Potential evapotranspiration
From Freepedia
Potential evapotranspiration, PET, is a measure of atmospheric demand for water vapour from evaporation and transpiration. This demand incorporates the energy available for evaporation and the ability of the lower atmosphere to transport evaporated moisture away from the land surface. PET is expressed in terms of a depth of water, an amount that would evaporate if there were abundant water to do so.
PET is higher in the summer, on less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy for evaporation. PET is also higher on windy days because the evaporated moisture can be quickly moved from the ground of plants, allowing more evaporation to fill its place.
Evapotranspiration can never be greater than PET, but can be lower if there is not enough water to be evaporated or plants are unable to readily transpire.
Average annual PET is often compared to average annal precipitation, P. The ratio of the two, P/PET, is the aridity index.
