Technical note: An assessment of the relative contribution of the Soret effect to open-water evaporation

Abstract

It is standard practice to assume that evaporation from open water depends on the gradient in water vapour concentration as per Fick's law. However, Fick's law is only true in an isothermal system. In general, we anticipate an additional mass flux due to the temperature gradient (in a non-isothermal system), and this is known as Soret diffusion or the Soret effect. Here we evaluate the magnitude of the Soret effect and find that, under exceptional circumstances, the Soret effect may be as high as ∼ 5 % of the classical concentration-dependent mass (“Fickian”) diffusion but that it will usually be less than 2 % of that same flux. Furthermore, we evaluate the magnitude of an additional advective (or “Stefan”) flux not usually considered in hydrologic studies that may be as high as ∼ 5 % of the concentration-dependent diffusion. Whether these small additional fluxes need to be considered will depend on the nature of the investigation.