Potential evapotranspiration
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.
Potential evapotranspiration is expressed in terms of a depth of water or soil moisture percentage.
Surface and air temperatures, insolation, and wind all affect this.
[1] 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.
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.
The difference between potential evapotranspiration and actual precipitation is used in irrigation scheduling.
Subarctic regions, between 50°N[2] 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.
is the average daily temperature (degrees Celsius; if this is negative, use
[3] Somewhat modified forms of this equation appear in later publications (1955 and 1957) by C. W. Thornthwaite and Mather.
Penman's equation requires daily mean temperature, wind speed, air pressure, and solar radiation to predict E. Simpler Hydrometeorological equations continue to be used where obtaining such data is impractical, to give comparable results within specific contexts, e.g. humid vs arid climates.
The Penman–Monteith equation refines weather based evapotranspiration (ET) estimates of vegetated land areas.
This equation was then derived by FAO for retrieving the potential evapotranspiration ET0.
[5] It is widely regarded as one of the most accurate models, in terms of estimates.
This is done by removing the aerodynamic terms from the Penman–Monteith equation and adding an empirically derived constant factor,
The assumption here is for vegetation with an abundant water supply (i.e. the plants have low moisture stress).
Areas like arid regions with high moisture stress are estimated to have higher
[6] The assumption that an air mass moving over a vegetated surface with abundant water saturates has been questioned later.
As water evaporates more easily into a dry atmosphere, evapotranspiration is enhanced.
This explains the larger than unity value of the Priestley-Taylor parameter
