## 3 Results
### Validation  
#### 3.2.1 Remote Sensing Reflectance  
According to the factors that cause the number of data points to decrease, as coincides with the image acquisition-respective quality flags and the match-up exercise for C2-Nets, once the different ACs were applied, a total of 81 samples for C2 RCC, 41 samples for C2X, 55 for C2 XC, and 92 for Polymer were used for validation. This indicates higher restrictiveness of C2X and C2 XC with respect to C2 RCC and Polymer for our dataset. The linear correlations between the in situ and remote sensing reflectance data for the different S2-MSI bands, together with the calculated validation statistics, are shown in Figure 3.  
Regarding to the fits, although the correlation coefficients vary between ACs and between bands, it is generally observed that the lines fit below the 1:1 line for the high values, indicating that all the validated ACs underestimate higher reflectance values, except C2X for the \(R740\) and \(R783\) bands. Considering the coefficients of determination for the blue bands (\(R443\) and \(R492\)), the lowest values of 0.47 for \(R443\) and 0.61 for \(R492\) were obtained with C2X, and although C2 RCC had high coefficients, the slope values for C2 XC and Polymer were closer to 1. The coefficients of the determination of the green and red bands improved for all the ACs, the lowest values of 0.58 for \(R560\) and 0.67 for \(R665\) were also obtained with C2X, and although for C2 RCC and Polymer, the R\({}^{2}\) was high, (more than 0.8), for C2 RCC, the slope value was closer to 1. On the other hand, for the Red-edge and NIR bands, the lowest values were obtained with C2 RCC, with an R\({}^{2}\) of 0.74 for \(R704\), 0.57 for \(R740\), 0.61 for \(R783\), and 0.54 for \(R865\), while high coefficients of determination (more than 0.8) and slopes values very close to 1 were obtained with C2X, similar to results obtained with C2 XC and Polymer, except for Polymer for \(R865\) with an R\({}^{2}\) of 0.77. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(p\) **\textless{} 0.001** & **CDOM** & **TSS** & **Chl\_a** & **PC** \\ \hline
**SDD** & \textless{}0.571 (102) & \textless{}0.957 (92) & \textless{}0.789 (131) & \textless{}0.718 (96) \\
**CDOM** & & 0.600 (90) & 0.604 (104) & 0.573 (69) \\
**TSS** & & & 0.768 (98) & 0.749 (59) \\
**Chl\_a** & & & & 0.749 (105) \\ \hline \end{tabular}
\end{table} 
Table 4: Spearman correlation coefficients, probability value and data points (in parenthesis). Figure 2: Boxplot of the values range for the water quality parameters. The box bounds the interquartile range (IQR; 25–75 percentile), the horizontal line inside the box indicates the median, and the whiskers (error bars) indicate the 90 th above and 10 th below percentiles. Dots indicate the outliers. Figure 3: Linear correlations between the in situ and remote sensing reflectance data for the different S2-MSI bands and respective validation statistics results, using 81 samples for C2 RCC, 41 for C2X, 55 for C2 XC, and 92 for Polymer. RMSE and bias in sr\({}^{-1}\) and RRMSE in %. Regarding the bias for the blue and green bands, the AC with the lowest values is Polymer, with 0.00001 sr\({}^{-1}\) for \(R443\), 0.00041 sr\({}^{-1}\) for \(R492\), and 0.00146 sr\({}^{-1}\) for \(R560\). For the red band, the lowest value of 0.00049 sr\({}^{-1}\) was obtained with C2 XC. For the Red-edge and NIR bands, the lowest bias was obtained with C2X with 0.00015 sr\({}^{-1}\) for \(R704\), 0.00019 sr\({}^{-1}\) for \(R740\), and 0.00022 sr\({}^{-1}\) for \(R865\), except for the \(R783\) band, for which the lowest bias of 0.00007 sr\({}^{-1}\) was obtained with Polymer. Coinciding with the bias results, the lowest RMSE for the blue and green bands were obtained with Polymer, with 0.00245 sr\({}^{-1}\) for \(R443\), 0.00290 sr\({}^{-1}\) for \(R492\), and 0.00315 sr\({}^{-1}\) for \(R560\). For the red band, the lowest RMSE of 0.002 sr\({}^{-1}\) was obtained with Polymer and C2 XC.