The crop water need (ET crop) is defined as the depth (or amount) of water needed to meet the water loss through evapotranspiration. In other words, it is the amount of water needed by the various crops to grow optimally.

The crop water need always refers to a crop grown under optimal conditions, i.e. a uniform crop, actively growing, completely shading the ground, free of diseases, and favourable soil conditions (including fertility and water). The crop thus reaches its full production potential under the given environment.

The crop water need mainly depends on:

· the climate: in a sunny and hot climate crops need more water per day than in a cloudy and cool climate

· the crop type: crops like maize or sugarcane need more water than crops like millet or sorghum

· the growth stage of the crop; fully grown crops need more water than crops that have just been planted.

Section 3.1 indicates the influence of the climate on crop water needs: this influence is given by the reference crop evapotranspiration (ETo). Section 3.2 indicates the influence of the crop type and growth stage on crop water needs: the influence of the crop is expressed in the crop factor (Kc).