Daytime evaporation and the self-preservation of the evaporative fraction and the Bowen ratio

Under certain conditions several ratios of fluxes in the surface energy budget are known to remain relatively constant during the daytime hours. This 'self-preservation' has allowed the use of midday values of the evaporative fraction, EF, multiplied by the daytime total of net radiation minus the ground heat flux, to provide a good estimate of total daytime evaporation. A similar method is discussed, in which the daytime total sensible heat flux is divided by the noon Bowen ratio (beta = (1/EF)-1). Data from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) indicate that the two methods give rather different results. It is shown by propagating uncertainties that the uncertainty in the daytime total evaporation is likely to be smaller with the constant-EF method than with the constant-beta method for beta greater than about -1/2.