Significance of the WPL correction for the latent heat flux in semi-arid conditions

The estimation of surface turbulent fluxes with the eddy-covariance (EC) technique is done through instrumentation that accounts for the fast response of primary meteorological variables measured at a certain distance from the surface. In practice, these measurements are typically subjected to additional corrections to mitigate the source of errors related to known micrometeorological effects. One of these effects is related to density fluctuations produced by temperature and humidity variations, a process that has to be considered when determining the flux of any air constituent. In consequence, this correction (called WPL after Webb, Pearman and Leuning work in 1980) affects the estimation of CO2 and water vapour fluxes. However, when comparing the impact of WPL correction respect to other error sources, it is usually considered relevant for CO2 but rather small on water vapour.

In the current study, the impact of the WPL correction on the latent heat flux has been analysed in semi-arid conditions where the water availability is scarce in Summer. Experimental data is taken from Els Plans site, an instrumental facility deployed in the framework of the 2021 LIAISE field campaign (Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment), located over a rainfed field close to a wide irrigated area in the Eastern Ebro basin, Catalonia.

The characterization of the latent heat flux through the EC systems has been evaluated for 13 months and their results are compared against the observations from a smart-field-lysimeter. In Summer, the WPL correction in daytime accounts for a couple of tens of W/m² and doubles the initial value provided by the EC system, getting closer to the the lysimeter. The WPL correction is less important after a rain event, when the latent heat flux is larger due to enough water availability. When analysing the entire yearly cycle, the relative importance of WPL correction respect to the total water vapour flux increases with Bowen ratio. These results suggest that WPL influence cannot be neglected when dealing with arid and dry conditions.