DETERMINATION OF HEAT AND WATER-VAPOR FLUXES ABOVE A SPRUCE FOREST BY EDDY-CORRELATION

Heat and water vapour fluxes have been measured by eddy correlation at three levels above a spruce forest (tree height 31 m) in central Germany. A number of methodical problems are discussed, and necessary data corrections are presented. Stationarity of the fluxes is proven by cospectral analysis, confirming that averaging times of 10-30 min are adequate to describe the transport processes. Sensible and latent heat fluxes do not vary with height in the layer from (z - d) = 6.5z0 to (z - d) = 11.7z0. Based on footprint estimates it is concluded that, in general, the measured fluxes may be taken as representative of the spruce canopy. Even in the worst-case situations, when the wind comes from a beech-spruce transition 200 m away, no height dependence of the fluxes is detected. On the selected days, which were mainly bright, a Bowen ratio near one is found during the hours of highest available energy, whereas at night no evapotranspiration is observed. The turbulent diffusion coefficients of heat and water vapour follow a distinct diurnal pattern, with a maximum of about 10 m2 s-1 shortly after noon. The heat diffusion coefficient tends to exceed that of water vapour by a factor of about 1.2, showing daytime-dependent variations. Owing to some unexplained deficits in the energy balance this value is not very certain, but the results are consistent with some other published data. At least they demonstrate that uncritical application of the similarity hypothesis (D(h) = D(v)) above high vegetation may lead to erroneous flux calculations.