## 5 Results and Discussion  
### Water Depth Estimation  
The correlation between the reflectance of all the three bands, green, red, and NIR, for both SLSTR and OLCI with in situ measured depth is presented in (Figures 9 and 10). The datasets were selected relatively far from the edges of the Nubia lake shore and Allaki Khor shore with the aim of avoiding coastal pixels due to the spatial resolution of the imaging sensors. The spatial position of the Nubia Lake dataset was used to build a simple model based on a quadratic equation in the calibration phase. Those points were called \"test set\" and their number was 45 points, while the spatial position of 22 points assigned to the Khor Allaqi dataset, represented the model's validity relationship between in situ measured depth and corresponding reflectance from green, red, and NIR band for SLSTR and OLCI instruments in the validation phase; those points are called the \"Validation set\". The spatial position of all 67 points was from the Sentinel-3 satellite image on 21 September 2018 of Lake Nubia data and 27 September 2013 of Allaqi Khor data, as well as the water level used to estimate the depth, on the same Lake Nubia data date. Figure 8: Water surface area variation with time series from 2016 to 2019. Figure 9: Regression model of SLSTR reflectance and depth; (**a**) In situ measured depth and reflectance of S1 (Green band) relationship, and result of correlation and polynomial equation (**b**) Model results validation of S1 (Green band), NRMSE between estimated and in situ measured depth (**c**) In situ measured depth and reflectance of S2 (Red band) relationship, and results of correlation and polynomial equation (**d**) Model results validation of S2 (Red band), NRMSE between estimated and in situ measured depth (**e**) In situ measured depth and Reflectance of S3 (NIR band) relationship, and result of correlation and polynomial equation (**f**) Model results’ validation of S3 (NIR band), NRMSE between estimated and in situ measured depth. Figure 10: Regression model of OLCI reflectance and depth; (**a**) In situ measured depth and reflectance of Qa6 (Green band) relationship, and result of correlation and polynomial equation (**b**) Model results’ validation of Qa6 (Green band), NRMSE between estimated and in situ measured depth (**c**) In situ measured depth and reflectance of Qa8 (Red band) relationship, and result of correlation and polynomial equation (**d**) Model results validation of Qa8 (Red band), NRMSE between estimated and in situ measured depth (**e**) In situ measured depth and reflectance of Qa17 (NIR band) relationship, and result of correlation and polynomial equation (**f**) Model results’ validation of Qa17 (NIR band), NRMSE between estimated and in situ measured depth. Transformation from radiance to reflectance of all bands for both OLCI and SLSTR was carried out with the SNAP software. The three bands, green, red, and NIR, of two instruments SLSTR and OLCI, hosted by the Sentinel-3A platform, were used to make preliminary study for investigating the relationship between optical measurements (i.e., reflectance, in our case) and in situ measured water depth data. The correlation between in situ depth measurements and the corresponding reflectance values for each band was determined and their relationship could be approximated by a quadratic equation. The green band resulted in a slightly higher accuracy than the red band; R\({}^{2}\) of the green band is equal to 0.93 for the SLSTR instrument and 0.92 for the OLCI instrument, while R\({}^{2}\) in the red band is equal to 0.95 and 0.92 for SLSTR and OLCI, respectively. The NIR band is characterized by a much lower accuracy of the model (R\({}^{2}\) = 0.53 for SLSTR instruments and R\({}^{2}\) = 0.59 for OLCI instruments).