Evapotranspiration — How good is the Bowen ratio method?

Abstract Under moist conditions, the energy balance approach to determining evapotranspiration from plant communities can give good results, but the method may not be nearly so accurate under very dry conditions, or with considerable advection of energy in moist conditions. In the former case, error analysis shows that the relative error in evapotranspiration can only be kept small provided that the relative error in the Bowen ratio is likewise small. In dry conditions, however, the absolute error in evapotranspiration is always fairly small, because of the small value of evapotranspiration itself. Analysis of the effect on the Bowen ratio of errors in the dry- and wet-bulb temperature gradients shows that in very dry conditions the required accuracy in the measurement of these gradients is an order of magnitude greater than could reasonaly be expected for most Bowen ratio equipment. In the latter case, experimental results show that Bowen ratio measurements considered to be made within the boundary layer, can give by day too small a value of evapotranspiration, and by night a latent heat flux direction which is inconsistent with the direction of the vapour pressure gradient. This raises the question as to what exactly is meant by adjustment of atmospheric properties to those of a new underlying surface when air passes over a boundary between two types of surface.