Water & Drought

Recharge method gives hope to increasing central San Joaquin Valley aquifers

Researchers on Tuesday estimated that groundwater levels could rise 12 percent to 20 percent through intentional flooding of selected farmland between Fresno and Merced counties.

The report from the California Water Foundation said the method, using excess river flows during wet winters, also could be used in other counties with concerns about overpumping.

It came out a day after the Almond Board of California, based in Modesto, announced a similar research project in San Joaquin Valley nut orchards. Both efforts would direct the river water to farms with well-draining soil and crops that would not be harmed by a winter soaking.

The report from the Sacramento-based foundation said the recharge would boost not just the groundwater in the study area, but aquifers and rivers that connect with it via seepage.

“This is a winner for agriculture because they can utilize active farmland to recharge groundwater basins that they rely on, and it is a winner for stream flow and fish,” Deputy Director Andrew Fahlund said.

Water tables have dropped because of four years of drought and, in some places, pumping that exceeds natural recharge from rain and subsurface flows. Farmers face increased power costs for pumping from deeper levels, and some land has sunk because of the missing water.

The report covers parts of Fresno, Madera and Merced counties, generally between the San Joaquin River and the lower Sierra Nevada foothills. It has average annual overdraft of about 250,000 acre-feet, the authors said.

Reservoirs could not provide much recharge water in some years, such as 2015, but the report said the long-term average for the study area could be 31,000 to 52,000 acre-feet. The lower figure assumes recharge from December through February. The higher figure adds November and March.

The researchers project that another 34,200 to 55,500 acre-feet of the recharge water would end up in streams and 14,000 to 22,700 would flow underground to neighboring aquifers.

They suggest spreading the practice to land irrigated by the Stanislaus, Tuolumne and other rivers. This could take care of a sizable part of the Valley’s overdraft, estimated at 1.2 million acre-feet in an average year.

Recharge is not a new idea: It can happen when farmers used flood irrigation in summer, though that method is declining as drip and microsprinklers increase. It can happen through the unlined bottoms of reservoirs and canals. Some agencies have built basins specifically for recharge.

The report calls for a coordinated effort to capture excess river water, which was abundant in 2010 and 2011.

The Merced Irrigation District provided input to the researchers and is at work on its own plans for “conjunctive use” of river supplies and groundwater, spokesman Mike Jensen said.

“The district believes that in fact this is a tool that could benefit local groundwater, in addition to the recharge we already do through recharge basins and unlined waterways,” he said.

It just might not happen soon. The district’s McClure Reservoir is so low that farmers this year got only 2 percent of the 500,000 or so acre-feet available in better times.

The foundation study drew praise from Sustainable Conservation, a San Francisco group that will work with the Almond Board on the upcoming research.

“The groundwater report is great news for a struggling California and identifies the important role farming can play in helping create a sustainable water future for the state,” Executive Director Ashley Boren said in a news release.

John Holland: 209-578-2385

By the numbers

250,000: Acre-feet of groundwater overdraft in an average year in the study area (parts of Fresno, Madera and Merced counties)

31,000: Acre-feet of potential recharge if done with excess river flows from December through February

52,000: Acre-feet if recharge is done from November through March

Source: California Water Foundation. The report is at www.californiawaterfoundation.org/news.