Determination of orchard water requirement by a combined trunk sap flow and meteorological approach

Abstract

Sap flow measurements in the trunk and potential transpiration computed from meteorological data were used during two irrigation seasons to determine orchard water requirement. Standard commercial irrigation was applied except for two trees which were irrigated every 3 days to maintain unlimited soil water availability and used as reference trees. Measured transpiration was proportional to potential transpiration. The ratio between the two, averaged for all trees, was approximately 0.3 when soil water availability was not limiting falling to 0.2 when soil water potential in the main root zone dropped to -90 kPa. This ratio was successfully used to schedule orchard irrigation at an arbitrarily chosen reduction of 20% in the ratio as measured on the first day after irrigation. Yield of individual trees was highly correlated with their seasonally accumulated transpiration. Analysis of variability in transpiration between trees led to the conclusion that orchard transpiration may be determined with acceptable accuracy by this approach which can, therefore, be used to schedule orchard irrigation.