## 2 Study Area and Database  
### Study Area  
Our study area encompassed the reservoirs and rivers in the Ebro River basin (Figure 1). The Ebro River, which has a length of approximately 910 km and a drainage basin with an area of approximately 85,362 km\({}^{2}\), is one of the most important rivers on the Iberian Peninsula [68]. The river flow is irregular throughout the year, with low levels at the end of summer and high levels during the spring due to melt runoff in the Pyrenees, leading to a danger of flooding. The annual variation of the river flow is more than 800 m\({}^{3}\) s\({}^{-1}\) at Ribarroja dam, which is the last dam controlling the water flow entering the Mediterranean Sea [69]. The Ebro River is of great importance for agriculture in summer, during which drought often occurs due to the continental Mediterranean climate. Nevertheless, the mean annual flow has decreased by approximately 29% during the 20 th century due to many causes: the construction of dams, the increasing demands for irrigation, and evaporation, the latter being higher than the precipitation due to low rainfall amounts, high sunshine intensities and strong, dry winds, from reservoirs within the river basin [68].  
The Pyrenees mountain range lies to the north of the Ebro River basin. Consequently, it is challenging to retrieve the water surface heights of the studied area as the water bodies are relatively small and are greatly influenced by the mountainous terrain. The selected water bodies, for which we have gauging stations for validation, are shown in Figure 1. The coordinates, the widths of the water bodies covered with satellite tracks, the approximate distances between the gauging station and satellite tracks, and the average slope within 5 km along the satellite track are listed in Table 2. The water bodies that can be retracked properly, along with their DEMs, are shown in Figure 2 along with the water masks, Sentinel-3 tracks and the location of gauging stations. Only tracks overpassing the upper stream of the river (before the dam) were considered, ensuring the results could be validated against the in situ measurements.  
Figure 1: The Ebro River basin: (**a**) the Ebro River network in the Iberian Peninsula; and (**b**) the water bodies covered by Sentinel-3 satellite tracks with all available gauging stations.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Water** & **Coordinates** & **Width** & **Satellite** & **Tracking** & **Gauging Station** & **Average Slope** \\
**Bodies** & & & & **Tracks** & **Mode** & **Distance** & **(5 km)** \\ \hline Ebro & (43.0\({}^{\circ}\)N, & 1.8 km & 014 & Closed loop & 8 km & 4\% \\ Reservoir & 3.96\({}^{\circ}\)W) & & 400 m–2.7 km & & & \\ Itoiz & (42.81\({}^{\circ}\)N, & 400 m–2.7 km & & & \\ Reservoir & 1.37\({}^{\circ}\)W) & & depends on satellite & 165 & Closed loop & 2 km & 19\% \\ Irabia & (42.99\({}^{\circ}\)N, & 130 m & 186 & Closed loop & 450 m & 20\% \\ Reservoir & 1.15\({}^{\circ}\)W) & & 130 m & 186 & Closed loop & 450 m & 20\% \\ Sotonera & (42.12\({}^{\circ}\)N, & 4.5 km & 222 & Closed loop & 1.5 km & 3\% \\ Reservoir & 0.68\({}^{\circ}\)W) & & 4.5 km & 222 & Closed loop & 1.5 km & 3\% \\ Ribarroja & (41.24\({}^{\circ}\)N, & 400 m & 242 & Open loop & 3.5 km & 24\% \\ Reservoir & 0.40\({}^{\circ}\)E) & & & & & \\ Mequinnza & (41.26\({}^{\circ}\)N, & 600 m & 279 & Open loop & 30 km & 3.5\% \\ Reservoir & 0.04\({}^{\circ}\)W) & & & & & \\ Cavallers & (42.59\({}^{\circ}\)N, & 800 m & 299 & Open loop & 500 m & 27\% \\ Reservoir & 0.86\({}^{\circ}\)E) & & & & & \\ San Salvador & (41.78\({}^{\circ}\)N, & 1.2 km & 242 & Open loop & 2.5 km & 4.5\% \\ Reservoir & 0.20\({}^{\circ}\)E) & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Selected water bodies.  
Figure 2: _Cont._