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Sensors 2016, 16(12), 2042;

Comparing ∆Tmax Determination Approaches for Granier-Based Sapflow Estimations

Department of Geography, University of Bonn, Meckenheimer Allee 166, 53115 Bonn, Germany
Center for Development Research, University of Bonn, Walter-Flex-Straße 3, 53113 Bonn, Germany
Dendro-Labor Windeck, Tree-Ring Analytics, 51570 Windeck, Germany
Author to whom correspondence should be addressed.
Academic Editor: Russell Binions
Received: 19 October 2016 / Revised: 22 November 2016 / Accepted: 28 November 2016 / Published: 1 December 2016
(This article belongs to the Special Issue Sensors for Environmental Monitoring 2016)
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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. ∆Tmax represents a state of zero sap flux, which was originally assumed to occur each night leading to a ∆Tmax determination on a daily basis. However, researchers have proven that, under certain conditions, sapflow may continue throughout the night. Therefore alternative approaches to determining ∆Tmax have been developed. Multiple ∆Tmax approaches 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 ∆Tmax 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 ∆Tmax determination procedure has a major influence on absolute ∆Tmax values and the respective sap flux density computations. Consequently, the choice of the ∆Tmax determination approach may be a significant source of uncertainty in sapflow estimations. View Full-Text
Keywords: heat dissipation; thermal dissipation; maximum temperature gradient; data processing; transpiration; Norway spruce; Picea abies heat dissipation; thermal dissipation; maximum temperature gradient; data processing; transpiration; Norway spruce; Picea abies

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Rabbel, I.; Diekkrüger, B.; Voigt, H.; Neuwirth, B. Comparing ∆Tmax Determination Approaches for Granier-Based Sapflow Estimations. Sensors 2016, 16, 2042.

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