Growing grapes with less water

Data on deficit irrigation treatments on juice and wine grapes will be collected by WSU’s Joan Davenport.

A broad research coalition has embarked on a comprehensive, five-year project to develop sustainable water management strategies for vineyards. The aim is to maintain or improve yield and fruit quality, while using less and lower quality irrigation water.

Directed by Dr. James Ayars, agricultural engineer at the U.S. Department of Agriculture’s Water Management Research Unit in Parlier, California, the $562,000 project is funded by USDA’s Specialty Crop Research Initiative and involves researchers, extension educators, and grower cooperators from California, Oregon, Washington, and Texas. Additionally, the project has support of the National Grape and Wine Initiative, an industry coalition.

The project tackles many areas related to limited water resources and impaired water quality, from salinity management strategies and development of grapevine rootstocks that better resist drought and tolerate salinity to basic research to improve techniques for estimating vine evapotranspiration and crop coefficients. But ultimately, Ayars said, the goal is to help all grape growers improve their water management and irrigation strategies.

Historically, irrigated agriculture in the western United States has been all about availability and quality of water. The region depends on snowpack and water storage in reservoirs for irrigation in the dry summer months. ­Competition for limited water resources between municipal, industrial, environmental, and agricultural entities is a common theme, especially in drought years. But supplies are expected to become more limited in the future if ­climate change scenarios play out.

During a recent phone interview with the Good Fruit Grower, Ayars explained that one of the project’s main objectives will be to determine the effects and limits of deficit irrigation strategies on the yield and quality of wine, juice, raisin, and table grapes and to develop technologies to remotely quantify, in real time, ­vineyard-scale evapotranspiration and vine water use.

He noted that water use varies widely, depending on the type of grape grown. In Washington State, most wine grape growers use regulated deficit irrigation strategies to improve the sensory aspects of fruit. However, table and raisin grapes are often fully irrigated as growers strive to grow large berries, while juice grapes are irrigated to raise soluble solids to at least 16° Brix.

“Any potential water savings that could be applied at specific growth periods must factor in the quality ­requirements of the various end products,” he said.

Deficit treatments

Extensive research has focused on the effect of deficit irrigation on wine grapes, but not juice, raisin, or table grapes.

Dr. Larry Williams, plant physiologist at the University of California, Davis, recently demonstrated, using a weighing lysimeter, that Thompson seedless grapes could be grown with a 20 percent reduction in the total water requirement without noticeable yield losses.

“This finding is significant,” said Ayars, adding that irrigation scheduling has long been based on the premise of balancing soil water supply and crop water use, utilizing evapotranspiration curves and crop coefficients.

“Maybe we’ve been overirrigating all along,” he said. “Maybe when we apply 80 percent of full water, it’s really 100 percent.”

Part of the research will look at ways to improve evapo­transpiration estimates developed on a regional basis that may not be accurate enough to tell growers what their specific water requirement are.

Washington State University’s Dr. Joan Davenport, a member of the project’s scientific team, will collect data on Concord juice and wine grapes from a Yakima Valley grower-cooperator. Deficit irrigation treatments will be imposed at different growth stages on juice, wine, raisin, and table grapes in the trials from Washington and ­California vineyards.

Economics

The economic impacts of the research findings will also be studied.

“Ultimately, I think where we will see impact will not be in direct water saving costs but in other vineyard practices that evolve from reducing water use,” he said. “What I think we’ll see at the end of the day will be improved yields and quality, reduced labor costs from less pruning, shoot thinning, and canopy management costs, and more efficient irrigation management.”