Alexander Levin

Alexander Levin, professor and viticulturist, studies the irrigation needs of Southern Oregon winegrapes.

ROGUE VALLEY, Ore. — Winegrape growers in Southern Oregon may be able to slash water usage nearly in half while still producing high-quality, high-yield fruit, according to an Oregon State University study.

Many Southern Oregon growers consult water management data OSU researchers believe is either inaccurate or designed for a different region, leading some growers to irrigate too much.

“Watering twice as long as you need to is a big deal,” said Alexander Levin, the study’s leader and assistant professor and viticulturist at the Southern Oregon Research and Extension Center.

Overwatering, Levin said, costs more, hurts the environment, can leach nutrients from soil and wash fertilizer beyond roots. It can also create leafy canopies that attract pests and create dark, humid habitat for fungi. And in a drought, every drop of water is precious.

In Oregon, many growers use AgriMet, a free weather station sponsored by the U.S. Bureau of Reclamation, to determine how much water they need for specific crops.

The problem is AgriMet appears to overestimate how much to irrigate winegrapes by 44%.

AgriMet estimates winegrapes in the region need 20.2 inches of water per year. Levin’s study found winegrapes need only 11.4 inches annually to thrive.

That number — 11.4 inches — represents the ideal amount of water vines need to produce good-quality fruit and good yields. Winegrapes could survive with less water, such as 6 to 8 inches, but will perform better with around 11 inches.

Along with using AgriMet, some growers also use crop coefficients — properties of plants used to predict evapotranspiration for irrigation scheduling — that were developed in California, where crop growth, water requirements, latitude and weather patterns differ.

By using OSU’s numbers, Levin estimates Southern Oregon growers can conserve 11% of their water compared to using California numbers.

Levin said his findings may even impact growers who already practice “deficit irrigating” — applying less water than is required for a crop. That’s because, Levin said, even some deficit irrigators may be irrigating too much.

A “deficit irrigator” watering 25% less than AgriMet’s suggested 20.2 inches, for example, is still applying 15.5 inches per year, above OSU’s suggested 11.4 inches.

Levin arrived at his 11.4-inch “ideal” by conducting research at vineyards in Jackson County starting in 2017. He used solar panels to estimate vines’ evapotranspiration and to find the “crop coefficient,” which can be plugged into a mathematical equation for irrigation adjustments.

AgriMet staff have not yet confirmed, either to Levin or the Capital Press, whether they plan to use Levin’s findings at the weather station. In the meantime, Levin encourages growers to email him at alexander.levin@oregonstate.edu for the new crop coefficient.

Michael Moore, general manager at Quail Run Vineyards and one of the growers who participated in Levin’s trials, said he thinks Levin’s numbers are accurate.

Moore irrigates his winegrapes about 12 inches annually, which he said “feels really appropriate for our area.”

Although Levin’s study will likely prove useful in the future, it may have little impact this year, because growers would be lucky to have 11.4 inches of water. Some water districts are telling growers just to fight for vine survival.

“The scope of the drought going on right now is monumental,” said Moore.

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