Farm Progress

“The information in these tips will be of practical use for growers and other water-related stakeholders now and into the future as our agricultural community continues to adapt to climate variability and to a changing water supply situation,” said Daniele Zaccaria, UC ANR Cooperative Extension agricultural water management specialist.

November 18, 2015

5 Min Read
California's walnut crop in 2018 is projected at 690,000 tons, according to the National Agricultural Statistics Service.

Drought strategies for managing almonds, walnuts, and other crops are available free from University of California (UC) Agriculture and Natural Resources (UC ANR).

The new and updated tips were developed by UC ANR scientists and funded by the California Department of Water Resources.

“The information in these tips will be of practical use for growers and other water-related stakeholders now and into the future as our agricultural community continues to adapt to climate variability and to a changing water supply situation,” said Daniele Zaccaria, UC ANR Cooperative Extension agricultural water management specialist.

Doug Parker director of UC ANR’s California Institute for Water Resources says even if El Nino brings rain this fall that water scarcity will continue to impact California farmers.

“As climate change continues to reduce the average annual snowpack, it is likely that droughts in California will become more frequent and severe in the years to come,” Parker said.

The online drought tips currently available for free download are located at The tips focus on almonds and walnuts, management strategies for alfalfa, use of shallow groundwater for crop production, fog contribution to crop water use, and reclaiming saline, sodic, and saline-sodic soils.

Several more drought tips for dozens of commodities and situations will be published and posted online soon.

Should those El Nino storms arrive, users of computers and mobile devices can also come in out of the rain or snow to watch a series of videos featuring water experts from UC and other agencies and institutions.

They can be accessed at

The tips on drought management for California almonds came from David Doll, UC Cooperative Extension farm advisor for Merced County, and Kenneth Shackel, professor with the Department of Plant Sciences at UC Davis.

Those on strategies for walnut production are from Alan Fulton, UC Cooperative Extension irrigation and water resources farm advisor for Tehama, Glenn, Colusa, and Shasta Counties; and Richard Buchner, UC Cooperative Extension orchard farm advisor for Tehama County.

The tips on drought strategies for walnut production include the observation that precision irrigation technology has allowed planting at locations previously thought to be unsuitable for walnut production and that successful California walnut production depends on irrigation.

Walnut trees that produce economical yields with good kernel quality require about 40 to 42 inches of water annually. Most of the water is transpired through the leaves, while some is lost through evaporation from the soil surface.

Fulton and Buchner say that leaves trade water for carbon under full sun to make carbohydrates, the raw material for tree growth and fruit development. Maximum carbohydrate production occurs in leaves exposed to full sunlight with an adequate water supply.

If transpiration is limited by the lack of water, then photosynthesis declines which compromises tree growth, walnut yield, and nut quality.

Regulated deficit irrigation strategies, which withhold water at specific growth stages, have not proven to be as effective in walnuts as in other perennial crops such as almonds, pistachios, prunes, wine grapes, or olives for oil.

In walnuts, water curtailments at any growth stage can affect crop production and quality.

A new experiment is ongoing in a Chandler orchard in Tehama County. This experiment is designed to evaluate withholding irrigation water from pollination through the spring period of rapid shoot growth and external nut sizing.

In the first season, the initial irrigation was delayed as much as 50 days and total applied water was reduced to 24 inches. Yield was not immediately affected, but the nut size was reduced by 10 percent in the most extreme cutbacks.

If water is applied to satisfy 100 percent of evapotranspiration demands, irrigation should be managed to prevent the measured plant available soil moisture from dropping below 50 percent in a root zone of approximately five feet.

A 10 to 20 percent reduction in irrigation water supply to walnuts equals to a shortfall of 4 to 8 inches. To minimize impacts on walnut production, Fulton and Buchner recommend gradually withholding water and allowing slightly more crop stress during kernel development, nut maturity, harvest, and postharvest.

The researchers say the production history of each orchard should be evaluated, along with the cost of production. The goal is to identify orchards that consistently produce higher yields of large, sound walnuts per acre foot of water, and other production inputs.

Decisions would then be made to prioritize limited water supplies to the more efficient orchards to sustain production.

Research has shown that almond trees are able to survive on as little as 7.6 inches of water, but produce best with 54 to 58 inches in California.

Doll and Shackel say that moderate levels of plant stress often occur within orchards and may be beneficial, particularly at the onset of hull split, when it may help to reduce the fungal disease hull rot and synchronize hull split.

They add that one year of reduced spur growth resulting from irrigation deficits will not necessarily lead to a dramatic decrease in next year’s yield, but the effect can be cumulative if consecutive years of deficit irrigation occur and the number of fruiting spurs decrease.

The researchers say it is critical to maintain irrigation through the post-hull-slit-postharvest period, since a water stress level that is too high reduces kernel weight and quality.

Severe stress from deprivation of postharvest irrigation has been found to decrease the next year’s crop yield more than a preharvest water deficit.

Doll and Shackel say growers get the most out of water use when they irrigate just before water stress is low enough to cause a significant reduction in yield. This method of applying water during critical almond development periods and limiting water application during less-critical periods is called strategic deficit irrigation.

A pressure chamber can be used to measure plant stress.

Due to the negative impacts of severe drought, it may be beneficial to redirect water from older orchards to younger or higher producing blocks, with the intent of removing the older block.

Doll and Shackel recommend removing vegetation on the orchard floor or managing it to eliminate water loss through transpiration. They also say nitrogen applications should be reduced during periods of drought.

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