Correcting eddy-covariance flux underestimates over a grassland

Independent measurements of the major energy balance flux components are no t often consistent with the principle of conservation of energy. This is re ferred to as a lack of closure of the surface energy balance. Most results in the literature have shown the sum of sensible and latent heat fluxes mea sured by eddy covariance to be less than the difference between net radiati on and soil heat fluxes. This under-measurement of sensible and latent heat fluxes by eddy-covariance instruments has occurred in numerous field exper iments and among many different manufacturers of instruments. Four eddy-cov ariance systems consisting of the same models of instruments were set up si de-by-side during the Southern Great Plains 1997 Hydrology Experiment and a ll systems under-measured fluxes by similar amounts. One of these eddy-cova riance systems was collocated with three other types of eddy-covariance sys tems at different sites; all of these systems under-measured the sensible a nd latent-heat fluxes. The net radiometers and soil heat flux plates used i n conjunction with the eddy-covariance systems were calibrated independentl y and measurements of net radiation and soil heat flux showed little scatte r for various sites. The 10% absolute uncertainty in available energy measu rements was considerably smaller than the systematic closure problem in the surface energy budget, which varied from 10 to 30%. When available-energy measurement errors are known and modest, eddy-covariance measurements of se nsible and latent heat fluxes should be adjusted for closure. Although the preferred method of energy balance closure is to maintain the Bowen-ratio, the method for obtaining closure appears to be less important than assuring that eddy-covariance measurements are consistent with conservation of ener gy. Based on numerous measurements over a sorghum canopy, carbon dioxide fl uxes, which are measured by eddy covariance, are underestimated by the same factor as eddy covariance evaporation measurements when energy balance clo sure is not achieved. Published by Elsevier Science B.V.