Research project in Stanislaus County will see if winter flood irrigation refills aquifers

July 9, 2015

Some of those concerned with the groundwater debate maintain that flood irrigation of crops can be an effective way of refilling aquifers.

The University of California Cooperative Extension in Stanislaus County is working on a pilot project to test the theory. The study could advance our understanding of how aquifers are replenished and show how agriculture could contribute to recharging groundwater in Stanislaus and other counties in the San Joaquin Valley.

Groundwater may seem a dull topic for readers, but every soul in the county has a stake in maintenance of a good supply of groundwater for drinking water, business, industry and farming.

The two- or three-year study is in the planning stage and has a $97,000 budget. The cooperative extension has asked for an Almond Board research grant to fund it but has not received official word yet. The cooperative extension is talking with a few almond growers about volunteering their orchards for the experiment.

Roger Duncan, a farm adviser for the Cooperative Extension, said the study will look at the effects of December-to-January flood irrigation of almonds. Researchers want to test two things:

  • What effect winter irrigation will have on the growth of trees. Local growers usually don’t irrigate in winter. It’s not clear what the benefits would be, and applying one or two feet of water to orchards in winter could conceivably damage the roots.
  • How well winter flood irrigation replenishes groundwater. A hydrologist from University of California Davis will use instruments to measure the movement of water through the soil.

Duncan said they hope to begin the research on one orchard in the Modesto area and a second orchard in another county in the San Joaquin Valley. The local test plot will be 10 to 15 acres of fairly sandy soil. The experiment will use water taken from the ground. Irrigation districts clean and repair their canals during the winter but have pumped water available.

Leading the research team will be Professor Ken Shackel of the UC Davis College of Biological Sciences and hydrologist Helen Dahlke of UC Davis.

“If it works well, we can expand and potentially look at other locations, other soil types and other cropping systems,” Duncan said.

Some preliminary results will likely be reported in 2016. Duncan expects the final results will be published in scientific journals and should be of interest to local and state government officials, the county’s Water Advisory Committee, the almond industry and general public.

Commercial almond orchards are not usually irrigated in winter in Stanislaus County because there’s enough rainfall to keep the ground moist. Regarded as a wasteful practice from the era of cheap and plentiful water, flood irrigation has been largely replaced by sprinkler irrigation. But it could bounce back as a strategic tool as local jurisdictions try to manage their groundwater levels.

Retired hydrologist Vance Kennedy of Modesto said he is glad to see the research project move forward. He said winter flood irrigation is potentially an important tool for replenishing aquifers under climate change models for California.

During wintertime, stormwater is released from Sierra reservoirs and it flows out to the ocean. If growers and irrigation districts in the valley would cooperate, some of the water could be delivered to farmers for winter irrigation in areas with permeable soils, which allow water to filter down through the ground to aquifers.

It could help replenish the groundwater for urban users as well as agriculture.

Kennedy said that underground aquifers have more storage capacity than do reservoirs in the Sierra. Some climatologists believe that the days of large snowpacks in the Sierra are behind us. Instead, climate change will bless California with massive rainstorms that cause rivers to swell during the winter.

Kennedy said it would make sense to divert that water from rivers and store it underground.

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