Estimation of evapotranspiration from thermal infrared (IR)as been widely studied in recent years and in particular by methods using the simplified relation ETR − Rn = A − B(Ts−Ta) proposed by Jacksonet al.(1977) and thermal IR data from the NOAA satellite obtained near midday (local time). The representativeness of instantaneous temperature for computing daily fluxes is discussed and a one-layer model is presented for a developed crop. A new theoretical expression of the simplified relation which can be applied to developed crops is presented and the sensitivity of the parameters in this relation is examined.