Validation of eddy flux measurements above the understorey of a pine forest

Measurements of turbulent exchange in the understorey of a forest canopy are necessary to understand and model both the functioning of the lower stratum and its contribution to turbulent exchange at the whole canopy scale. Eddy covariance measurements of eddy fluxes just above the floor of a forest canopy must be thoroughly validated, given the particular conditions prevailing there. Our objective is two-fold: (i) check the overall quality of such eddy flux measurements through the analysis of the understorey energy balance closure and (ii) define quality criteria for each half-hourly sample, based on the residual term of the energy balance. A subset of the EUROFLUX data base, collected within a pine forest canopy in south-west France, was used for this purpose. During this experiment, all heat storage terms were carefully measured, which allowed the closure of the understorey energy balance to be rigorously tested. As in most experiments storage term measurements are not available, we also developed a method to estimate them, in order to apply the above-mentioned data selection method. The energy balance closure was found to be quite satisfactory (the slope of the sum of eddy fluxes and storage terms versus transmitted net radiation is 0.99, the intercept is less than 1Wm−2, r2 is 0.94, there is no deviation from a linear trend). The data selection procedure allows a fair description of the daily and day-to-day variation of turbulent fluxes while rejecting the most dubious data, whether experimental or estimated storage terms are used. This analysis proves the validity of eddy flux measurements in the lower part of the forest and offers tools for flux data selection, depending on the type of studies such data are intended for.