Potential evapotranspiration (PET) refers to the amount of water that could evaporate from soil and transpire from plants if sufficient water were available. It represents the atmospheric demand for moisture, influenced by meteorological factors like temperature, humidity, wind speed, and solar radiation, rather than soil moisture. As a theoretical concept, PET assesses the potential for water loss from surfaces like soil or vegetation. When actual evapotranspiration (AET) falls below PET, it typically indicates water stress, as plants and the environment do not have enough water to meet atmospheric demand.

PET is crucial for assessing crop water requirements and irrigation needs, particularly in water-scarce regions. It is also used in climate classifications, such as UNESCO’s classification of Arid Zones, and in methods for calculating the growing period.

When full meteorological data are available (temperature, radiation, relative humidity, and wind speed), the Penman-Monteith method is recommended for calculating PET. This equation provides a more accurate estimate for specific crops or surfaces, making it ideal for agricultural applications and climate modeling.

The Hargreaves method is a popular alternative when data are limited, requiring only mean air temperature, daily temperature range (difference between maximum and minimum), and extraterrestrial radiation (varied by latitude and time of year). PET calculated using Hargreaves shows a strong correlation with Penman data, making it suitable for large-scale hydrological and climatological studies that don’t require daily data.

Other PET calculation methods exist, though their limitations make them less relevant for irrigation planning or research. Detailed procedures for different PET methods can be found in this training document.