface water could be directed from streams via existing water conveyance systems onto dormant or fallow agricultural fields, which would then serve as infiltration basins. Solutions need to be developed to add significant recharge to California's aquifers, often during relatively short periods when excess surface water is available. 

A 3-year project, funded by UC Division of Agriculture and Natural Resources, aims to look at the feasibility of such groundwater recharge activities by setting up pilot groundwater recharge field experiments, which would provide valuable data to address concerns about the costs and risks to crops, the influence these projects may have on groundwater levels and flows, and the possibility of recharging contaminated water or degrading groundwater quality by leaching contaminants such as nitrate from the vadose zone. Data collected could serve as a foundation for developing economic incentives at the local, state or federal level to acknowledge the landowner's service to the local community and California's water supply reliability.

To read their entire article, "Out of sight but not out of mind: California refocuses on groundwater,” and the special "Water efficiency" edition of California Agriculture, visit http://californiaagriculture.ucanr.edu and  http://ucanr.edu/repositoryfiles/cav6803p54-136027.pdf.

Further reading:

UC Cooperative Extension Groundwater Program http://groundwater.ucdavis.edu

Helen Dahlke's groundwater banking project http://dahlke.ucdavis.edu/research/groundwater-banking

California Water Action Plan: Improving Groundwater Management (links to state policy and emerging legislation) http://groundwater.ca.gov

California Department of Water Resources report to the Governor's Drought Task Force http://www.water.ca.gov/waterconditions/docs/Drought_Response-Groundwater_Basins_April30_Final_BC.pdf

California Water Plan Update 2013 http://www.waterplan.water.ca.gov/cwpu2013

Association of California Water Agencies Recommendations for Achieving Groundwater Sustainability http://www.acwa.com/sites/default/files/post/groundwater/2014/04/final_acwa-groundwater-sustainability-recommendations.pdf