Often, scientists distinguish between two different aspects of evapotranspiration: potential evapotranspiration and actual evapotranspiration.
Potential evapotranspiration or PE is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control on water supply. Actual evapotranspiration or AE is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration.
Scientists consider these two types of evapotranspiration for the practical purpose of water resource management. Around the world humans are involved in the production of a variety of plant crops. Many of these crops grow in environments that are naturally short of water. As a result, irrigation is used to supplement the crop's water needs. Managers of these crops can determine how much supplemental water is needed to achieve maximum productivity by estimating potential and actual evapotranspiration. Estimates of these values are then used in the following equation:
crop water need = potential evapotranspiration - actual evapotranspiration
The following factors are extremely important in estimating potential evapotranspiration:
- Potential evapotranspiration requires energy for the evaporation process. The major source of this energy is from the Sun. The amount of energy received from the Sun accounts for 80% of the variation in potential evapotranspiration.
- Wind is the second most important factor influencing potential evapotranspiration. Wind enables water molecules to be removed from the ground surface by a process known as eddy diffusion.
- The rate of evapotranspiration is associated to the gradient of vapor pressure between the ground surface and the layer of atmosphere receiving the evaporated water.