Potential Evapotranspiration, Priestley-Taylor Model and the Decoupling Coefficient

As shown by Priestley and Taylor (1972), the Priestley-Taylor equation provides robust estimates of potential evapotranspiration in the absence of advection from a dense well-watered canopy or a free-water surface in the absence of advection. Evaporation from such a wet surface is representative of the condition when the surface resistance r

_{s}=0. Defining this rate as LE

_{0}, it follows that the Priestley-Taylor rate

LE

_{PT}≈LE_{0}Eqn 1If this were not true, the Priestley-Taylor model would not perform as well as it did for free-water or ocean surfaces (in the absences of advection), as shown by Priestley and Taylor (1972). Further, since the surface-atmosphere decoupling coefficient

Ω=LE/LE

_{0}Eqn 2(Monteith 1965), it follows that

Ω=LE/LE

_{PT}Eqn 3This should be valid for non-advective conditions, and/or when the influence of air dryness (i.e. VPD) varies in proportion to net radiation. The Penman equation or the Penman-Monteith equation with r

_{s}=0 should provide a more accurate estimate of LE

_{0}we=hen such conditions are not met. Considering that the Priestley-Taylor model is given by

LE

_{PT}=α_{PT}LE_{eq}Eqn 4it follows from Eqn 3 and 4 that

Ω=1/α

_{PT}*LE/LE_{eq}Eqn 5_{}Therefore, Ω and α

_{PT}are inversely related, and when LE=LE

_{eq}

Ω

as shown by Pereira (2004). _{eq}=α_{PT}^{-1}Eqn 6References

Monteith, J.L., 1965. Evaporation and environment. Symp. Soc. Exptl. Biol. 19, 205–234.

Pereira, A.R., 2004. The Priestley-Taylor parameter and the decoupling factor for estimating reference evapotranspiration. Agric. For. Meteorol. 125 (3-4),

305-313.

Priestley, C.H.B., Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon.Weather Rev. 100 (2), 81–92.