Performance of direct root-zone deficit irrigation on Vitis vinifera L. cv. Cabernet Sauvignon production and water use efficiency in semi-arid southcentral Washington

The direct root-zone irrigation system is a new technique to deliver water directly to the root zone of grapevines at specific irrigation rates and delivery depths. The response of Vitis vinifera L. cv. Cabernet Sauvignon to direct root-zone deficit irrigation was investigated in a commercial vineyard with loamy sand soil in the semi-arid lower Columbia Basin of southcentral Washington State, USA. Three irrigation rates and three delivery depths were compared for effects on grape yield and crop water use efficiency during three consecutive growing seasons. Furthermore, we investigated the influence of irrigation rate on grape quality, and used an in situ root imaging system for root phenotyping under different delivery depths for two years. Results showed that delivery depth had no significant effect on grape yield and crop water use efficiency; however, increases in delivery depth from 30 cm to 90 cm resulted in 60-70% decreased root number and 46-59% decreased root length in the upper soil profile (0-60 cm). Moderate irrigation rates (35% less water use on average) improved crop water use efficiency by 14-23% and only reducing grape yield 15-18% per individual grapevine, without sacrificing grape quality or inducing early defoliation for the first two years. Surprisingly, no significant reduction of grape yield was found in the third year, which can be attributed to cooler and wetter weather and potentially vine acclimation to the direct root-zone irrigation strategy. We conclude that direct root-zone deficit irrigation could be a convenient and efficient subsurface irrigation method to improve crop water use efficiency without reducing grape quality under seasonal drought. Future studies should compare the economic and environmental returns of direct root-zone deficit irrigation with other traditional irrigation strategies under different environmental conditions to optimize the direct root-zone deficit irrigation method.