Once the soil surface is dry, 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 ET is due to transpiration and evaporation from the crop canopy where most of the solar radiation is intercepted. Prevailing weather conditions, available water in the soil, crop species, and growth stage influence crop water use.
Different crops reach full cover at different growth stages and times after planting.
In humid and semihumid areas where water usually is not a limiting factor, grass is used as a reference ET crop. In arid or semiarid areas, alfalfa is more suitable as a reference ET crop because it has a deeper root system, which reduces its susceptibility to water stress resulting from dry weather. The standard is a large pan of water in or near a field. Figures 1 and 2 show corn and soybean ET factors, respectively, over a growing season. The ET a is the water use of a particular crop at a given time.