## 3 Data Sets
### Altimetry Data  
SRAL is the instrument that (hosted by Sentinel-3 satellite) we used for generating the water level time series in this paper. AHDL surface water level time series can be used as an input to [PERSON]'s algorithm to estimate water volume variations in AHDL. The SRAL is a versatile altimeter working with a carrier frequency in the Ku-band (20 GHz) and a chirp (the wave shape of the transmitted signal) band width of 350 MHz. Substantially, the instrument is a radir-pointing SAR (Synthetic Aperture Radar). Data used in this paper were processed by the ESA GPOD service (Grid Processing on Demand), running the SARvatore processor (SAR Versatile Altimetry Toolkit for Ocean Research and Exploitation). Water levels with respect to the earth reference ellipsoid were calculated with the aid of the SARvatore processor. The nominal track of the relative orbit #249 of Sentinel-3A is shown in (Figure 4), together with the ground projections of the actual measurement points. Water levels have been calculated in terms of \"orbit-range\" with no corrections, and were taken by the output (L2 level) product of SARvatore. A comparison between in situ water level time series and those of altimetry must be evaluated to check the precision of altimetry measurements, thus the possibility of calibrating altimetry measurements properly. In (Figure 5), both time series are plotted, showing a RMSE = 38 cm. We also demonstrated [37], that the simultaneous acquisition by optical imaging radiometers and SRAL offers the opportunity to cross-calibrate radiometric imagers as \"optical altimeters\" by estimating the percent coverage of water in coastline pixels. In this paper, we build on the same altimetry dataset. It is worth mentioning that all data from altimetry and  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **SLSTR** & **OLCI** \\ \hline Spatial resolution & 500 m or 1 km per pixel & \(\sim\)300 m per pixel \\ \hline \multirow{4}{*}{Sensor} & Sea and Land Surface Temperature & Ocean and Land Color Instrument \\  & Radiometer (SLSTR), 11 bands: 3 & (OLCI), 21 bands: 16 visible bands, 5 Near-Infrared bands \\ \hline \multirow{2}{*}{Units} & Radiance: (mW.m\({}^{-2}\).sr\({}^{-1}\).nm\({}^{-1}\)) & Radiance (mW.m\({}^{-2}\).sr\({}^{-1}\).nm\({}^{-1}\)). \\  & Reflectance (N/A) & Reflectance (N/A). \\ \hline \multirow{4}{*}{Common usage/purpose} & Climate change monitoring, vegetation monitoring, active fire detection, land, and sea surface temperature monitoring. & Maritime, land, atmospheric, and climate change monitoring \\ \cline{1-1} \cline{2-4}  & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main characteristics of the SLSTR and OLCI instruments onboard Sentinel-3 [4].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Name** & **Wave Length (nm)** & **Band Width (nm)** & **Resolution (m/px)** \\ \hline \multirow{4}{*}{SLSTR} & S1 & \multirow{4}{*}{554.27} & \multirow{4}{*}{19.26} \\  & (Green) & & & \\ \cline{1-1} \cline{2-4}  & S2 & & & \\ \cline{1-1} \cline{2-4}  & (Red) & & & \\ \cline{1-1} \cline{2-4}  & S3 & \multirow{4}{*}{868} & \multirow{4}{*}{20.60} \\  & (NIR) & & & \\ \hline \multirow{4}{*}{OLCI} & Oa06 & \multirow{4}{*}{665} & \multirow{4}{*}{10} & \multirow{4}{*}{300} \\  & (Green) & & & \\ \cline{1-1} \cline{2-4}  & Oa08 & \multirow{4}{*}{865} & \multirow{4}{*}{20} & \multirow{4}{*}{860} \\ \cline{1-1} \cline{2-4}  & (Red) & & & \\ \cline{1-1} \cline{2-4}  & Oa17 & \multirow{4}{*}{865} & \multirow{4}{*}{20} & \multirow{4}{*}{860} \\ \cline{1-1} \cline{2-4}  & (NIR) & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Main characteristics of green, red and NIR bands for both SLSTR and OLCI instruments [4].  
determination \(R^{2}>0.8\), which highlights a good relationship between the reflectance and measured field.