# 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-2-hydrology/chapter-2-2-evapotranspiration/2-2.2-potential-evapotranspiration/2-2.2.2-priestley-taylor-method.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.