## 5 Discussion  
In this study, the size of the target water bodies vary from 130 m to 4.5 km and change with seasons. The geometries of the water bodies are complex with irregular lake shores and various sizes,  
Figure 7: Water level time series over different water bodies: (**a**) Itoiz Reservoir; (**b**) Irabia Reservoir; (**c**) Sotonera Reservoir; and (**d**) Ribarroja Reservoir. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Water Bodies**} & \multirow{2}{*}{**Width**} & **Tracking** & \multicolumn{3}{c}{**RMSE/abRMSE (m)**} \\ \cline{3-6}  & & **Mode** & **Two-Step Physical** & **OCOG** & **Threshold** & **Level-2 Ocean** \\ \hline Ebro Reservoir & 1.8 km & Closed loop & 0.32/0.29 & 0.30/0.28 & 0.30/0.28 & 2.18/1.76 \\ Itoiz Reservoir & 400 m-27 km & Closed loop & 1.18/1.02 & 1.10/1.00 & 1.10/1.00 & 1.41/1.03 \\ Irabia Reservoir & 130 m & Closed loop & 1.39/1.39 & 1.39/1.38 & 1.39/1.38 & 1.44/1.39 \\ Sotonera Reservoir & 4.5 km & Closed loop & 0.60/0.43 & 0.49/0.38 & 0.48/0.44 & 1.65/1.19 \\ Ribarroja Reservoir & 400 m & Open loop & 0.18/0.16 & 0.29/0.28 & 0.31/0.16 & 0.44/0.20 \\ Mequinenza Reservoir & 600 m & Open loop & & & Off-track \\ Cavallers Reservoir & 800 m & Open loop & & & Off-track \\ San Salvador Reservoir & 1.2 km & Open loop & & & Off-track \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of the water level validation results over all monitored water bodies. making the along-track width (the width as seen by the satellite flying along the water body) very different from one satellite overpass to another due to the drifting satellite ground tracks. As a result, continuous clean waveforms rarely exist in our study area. The shore of the Ebro Reservoir is irregular, and lands exist within water, resulting in no clean waveforms available over many tracks. Figure 6b shows the water level time series derived using the whole waveform, while Figure 6a shows the water level time series derived using the waveform portion selection. Retracking of the waveform portion accordingly reduced land contamination effects on the waveform. Figure 6a shows higher consistency with in situ time series, and thus shows that most of the outliers were caused by the land contamination. The land contaminated waveforms can be originated by surface area changes or satellite track drifting, making the altimeter footprint closer to the side bank, introducing outliers in the time series. Over the water bodies where Sentinel-3 is operated in closed loop, the water levels of all water bodies could be tracked. Over the water bodies where Sentinel-3 is operated in open loop, three water bodies were missed and only one was tracked accurately. The results over the Ebro Reservoir, Sotonera and Ribarroja Reservoirs are satisfactory. The Ebro Reservoir is located in the north near the Pyrenees mountain range with elevations exceeding 820 m, and an average slope within 5 km along track of about 4%. The RMSE was 30 cm compared with the in situ measurements. The Sotonera Reservoir is the biggest reservoir studied, and is located in a relatively flat area with a slope of about 3% and an elevation of about 410 m. The result is good as there is no influence from the mountainous terrain, and it is big enough to be well monitored by satellite. The Ribarroja Reservoir is only 400 m wide and with a slope of more than 20%, but an elevation of only around 70 m. Its small width results in only one altimeter footprint per track available within the water body. The ubRMSE of Ribarroja Reservoir shows very good performance with a value of about 16 cm. However, the results for a few tracks were excluded due to poor waveform quality, which may be influenced by the rapid changes of surrounding terrain. The RMSEs over the Itoiz Reservoir and Irabia Reservoir exceeds 1 m. One reason for this could be the rapid change in the elevations along the satellite track as both water bodies are located in the Pyrenees, making it easy for a satellite to lose its track.