Despite cutbacks in deliveries of surface water, most walnut growers in California’s major northern walnut production areas should be able to meet the crop’s water needs this season, assuming growers continue to use their available water supplies efficiently, reports Allan Fulton, University of California Cooperative Extension irrigation and water resources advisor for the northern Sacramento Valley. 

Those reductions have been prompted by low mountain snowpack and persistent dry weather since the first of this year. However, last fall 2012 rains should help make up the difference in the Sacramento Valley, he notes.

Research shows that a walnut tree consumes about 3.5 acre-feet of water in a season, notes Fulton, based in Red Bluff, Calif., he serves growers in Tehama, Glenn, Colusa and Shasta counties.

In the northern Sacramento Valley, stored soil moisture in the terrace soils, those generally west of Interstate 5, provides about 10 percent to 20 percent of a walnut crop’s total seasonal needs, he says. By contrast, soil moisture stored in the deeper, less-stratified soils farther east and closer to the Sacramento River, may supply as much as 35 percent of a walnut orchard’s total annual water requirements.

Northern Sacramento Valley received about 12 to 16 inches of rain in November and December. That compares to the typical 14 to 20 inches for the entire fall and winter period. Growers here are in better shape for stored soil moisture this year than those farther south, where winters are generally drier. For example, in Tulare County in the San Joaquin Valley, rainfall since early November has totaled no more than about 4 inches when the average is often closer to 7 inches.

One way to stretch the available supply through the end of the irrigation season is to ration the amount of water applied with each irrigation. Soil moisture sensors placed at various levels of the soil profile can help with that or any irrigation regime.

“Rationing is pretty simple,” Fulton says. “And, research shows that it does a reasonably good job of minimizing stress on the trees.”  For example, if a grower estimates that the combination of stored soil moisture and surface water allocation will only supply 75 percent of the seasonal water consumption by a walnut tree, ration the applied irrigation water at 75 percent across all phases of tree growth and crop development during the season.”

Pressure chamber

Another, more accurate, but also more expensive and more laborious approach is to use a pressure chamber. This device, also called a pressure bomb, applies air pressure to a leaf cut from a tree. The amount of pressure required to push water to the cut surface of the petiole indicates the tension or stress the leaf is experiencing. The higher the pressure, the harder for the tree to take up enough water to replace the amount lost through evapotranspiration (ET). As this stress increases, so does the tree’s need for more water.

“The pressure chamber can also be used to identify areas within an orchard or across a ranch where different soil and rooting conditions result in different levels of water stress,” Fulton explains. “Then, you might be able to divert water from blocks with little or no water stress to blocks trees are under higher stress and requiring more water. That can help make more efficient use of a limited supply of water and minimize unwanted effects.”

The pressure chamber is designed for use between noon and 4 p.m., he notes. Also, it requires sampling trees that represent conditions throughout the orchard. The more samples tested, the more reliable the results. However, testing as few as three or five representative trees in an orchard, periodically, throughout the season can help improve irrigation efficiency, he says.

“With some experience, a grower should be able to get useful orchard water status indicators by spending no more than about an hour a week per orchard doing this testing,” Fulton says.

Limited surveys suggest that about one of every two California growers use some method to monitor soil moisture or track real-time ET, while about one of every four uses a pressure chamber to help make irrigation decisions, he reports.

Pressure chambers cost about $1,500 for one with a hand pump to provide pressure. Gas-pressurized models cost $3,500 to $5,000.

“At first glance, investing in irrigation management tools may seem expensive,” Fulton says. “But, with a high-value crop like walnuts, they can pay back in one season in the form of higher yields or improved quality by minimizing water stress.”

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