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Knowledge of the photosynthetic photon flux density (Q(p)) is critical in different applications dealing with climate change, plant physiology, biomass production, and natural illumination in greenhouses. This is particularly true regarding its diffuse component (Q(pd)), which can enhance canopy light-use efficiency and thereby boost carbon uptake. Therefore, diffuse photosynthetic photon flux density is a key driving factor of ecosystem -productivity models. In this work, we propose a model to estimate this component, using a previous model to calculate Q(p) and furthermore divide it into its components. We have used measurements in urban Granada (southern Spain), of global solar radiation (R-s) to study relationships between the ratio Q(pd)/R-s with different parameters accounting for solar position, water-vapour absorption and sky conditions. The model performance has been validated with experimental measurements from sites having varied climatic conditions. The model provides acceptable results, with the mean bias error and root mean square error varying between -0.3 and -8.8% and between 9.6 and 20.4%, respectively.

Relevance: Knowledge of the photosynthetic photon flux density (Q(p)) is very important in different applications dealing with climate change, plant physiology, biomass production and natural illumination in greenhouses.

Direct measurements of this flux are very scarce so that modelling simulations are needed, this is particularly true regarding its diffuse component.

We propose a new parameterization to estimate this component using only measured data of solar global irradiance, which facilitates its use for the construction of long-term data series of PAR in regions where continuous measurements of PAR are not yet performed.