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Abstract
The main objectives of this work are to address the analysis of the spatial and temporal variability of the ratio between photosynthetically active radiation (PAR) and global horizontal irradiance (GHI), as well as to develop PAR 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 PAR/GHI ratio and the location; the oceanic climate showed higher values of PAR/GHI compared with Mediterranean climates. Further, the temporal variability of PAR/GHI 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 PAR as a function of GHI 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%).
Highlights
Active radiation (PAR) is the part of the solar spectrum included in the 400–700 nm range
Focusing on the ground measurements, at the three stations considered, there was a reduction in the relationship of Photosynthetically active radiation (PAR)/global horizontal irradiance (GHI) when kt increased
The logarithmic term was higher at Santiago-EOAS and, the rate of decrease of the PAR/GHI ratio was higher for this station
Summary
Active radiation (PAR) is the part of the solar spectrum included in the 400–700 nm range. PAR is a source of energy responsible for photosynthesis. Depending on the number of photons that plants can absorb, photosynthesis rate is affected, that is, the quanta of energy necessary to create the products of photochemical reactions used in the process of CO2 assimilation [1]. The radiation intensity changes along the daytime and plants try to keep a balance between converting radiation energy, and protecting the photosynthetic system against photoinhibition or fixing possible plant damage. The photosynthetic rate increases proportionally to the light intensity, until reaching a maximum value. The mechanism responsible for light collecting is supplied with an excessive number of photons, producing photoinhibition over light-dependent photosynthesis
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