2.3.3. *ET*<sup>0</sup> Calculation Model

The Hargreaves model, which can reveal the physics of the evaporative process, was used for calculating monthly *ET*<sup>0</sup> based on a future climate. It has been widely demonstrated to be able to provide reliable estimations [31,46,47]. The Food and Agriculture Organization of the United Nations (FAO) [4] also suggests it as the simplified standard algorithm for *ET*<sup>0</sup> under the missing meteorological observations as required by the Penman– Monteith formula. Several improvements were made to the original equation [48,49]. The form used in this study was published in 1985 [50] and expressed as follows:

$$ET\_{0(Har)} = 0.0023 R\_a (T\_{max} - T\_{min})^{0.5} (T\_{mean} + 17.8) \tag{2}$$

where *ET*0(*Har*) is the *ET*<sup>0</sup> calculated by the Hargreaves empirical formula (mm/d); *Tmax* and *Tmin* are the mean maximum and minimum temperatures for the calculation time interval (◦C), respectively; *Tmean* is the mean temperature for the calculation time interval ( ◦C); and *Ra* is the zenith radiation (mm/d); the applicable time scale is 30 or 10 days.
