Photosynthetically active radiation (PAR) comprises the spectral range of global solar radiation (Rs) that is highly related to vegetation productivity. The study aimed to evaluate the relationship between PAR and Rs in Petrolina, PE, and Brasília, DF, Brazil, with data measured in 2011 and 2013 at two stations of the Sistema Nacional de Organização de Dados Ambientais located in Petrolina, PE and Brasília, DF, Brazil, and the obtained models were evaluated using the measurements of 2014. It was verified that the PAR, in instantaneous values (μmol m-2 s-1), can be estimated at 2.31 times the Rs (W m-2) measured in Petrolina, while for daily values of PAR (MJ m-2) is equal to 50% of Rs (MJ m-2). In Brasília, PAR (μmol m-2 s-1) is 2.05 times the Rs (W m-2) and, in daily values, equal to 44% of Rs (MJ m-2). The variability of the PAR/Rs ratio followed the local variations of clearness index (Kt) and Rs. The models presented an adequate performance based on statistical indices mean absolute error, mean relative error, and root mean square error and can be used to estimate PAR.
