Evapotranspiration, commonly abbreviated ET, is the combined transfer of water from land and inland water surfaces to the atmosphere through evaporation and transpiration. It includes evaporation from soil, lakes, and wet vegetation, together with water absorbed by roots and subsequently released as vapor by plants. A major component of the water cycle, evapotranspiration connects terrestrial water availability with atmospheric moisture and surface energy exchange. (usgs.gov)
Components and physical controls
Evaporation converts liquid water into vapor and removes it from an exposed surface. Water retained on leaves after rainfall can evaporate without passing through plant tissues. Transpiration, by contrast, involves water uptake by roots, movement through the plant, and vapor release primarily through stomata. The two processes often occur simultaneously, making their separate measurement more difficult than measurement of their combined flux. (usgs.gov)
The energy needed for vaporization comes principally from solar radiation, with additional energy supplied by surrounding air and other heat exchanges. Evapotranspiration consumes latent heat, linking it to the Earth’s energy budget. Vapor transfer also depends on the difference in vapor concentration between an evaporating surface and the surrounding air, and on the efficiency with which air movement removes moisture. (fao.org)
The principal meteorological controls are radiation, air temperature, humidity, and wind speed. Surface controls include vegetation height, leaf area, ground cover, and water availability. A dry soil surface restricts evaporation; limited root-zone moisture can increase stomatal resistance and restrict transpiration. Consequently, atmospheric demand alone does not determine the actual flux. Crop development and management practices also change these controls over a growing season. (fao.org)
Actual, potential, and reference evapotranspiration
Actual evapotranspiration, often denoted AET or ETₐ, is the flux occurring under existing environmental and water-supply conditions. Potential evapotranspiration, or PET, describes evaporation and transpiration that could occur with adequate water supply under specified surface conditions. PET expresses evaporative demand, whereas AET also reflects limitations on available water. The surface assumptions used to calculate PET matter when comparing estimates. Both quantities are used in hydrological assessments. (pubs.usgs.gov)
Reference evapotranspiration, ET₀, provides a more explicitly standardized benchmark. In the Food and Agriculture Organization’s FAO-56 framework, its reference surface is hypothetical, well-watered grass 0.12 meters high, with surface resistance of 70 seconds per meter and albedo of 0.23. These fixed characteristics allow evaporative demand to be compared between locations and seasons without changing the reference vegetation. (fao.org)
The FAO Penman–Monteith equation calculates ET₀ from radiation, temperature, humidity, and wind measurements. It combines energy availability with aerodynamic transport and surface resistance. Reference ET is not a direct measurement of water use by a particular field, and actual crop ET can exceed or fall below the grass-reference value. (fao.org)
Units and estimation methods
ET is commonly expressed as an equivalent water depth over a specified period, such as millimeters per day or millimeters per year. One millimeter over one hectare represents 10 cubic meters of water. Expressing ET as depth allows comparison with rainfall, soil-water storage, and irrigation applications. It may also be expressed as an energy flux when studying surface heat exchange. (fao.org)
Several complementary approaches are used:
- Lysimeters determine water loss from an isolated soil volume, often by weighing it while accounting for water inputs and drainage.
- Water-balance methods estimate ET from measured inputs, outputs, and changes in water storage. Their accuracy depends on adequately accounting for the other balance components.
- Eddy covariance measures turbulent water-vapor transport using rapid observations of vertical wind velocity and vapor fluctuations above a surface.
- Meteorological models calculate ET from weather observations and assumptions about surface properties.
- Remote sensing supports spatial estimates through satellite observations and models; ground-based flux measurements provide benchmarks for their evaluation. (fao.org)
These methods represent different spatial areas and time intervals. Satellite estimates and flux-tower observations therefore require careful comparison rather than being treated as interchangeable measurements. In constructing benchmark datasets, researchers assess measurement quality, energy-balance closure, and the land area represented by each tower. (usgs.gov)
Agricultural applications
Under standard, non-stressed conditions, crop evapotranspiration is commonly estimated as
[ ET_c=K_cET_0, ]
where (K_c) is a crop coefficient. It represents differences between the crop and reference surface, including canopy structure and growth stage. A single coefficient combines soil evaporation and plant transpiration; a dual-coefficient approach treats them separately. (fao.org)
With soil-water stress included, the dual approach becomes
[ ET_{c,\mathrm{adj}}=(K_sK_{cb}+K_e)ET_0. ]
Here (K_{cb}) is the basal crop coefficient, (K_e) describes soil evaporation, and (K_s) reduces transpiration when root-zone water becomes limiting. Applying the stress factor only to transpiration preserves the distinction between plant water stress and evaporation following a wetting event. ET estimates are used in irrigation scheduling and system design, but ET is not itself the total volume that must be delivered to a field. (fao.org)
Hydrological significance
Within a drainage basin, evapotranspiration is a major pathway returning precipitation to the atmosphere. Water-balance models represent it alongside runoff, soil moisture, and other storage changes. Distinguishing atmospheric demand from actual water loss is important when assessing water availability under different climates or projected climate change. (water.usgs.gov)
Evapotranspiration can also draw on groundwater. Where roots reach the saturated zone or obtain water supplied from a shallow water table, transpiration removes water from groundwater storage and can lower the water table locally. Its source therefore depends on vegetation, rooting depth, and the relationship between soil moisture and groundwater. (usgs.gov)