Comparing ΔTmax determination approaches for Granier-based sapflow estimations

Abstract

Granier-type thermal dissipation probes are common instruments for quantifying tree water use in forest hydrological studies. Estimating sapflow using Granier-type sapflow sensors requires determining the maximum temperature gradient (ΔTmax) between the heated probe and the reference probe below. ΔTmax represents a state of zero sap flux, which was originally assumed to occur each night leading to a ΔTmax determination on a daily basis. However, researchers have proven that, under certain conditions, sapflow may continue throughout the night. Therefore alternative approaches to determining ΔTmax have been developed. Multiple ΔTmax approaches are now in use, however, sapflow estimates remain imprecise because the empirical equation that transfers the raw temperature signal (ΔT) to sap flux density (Fd) is strongly sensitive to ΔTmax. In this study, we analyze the effects of different ΔTmax determination approaches on sub-daily, daily and (intra-)seasonal Fd estimations. On this basis, we quantify the uncertainty of sapflow calculations, which is related to the raw signal processing. We show that the ΔTmax determination procedure has a major influence on absolute ΔTmax values and the respective sap flux density computations. Consequently, the choice of the ΔTmax determination approach may be a significant source of uncertainty in sapflow estimations.