A model of canopy resistance rc is presented for the determination of actual crop evapotranspiration using the Penman-Monteith type of model at hourly and daily scales. Here rc is deduced from weather variables (net radiation, air temperature and humidity, and wind speed) and by an indicator of the crop water status measured on the plants: the predawn leaf water potential (i.e., measured before dawn). The model needs a calibration per crop and not per site; that is, once it is calibrated for a given crop in a certain locality, then it can be used anywhere without further calibration. The model was tested on sorghum crops in different water conditions at two sites in southern Italy for several years. The results showed that it performs well at hourly, daily, and seasonal scales. In fact, the difference between the cumulative values of the actual evapotranspiration measured and the one calculated by the model ranges between -9% and +11%. We concluded that it is not sensitive to climate and soil type or to crop species and variety. ¿ 2001 American Geophysical Union