> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/chapter-2-2-evapotranspiration/2-2.2-potential-evapotranspiration/2-2.2.3-hargreaves-method.md).

# 2:2.2.3 Hargreaves Method

The Hargreaves method was originally derived from eight years of cool-season Alta fescue grass lysimeter data from Davis, California (Hargreaves, 1975). Several improvements were made to the original equation (Hargreaves and Samani, 1982 and 1985) and the form used in SWAT+ was published in 1985 (Hargreaves et al., 1985):

$$\lambda E\_o=0.0023*H\_0*(T\_{mx}-T\_{mn})^{0.5}\*(\overline T\_{av}+17.8)$$    2:2.2.24

where $$\lambda$$ is the latent heat of vaporization (MJ kg$$^{-1}$$), $$E\_o$$ is the potential evapotranspiration (mm d$$^{-1}$$), $$H\_0$$ is the extraterrestrial radiation (MJ m$$^{-2}$$ d$$^{-1}$$), $$T\_{mx}$$ is the maximum air temperature for a given day (°C), $$T\_{mn}$$ is the minimum air temperature for a given day (°C), and $$\overline T\_{av}$$is the mean air temperature for a given day (°C).


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