The Angström-Prescott (AP) model is widely suggested for estimating global solar radiation (Rs), especially for reference crop evapotranspiration (ET0) calculations. Many previous studies indicated that the coefficients of the AP model depend on the climatic and geographical characteristics of study sites. In the present work, calibrated AP models were developed in some climates in Iran. Different radiation scenarios from estimated and measured Rs were applied to calibrate the AP coefficients. Radiation scenarios included measured Rs (M) and estimated Rs from the models of Daneshyar (D) and Hargreaves (H). Daily reference evapotranspiration was calculated using the coefficients obtained from different Rs scenarios. The estimated daily ET0 values were compared with field lysimeter data for validation. Results showed that scenarios D and H present the minimum ET0 deviation from lysimeter data. Findings suggested that at the majority of the study sites, calibration of the AP model improved the accuracy of estimated ET0. Results also indicated the applicability of reliable Rs estimation methods to calibrate the AP model. Apart from applied Rs scenarios, the daily ET0 is underestimated by the FAO56 Penman-Monteith model.
