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 ba lance. This method has given good results over a full grass canopy cov er with unstressed transpiration, in spite of the not well understood meaning of the aerodynamic resistance needed. A simple and more accura te method, which determines the Bowen ratio (beta) and the latent heat flux (lambda E) over a reference grass area without knowledge of surf ace resistances, is proposed. The proportionality between beta and a r adiative Bowen ratio beta(s) is shown. The radiative Bowen ratio is de rived from surface and air temperatures and vapor pressure. This relat ionship allows the estimation of lambda E from temperatures, vapor pre ssure, and solar radiation measurements at one reference level. The ne w method gives an estimated error of around 10% in front of the residu al one, where the wind speed has to be known, with an error of above 2 0%. The proposed method requires further verification for different cl imatological conditions.