Examination of the Surface Energy Budget: A Comparison of Eddy Correlation and Bowen Ratio Measurement Systems

Abstract

A reliable method for monitoring the surface energy budget is critical to the development and validation of numerical models and remote sensing algorithms. Unfortunately, closure of the energy budget remains difficult to achieve among measurement systems. Reasons for nonclosure still are not clearly understood, and, until recently, few long-term datasets were available to address this issue of nonclosure. This contribution examined 108 days of a year dataset collected from collocated eddy correlation (EC) and Bowen ratio (BR) systems. Differences between systems were examined across seasonal and diurnal cycles to better understand nonclosure of the energy budget. Closure by the EC system was observed to vary with season and with time of day, primarily as a function of latent heat flux. Furthermore, the EC and BR methods partitioned energy differently, with the EC system favoring latent heat flux and the BR system favoring sensible heat flux. Instrument error, surface heterogeneity, and the theoretical assumptions behind the EC and BR methods are discussed to explain observed patterns in closure and the differences between measurement systems. Sensor error and variability in net radiation and soil moisture data increased uncertainty in measurements of net radiation and ground heat flux. Significant differences in soil temperature and flux between sites appear to be caused by the heterogeneity of vegetation and soil type. Finally, several assumptions of the BR method are examined to explain observed differences in sensible and latent heat flux between systems. Recommendations for future observational studies are proposed.