Submitted to: Agricultural Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 13, 2000
Publication Date: July 23, 2000
Interpretive Summary: A better understanding of how energy is partitioned at the earth's surface is necessary for the improvement of regional weather, global climate models, and assessing soil and plant water use or evapotranspiration. Accurate measurements of surface energy components are imperative for accurate modeling of energy and water balances. Many different types of instruments and methods have been developed to measure surface energy fluxes. Numerous field experiments have indicated that one of the most common measurement techniques consistently underestimates evapotranspiration. A method is used to correct for the underestimation and is evaluated for its consistency among different manufacturers of instruments. When the fluxes appear to be underestimated, steps should be taken to close the energy budget. If such steps are not undertaken, there are likely to be significant discrepancies in long-term water and energy balances between models and observations for different land surfaces.
Independent measurements of all the energy balance flux components are not often consistent with the principle of conservation of energy. This is referred to as a lack of surface energy balance closure. Most results in the literature have shown the sum of sensible and latent heat fluxes measured by eddy covariance to be less than the difference between net radiation and soil heat flux. This under-measurement of sensible and laten heat fluxes by eddy-covariance instruments has occurred in numerous field experiments and among many different manufacturers of instruments. Four eddy-covariance systems consisting of the same models of instruments were set up side-by-side during the Southern Great Plains 1997 Hydrology Experiment, and all systems under-measured fluxes by similar amounts. One of these eddy-covariance systems was collocated with three other types of eddy-covariance systems at different sites; all of these systems under-measured the sensible and latent-heat fluxes. The net radiometers an soil heat flux plates used in conjunction with the eddy-covariance systems were calibrated independently, and measurements of net radiation and soil heat flux showed little scatter for various sites. The 10 percent uncertainty in available energy was considerably smaller than the systematic closure problem in the surface energy budget. When available-energy measurement errors are known and modest, then eddy-covariance measurements of sensible and latent heat fluxes should be adjusted for closure. The method for obtaining closure appears to be less important than assuring that eddy-covariance measurements are consistent with conservation of energy.