Pete Jacoby* and Xiaochi Ma
*Washington State University, P.O. Box 646420, Department of Crop and Soil Sciences, Pullman, WA 99164-6420 (jacoby@wsu.edu)

Increased crop water use efficiency of winegrapes was achieved through DRZ drip irrigation during a three-year study near Benton City, WA. Rather than using buried lines to deliver subsurface drip irrigation, DRZ is applied through vertical PVC delivery tubes inserted into the soil 0.5 m either side of the trunk and in line with the trellis system. DRZ allowed water to be applied into the lower root-zone at 30, 60, or 90 cm depths. This technique saved at least 40% of the water used by surface drip irrigation and yielded from 70 to 95% of commercial production. These results confirm our hypothesis that DRZ deficit irrigation delivery can conserve water while enhancing use of water to regulate vine activity to achieve grape quality and quantity goals. DRZ was initiated at fruit set and maintained on the same schedule as commercial surface drip irrigation through harvest. Field and greenhouse observations using mini-rhizotrons verified that own-rooted winegrapes (Cabernet Sauvignon) developed the mass of roots near and below the depth of water delivery in the soil profile. Results to date have not documented a particular advantage for a specific depth of delivery or for pulsed water delivery rather than continuous flow during irrigation sets. During the 2017 growing seaons, vines receiving DRZ delivery demonstrated greater photosynthetic capacity, as measured by rates of stomatal conductance and CO2 assimilation, than vines irrigated at equal rates by surface drip. Quality of grapes, measured as higher Brix, tannins, anthocyanin, and lower acidity, increased in direct proportion with increasing rates of deficit irrigation and resulting plant water stress. All irrigation events and soil moisture content were tracked with recording electric capacitance probes, and actual water delivery was quantified with mechanical meters read after irrigation sets.

Funding Support: NW Center for Small Fruit Research, WSDA Specialty Crop Block Grant Program, WA Winegrowers, Western Sustainable Agriculture Research and Education (WSARE)