Vines receiving drip irrigation delivered directly into the lower root zone yielded 70% of commercial production while receiving only 15% as much water as vines that received full commercial rates of surface drip irrigation during 2015, the hottest and driest year on record in south-central Washington. Preliminary data supports our hypothesis that water use efficiency can be significantly improved through deep sub-subsurface irrigation. Vines receiving direct root zone irrigation at rates reduced to 60, 30, or 15% of commercial drip irrigation produced individual clusters with more, but smaller, berries than did clusters from vines receiving full rates of surface drip irrigation. Preliminary findings from a replicated, randomized complete block experimental design conducted in a commercial vineyard in Washington’s Red Mountain AVA suggest that this form of subsurface microirrigation has the potential to conserve at least half of the water required for quality grape production under contemporary surface drip irrigation. Additionally, we hypothesize that this form of microirrigation can produce grapes of more desirable quality for producing premium red wines. Grape production was increasingly higher with depth and amount of applied water, yielding 90% of commercial production in plots receiving 60% of the commercial irrigation rate delivered at slightly less than a meter below the soil surface. Interrupted irrigation delivery provided no advantage over uninterrupted delivery during 2015; however, this could have been influenced by the soil texture at this site. Vines receiving direct root zone irrigation at reduced volumes of total water showed more plant water stress than those receiving full commercial rates of irrigation via surface drip irrigation, based on samples of midday stem xylem pressure potential. This experiment will be expanded to other sites and grape varieties in 2016.

Funding Support: Washington Wine Commission Wine Advisory Committee, NW Center for Small Fruit Research, Washington State Dept. of Agriculture Specialty Crop Block Grant Program