# 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).

