A simple method for estimating the latent heat flux over grass from radiative Bowen ratio

The evaluation of crop evapotranspiration from infrared temperature is usually calculated as the residual component of the surface energy balance. This method has given good results over a full grass canopy cover with unstressed transpiration, in spite of the not well understood meaning of the aerodynamic resistance needed. A simple and more accurate method, which determines the Bowen ratio (beta) and the latent heat flux (lambda E) over a reference grass area without knowledge of surface resistances, is proposed. The proportionality between beta and a radiative Bowen ratio beta(s) is shown. The radiative Bowen ratio is derived from surface and air temperatures and vapor pressure. This relationship allows the estimation of lambda E from temperatures, vapor pressure, and solar radiation measurements at one reference level. The new method gives an estimated error of around 10% in front of the residual one, where the wind speed has to be known, with an error of above 20%. The proposed method requires further verification for different climatological conditions.