Evapotranspiration (
ET) is the sum of
evaporation and
plant transpiration from the Earth's land surface to
atmosphere. Evaporation accounts for the movement of water to the air from sources such as the
soil,
canopy interception, and
waterbodies. Transpiration accounts for the movement of water within a
plant and the subsequent loss of water as vapor through
stomata in its
leaves. Evapotranspiration is an important part of the
water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an
evapotranspirator.
Potential evapotranspiration (
PET) is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop, completely shading the ground, of uniform height and with adequate water status in the soil profile. It is a reflection of the
energy available to evaporate water, and of the
wind available to transport the water vapour from the ground up into the lower
atmosphere. Actual evapotranspiration is said to equal potential evapotranspiration when there is ample water.