Fetch requirements for bowen ratio measurements of latent and sensible heat fluxes

Abstract Simultaneous measurements of the Bowen ratio were made at different fetch-to-height ratios over an aerodynamically smooth plot of bermudagrass which was downwind of an aerodynamically rough field of cotton. The measurements were used to evaluate effects of fetch and measurement height on Bowen ratio estimates of latent and sensible heat fluxes. The measurements indicated that significant boundary-layer adjustment occurred within the first 15 m of fetch. For the conditions encountered, the Bowen ratio and its variability increased with measurement height. However, departure from ideal fetch (measurement within the equilibrium sublayer) had only a small effect on flux estimates. The data suggest that when the Bowen ratio is small, the method can be used successfully at fetch-to-height ratios as low as 20:1, much less than the often-quoted value of 100:1.

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