title: Comparing ∆TmaxDetermination 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 (∆Tmax) between the heated probe and the reference probe below. ∆Tmaxrepresents a state of zero sap flux, which was originally assumed to occur each night leading to a ∆Tmaxdetermination on a daily basis. However, researchers have proven that, under certain conditions, sapflow may continue throughout the night. Therefore alternative approaches to determining ∆Tmaxhave been developed. Multiple ∆Tmaxapproaches are now in use; however, sapflow estimates remain imprecise because the empirical equation that transfers the raw temperature signal (∆T) to sap flux density (Fd) is strongly sensitive to ∆Tmax. In this study, we analyze the effects of different ∆Tmaxdetermination approaches on sub-daily, daily and (intra-)seasonal Fdestimations. On this basis, we quantify the uncertainty of sapflow calculations, which is related to the raw signal processing. We show that the ∆Tmaxdetermination procedure has a major influence on absolute ∆Tmaxvalues and the respective sap flux density computations. Consequently, the choice of the ∆Tmaxdetermination approach may be a significant source of uncertainty in sapflow estimations.
authors.affiliation: Department of Geography, University of Bonn, Meckenheimer Allee 166, 53115 Bonn, Germany. irabbel@uni-bonn.de.
authors.forename: Inken
authors.initials: I
authors.lastname: Rabbel
authors.affiliation: Department of Geography, University of Bonn, Meckenheimer Allee 166, 53115 Bonn, Germany. b.diekkrueger@uni-bonn.de.
authors.forename: Bernd
authors.initials: B
authors.lastname: Diekkrüger
authors.affiliation: Center for Development Research, University of Bonn, Walter-Flex-Straße 3, 53113 Bonn, Germany. hvoigt@uni-bonn.de.
authors.forename: Holm
authors.initials: H
authors.lastname: Voigt
authors.affiliation: Dendro-Labor Windeck, Tree-Ring Analytics, 51570 Windeck, Germany. b.neuwirth@online.de.
authors.forename: Burkhard
authors.initials: B
authors.lastname: Neuwirth
keywords: Norway spruce; Picea abies; data processing; heat dissipation; maximum temperature gradient; thermal dissipation; transpiration