Independent measurements of the major energy balance flux components are not often consistent with the principle of conservation of energy. This is referred 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 measured by eddy covariance to be less than the difference between net radiation and soil heat fluxes. This under-measurement of sensible and latent 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 and 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% absolute uncertainty in available energy measurements 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 sensible 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 energy. Based on numerous measurements over a sorghum canopy, carbon dioxide fluxes, which are measured by eddy covariance, are underestimated by the same factor as eddy covariance evaporation measurements when energy balance closure is not achieved.
Bibliographical noteFunding Information:
Jerry L. Hatfield from USDA-ARS National Soil Tilth Laboratory and Tom J. Sauer from USDA-ARS Biomass Research Center provided valuable assistance in the maintenance of the four flux towers operating at the El Reno facility during SGP97. Analyses of ARM Bowen-ratio data were supported by the US Department of Energy under Contract No. W-31-109-Eng-38 as part of the Atmospheric Radiation Measurement Program of the Office of Energy Research, Office of Health and Environmental Research.
This research was supported by NASA Grant NAGW-4138, USDA Cooperative Agreement No. 58-1270-7-008, and the University of Wisconsin Experiment Station. The authors appreciate the efforts of Dr. Tom Jackson of USDA Hydrology Laboratory in Beltsville, MD for his leadership of the SGP97 experiment.
- Evapotranspiration flux
- Friction velocity