Many researcher have done research to modify the Hargreaves method by altering the constant value (Bachour et al., 2013), (Berti et al., 2014) and (Majeed et al., 2017). The ETo estimated by the modified Hargreaves (HG) method was again examined against the ETo calculated by the Penman-Monteith (PM) method. The monthly ETo was improved by the modified Hargreaves (HGmod) method. There was an improvement in RMSE and percentage error was also reduced as shown in the Table 1 and 2. The calculation of ETo from that modified method reduced the percentage error up to 0.87% and 1.89% with RMSE of 0.29 mm/day and 0.27 mm/day at Quetta and Zhob, respectively as shown in the Fig. 1 (b) and 2 (b) and also in the Tables 2 and 3.
The comparison of ETo by HG org and standard Penman-Monteith (PM) method showed that the original Hargreaves (HG org) method overestimated ETo in hot semi-arid environment of Zhob and underestimated ETo in cold semi-arid environment of Quetta. The modified Hargreaves (HG mod) showed better estimation of ETo at both the stations. So, it is recommended that before using original Hargreaves method it must be modified according to the regional environment.