When to water? Assessment of plant-based measurements to indicate irrigation requirements

Abstract

A range of plant-based methods were assessed for their ability to predict vine performance and irrigation requirement, taking into account spatial and temporal variation in these measures. Pre-dawn leaf water potential and canopy conductance index based on leaf canopy temperature measured with an infrared thermometer best predicted moisture needs of plants.

Summary

The purpose of this project was to provide grape growers with an assessment of plant-based instruments as ways of determining the water needs of vines. Growers have been aware of the incomplete picture given by soil moisture monitors and inconsistencies that can arise from variations in soil depth and soil type. Particular interest has been shown in the capacity of plant-based measurements of vine performance which can be used in understanding short and long term responses to irrigation and environmental conditions.

Importance of various things that can be measured (including stomatal conductance, shoot and trunk diameter, leaf and canopy temperatures, sap flow, and leaf and stem water potential) was considered and performance of available instruments was evaluated. Practicalities were also taken into account.

Two approaches in particular were able to reliably differentiate between vines receiving different irrigation treatments. These were measurement of pre-dawn leaf water potential with a pressure bomb, and the calculation of a canopy conductance index based on the temperatures of the leaf canopy and filter paper reference surfaces measured with an infrared thermometer. Other techniques assessed included measurement of stomatal conductance with a porometer (which, like a pressure bomb, uses portable equipment allowing both spatial and temporal variation to be measured). Fixed pieces of equipment like sap flow monitors and devices to record trunk diameter changes, have the disadvantage of being expensive and fixed to a relatively small number of vines.

This assessment has delivered to grape growers a comprehensive comparison of plant-based techniques. Two of these have enough merit to be developed into practical irrigation scheduling tools.