All these terms refer to the same process, ET, in which the plant extracts water from the soil for tissue building and cooling purposes, as well as soil evaporation. Once the soil surface is dried out, evaporation decreases sharply. Thus significant evaporation occurs after rain or irrigation. Furthermore, as the growing season progresses and canopy cover increases, evaporation from the wet soil surface gradually decreases. When the crop reaches full cover, approximately 95 percent of the ET is due to transpiration and evaporation from the crop canopy where most of the solar radiation is intercepted.
Full cover is a growth stage at which most of the soil is shaded by the crop canopy.
In a more technical term, the crop is at full cover when the leaf area is three times the soil surface area under the canopy. At this growth stage, the crop canopy intercepts most of the incoming solar radiation, thereby reducing the amount of energy reaching the soil surface. In humid and semi-humid areas where water usually is not a limiting factor, grass is used as a reference ET crop. In arid or semi-arid areas, alfalfa is more suitable as a reference ET crop because it has a deep root system, which reduces its susceptibility to water stress resulting from dry weather. ET a of an annual crop reaches its maximum at full cover, and can be higher or lower than ET r , depending on the crop.