| Summary: | The main objectives of this work are to address the analysis of the spatial and temporal variability of the ratio between photosynthetically active radiation (<i>PAR</i>) and global horizontal irradiance (<i>GHI</i>), as well as to develop <i>PAR</i> models. The analysis was carried out using data from three stations located in mainland Spain covering three climates: oceanic, standard Mediterranean, and continental Mediterranean. The results of this analysis showed a clear dependence between the <i>PAR/GHI</i> ratio and the location; the oceanic climate showed higher values of <i>PAR/GHI</i> compared with Mediterranean climates. Further, the temporal variability of <i>PAR/GHI</i> was conditioned by the variability of clearness index, so it was also higher in oceanic than in Mediterranean climates. On the other hand, Climate Monitoring Satellite Facility (CM-SAF) and Moderate-Resolution Imaging Spectroradiometer (MODIS) data were used to estimate <i>PAR</i> as a function of <i>GHI</i> over the whole territory. The validation with ground measurements showed better performance of the MODIS-estimates-derived model for the oceanic climate (root-mean-square error (RMSE) around 5%), while the model obtained from CM-SAF fitted better for Mediterranean climates (RMSEs around 2%).
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