ELIMINATING THE HEAT INPUT AS PARAMETER IN THE SAPFLOW+ METHOD

Abstract

The recently developed Sapflow+ method to determine sap flux density and sapwood water content necessitates a correct estimation of the heat generated by the heater needle. Even though this can theoretically be calculated based on the resistance of the heater and the applied voltage and current, in practice, however, these theoretical calculations overestimate the actual heat output due to probe limitations. Originally, the heater was therefore calibrated in a medium with known thermal characteristics, leading to an accurate correction factor. Elimination of this heat input from the model is, nevertheless, much more convenient as sensor specific calibration is labour-intensive and has to be conducted meticulously as the needle distances during calibration have to be equal to those applied during field measurements. Dividing the original equations is theoretically possible to eliminate the heat input, but has practical limitations. Adapting the equations and grouping heat input and volumetric heat capacity into a single parameter, however, enables in-situ estimations of heat input if a single measurement of dry wood density and water content is conducted. Our results show that, based on a single wood core measurement, the Sapflow+ sensor can be calibrated in-situ, pre-empting sensor specific calibration in a separate medium.