Crops need nutrients, energy (light) and water for their growth. The water requirements depend on the climatic conditions, on the characteristics of the particular species and cultivar, and on the growth stage of the crop.
A worldwide accepted and applied method to calculate crop water requirements is provided in FAO (1986): Crop evapotranspiration - Guidelines for computing crop water requirements. Irrigation and drainage paper 56 and Irrigation Water Management, Training manual no. 3.
See also: Adapting water use for crop production to regional and local conditions