University of California scientists recently identified 3.6 million acres of California cropland suitable for replenishing the state’s groundwater reserves, which are at a record low. Walnut orchards may be  viable sites for groundwater recharge, though the potential for water damage to such high-value crops adds risk. Photo by David Doll. 

By Philip Bachand, Helen Dahlke, William Horwath, Thomas Harter and Toby O’Geen

A much-anticipated “Godzilla” El Niño this winter may refill California’s drought-diminished reservoirs, but it won’t do much to restock the severely depleted aquifers we rely upon to get by during droughts.

The California Department of Water Resources (DWR) recently reported that groundwater levels across most of the state have dropped 50 feet below historical lows, with levels in many areas of the San Joaquin Valley more than 100 feet below previous record lows. Source: DWR

One reason for this is the sheer depth of California’s precipitation deficit – the deepest of any drought in 120 years of recordkeeping. The state has been drier than normal for 10 of the past 14 years.

The above-average amount of pumping during all those dry years has led to the deepest groundwater levels ever recorded in most areas of the state. In some areas water tables have plummeted by more than 100 feet. Unlike surface water reservoirs, aquifers do not refill with just one or two wet years, no matter how big the El Niño

A strong El Niño nonetheless presents a precious opportunity to replenish some of these vital underground reserves faster than would otherwise occur.

California’s vast acreage of irrigated farmland holds the key. Using the existing irrigation network, we can capture flood flows from our rivers onto suitable dormant or fallow agricultural fields, allowing the surplus water to infiltrate aquifers.

A team of University of California scientists has already identified 3.6 million farm acres with soils most conducive to groundwater recharge – those with high percolation rates. They developed an interactive map where growers can obtain site-specific information on the suitability of soils for on-farm recharge.

In other words, more than one-third of the state’s nearly 10 million acres of irrigated cropland could potentially be re-purposed as groundwater infiltration basins during winter and early spring – outside the usual growing season.

Likewise, the same infrastructure that conveys about 20 million acre-feet of water to irrigate California crops during the growing season could be used to divert high river flows onto fields in the off season, mimicking natural floodplains.

UC researchers recently developed a soil suitability index for groundwater recharge on cropland, based on such factors as (A) deep percolation, (B) root zone residence time and (C) chemical limitations. Source: UC Davis California Soil Resource Lab 

During storms and flood-control releases, excess river water could be routed through irrigation canals onto farms, where the surplus would seep underground to replenish groundwater. This action could also mitigate downstream flood risks.

Farmers stand to benefit from this off-season arrangement. The infiltration of floodwaters  would restore the drought-depleted soil moisture in crop root zones while accelerating recharge of regional aquifers. Greater availability of groundwater saves farmers the high expense of pumping deeper and drilling more wells.

Changes in groundwater storage in the Central Valley (dark blue) and its subregions, 1922-2009 . The largest depletions have occurred in the Tulare Lake Basin, which includes the southern Valley from Fresno to Bakersfield. Source: Thomas Harter/California Agriculture. (Adapted from Brush 2014)

Researchers with UC Davis and UC Agriculture and Natural Resources and their team of consultants are working to bring this new drought management and groundwater banking strategy into the mainstream. They have been test-flooding a number of vineyards, almond orchards and other cropland recently, looking at infiltration rates, plant physiology, groundwater quality and costs. So far, results look promising.

To take advantage of a strong El Niño this winter, farmers would need to determine how much floodwater they would want to divert and when they would want it. This would depend on the crop, soil preparation and planting dates, the soil moisture status and other agronomic considerations.

Dam operators and irrigation district managers also would need to make a number of strategic determinations, including:

• The amount of investment needed for diverting and distributing flood flows into irrigation systems
• Where that investment would reap the greatest benefits in groundwater and soil moisture replenishment
• Whether to postpone wintertime maintenance of diversion and distribution systems to accommodate floodwater delivery
• The type and level of incentives and outreach needed to gain landowner participation.

Jim Morris, a Siskiyou County rancher shown here, opened his alfalfa field last spring to UC researchers who are testing the groundwater recharge potential in various croplands around the state. “It was amazing to see how well the land absorbed the water and how quickly the water table rose,” Morris told UC Davis News. Photo by Steve Orloff.

Farmers and water managers should make preparations now to reap the most benefits of the predicted winter storms, including:

• Identify fields and crops suitable for winter and early spring flooding
• Minimize fall applications of fertilizers and pesticides to protect groundwater quality
• Do your irrigation canal repairs and clearance this fall, before the storms
• Clarify water rights as they pertain to capturing and applying large amounts of floodwater to cropland

To be sure, groundwater banking on farmland is several big steps away from becoming widespread. Several agronomic, policy and institutional questions remain to be investigated.

But flooding just 10 percent of the state’s irrigated cropland a few feet deep this winter would result in an additional 3 million acre-feet of groundwater. That’s half of the additional 6 million acre-feet of groundwater that California farmers are expected to pump this drought year to offset the drastic shortfall in surface water deliveries.

After four years of drought, a statewide effort by farmers, government agencies and water districts to capture flood flows to replenish our depleted groundwater is needed to seize what is expected to be rare opportunity – a monster El Niño.

Philip Bachand is an environmental engineering consultant in Davis. Helen Dahlke, William Horwath, Thomas Harter and Toby O’Geen are on the faculty at UC Davis’ Department of Land, Air and Water Resources.

Further reading

Agricultural Groundwater Recharge and Banking. Project Website. UC Davis. Posted October 2015

Alvarez, Felicia. Yolo farmers bank water for the future. The Davis Enterprise. Oct. 14, 2015

Bachand P, Horwath WR, Roy SB, Choperena J and Cameron D. 2014. “Implications of using on-farm flood flow capture to recharge groundwater and mitigate flood risks along the Kings River , CA.” Environmental Science & Technology. 48 (23), pp 13601–13609 DOI: 10.1021/es501115c. Nov. 13, 2014

Dahlke HE. Summary of on-going research activities on groundwater banking on cropland. UC Davis Department of Land, Air and Water Resources, UC Davis

Howitt R, MacEwan D, Medellín-Azuara J, Lund J, Sumner D. “Economic analysis of the 2015 drought for California Agriculture”. Center for Watershed Sciences, UC Davis. August 2015

“Flooding farms in winter may help replenish groundwater”. UC Davis news release. Sept. 9, 2015

“Flooding farms in winter may help replenish groundwater”. UC Davis news release. Sept. 9, 2015

Khokha S. “How flooding fields could alleviate water supply stress.” KQED. March 29, 2013

O’Geen AT, et all. 2015. “Soil suitability index identifies potential areas for groundwater banking on agricultural lands.” California Agriculture. 69(2):75-84. DOI: 10.3733/ca.v069n02p75.April-June 2015.

Quinton A. “Flooding California farmlands might restore groundwater.” Capital Public Radio (Sacramento). Sept. 22, 2015

Swain D. “El Nino now among strongest in modern history”. California Weather Blog. Oct. 8, 2015