On evapotranspiration and eddy covariance measurements corrections

Abstract. Eddy Covariance (EC) technique is one of the most used technique monitoring Green House Gases (GHG) fluxes such as H₂O, CO₂, CH₄. Water vapor movement and corresponding air density fluctuations were corrected by Webb et al. (1980) but not water vapor formation. Classic EC technique supposes mean air vertical speed nullity when it cannot be the case because of water evaporation. Water is falling as a liquid, evaporating directly from soil surface, from shallow soil subsurface or either through vegetation transpiration and becomes a gas which corresponds to a notable volume expansion. Water vapor is mounting through atmosphere, compensating in a cloud and falling as a rain (liquid) again. Evaporation and corresponding volume expansion make mean vertical air speed positive (upgoing) and influence more or less a flux balance following concerned gas or energy. A simple accessing and corresponding correction for the half hourly summation is given and applied to a 11-month real set of data. These corrections allow to explain, in part, most observed eddy covariance discrepancies.