Pressure bombs a popular tool to determine water needs

Published online: May 05, 2014

RED BLUFF, Calif.—With drought drastically reducing water availability for many farms, fruit and nut growers are making use of technology that helps them maximize their irrigation efficiency, university experts say.

The Fruit and Nut Center at the University of California-Davis has launched a website to help growers interpret their readings from pressure bombs, which advisors describe as sort of like blood pressure tests for tree leaves, to determine how much water their trees need.

Such technology is proving vital to production of such crops as walnuts, almonds and prunes, as growers struggle to conserve water, control weeds and make sure nitrogen doesn’t seep into aquifers, UC advisors say.

The bombs appear to be gaining in popularity as producers grapple with state and federal water allocations that have been drastically reduced or zeroed out, forcing them to rely on wells.

“If you really want to do a good job of deficit irrigation, you need a tool to do that and the pressure chamber is the only technology available for almonds, walnuts and prunes,” said Rick Buchner, a UC Cooperative Extension crop advisor here. “We have very good numbers to interpret what the bomb is telling you.”

The bombs are a great tool for ensuring you’re watering your trees only when they need it, asserted Jerry Sneed, a field representative for Crain Ranch in nearby Los Molinos, Calif. The ranch is one of Northern California’s leading walnut growers and processors.

“I have total faith in them,” Sneed said in a UC news release. “They’ve never steered us wrong.”

Extension advisors said they launched the website because more growers are making the $1,200 to $3,000 investment in pressure chambers but some of them were having trouble interpreting the water-stress readings.

The extension has weather stations positioned throughout the state to tell growers how hard their trees should normally be working to pull water under the temperature and humidity for that day. Baseline conditions can vary; “normal” stress levels for almonds would be considered severe for grapes or deadly for walnuts, the release explains.

“We’ve got guys to tell those guys what the numbers mean,” Buchner said. “Then it’s a matter of deciding what they want to achieve and set thresholds accordingly.”

UC-Davis plant physiology professor Ken Shackel has been using pressure chambers to measure plant water stress since the early 1990s. Extension experts have been pushing the bombs and drip-irrigation technology in recent years as more tree plantings on marginal soil, dry weather and restrictions on nitrogen in groundwater have increased the need for super-efficient watering.

The new website is the latest in a series of UC sites designed to help growers deal with drought.


Calculating stem water potential:

UC Drought Management:

UCCE Water/Irrigation Program:

UC-Davis Fruit and Nut Research and Information Center: