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.

Variable Name	Definition	File Name
IPET	Potential evapotranspiration method	.bsn
WND_SP	$u_z$: Daily wind speed (m/s)	.wnd
CO2	$CO_2$: Carbon dioxide concentration (ppmv)	.sub
MAX TEMP	$T_{mx}$: Daily maximum temperature (°C)	.tmp
MIN TEMP	$T_{mn}$: Daily minimum temperature (°C)	.tmp
GSI	$g_{\Box,mx}$: maximum leaf conductance (m s$^{-1}$)	crop.dat
FRGMAX	$fr_{g,mx}$: Fraction of maximum leaf conductance achieved at the vapor pressure deficit specified by $vpd_{fr}$	crop.dat
VPDFR	$vpd_{fr}$: Vapor pressure deficit corresponding to value given for $fr_{g,mx}$(kPa)	crop.dat