Determination of transpiration in irrigated grapevines: comparison of the heat-pulse technique with gravimetric and micrometeorological methods

The use of a heat-pulse technique to monitor sap flow from which transpiration can be deduced was evaluated in ungrafted grapevines (Vitis vinifera L., cv. Sultana) under glasshouse and field conditions. There was a significant degree of agreement between daily transpiration deduced from heat-pulse velocity (T hp) and that determined directly by gravimetry in the glasshouse and by calibration using the Penman-Monteith equation in the field. Comparison throughout the growing season of T hp to transpiration calculated with the full Penman-Monteith equation produced a high coefficient of determination (r 2=0.69). A similar comparison of T hp with transpiration calculated with only the aerodynamic component of the Penman-Monteith equation produced a non-linear relationship, due to the equation over-estimating transpiration relative to T hp at high vapour pressure deficits (i.e. above 2.5 kPa). Values for total seasonal transpiration measured with heat-pulse sensors or calculated with either the full Penman-Monteith equation or with only the modified aerodynamic component of the equation were within 10% of each other. Transpiration (T hp) in grapevines with an adequate supply of soil water was shown to be coupled to ambient air conditions.