Abstract A thermal infrared image provides information about crop evapotranspiration in a certain area at flighttime. A simple method has been developed to convert instantaneous crop temperatures into 24-h evapotranspiration values. For clear days good results were obtained with a linear relationship between relative 24-h evapotranspiration (LE 24/LE p 24) and the increase in crop temperature around midday: LE 24 / LE p 24 = 1 − B r(T c − T c ∗ ). The crop temperature T c is related to the temperature of the crop that is potentially transpiring, T c ∗. For the typical flight days to detect crop stress conditions, the empirical coefficient B r appeared to be almost insensitive to variations in air temperature, relative humidity, and incoming radiation flux. On the other hand, B r was strongly dependent on wind velocity, crop type, and crop height. Information about crop type and crop height can be derived from reflection images. The wind velocity can be obtained from a nearby meteorological station. The present method appears to be suited for automatic mapping of 24-h evapotranspiration values with the aid of digital reflection and thermal infrared images. However, application of the method requires plots with full soil overage.