The original Hargreaves (HG) method examined against standard Penman-Monteith (PM) method for monthly calculation of ETo. The HGorg method indicated underestimation of ETo by 35.02% for cold semi-arid conditions of Quetta and overestimation of ETo by 15.82% at hot semi-arid conditions of Zhob as shown in Fig.1 (a) and 2 (a) and also in Tables 2 and 3. Therefore, the HGorg method is not suggested for execution in cold and hot semi-arid conditions of Quetta and Zhob stations, respectively without being modified.
Modification of Hargreaves (HG) method
The modification to the Hargreaves (HG) method is by determining value of constant term that lessen the RMSE and Percentage error between ETo value calculated by standard Penman-Monteith (PM) method. The resultant modified HG (HGmod) equation arrangements are given below
Modification at Quetta station
ETo HG mod = 0.0031 Ra (Tmax – T min) 1/2 T + 17.8 (6)
Modification at Zhob station
ETo HG mod = 0.0019 Ra (Tmax – T min) 1/2 T + 17.8 (7)
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.