While transpiration takes place, the land area in which plants are grown also lose moisture by evaporation of water from soil and water bodies. In hydrology and irrigation, it is normal practice to considered these two under one head as evapotranspiration. The term consumptive use is also used to denote this loss. For a given set of atmospheric conditions, it depends on the availability of water.
Potential Evapotranspiration (PET)
If sufficient moisture is always available to completely meet the needs of the vegetation fully covering the area, then the evapotranspiration occurs after this will be the Potential.
As the moisture present is sufficient for the plant needs, so the PET does not depend on soil and plant factors. It depends on climatic factors.
Actual Evapotranspiration (AET)
The real evapotranspiration occurring in a specific situation is called actual (AET).