ABSTRACT
Several methods are available for estimating reference evapotranspiration (ETo) which require many weather variables that are not always available at all weather stations specially in developing countries.The Hargreaves equation(HG)requires only daily air temperature data and extraterrestrial radiation for ETo estimates. The HG method often tends to overestimate or underestimate ETo, so it become necessary before using HG method it must be calibrated according to local conditions.The HG equation was evaluated under semi-arid conditions by using15, 10 and 9-years of complete daily climatic data from the Bahawalpur, Bahawalnagar and Khanpur weather stations of Southern Punjab, Pakistan, respectively.The HG method was compared to FAO56 Penman Monteith equation(PM). The original HG equation overestimated for all time steps of three weather stations. The original HG equation overestimated ETo by14,12,and8 % for daily,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 Bahawalnagar 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 Bahawalpur 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.
