Factors That Affect ET Weather. The power of the atmosphere to evaporate water is the driving force for soil evaporation and crop transpiration. High air temperatures, low humidity, clear skies and high winds cause a large evaporative demand by the atmosphere. Example of daily variation in ET for corn. This potential for ET, governed by weather factors, is the starting point for estimating ET from weather data.
Different crops use different amounts of water over the course of the growing seasons.
I shows total growing season ET requirements for Nebraska crops. Crop planting times and water use patterns are somewhat different among the crops listed. Alfalfa is harvested 3-4 times each season and is unique since it is always in the vegetative stage. Winter wheat requires 2-3 inches of water from emergence to dormancy. Differences in water use among corn, sorghum and soybeans are mainly due to planting time and days to maturity.