## 3 Results  
### Validation of Pleiades-Retrieved Water Reflectances  
According to the atmospheric corrections applied, the water reflectance values retrieved from Pleiades imagery (rhos) in the MTZ (test site 1) contribute up to 57% of the signal recorded at the top of the atmosphere (rhot) in the blue band; this contribution progressively increases in the green and red bands and reaches 89% in the NIR band (Figure 4a). The contribution of rhos to rhot is significantly lower in the moderately turbid waters in the estuary mouth (test site 2), varying from 16% in the NIR to ~50% in the blue and green bands (Figure 4b). This indicates that the top of atmosphere signal recorded over the turbid part of the estuary largely consists of the water signal while in the less turbid waters (test site 2), the water contribution is significantly lower. Higher uncertainties may be introduced in the turbidity product in clearer waters due to the higher relative contribution of atmospheric correction errors. Figure 4: Examples of Pleiades-derived reflectance signal at the top of atmosphere (rhot), Rayleigh corrected reflectance (rhorc) and surface reflectance (rhos) after ACOLITE-DSF processing for (**a**) test site 1 on 24 May 2017, and (**b**) test site 2 on 7 April 2017. The relationships between the Pleiades-derived water reflectance, respectively at 495, 558, 655 and 842 nm, and synchronous in situ turbidity values measured at the Pauillac autonomous station were compared to the relationships previously established based on in situ data (SeaSWIR dataset) for test site 1. The relationships retrieved from Pleiades data were comparable to those obtained from the field dataset (Figure 5). The in situ (blue points) and satellite (red points) water reflectance values vs. turbidity values overlap at 495 and 558 nm bands. The Pleiades-derived water reflectance signal at these two wavelengths tends to saturate in the highly turbid waters of the GE [44]. The \(\rho\)495 signal is apparently not sensitive to turbidity variations, and the same was observed at 558 nm. The saturation reflectance can be determined by a Nechad-type function [44; 45], and was found to be 0.087 at 495 nm and 0.116 at 558 nm. The saturation effect had similar values of 0.081 and 0.123 in the in situ data. At higher turbidities of around 500 NTU, the Pleiades-derived water reflectance in the red band also saturates (0.160), again close to the in situ observation (about 0.164). The Pleiades-derived water reflectance at 865 nm shows a logarithmic increase with increasing turbidity without saturation, also as previously observed based on in situ measurements. An indirect validation of the atmospheric correction is obtained by retrieval of similar spectral relationships (i.e., saturation reflectances) from in situ and satellite data and hence, the atmospheric correction yields satisfactory results for turbidity retrieval in the MTZ area where the water signal dominated at the top of the atmosphere. Pleiades-derived green and red reflectances are then plotted as function of the corresponding OLI (Figure 6) and MODIS (Figure 7) water reflectances over test site 2 (data recorded on 7 April 2017). Figure 5: The red triangles the present relationships between Pleiades-derived water reflectance, respectively at 495, 558, 655 and 842 nm, and synchronous in situ turbidity values measured at the Pauillac autonomous station in 2017. The black dashed lines are the logarithmic relationships. The vertical bars show the standard deviation of the water reflectance values derived from Pleiades data over the ten pixels closest to the Pauillac field station. The horizontal bars show half the turbidity range between the two in situ measurements bounding the satellite overpass (i.e., their standard deviation). The blue points represent the relationships between the water reflectance and turbidity established using the in situ measurements (SeaSWIR dataset).