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The first year of a study into subsurface irrigation of Concord grapes — delivering water underground, rather than on the soil surface — shows the technology’s potential to produce comparable quantities and quality of grapes while saving water at the same time.

And those results came during one of the driest and hottest summers in recorded history in Washington.

Dr. Pete Jacoby, Washington State University professor and plant ecologist, conducted the first year of the multiyear study on a block of Concord grapes, established in 2003, at WSU’s Irrigated Agriculture Research and Extension Center near Prosser, Washington.

The block was divided into 30 plots, each comprised of 30 vines planted on 6-foot (in row) by 9-foot (between rows) spacing. Jacoby placed emitters 18 inches on either side of each vine to deliver water at depths of 1, 2, 3 and 4 feet below the soil surface.

Drought conditions in 2015 forced some irrigation districts to ration water to users with junior water rights, and water was withheld at this particular site from mid-May until the second week of June. For that reason, some planned water treatments were deferred in order to saturate the soil with water until normal delivery resumed.

However, for the remainder of the growing season, all plots received no less than 75 percent of the replacement evapotranspiration rate during each scheduled irrigation, Jacoby said.

At the mid-veraison stage of development, plant water stress was substantially less in plots receiving subsurface irrigation at the 2-foot and 3-foot depths, the study showed, as opposed to the 1-foot and 4-foot depths.

In a progress report, Jacoby attributed the difference to excess moisture loss to the understory vegetation as well as to surface evaporation at the surface and 1-foot depth and to the plant’s inability to uptake water from 4 feet below the surface due to hardpan conditions.

The research team collected grape clusters in late July. They were consistently heavier with increased depth of irrigation delivery.

The total numbers of berries were higher for the subsurface treatments than for the control surface drip treatment, and the average berry weight was greatest at the 2-foot and 3-foot depths of water delivery. They collected clusters again Sept. 12 and had similar results, with total berry numbers slightly higher for all subsurface treatments over the surface drip treatment.

The site has a highly variable soil type, including an extremely hard layer of solidified calcium carbonate below the upper 2 feet of the soil profile.

This layer may reduce the availability of soil moisture and create the need for more frequent irrigation, but that may mean other parts of the vineyard are overwatered at the same time.

Vineyards with similar conditions lend themselves to site-specific irrigation technologies as they become available, the report showed. •

– by Shannon Dininny