evapotranspiration

The release of water vapour from the earth's surface by evaporation and transpiration. Transpiration is the biological process whereby plants lose water vapour, mainly through pores in their leaves. This water is usually replaced by a continuous flow of water moving upwards from the roots. Rates of evapotranspiration vary with factors such as the temperature and humidity of the air, wind speed, plant type and the nature of the land surface. Since evapotranspiration is so variable, physical geographers prefer to use the concept of potential evapotranspiration (PE). This is the greatest amount of water vapour which could be diffused into the atmosphere given unlimited supplies of water.

Lysimeters may be used to measure PE, and various formulae have been devised to predict it.