## 4 Temporal Variability of the \(Q_{p}/R_{s}\) Ratio  
The seasonal characteristics of the \(Q_{p}/R_{s}\) ratio were studied at differing intervals: ten-minute, hourly, daily and monthly. Figure 7 shows the high positive correlation between \(Q_{p}\) and \(R_{s}\) at ten-minute intervals (\(R^{2}~{}=~{}0.992\)) with a slope of \(1.893\pm 0.001~{}\upmu\)mol\(\cdot\)J\({}^{-1}\). This value is close to the mean value \(1.93\pm 0.15~{}\upmu\)mol\(\cdot\)J\({}^{-1}\), with a standard deviation of \(\pm 0.15~{}\upmu\)mol\(\cdot\)J\({}^{-1}\). The \(Q_{p}/R_{s}\) ratio had similar values to those reported by other researchers [15], ranging between 1.21 and 2.84 \(\upmu\)mol\(\cdot\)J\({}^{-1}\). Figure 8 shows the box-plot of the mean hourly values of the \(Q_{p}/R_{s}\) ratio, calculated from the average of the ten-minute data, from sunrise to sunset, using the whole data base. The graph represents the mean value, the median, the three quartiles and both the maximum and the minimum values of the data, as well as the outlier values. Rising in the early hours of the day, \(Q_{p}/R_{s}\) stabilized in the central hours and tended to decrease at sun set. Higher dispersion of the values in the first and last hours than in the central hours of the day may be observed, as the interquartile range shows. The standard deviation ranged from 0.11 \(\upmu\)mol\(\cdot\)J\({}^{-1}\) within the hourly interval starting at 7:00 to 0.17 \(\upmu\)mol\(\cdot\)J\({}^{-1}\), within the hourly interval starting at 14:00. The average values were higher than the median values in all hours of the day except for the hourly interval starting at 06:00. The hourly average of the  
Figure 6: Daily frequency of occurrence of clear, partial, and overcast sky conditions in Burgos between April 2019 and January 2020. Figure 7: Photosynthetic photon flux density, \(Q_{p}\) (\(\upmu\)mol\(\cdot\)s\({}^{-1}\).m\({}^{-2}\)), and broadband solar irradiance, \(R_{s}\) (W\(\cdot\)m\({}^{-2}\)), measured in Burgos, between April 2019 and January 2020.  
ratio was \(1.910\pm 0.016\)\(\upmu\mathrm{mol\cdot J^{-1}}\), with maximum and minimum values of \(1.98\pm 0.11\)\(\upmu\mathrm{mol\cdot J^{-1}}\) and \(1.75\pm 0.16\)\(\upmu\mathrm{mol\cdot J^{-1}}\), respectively, at 07:00 and at 19:00 h.  
Figure 9 presents the monthly values of the \(Q_{p}/R_{s}\) ratio calculated with the average of the ten- minute data, using the whole data base. Figure 8 shows that the monthly data were almost constant throughout the central months of the year, from May to October, with a standard deviation between 0.11 and 0.16\(\upmu\mathrm{mol\cdot J^{-1}}\), and an interquartile range between 0.10 and 0.15\(\upmu\mathrm{mol\cdot J^{-1}}\). November was the measurement campaign month with the highest data dispersion: the interquartile range was 0.24\(\upmu\mathrm{mol\cdot J^{-1}}\), with a standard deviation of 0.20\(\upmu\mathrm{mol\cdot J^{-1}}\). The maximum value was recorded in April (\(1.98\pm 0.15\)\(\upmu\mathrm{mol\cdot J^{-1}}\)), while the minimum was reached in December (\(1.91\pm 0.17\)\(\upmu\mathrm{mol\cdot J^{-1}}\)). The monthly average of \(Q_{p}/R_{s}\) was \(1.930\pm 0.025\)\(\upmu\mathrm{mol\cdot J^{-1}}\). Based on the results of the monthly average of \(Q_{p}/R_{s}\), some authors have suggested the existence of a seasonal dependence of this term. [PERSON] et al. (2018) recorded higher values in the summer months and lower values from November to January, in Granada, Spain. However, in Greece, [PERSON] et al. (2018) recorded the highest values for autumn (September) while the lowest averages (March) were recorded in spring, with intermediate values for summer and winter. In Midwestern US (Midwestern, 2018), the lower values with smaller deviations were recorded in the summer months while the winter months showed higher values of the ratio with larger deviations.