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Assessment of Reference Evapotranspiration by the Hargreaves Method in Southern Punjab Pakistan
Authors: Adnan Majeed, Sajid Mehmood, Kaleem Sarwar, Ghulam Nabi, M Ashiq Kharal
Number of views: 256
Several methods are available for estimatingreference evapotranspiration (ETo) which requiremany weathervariablesthatare notalways availableatallweatherstations specially in developing countries.TheHargreaves equation(HG)requiresonly daily air temperature data and extraterrestrial radiation for ETo estimates. The HG method oftentendstooverestimateorunderestimateETo, so it become necessary before using HG method it must be calibrated according to local conditions.The HG equationwasevaluated undersemiarid conditionsby using15, 10 and 9-years ofcomplete daily climaticdatafromtheBahawalpur, Bahawalnagar and Khanpurweatherstations of Southern Punjab, Pakistan, respectively.TheHG methodwascomparedtoFAO56PenmanMonteithequation(PM).TheoriginalHG equation overestimated for all time steps of three weather stations. TheoriginalHG equation overestimated ETo by14,12,and8 % fordaily,10-daily (decade),and monthly ETo for Bahawalpur and by 8, 6 and 5 % of Bahawalnagar and by 25, 10 and 8% of Khanpur stations, respectively, as compared to PM equation. A simple mathematical logic applied to obtain the modified HG coefficients for all time steps showed that the modified HG equation improved the results of ETo estimation to 4, 3, and 2% for daily, 10-daily and monthly time steps of Bahawalpur station 4, 2.5 and 2% of Ba-hawalnagar station and 6, 3.5 and 3% of Khanpur station, difference from ETo computed by the PM method, with root mean square error (RMSE) of 1.411, 1.105 and 0.950 mm d-1 for daily, 10-daily, and monthly ET of Bahawal-pur and 1.800, 1.121 and 0.865 mm d-1 of Bahawalnagar and 1.13, 0.971 and 0.595 mm d-1 of Khanpur stations, respectively. For more accuracy, further improvement in modified HG method achieved by adding the wind speed, reaching an average difference of 1% for all timescales. The original HG equation overestimated for all time steps before modification.