Increasingly more San Joaquin Valley raisin growers are converting to dried-on-the-vine production to reduce labor costs. The new arbor or gable systems may also helping reduce late season water use.

Larry Williams, University of California, Davis viticulture professor, who specializes in grapevine physiology and vineyard irrigation, compared water use for grapevines on overhead trellis systems for dried-on-the-vine raisins to standard trellis systems for tray-dried Thompson Seedless grapes. He is using the information to develop seasonal crop coefficient values for DOV raisins grown on overhead trellis systems to help growers better schedule irrigations.

Seasonal vine water use can be determined by multiplying evapotranspiration rates from CIMIS data by a crop coefficient for raisin and wine grapes in the San Joaquin Valley. The crop coefficients were developed in past research based on traditional single-wire or crossarm trellises used in the San Joaquin Valley, and assume vines are not stressed for water.

Several factors can impact crop coefficients, including canopy size, seasonal growth, and spacing between rows. Overhead trellis systems generally develop larger canopies intercepting more solar radiation and higher vine water use compared to standard trellis systems.

Williams’ most recent research at the UC Kearney Agricultural Center in Parlier during 2006 and 2007 has shown that water use declines significantly once DOV fruiting canes are cut.

Williams said this could potentially save on water and pumping costs during a period of typically high demand for grapes.

Using a weighing lysimeter to measure plant transpiration and soil evaporation, Williams determined that water use of DOV vines fell from 12 gallons of water per day at veraison to about 5.3 gallons of water per day after canes were severed.

“In July or August at mid-day, vines are using in excess of 2 gallons of water per hour, so it’s like water being sucked through the vines and into the atmosphere — it’s quite a lot of water,” he said. “As we expected, severing canes reduces water use quite dramatically since a grower is reducing the transpiration surface of the vine, so water use is cut in half.”

As a result of these findings, Williams believes that growers with overhead type systems can calculate water use of DOV vines using a crop coefficient of about 1.3 prior to cane cutting and a crop coefficient of about 0.6 after cutting the fruiting canes.

“You can deficit irrigate your vines and still maximize yields, especially if you deficit irrigate after veraison. So, if you want to reduce pumping costs do you need to apply full ET throughout the growing season to maximize yield on this trellis system? The answer is, probably not,” he said.