Evapotranspiration (ET) is the combined process by which water is transferred from the earth's surface to the atmosphere. It includes evaporation of liquid or solid water from soil and plant surfaces plus transpiration of liquid water through plant tissues expressed as the latent heat transfer per unit area or its equivalent depth of water per unit area. Potential evapotranspiration (PET or ETp) is the rate at which water, if available, would be removed from the soil and plant surface expressed as the latent heat transfer per unit area or its equivalent depth of water per unit area. It is important to consider the relationship between the ET of a specific crop at a specific time in its growth stage for determining irrigation need and PET is important as ET estimates are often made based on PET.

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