Groundwater overdraft in the San Joaquin Valley – producer of half the state’s agricultural output – has averaged roughly 1.8 million acre-feet annually since the mid-1980s. Even before the start of the most recent drought in 2011, a few San Joaquin farmers recognized the dire need for sustainable water management and started individually pioneering a groundwater recharge practice that has since gained statewide traction.
On-farm groundwater recharge involves intentionally diverting surface or stormwater to agricultural fields for percolation into the aquifer during times of excess. The practice holds tremendous potential for increasing water storage and offsetting groundwater overdraft, but to scale efforts, some serious obstacles will need to be overcome.
Lodi wine-grape grower Al Costa, in partnership with North San Joaquin Water Conservation District and the nonprofit Sustainable Conservation, this year launched a groundwater demonstration project on a 13.7-acre parcel of old-Zinfandel grapes to study the benefits of flooding agricultural fields with surface water to refill the aquifer below. Thus far, 145 acre-feet of Mokelumne River water has inundated the field and percolated into the subsurface, rejuvenating a small fraction of the estimated 100,000 acre-feet of water overdrafted from the aquifer each year. And all this happened with no damage to the grape vines, Costa said.
His project is just one of many projects implemented throughout the San Joaquin Valley that helped capture a share of the past winter’s near record rainfall. A recent survey found that about three-quarters of the 81 San Joaquin water districts surveyed were actively recharging this year. The majority of districts were engaging in some type of on-farm recharge, including extra irrigation on active cropland, inundation of fallowed land or substituting surface water instead of groundwater for irrigation (a method known as in-lieu recharge), said Ellen Hanak, director of the Public Policy Institute of California’s Water Policy Center, which conducted the survey.
Despite an increasing number of districts and growers adopting this practice, its full potential has yet to be realized, as policymakers create frameworks for this emerging method and researchers quantify its value.
“There is not a lot of on-farm recharge being done today, but it’s growing and will continue to grow,” said Joe Choperena, Sustainable Conservation’s senior project manager.
Understanding an Emerging Method
At first glance, this technique seems to have no drawbacks. On average, it’s cheaper than surface water storage, like using reservoirs, and has a huge capacity for replenishing water supplies. Plus, there’s plenty of farmland available for recharge.
A 2015 University of California study identified 3.6 million acres of farmland where water can safely percolate deep into the underlying aquifer with low risk of crop damage or groundwater contamination, and a preliminary calculation showed that this farmland could soak in as much as 1.2 million acre-feet of water per day. Groundwater recharge projects could provide about six times more storage capacity than surface water storage for the same price, reported Stanford University’s Water in the West in 2014. A 2016 study estimated the price of on-farm recharge at at $36 per acre-foot for a site in the Kings River Basin, which is significantly cheaper than surface water storage and dedicated recharge basins.
With that amount of land and relatively inexpensive recharge potential, why hasn’t this practice been more widely adopted?
Several obstacles, notably infrastructure and surface water availability, have limited the widespread implementation of on-farm groundwater recharge.
Water agencies in the San Joaquin Valley considered infrastructure issues to be the most significant barrier to recharge this year, according to the recent PPIC survey. On-farm groundwater recharge often requires flood irrigation infrastructure, which many farmers replaced with more efficient systems like drip irrigation during past dry spells to save water.
A report for Sustainable Conservation estimated the cost of installing a flood-irrigation system on a 160-acre farm to be $850,000. While this may sound like a sizable expense for farmers, the organization’s marketing and communications director Alex Karolyi pointed out that, when amortized over 20 years, it equates to storing water for future use at a cost of $98 per acre-foot, which is a lot cheaper than the $200–$2,000 per acre-foot that farmers ended up paying for imported surface water during the last drought.
“I think it is safe to say that if infrastructure were in place we could begin to replenish what is typically pumped from groundwater in most years if floodwaters are available,” said Anthony (Toby) O’Geen, soil resource specialist at Cooperative Extension at the University of California, Davis, and lead author on the 2015 U.C. study.
