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).

Table 2:2-2: SWAT+ input variables used in potential evapotranspiration calculations summarized in this section.

Definition	Source Name	Input Name	Input File
$PET$: Potential evapotranspiration method	pet	pet	codes.bsn
$u_z$: Wind speed (m/s)	windav	wind_ave	weather-wgn.cli
$CO_2$​: Carbon dioxide concentration (ppm)	co2	co2	parameters.bsn
$T_{mx}$​: Daily maximum temperature (°C)	tmpmax	tmp_max_ave	weather-wgn.cli
$T_{mn}$​: Daily minimum temperature (°C)	tmpmin	tmp_min_ave	weather-wgn.cli
$g_{l,mx}$​: maximum leaf conductance (m s−1−1)	gsi	stcon_max	plants.plt
$g_l$: Fraction of maximum leaf conductance achieved at the vapor pressure deficit specified by $vpd$	gmaxfr	frac_stcon	plants.plt
$vpd$: Vapor pressure deficit corresponding to value given for $g_{l,mx}$ (kPa)	vpdfr	vpd	plants.plt