Evapotranspiration refers to the vaporization of water from both non-living and living surfaces on a landscape. Evapotranspiration is a composite of two words: evaporation and transpiration. Evaporation refers to the vaporization of water from surface waters such as lakes, rivers, and streams, from moist soil and rocks, and any other substrates that are non-living. Transpiration refers to the vaporization of water from any moist living surface, such as plant foliage, and the body or lung surfaces of animals.
Evapotranspiration is an important component of Earth's energy budget, accounting for a substantial part of the physical dissipation of absorbed solar radiation. In any moist ecosystem, air and surface temperatures would be much warmer than they actually are, if evapotranspiration was not operating to consume thermal energy during the vaporization of water. Because of vigorous transpiration of water in moist forests during the day, air in and beneath the canopy is considerably cooler than it would be if this process did not occur.
Evapotranspiration from forests has a large influence on the water budget, both globally and locally. In the absence of forest transpiration, an equivalent quantity of water would have to leave the watershed as seepage to groundwater and streamflow. In temperate forests, evapotranspiration typically accounts for 10-40% of the total input of water by precipitation. However, there is a distinct seasonality in the patterns of evapotranspiration from forests. In a temperate region, evapotranspiration is largest during the growing season, when air temperatures are highest and the amount of plant foliage is at its annual maximum. During this season, evapotranspiration rates are often larger than the inputs of water by rainfall, and as a result groundwater is depleted, and sometimes the surface flow of forest streams disappears. Evapotranspiration is very small during the winter because of low temperatures, although some direct vaporization of water from snow or ice to vapor does occur, a process known as sublimation. Evapotranspiration is also relatively small during the springtime, because deciduous trees do not yet have foliage, and air temperatures are relatively cool.
The disturbance of forests disrupts their capacity to sustain evapotranspiration, by reducing the amount of foliage in the canopy. Consequently, clearcutting and wildfire often lead to an increase in streamflow, and sometimes to an increase in height of the water table. In general, the increase in streamflow is roughly proportional to the fraction of total foliage that is removed. In the first year after clear-cutting an entire watershed, the increase in stream-flow can be as large as 40%, but most of this effect rapidly diminishes as vegetation re-grows after the disturbance.
See also Energy budgets.