The availability of surface water, whether in a river or a canal, is another issue. Some of the regions with the worst groundwater overdraft and best suitability for on-farm recharge, like the Tulare Basin, have no access to surface water, according to U.C. Davis hydrologist Helen Dahlke, whose research pioneers the study of this technique.
Crop tolerance for excess levels of saturation also determines site suitability. Dahlke and her team currently have five experimental sites across the state testing the impacts of on-farm recharge on various crops, including alfalfa, almonds and pistachios. “So far, it has looked pretty good [for] alfalfa,” said Dahlke, but the research is ongoing with other crops.
Between 2014 and 2017, her team applied 4–26ft of water to alfalfa fields on two farms in Northern California for an average of six to eight weeks between January and April with no negative impacts on crop yield.
These results show farmers that on-farm groundwater recharge will not damage their crops while also indicating to water districts and environmental organizations like Sustainable Conservation the types of agriculture they should target.
Sustainable Conservation is studying how 11 crops, including grapes, pistachios and walnuts, can handle flooding in spring and early summer when large releases from reservoirs offer water for recharge. During wet and above-normal precipitation years, the most optimal times for on-farm recharge in California are from December to May, when farmers can capitalize on flood pulses or on reservoir releases.
A 2017 study looked at the availability of high magnitude streamflow – flows above the 90th percentile that exceed environmental flow requirements and current surface water allocations under California water rights – in the Sacramento, San Joaquin and Tulare basins. The researchers found “that there is sufficient unmanaged surface water physically available to mitigate long-term groundwater overdraft in the Central Valley.”
Financing the Future
But the cost of capturing that excess water can be prohibitive. Even though on-farm recharge’s mean price is cheaper than other water storage options, Dahlke cautioned that comparing the cost of on-farm recharge projects with other forms of water storage is limited because the actual price of any given project can vary and is contingent on the state of the infrastructure not only on the farm but also the canal or pipeline delivering the surface water.
The cost of water obtained from managed aquifer recharge projects – of which on-farm recharge is a type – in California could range from $80–$960 per acre-foot per year, wrote Bea Gordon of Stanford’s Water in the West. Several factors influence the price, including land cost, lack of available data and changes to the cost of environmental compliance.
“The fact of the matter is, with cost, it’s more complicated than just [an] amount,” said Hanak of PPIC. “You need to factor in the bigger costs for the value of expanding capacity.”
Even with sufficient infrastructure, the on-farm recharge project on Lodi grower Costa’s property was hampered by the cost of electricity. The project had $5,000 set aside for paying for electricity to pump water from the Mokelumne River to the site, and the funds were eaten up after running the pump all day for 12 days, said Sustainable Conservation’s Choperena.
Even in that small amount of time, though, John Podesta, manager of North San Joaquin Water Conservation District, was amazed at how much water Costa was able to put in the ground on a small section of his vineyard.
“There is so much potential on this property and there [are] a lot of long-terms plans … to make this site a long-term recharge site,” he added.
Since the passage of the 2014 Sustainable Groundwater Management Act (SGMA), water districts, growers and the state government have invested more in groundwater recharge to halt overdraft and balance out aquifer levels. Recent water bonds – namely Proposition 1 – and other government funding mechanisms have allocated billions of dollars for improving water storage infrastructure, including groundwater recharge projects, but the competition for the funds is high and permitting of groundwater recharge projects remains complex and time-consuming.
Dahlke said that many of the challenges associated with on-farm recharge will resolve as water agencies comply with SGMA by working to manage groundwater more sustainably and more research sheds lights on the benefits of this emerging technique.
With the Sierra Nevada snowpack projected to substantially decrease by the end of the century because of climate impacts, California’s current water infrastructure will need to adapt. The key to future water storage is groundwater, contends U.C. Davis hydrogeologist Graham Fogg.
“We used to have more snow,” said Fogg. “We need to find another storage mechanism. Groundwater is a great place for that.”