In this work, the applicability of twelve solar radiation (R S ) estimation models and their impacts on daily reference evapotranspiration (ET o ) estimates using the Penman-Monteith FAO-56 (PMF-56) method were tested under cool arid and semi-arid conditions in Iran. The results indicated that the average increase in accuracy of the ET o estimates by the calibrated R S models, quantified by the decrease of RMSE, was 2.8% and 6.4% for semi-arid and arid climates, respectively. Mean daily deviations in the estimated ET o by the calibrated R S equations in semi-arid climates varied from -0.283 mm/day for the Glower-McCulloch model to 0.080 mm/day for the El-Sebaii model, with an average of -0.109 mm/day. For arid climates, mean daily deviations in the estimated ET o by the calibrated R S equations ranged from -0.522 mm/day for the Samani model to 0.668 mm/day for the El-Sebaii model, with an average of 0.125 mm/day.