evapotranspiration

Evapotranspiration is the combined flux of water from land surfaces to the atmosphere, arising from evaporation of water from soil, surface water, and plant transpiration. It is a major component of the hydrological cycle and a key measure of landscape water use. Evapotranspiration is typically expressed as depth over a period, such as millimeters per day or per month.

ET depends on energy supply, climatic conditions, and soil and vegetation characteristics. It includes potential evapotranspiration, representing the amount of water that would be lost with ample soil moisture, and actual evapotranspiration, which is reduced when soil moisture or plant water availability is limiting. Major drivers are solar radiation, temperature, relative humidity, wind speed, and soil moisture status. Vegetation type, canopy structure, and stomatal behavior also influence ET.

Measurements and estimation methods vary. Direct methods include lysimeters, eddy covariance, and soil moisture balance approaches; remote sensing can provide spatially distributed estimates. The FAO-56 Penman–Monteith equation is widely used to estimate reference evapotranspiration (ET0). Inputs typically include net radiation, soil heat flux, air temperature, humidity, and wind speed. Crop evapotranspiration is commonly computed as ETc = ET0 × Kc, where Kc is a crop coefficient that changes with crop type and growth stage.

Applications and significance include irrigation planning, agricultural water management, drought assessment, and hydrological and climate studies. ET links meteorological conditions to plant water use and informs water resource design and management. Uncertainties arise from input data quality, model assumptions, and spatial-temporal variability in soil moisture and vegetation.