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.