Two other tools can be used to improve scheduling and monitoring of vineyard irrigation events. To measure water tension, Zhuang recommends placing two tensiometers directly beneath the drip line, side by side and 12 to 18 inches to the side of an emitter. Use one to monitor the 1- to 2-foot depth and the other to measure moisture 3- to 4-feet deep below the soil surface.

Generally, a pair of tensiometers for every 20 acres is adequate, he notes. However, he advises using more in variable soils.

“Irrigation should be initiated once pre-determined values of soil tension have been reached at the different soil depths,” he says. “Usually, irrigation should begin before soil tensions at about the 2-foot depth approach -40 to -50 centibars. But, irrigation threshold values will depend on soil type.”

Neutron probes can be used to measure total water content of the soil. Two access tubes for this type of device can be placed side by side directly beneath the drip line to a depth of 6 to 9 feet, Zhuang notes. Placing more access tubes between vine rows provides more accurate data on water use by indicating water depletion across the vineyard floor, he notes.

Irrigation should start when the soil allowable depletion is less than the difference between neutron probe measurements and soil water-holding capacity, he says. Soil allowable depletion can vary from 1.4 inches in sandy soils to 4.4 inches in clay.

“Overall, soil water content is highly correlated with midday leaf-water potential.” Zhuang says. “Growers can use soil and vine measurements to validate irrigation scheduling based on experience and visual observation.”