It is proposed that a body of water required to dissipate a fixed quantity of heat (derived from all sources including insolation) will come to an equilibrium temperature essentially determined by the maximization of the evaporation rate. By using this hypothesis and one of several simple expressions for the Bowen ratio versus surface temperature dependency it is shown that long-term average fluxes of sensible and latent heat from clean water surfaces at equilibrium temperature can be expressed analytically in terms of the emissivity, the Stefan-Boltzmann constant, and the surface temperature only. Data obtained in two different field experiments are found to be in good agreement with predictions made by this simple model.