Potential evapotranspiration (PET) or potential evaporation (PE) is the amount of water that would be 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 actual evapotranspiration (AET) 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 (ET0). 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 0.5 m (1.6 ft) in height, rather than the general short green grass reference, due to the higher value of ET from the alfalfa reference. 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.
Measurements Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for lakes and oceans), the soil type for bare soil, and also the density and diversity of vegetation. Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally on short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying by a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and actual precipitation is used in irrigation scheduling. Average annual potential evapotranspiration is often compared to average annual precipitation, the symbol for which is P. The ratio of the two, P/PET, is the aridity index. A humid subtropical climate is a zone of climate with hot and humid summers, and cold to mild winters. Subarctic regions, between 50°N and 70°N latitude, have short, mild summers and freezing winters depending on local climates. Precipitation and evapotranspiration is low (compared to warmer variants), and vegetation is characteristic of the coniferous/taiga forest.
Estimates of potential evaporation
Thornthwaite equation (1948)
P E T = 16 ( L 12 ) ( N 30 ) ( 10 T d I ) α {\displaystyle PET=16\left({\frac {L}{12}}\right)\left({\frac {N}{30}}\right)\left({\frac {10T_{d}}{I}}\right)^{\alpha }}
Where
P E T {\displaystyle PET} is the estimated potential evapotranspiration (mm/month)
T d {\displaystyle T_{d}} is the average daily temperature (degrees Celsius; if this is negative, use 0 {\displaystyle 0} ) of the month being calculated
N {\displaystyle N} is the number of days in the month being calculated
L {\displaystyle L} is the average day length (hours) of the month being calculated
α = ( 6.75 × 10 − 7 ) I 3 − ( 7.71 × 10 − 5 ) I 2 + ( 1.792 × 10 − 2 ) I + 0.49239 {\displaystyle \alpha =(6.75\times 10^{-7})I^{3}-(7.71\times 10^{-5})I^{2}+(1.792\times 10^{-2})I+0.49239}
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