The effect of relative humidity on eddy covariance latent heat flux measurements and its implication for partitioning into transpiration and evaporation

Zhang, Weijie, Jung, Martin, Migliavacca, Mirco, Poyatos, Rafael, Miralles, Diego G., El-Madany, Tarek S., Galvagno, Marta, Carrara, Arnaud, Arriga, Nicola, Ibrom, Andreas, Mammarella, Ivan, Papale, Dario, Cleverly, Jamie R., Liddell, Michael, Wohlfahrt, Georg, Markwitz, Christian, Mauder, Matthias, Paul-Limoges, Eugenie, Schmidt, Marius, Wolf, Sebastian, Brümmer, Christian, Arain, M. Altaf, Fares, Silvano, Kato, Tomomichi, Ardö, Jonas, Oechel, Walter, Hanson, Chad, Korkiakoski, Mika, Biraud, Sébastien, Steinbrecher, Rainer, Billesbach, Dave, Montagnani, Leonardo, Woodgate, William, Shao, Changliang, Carvalhais, Nuno, Reichstein, Markus, and Nelson, Jacob A. (2023) The effect of relative humidity on eddy covariance latent heat flux measurements and its implication for partitioning into transpiration and evaporation. Agricultural and Forest Meteorology, 330. 109305.

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While the eddy covariance (EC) technique is a well-established method for measuring water fluxes (i.e., evaporation or 'evapotranspiration’, ET), the measurement is susceptible to many uncertainties. One such issue is the potential underestimation of ET when relative humidity (RH) is high (>70%), due to low-pass filtering with some EC systems. Yet, this underestimation for different types of EC systems (e.g. open-path or closed-path sensors) has not been characterized for synthesis datasets such as the widely used FLUXNET2015 dataset. Here, we assess the RH-associated underestimation of latent heat fluxes (LE, or ET) from different EC systems for 163 sites in the FLUXNET2015 dataset. We found that the LE underestimation is most apparent during hours when RH is higher than 70%, predominantly observed at sites using closed-path EC systems, but the extent of the LE underestimation is highly site-specific. We then propose a machine learning based method to correct for this underestimation, and compare it to two energy balance closure based LE correction approaches (Bowen ratio correction, BRC, and attributing all errors to LE). Our correction increases LE by 189% for closed-path sites at high RH (>90%), while BRC increases LE by around 30% for all RH conditions. Additionally, we assess the influence of these corrections on ET-based transpiration (T) estimates using two different ET partitioning methods. Results show opposite responses (increasing vs. slightly decreasing T-to-ET ratios, T/ET) between the two methods when comparing T based on corrected and uncorrected LE. Overall, our results demonstrate the existence of a high RH bias in water fluxes in the FLUXNET2015 dataset and suggest that this bias is a pronounced source of uncertainty in ET measurements to be considered when estimating ecosystem T/ET and WUE.

Item ID: 78394
Item Type: Article (Research - C1)
ISSN: 1873-2240
Keywords: Eddy covariance, Energy balance closure, Evapotranspiration, FLUXNET, Latent energy
Copyright Information: © 2022 Published by Elsevier B.V.
Date Deposited: 08 Aug 2023 02:15
FoR Codes: 41 ENVIRONMENTAL SCIENCES > 4102 Ecological applications > 410203 Ecosystem function @ 50%
37 EARTH SCIENCES > 3707 Hydrology > 370702 Ecohydrology @ 50%
SEO Codes: 18 ENVIRONMENTAL MANAGEMENT > 1806 Terrestrial systems and management > 180601 Assessment and management of terrestrial ecosystems @ 100%
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