The Priestley–Taylor parameter and the decoupling factor for estimating reference evapotranspiration

Using two extensive data sets of daily lysimetric measurements of reference evapotranspiration (E0, mm day−1) under climatic conditions ranging from humid tropical (Piracicaba, SP, Brazil) to semi-arid (Davis, CA, USA) it is here presented an evidence that the Priestley–Taylor α parameter can be set equal to the inverse of the McNaughton–Jarvis decoupling factor Ω, that is, α=Ω−1. This approach does not require local calibrations and it allows for a variable α. Several statistical measures of the goodness of fit indicate that this simplification represents a good alternative to the theoretically more robust Penman–Monteith with the parameterization scheme of the FAO-56. Selected days with almost perfect match between FAO-56 estimates and observations, with E0 varying from 0.60 to 7.0 mm day−1, showed Ω values from 0.69 to 0.93. Ω decreased linearly as a function of the wind run. The Perrier–Katerji–Goose–Itier model for α did not work well for the present data sets. The main problem here is the determination of a critical resistance with daily averages of vapor pressure deficit.