# 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.

<table><thead><tr><th width="227">Definition</th><th>Source Name</th><th>Input Name</th><th>Input File</th></tr></thead><tbody><tr><td><span class="math">PET</span>: Potential evapotranspiration method</td><td>pet</td><td>pet</td><td><a href="../../../../introduction-1/basin-1/codes.bsn">codes.bsn</a></td></tr><tr><td><span class="math">u_z</span>: Wind speed (m/s)</td><td>windav</td><td>wind_ave</td><td><a href="../../../../introduction-1/climate/weather-wgn.cli">weather-wgn.cli</a></td></tr><tr><td><span class="math">CO_2</span>​: Carbon dioxide concentration (ppm)</td><td>co2</td><td>co2</td><td><a href="../../../../introduction-1/basin-1/parameters.bsn">parameters.bsn</a></td></tr><tr><td><span class="math">T_{mx}</span>​: Daily maximum temperature (°C)</td><td>tmpmax</td><td>tmp_max_ave</td><td><a href="../../../../introduction-1/climate/weather-wgn.cli">weather-wgn.cli</a></td></tr><tr><td><span class="math">T_{mn}</span>​: Daily minimum temperature (°C)</td><td>tmpmin</td><td>tmp_min_ave</td><td><a href="../../../../introduction-1/climate/weather-wgn.cli">weather-wgn.cli</a></td></tr><tr><td><span class="math">g_{l,mx}</span>​: maximum leaf conductance (m s−1−1) </td><td>gsi</td><td>stcon_max</td><td><a href="../../../../introduction-1/databases/plants.plt">plants.plt</a></td></tr><tr><td><span class="math">g_l</span>: Fraction of maximum leaf conductance achieved at the vapor pressure deficit specified by <span class="math">vpd</span> </td><td>gmaxfr</td><td>frac_stcon</td><td><a href="../../../../introduction-1/databases/plants.plt">plants.plt</a></td></tr><tr><td><span class="math">vpd</span>: Vapor pressure deficit corresponding to value given for <span class="math">g_{l,mx}</span> (kPa)</td><td>vpdfr</td><td>vpd</td><td><a href="../../../../introduction-1/databases/plants.plt">plants.plt</a></td></tr></tbody></table>