The use of sap flow measurements for scheduling irrigation in olive, apple and Asian pear trees and in grapevines

We evaluated three approaches for scheduling irrigation in wine grape vineyards and in olive, apple and Asian pear tree orchards, based on sap flow measurements and models of plant transpiration. In the first approach, we analysed how the shape of the sap-flow profile changed in response to root-zone soil water conditions and potential evaporative demand. The second approach was based on a transpiration ratio, as defined from the actual daily water use of a target plant divided by the potential daily water use of similar-sized plants under non-limiting soil water conditions (well-irrigated plants). Values of the actual plant water use were always determined from measured sap flow. Two independent methods were assessed for the calculation of potential plant water use; either sap flow was measured in well-irrigated plants or we used a leaf-area based model of plant transpiration. On some occasions water stress was found to modify the shape of the sap velocity profile. However, most of the time the velocity profile was found to be an insensitive indicator for triggering irrigation. The transpiration ratio method, using measured sap flow in well-irrigated plants, was more useful for irrigation scheduling, at least for the two species (i.e. olive and grape) that were investigated here. Nonetheless, realization of such an approach in a commercial orchard may not be practical due to problems associated with irrigation management e.g. excessive vegetative growth may occur on the reference plants over time. Besides, irrigating the orchard to maintain non-limiting soil water conditions is not always the best option for water and nutrient management. The alternative transpiration ratio method based on a leaf-area based model of plant water use, yielded the best results. Modelled transpiration rates always provided reliable information not only for well-irrigated plants, but also for deficit-irrigated plants. This result lends support to the use of the method for irrigation scheduling of vineyard and orchard trees. However, the use of models does require detailed microclimate data as well as a user-friendly technique to quantify plant leaf area. From a practical viewpoint the method should encompass the spatial variability of the soil and plants within the orchard. Accurate quantification of these factors is a cornerstone of precision horticulture and such information would help to minimise risks associated with insufficient as well as excessive irrigation applications.