Assessment of Hargreaves and Blaney-Criddle Methods to Estimate Reference Evapotranspiration Under Coastal Conditions

There are a lot of weather stations of the world like Pakistan where all the metrological parameters are not accessible to evaluate PM method for the approximation of ETo. So ablatives methods like Hargreaves and Blaney-Criddle methods are used which required small numbers of metrological parameters. A study has been directed to assess the accuracy of Hargreaves and Blaney-Criddle methods against standard PM method for approximation of reference evapotranspiration (ETo) in coastal arid locations of Sindh and Baluchistan. The statistical evaluation of the study indicated that the Hargreaves method underestimated ETo by 44.61%, 30.86%, 27.03% and 37.8% at Karachi, Gawadar, Jiwani and Pasni, respectively. The Blaney-Criddle method underestimated ETo in winter and overestimated ETo in summer by 14.91% at Karachi station, overestimated ETo by 23.98% at Gawadar station, underestimated ETo in January and December and overestimated ETo in remaining months by 22.38% at Jiwani station and overestimated ETo by 27.25% at Pasni station. The difference of variations of Hargreaves (HG) method with PM method with RMSE were 2.03 mm/day, 1.46 mm/day, 1.21 mm/day and 1.65 mm/day at Karachi, Gawadar, Jiwani and Pasni stations, respectively. The difference of variations of BC method with PM method with RMSE were 1.98 mm/day, 2.42 mm/day, 2.16 mm/day and 2.89 mm/day at Karachi, Gawadar, Jiwani and Pasni stations, respectively. The R 2 with Hargreaves method were 0.95, 0.92, 0.89 and 0.97 at Karachi, Gawadar, Jiwani and Pasni, respectively. The R 2 with BC method were 0.59, 0.78, 0.86 and 0.78 at Karachi, Gawadar, Jiwani and Pasni stations, respectively.