The combined process of water evaporation from soil and plant surfaces and transpiration from vegetation. It is the largest component of the water budget in most catchments.
Evapotranspiration (ET) is the process by which water is transferred from the land surface to the atmosphere through the combined mechanisms of evaporation from soil, water bodies, and plant surfaces, and transpiration through plant stomata. It is typically the second-largest component of the hydrologic cycle after precipitation, accounting for roughly 60-70% of precipitation in many regions. ET is driven by solar radiation, temperature, humidity, wind speed, and vegetation characteristics. Reference evapotranspiration (ET0) represents the ET rate from a standardized reference surface and is calculated using methods such as the Penman-Monteith equation recommended by the FAO. Actual evapotranspiration (ETa) depends on soil moisture availability and crop coefficients. Measurement techniques include lysimeters, eddy covariance systems, Bowen ratio stations, and remote sensing-based energy balance models such as SEBAL and METRIC. Accurate ET estimation is critical for irrigation scheduling, drought monitoring, water resource planning, and climate change impact assessment.