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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