Evapotranspiration is defined as “the total quantity of water used by the vegetation growth of a given area in transpiration and building of a plant tissue, and that evaporated from the adjacent soil in the area in any specified time.” It therefore includes the water removed from the soil by transpiration and evaporation. Transpiration is the process in which the water that enters the plant roots and is used in building plant tissue finally passes into the atmosphere in the vaporous from through the leaves of the plants.
Evaporation is the process in which water from the adjacent soil passes into the atmosphere in the vaporous form. For a given set of conditions, Evapotranspiration obviously depends on the availability of water. If sufficient moisture is always available to completely meet the needs of the plants the resulting Evapotranspiration is called potential Evapotranspiration (PET). Potential Evapotranspiration no longer critically depends on soil and plant factors but depends essentially on climatic factors. The real Evapotranspiration occurring in a specified situation is called actual Evapotranspiration (AET). At the field capacity, since the water supply to the plants is adequate AET will be equal to PET, so that the ratio (AET/PET) is equal to 1. With the reduction in the available moisture (as the soil dries out) the ratio (AET/PET) would be less than unity. The decrease in the ratio (AET/PET) with available moisture depends upon the type of soil and the rate of drying.
Various Methods of Evapotranspiration Project Report