The main interacting factors determining the actual evapotranspiration from crops were analysed.The apparent diffusion resistance of the crop was introduced because of the geometry of the evaporative surface. Its relation to light intensity and to suction in the leaves had been estimated for grass.The precipitation intercepted by the crop increased evapotranspiration when the diffusion resistance of the crop was not zero.The relation between capillary conductivity and suction was discussed. The relation between diffusion resistance of the crop and the soil physical conditions was calculated with a flow model.A mathematical model of the factors determining evapotranspiration was derived. The possibility of relating evapotranspiration from crops and evaporation from a free water surface was discussed, and the transfer of such a relation from one area to another different in climate was considered.