Performance Evaluation of Reference Evapotranspiration Equations Under Semiarid Pakistani Conditions

Abstract

Five ETo estimation methods viz. Hargreaves (HAR), modified Hargreaves (MHG), FAO-Radiation (RAD), reduced-set Penman–Monteith (RPM), combination method FAO–Penman with wind functions of Watts & Hancock (WH at Lahore, RAD method under-estimated ETo by 27 %, and at Bahawalpur HAR by 6 %. Overestimation ranges observed were: WH overestimation at D.I. Khan was 1 % against Jhelum where it was 0 %. This aspect highlighted the variant nature of ETo methods in terms of daily ETo estimates at different locations. At Sialkot, D.I. Khan and Khanpur, RPM method showed excellent performance with top rank; and being ahead of W&H combination method at those stations indicated its effectiveness that how a less data requiring method, if calibrated, could perform even better than a physically based combination method. The RPM method is useful where some of the climatic data are missing, and should be preferred to an alternative ETo calculation procedure, requiring only limited meteorological parameters. To be consistent with FAO recommendations, the global use of P_M method is upheld if the required climatic data are available.