Sap flow measurement by a single thermal dissipation probe in transient regime: implementation of the method and test under field conditions

Abstract

Recent works have showed the possibility of measuring sap flow density (u) by a single heated probe using the transient regime just after the heating current is switched off. The method (SP-T) is based on a thermal index (I) which involves the temperature of the probe at three different times of the cooling kinetics (beginning, intermediate time, and end), the latter representing wood temperature at equilibrium. The present paper re-examines the method to better estimate wood temperature and to account for possible variations in probe-wood thermal properties. After a recalibration of the method on an olive branch with a hydraulic bench, the SP-T method is used to measure sap flow on two olive trees under field conditions. Estimated flow densities are compared to those obtained with the transient thermal dissipation-dual probe method (TTD-DP) of Do and Rocheteau. Obtained results show that: (1) wood reference temperature is better estimated when using the probe temperatures just before heating and at the end of the cooling phase; (2) calibration relationships based on a “normalized” thermal index K t = (I − I o )/I o, where I o represents the value of I under zero flow conditions, better account for differences in the wood-probe thermal properties; (3) I o, taken as the lowest I over a 24-h span, has shown consistent stability for a given probe-tree complex; (4) the SP-T method used in its normalized form appears to be in good agreement with the TTD-DP method.