Editing Potential evapotranspiration (section)

{{Short description|Amount of evaporation}}
[[File:Potential Evaporation in North America Through 2100.ogv|thumb|300px|This animation shows the projected increase in potential evaporation in North America through the year 2100, relative to 1980, based on the combined results of multiple [[Climate model|climate models]].]]

'''Potential evapotranspiration''' ('''PET''') or '''potential evaporation''' ('''PE''') is the amount of water that would be [[Evapotranspiration|evaporated and transpired]] by a specific [[crop]], [[soil]] or [[ecosystem]] if there was sufficient water available. It is a reflection of the energy available to evaporate or transpire water, and of the wind available to transport the [[water vapor]] from the ground up into the lower atmosphere and away from the initial location. Potential evapotranspiration is expressed in terms of a depth of water or soil moisture percentage.

If the [[evapotranspiration|actual evapotranspiration]] is considered the net result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then PET is a measure of the demand side (also called ''evaporative demand''). Surface and air temperatures, [[insolation]], and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual [[precipitation]].

Often a value for the potential evapotranspiration is calculated at a nearby climatic station on a reference surface, conventionally on land dominated by short grass (though this may differ from station to station). This value is called the reference evapotranspiration (ET<sub>0</sub>). Actual evapotranspiration is said to equal potential evapotranspiration when there is ample water present. Evapotranspiration can never be greater than potential evapotranspiration, but can be lower if there is not enough water to be evaporated or plants are unable to transpire maturely and readily. Some US states utilize a full cover alfalfa reference crop that is {{Convert|0.5|m|ft|abbr=on}} in height, rather than the general short green grass reference, due to the higher value of ET from the [[alfalfa]] reference.<ref name="extension.uidaho.edu">{{cite web |title=Kimberly Research and Extension Center |url=http://extension.uidaho.edu/kimberly/files/2013/02/ascestzdetmain2005.pdf |url-status=dead |archive-url=https://web.archive.org/web/20160304080954/http://extension.uidaho.edu/kimberly/files/2013/02/ascestzdetmain2005.pdf |archive-date=4 March 2016 |access-date=4 May 2018 |website=extension.uidaho.edu}}</ref> 

Potential evapotranspiration is higher in the summer, on clearer and less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy (heat) for evaporation. Potential evapotranspiration is also higher on windy days because the evaporated moisture can be quickly moved from the ground or plant surface before it precipitates, allowing more evaporation to fill its place.