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

Measurements of turbulent exchange in the understorey of a forest canopy ar e necessary to understand and model both the functioning of the lower strat um and its contribution to turbulent exchange at the whole canopy scale. Ed dy covariance measurements of eddy fluxes just above the floor of a forest canopy must be thoroughly validated, given the particular conditions prevai ling there. Our objective is two-fold: (i) check the overall quality of suc h eddy flux measurements through the analysis of the understorey energy bal ance 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 bal ance to be rigorously tested. As in most experiments storage term measureme nts are not available, we also developed a method to estimate them, in orde r to apply the above-mentioned data selection method. The energy balance cl osure was found to be quite satisfactory (the slope of the sum of eddy flux es and storage terms versus transmitted net radiation is 0.99, the intercep t is less than 1 W m(-2), r(2) is 0.94, there is: no deviation from 3 linea r trend). The data selection procedure allows a fair description of the dai ly and day-to-day variation of turbulent fluxes while rejecting the most du bious 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. (C) 2001 Elsevier Science B.V. All rights reserved.