Editing Potential evapotranspiration (section)

=== Priestley–Taylor equation ===
The '''Priestley–Taylor equation''' was developed as a substitute to the Penman–Monteith equation to remove dependence on observations. For Priestley–Taylor, only radiation (irradiance) observations are required. This is done by removing the aerodynamic terms from the Penman–Monteith equation and adding an empirically derived constant factor, <Math>\alpha</Math>.

The underlying concept behind the Priestley–Taylor model is that an air mass moving above a vegetated area with abundant water would become saturated with water. In these conditions, the actual evapotranspiration would match the Penman rate of potential evapotranspiration. However, observations revealed that actual evaporation was 1.26 times greater than potential evaporation, and therefore the equation for actual evaporation was found by taking potential evapotranspiration and multiplying it by <Math>\alpha</Math>. 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 <Math>\alpha</Math> values.<ref>{{cite book |editor=M. E. Jensen, R. D. Burman & R. G. Allen |year=1990 |title=Evapotranspiration and Irrigation Water Requirement |series=ASCE Manuals and Reports on Engineering Practices |volume=70 |publisher=[[American Society of Civil Engineers]] |location=New York, NY |isbn=978-0-87262-763-5}}</ref>

The assumption that an air mass moving over a vegetated surface with abundant water saturates has been questioned later. The lowest and turbulent part of the atmosphere, the [[atmospheric boundary layer]], is not a closed box, but constantly brings in dry air from higher up in the atmosphere towards the surface. As water evaporates more easily into a dry atmosphere, evapotranspiration is enhanced. This explains the larger than unity value of the Priestley-Taylor parameter <Math>\alpha</Math>. The proper equilibrium of the system has been derived and involves the characteristics of the interface of the atmospheric boundary layer and the overlying free atmosphere.<ref>{{cite journal|last1=Culf|first1=A.|title=Equilibrium evaporation beneath a growing convective boundary layer|journal=Boundary-Layer Meteorology|date=1994|volume=70|issue=1–2|pages=34–49|doi=10.1007/BF00712522|bibcode=1994BoLMe..70...37C}}</ref><ref>{{cite journal|last1=van Heerwaarden|first1=C. C.|title=Interactions between dry-air entrainment, surface evaporation and convective boundary layer development|journal=Quarterly Journal of the Royal Meteorological Society|date=2009|volume=135|issue=642|pages=1277–1291|doi=10.1002/qj.431|display-authors=etal|bibcode=2009QJRMS.135.1277V}}</ref>