Parametric models to estimate photosynthetically active radiation in Spain

Different applications dealing with plant physiology, biomass production and natural illumination in greenhouses require knowledge of the photosynthetically active radiation. In absence of measurements of this flux, one must rely on parametric approaches. In this way, the radiant energy is computed using available atmospheric parameters. In the present work, we have developed a comparison among several cloudless sky parameterization schemes. For this purpose, data recorded at two radiometric stations are used. The first one is located at the University of Almerı́a, a seashore location, while the second one is located at Granada, an inland location. The performance of the models has been tested in relation to their predictive capability of direct, diffuse and global components of the photosynthetically active radiation. After our study, it appears that the information concerning the aerosol radiative effects is fundamental to obtain a good estimation, especially for the direct and diffuse components. In order to improve the fitting of the model to the experimental data, several modifications have been suggested. The modified version of the models provide estimates of direct and global components of the photosynthetically active radiation with mean bias deviation below 2%, and root mean square deviation close to experimental error, with slightly worse results for the diffuse component.