Evaporation and advection I: evaporation from extensive homogeneous surfaces

Abstract The process of modification of the Bowen ratio, with distance downwind of a change in surface wetness, is considered to establish the ratio of the fluxes of sensible and latent heat when unaffected by surface inhomogeneity. The case of steady‐state two‐dimensional advection is considered. It is shown that the coupled, simultaneous, diffusion equations for heat and moisture transfer from natural vegetated or wet surfaces can be used to generate two independent diffusion equations in composite variables. The vegetated surface is represented as an extensive single leaf. The height integral of the reciprocal effective eddy diffusivity is assumed to become indefinitely large with height so that heat and moisture continuously accumulate within the overpassing air. As a result equilibrium does not obtain. However, the surface latent heat flux approaches a quasi‐equilibrium value given, in conventional symbols, by LE = (s/(s+γ)) (Rn −G). For terrestrial surfaces, where the available energy depends only slightly on surface temperature, the Bowen ratio approaches γ/s. For deep water bodies and melting snow the heat flux into the surface also evolves with distance and the above relationship is approached only in the trivial sense that Rn = G and LE = 0 . This model for terrestrial surfaces is consistent with some recent experimental observations.