Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired if sufficient water were available. It represents the atmospheric demand for water and is a key parameter in water balance calculations.
Potential evapotranspiration (PET) represents the maximum rate of evapotranspiration that would occur from a large, uniform area of actively growing green vegetation with an unlimited water supply. It quantifies the atmospheric evaporative demand driven by solar radiation, temperature, humidity, and wind speed. PET is a theoretical upper limit that is rarely achieved in practice, as actual evapotranspiration (AET) is typically lower due to limited soil moisture or plant stress. Several methods exist for estimating PET, ranging from temperature-based approaches such as the Thornthwaite method and the Hargreaves equation to more physically based methods such as the Penman-Monteith equation, which is recommended by the Food and Agriculture Organization (FAO) as the standard reference method. PET is a critical input for water balance studies, irrigation scheduling, crop water requirement estimation, drought assessment, and watershed modeling. In arid and semi-arid regions, PET greatly exceeds precipitation, resulting in water deficit conditions. The ratio of PET to precipitation is used in climate classification systems and aridity indices. Climate change is expected to increase PET in most regions through higher temperatures, though changes in humidity, wind, and cloud cover may partially offset temperature effects.