Matching Irrigation to Vine Water-requirements: Limitations of Using Sap-flow Technology for Scheduling Irrigation

Mismatch between water applied during irrigation and that required by the crop for transpiration is a major cause of poor returns per unit water use and of land degradation. Advances in commercial sap-flow gauges should provide opportunities for improving water management in tree-horticulture, but this has not been widely explored. In this study we tested the capability of sap-flow gauges to detect water-stress and, hence a need for irrigation, by withholding water for two months from actively growing, mature, grapevines. Withholding irrigation reduced soil-water content in the 1.8 m soil-profile by almost 40% compared to where irrigation was maintained, but the reduced soil-water resulted in only a small decrease in the transpiration deduced from sap-flow throughout the 2-month period. This was despite a 3-fold increase in stomatal resistance and a rise of 1.0 °C in leaf temperature for the stressed compared to the continuously irrigated vines. Reasons for this response are not clear, but it is possible that the stressed-vines accessed water either directly from the watertable or from its capillary fringe. Withdrawal of irrigation also did not result in yield decline for the stressed-vines. The results showed the limitation of employing sap-flow systems in managing irrigation. Other implications of these findings are explored including proposals on using weighted crop factor to match irrigation with grapevine water-requirements.