EVALUATION AND CALIBRATION OF METHODS ESTIMATING SOLAR RADIATION AND ITS EFFECT ON REFERENCE CROP EVAPOTRANSPIRATION

Abstract

radiation and their effect on reference crop evapotranspiration in Mosul area . Theclimatological data are presented for average 30 years 1971-2000 of Mosul station ,which include monthly average maximum and minimum temperatures , mean relativehumidity, wind speed , sunshine hours, and measured incoming solar radiation . Thesedata are used to calculate monthly solar radiation by Angstrom and Hargreavesequations and reference evapotranspiration ETo by Penman-Monteith equation. Theresults of estimated monthly solar radiation are listed , plotted and compared withmeasured solar radiation . The coefficient of determination (R2) and the Root MeanSquare Error (RMSE) are calculated . The values of R2 and RMSE are 0.998 and 3.6MJ m-2 day-1 for Angstrom method and are 0.996 and 4.2 MJ m-2 day-1 for Hargreavesmethod . The results showed that the method of estimate of solar radiation by theAngstrom formula gave the highest R2 and smallest RMSE and , therefore it is the bestway to calculate the solar radiation . But in case of absence measured solar radiation incalculation ETo led to overestimated by 9% for Angstrom and 11% for Hargreaves .The two linear regression equations are used to calibrate the coefficients of Angstromand Hargreaves methods to Mosul area . The average ETo was 3.39 mm day-1 formeasured solar radiation .