Midday Stem Water Potential as a Basis for Irrigation Scheduling

There has been great progress in using midday stem water potential (SWP, measured with the pressure chamber) as a basis for irrigation scheduling, particularly for the management of regulated deficit irrigation (RDI) in trees and vines. In almonds, RDI is used during a one month period when the hulls are splitting, in order to reduce hull rot and improve harvestability. A plant-based RDI experiment was performed at a number of commercial almond orchard sites to evaluate the reliability of this method, and at one site, to test whether crop consumptive water use may be reduced without significant detrimental effects to almond production. In all sites, irrigation was reduced during the hull-split period to achieve a target SWP of -14 to -18 bars. Over many sites and years, RDI showed no significant reduction, or trend in reduction of yield, and in some cases allowed a substantial savings in applied water. Under favorable soil conditions (deep, well-drained and a good water holding capacity) an annual savings in applied water of about 50% was realized compared to the estimated ETc. Even on soils with very low water holding capacity, a plant-based RDI regime was used successfully and gave a modest (10%) reduction in consumptive use. Field irrigation experiments have also been performed on other species (walnut, pear, winegrapes) and SWP has proved to be an invaluable tool to provide feedback for the adjustment of irrigation amounts and timings in order to obtain a desired plant response.