A THEORETICALLY BASED DETERMINATION OF BOWEN-RATIO FETCH REQUIREMENTS

Determination of fetch requirements for accurateBowen-ratio measurements of latent- and sensible-heatfluxes is more involved than for eddy-correlationmeasurements because Bowen-ratio sensors are locatedat two heights, rather than just one. A simplesolution to the diffusion equation is used to derivean expression for Bowen-ratio fetch requirements,downwind of a step change in surface fluxes. Theserequirements are then compared to eddy-correlationfetch requirements based on the same diffusionequation solution. When the eddy-correlation andupper Bowen-ratio sensor heights are equal, and theavailable energy upwind and downwind of the stepchange is constant, the Bowen-ratio method requiresless fetch than does eddy correlation. Differences infetch requirements between the two methods aregreatest over relatively smooth surfaces. Bowen-ratiofetch can be reduced significantly by lowering thelower sensor, as well as the upper sensor. The Bowen-ratio fetchmodel was tested using data from a fieldexperiment where multiple Bowen-ratio systems weredeployed simultaneously at various fetches and heightsabove a field of bermudagrass. Initial comparisonswere poor, but improved greatly when the model wasmodified (and operated numerically) to account for thelarge roughness of the upwind cotton field.