Performance of Bowen ratio systems on a slope

Abstract

The Bowen ratio energy balance technique was used to assess the energy fluxes on inclined surfaces in several recent surface-atmosphere interaction studies. Since air flow over a sloping surface may differ from that over a flat terrain, it is important to examine whether Bowen ratio measurements taken on sloping surfaces are valid. In this study, the suitability of using the Bowen ratio technique on sloping surfaces was tested by examining the assumptions that the technique requires for valid measurements. This was accomplished by studying the variation of Bowen ratio measurements along a selected slope at a site of the First ISLSCP Field Experiment (FIFE). In September 1988, four Bowen ratio systems were set up in a line along the 22° north-facing slope with northerly air flow (air flowed up the slope). In July of 1989, six Bowen ratio systems were similarly installed with southerly air flow (air flowed down the slope). Results indicated that, at distances between 10 and 40 m from the top of the slope, no temperature or vapor pressure gradient parallel to the slope was detected. Uniform Bowen ratio values were obtained on the slope, and thus the sensible or latent heat flux should be similar along the slope. This indicates that the assumptions for valid flux measurements are reasonably met on the slope. The Bowen ratio technique should give good estimates of the energy fluxes on slopes similar to that in this study.