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

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. 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.

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;  LIDDELL Michael;  WOHLFAHRT G.;  MARKWITZ Christian;  MAUDER Matthias;  PAUL-LIMOGES Eugenie;  SCHMIDT Marius;  WOLF Sebastian;  BRUMMER Christian;  ALTAF ARAIN M.;  FARES Silvano;  KATO Tomomichi;  ARDO Jonas;  OECHEL Walter;  HANSON Chad;  KORKIAKOSKI Mika;  BIRAUD Sebastien;  STEINBRECHER R.;  BILLESBACH D.P.;  MONTAGNANI Leonardo;  WOODGATE William;  SHAO Changliang;  CARVALHAIS Nuno;  REICHSTEIN Markus;  NELSON Jacob; 

2023-03-23

ELSEVIER

JRC129194

0168-1923 (online),   

https://www.sciencedirect.com/science/article/pii/S0168192322004919?via%3Dihub,    https://publications.jrc.ec.europa.eu/repository/handle/JRC129194,   

10.1016/j.agrformet.2022.109305 (online),