Density Fluctuations and Use of the Krypton Hygrometer in Surface Flux Measurements

Abstract

The magnitude of the of the Webb et al. (1980) density corrections on water vapor fluxes due to the flux of sensible heat is examined in terms of the Bowen ratio. The correction due to latent heat is proportional only to the mole fraction of water vapor and is 5 times smaller than that for the sensible heat. The temperature regime within the path of the krypton hygrometer was measured to determine its effect in the Webb et al. (1980) corrections and the krypton O 2 absorption corrections. The results of this experiment are inconclusive, but confirm the need for additional measurements. Two methods for determining water vapor from krypton hygrometer measurements are reexamined. An approximate method endorsed by the manufacturer and having errors of less than 3 percent for fluctuation smaller than 2 gm -3 is shown to underestimate the vapor flux by 14% under specific conditions of light winds and irrigated surfaces in arid regions (K. Kunkel, 1992, personal communication).