The Angström-Prescott (AP) model is widely suggested for estimating global solar radiation (Rs), especially for reference crop evapotranspiration (ET0) calculations. Many studies indicated that the coefficients of the AP model are dependent on the climatic and geographical characteristics of study sites. In the present study, calibrated AP models were developed in some Iran’s climates. 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 Daneshyar (D) and Hargreaves (H) models. Precision of the estimated daily ET0 evaluated against field lysimeter data. Results showed that scenarios D and H can perform the minimum ET0 deviations from lysimeter data, respectively. Results revealed that at 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 for calibrating the AP model. Apart from applied Rs scenarios, the FAO56-PM model tends to under-estimate the daily ET0.
