Attenuation Correction Procedures for Water Vapour Fluxes from Closed-Path Eddy-Covariance Systems |
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Authors: | Benjamin R K Runkle Christian Wille Michal Gažovič Lars Kutzbach |
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Institution: | 1.Institute of Soil Science, Klima Campus,University of Hamburg,Hamburg,Germany;2.Institute for Botany and Landscape Ecology,Ernst Moritz Arndt University Greifswald,Greifswald,Germany |
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Abstract: | Evapotranspiration is a source of water vapour to the atmosphere, and as a crucial indicator of landscape behaviour its accurate
measurement has widespread implications. Here we investigate errors that are prevalent and systematic in the closed-path eddy-covariance
measurement of latent heat flux: the attenuation of fluxes through dampened cospectral power at high frequencies. This process
is especially pronounced during periods of high relative humidity through the adsorption and desorption of water vapour along
the tube walls. These effects are additionally amplified during lower air temperature conditions. Here, we quantify the underestimation
of evapotranspiration by a closed-path system by comparing its flux estimate to simultaneous and adjacent measurements from
an open-path sensor. We apply models relating flux loss to relative humidity itself, to the lag time of the cross-correlation
peak between the water vapour and vertical wind velocity signals, and to models of cospectral attenuation relative to the
cospectral power of simultaneous sensible heat-flux measurements. We find that including the role of temperature in modifying
the attenuation–humidity relationship is essential for unbiased flux correction, and that physically based cospectral attenuation
methods are effective characterizers of closed-path instrument signal loss relative to the unattenuated flux value. |
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