Evaluation of solar radiation estimation methods for reference evapotranspiration estimation (Case study: Khorram Abad)

Abstract: Solar radiation is a very important factor in the estimation equations for evapotranspiration and its proper estimation in the development of simulation models of plant growth is very important. In this study 10 solar radiation estimation models and their impact on the seven models of estimation of reference evapotranspiration were evaluated in the city of Khorramabad. The required solar radiation is calculated from the equations of Penman-Monteith, Hargreaves Samani, Irmak, Allen, Annandale, Self-Calibrating, Bahel, Glover and Mc Culloch, Bristow and Campbell , Angstrom-Prescott and used in reference evapotranspiration equations, FAO Penman-Monteith, Priestley–Taylor, Irmak, Turc, Doorenbos and Pruitt, McGuinness - Bordne and Abtew. The calculated evapotranspiration was compared with the measured values from four lysimeter in the growth period (March, April, May and June), and for the evaluation of the results were used the statistical criteria MBE, RMSE, R2, t and R2/ t. The results showed that the reference evapotranspiration calculated with all models of solar radiation and use of Irmac and McGuinness - Bordne equations were lower than the measured values and the Penman Montieth radiation model used in the FAO Penman Montieth, Priestley–Taylor and Doorenbos - Pruitt equations showed that the calculated evapotranspiration values would be closer to the lysimetric. Also, the self-calibrating radiation model in the Turc evapotranspiration equation with R2 /t = 0.38 had the best results and the Penman Montieth radiation model in the Doorenbos and Pruitt equation with R2 /t = 0.27 was in the second order compared to other models.