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.