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# 2:2.2.2 Priestley-Taylor Method

Priestley and Taylor (1972) developed a simplified version of the combination equation for use when surface areas are wet. The aerodynamic component was removed and the energy component was multiplied by a coefficient, $$\alpha\_{pet}$$ = 1.28, when the general surroundings are wet or under humid conditions

$$\lambda E\_o=\alpha\_{pet}*\frac{\Delta}{\Delta+\gamma}*(H\_{net}-G)$$                                                                                                                  2:2.2.23

where $$\lambda$$ is the latent heat of vaporization (MJ kg$$^{-1}$$), $$E\_o$$ is the potential evapotranspiration (mm d$$^{-1}$$), $$\alpha\_{pet}$$ is a coefficient, $$\Delta$$ is the slope of the saturation vapor pressure-temperature curve, $$de/dT$$ (kPa ˚C$$^{-1}$$), $$\gamma$$ is the psychrometric constant (kPa ˚C$$^{-1}$$), $$H\_{net}$$ is the net radiation (MJ m$$^{-2}$$ d$$^{-1}$$), and $$G$$ is the heat flux density to the ground (MJ m$$^{-2}$$ d$$^{-1}$$).

The Priestley-Taylor equation provides potential evapotranspiration estimates for low advective conditions. In semiarid or arid areas where the advection component of the energy balance is significant, the Priestley-Taylor equation will underestimate potential evapotranspiration.