Evapotranspiration (ET) combines two processes: direct evaporation of water from wet surfaces (soil, water bodies, wet vegetation) and plant transpiration, whereby plants release water vapour through leaf stomata during photosynthesis. Together, these processes constitute the primary pathway for water returning from the land surface to the atmosphere within the water cycle. Two key concepts are distinguished: potential ET (PET), the maximum amount of water that would evapotranspire if water availability were unlimited, and actual ET (AET), limited by the moisture effectively available in the soil. PET depends on solar radiation, temperature, air humidity, wind speed, and vegetation type. It is calculated using formulas such as Penman-Monteith (FAO standard) or Thornthwaite. Evapotranspiration is a fundamental parameter for water resource management and agriculture worldwide. In Mediterranean climates, PET can exceed 1,400 mm annually while precipitation barely reaches 500–600 mm, explaining the need for irrigation. Weather models include ET as a key variable for predicting soil moisture, drought risk, and water availability for crops.