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Forests 2017, 8(9), 350; https://doi.org/10.3390/f8090350

How Reliable Are Heat Pulse Velocity Methods for Estimating Tree Transpiration?

1
Edaphic Scientific Pty Ltd., Port Macquarie, NSW 2444, Australia
2
School of Agriculture and Food Sciences, University of Queensland, St. Lucia, QLD 4072, Australia
Received: 11 August 2017 / Revised: 7 September 2017 / Accepted: 16 September 2017 / Published: 18 September 2017
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Abstract

Transpiration is a significant component of the hydrologic cycle and its accurate quantification is critical for modelling, industry, and policy decisions. Sap flow sensors provide a low cost and practical method to measure transpiration. Various methods to measure sap flow are available and a popular family of methods is known as heat pulse velocity (HPV). Theory on thermal conductance and convection, that underpins HPV methods, suggests transpiration can be directly estimated from sensor measurements without the need for laborious calibrations. To test this accuracy, transpiration estimated from HPV sensors is compared with an independent measure of plant water use such as a weighing lysimeter. A meta-analysis of the literature that explicitly tested the accuracy of a HPV sensors against an independent measure of transpiration was conducted. Data from linear regression analysis was collated where an R2 of 1 indicates perfect precision and a slope of 1 of the linear regression curve indicates perfect accuracy. The average R2 and slope from all studies was 0.822 and 0.860, respectively. However, the overall error, or deviation from real transpiration values, was 34.706%. The results indicate that HPV sensors are precise in correlating heat velocity with rates of transpiration, but poor in quantifying transpiration. Various sources of error in converting heat velocity into sap velocity and sap flow are discussed including probe misalignment, wound corrections, thermal diffusivity, stem water content, placement of sensors in sapwood, and scaling of point measurements to whole plants. Where whole plant water use or transpiration is required in a study, it is recommended that all sap flow sensors are calibrated against an independent measure of transpiration. View Full-Text
Keywords: meta-analysis; heat pulse velocity; sap flow; compensation heat pulse method; heat ratio method; T-max method; Sapflow+; transpiration meta-analysis; heat pulse velocity; sap flow; compensation heat pulse method; heat ratio method; T-max method; Sapflow+; transpiration
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

Supplementary materials

  • Supplementary File 1:

    XLSX-Document (XLSX, 16 KB)

  • Externally hosted supplementary file 1
    Link: http://www.edaphic.com.au/sap-flow-digest/
    Description: A list of data and references that were collated for the meta-analysis on the accuracy of heat pulse velocity sap flow methods in estimating transpiration.
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Forster, M.A. How Reliable Are Heat Pulse Velocity Methods for Estimating Tree Transpiration? Forests 2017, 8, 350.

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