Calibrating Net Solar Radiation of FAO56 Penman-Monteith Method to Estimate Reference Evapotranspiration

Estimation of reference crop evapotranspiration (ETo) are widely used in irrigation scheduling with using the standardized FAO56 Penman–Monteith equation, which has been the most reasonable method in humid and arid climatic conditions, to provide reference crop evapotranspiration estimation in irrigation schedualing and water resources managment. The source of energy for ETo process is the radiation received from the sun. The goals of this study are to calibrate and evaluate the three empirical radiation equations in FAO56 Penman–Monteith model for the Tabriz region. Determination of radiation equation coefficients carried out by using the solver tool in EXCEL software to decrease the difference between measured (Lysimetric Data) and predicted amount of evapotranspiration. As a result, the Angstrom model coefficients were 0.22 and 0.61 that gained the least error in comparison with measured Rs for the study station. The calibrated net radiation (Rn) served as the basis to estimate ETo accurately, which would be underestimated about 24% if no local calibration is performed on the FAO56 Penman-Monteith model in Tabriz. Although the evaluation process showed that the FAO56-PM net radiation equation error was less than the two others, results of developed Rn equation were satisfied regarding to MEP, NRMSE, NSE and d statistical indexes as 0.34%, 18.44%, 0.74 and 0.95, respectively. The minimum error was in the application of new developed Rn equation to calculate reference evapotranspiration by FAO-PM method. The average ETo was 658 mm per season based on presented new net solar radiation model in this study. The reasonable application of the standardized FAO56-PM model provided a sustainable irrigation schedualing and water resources management if local calibration of radiation equations applied for Tabriz and similiar condition areas.