## 4 Results
### Surface Water Storage  
For the monitoring of surface freshwater resources, water storage and its changes are more important than the number of lakes or their surface area. However, optical images, as used in Section 3, \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Scenario** & **Number of Targets** & **Area of Targets in km\({}^{2}\)** & **Mean Size of Targets in km\({}^{2}\)** \\ \hline Jason only & 212 (4.7\%) & 9125 (31.3\%) & 43.0 \\ Sentinel-3A/B & 704 (15.5\%) & 20893 (71.7\%) & 29.7 \\ Past configuration & 612 (13.5\%) & 18090 (62.0\%) & 29.6 \\ Current configuration & 853 (18.8\%) & 23110 (79.3\%) & 27.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number and area of lakes/reservoirs larger than 0.1 km\({}^{2}\) covered by different altimetry constellations. The values in parentheses are the percentages of the total number and total area, respectively. Figure 4: Percentage of missed lakes/reservoirs for different altimetry configurations depending on minimal target size (from 1 to 100 km\({}^{2}\)) based on a water occurrence threshold of 50%. are not able to provide any information on water volume or lake bathymetry. First approaches exist to derive storage changes and lake bathymetry from remote sensing techniques (for example, [PERSON] et al. [8] and [PERSON] et al. [30]), even on a global scale [7],[31]. However, since these approaches also rely on satellite altimetry, they are not able to provide information for all water bodies. The most comprehensive databases for water volume are still global hydrological models, even if, in these records, the very small water bodies are missing. WGHM provides 127 lakes and reservoirs in the MRB with areas between 2.9 and 1126.8 km\({}^{2}\) (total sum of 14,671.2 km\({}^{2}\)) and water volume variations up to 18.2 km\({}^{3}\). From these targets, three reservoirs are not detected by the automated target detection method developed in this study. They have a total area of about 20 km\({}^{2}\) (2.9, 12.4 and 4.8 km\({}^{2}\)) and are all very narrow. In total, 124 of all detected targets are available in WGHM (2.7%), representing about 50% of the total detected surface area. From the 127 WGHM targets, 116 can be mapped by one of the altimetry missions handled in this study; however, not at the same time. Eleven are not covered at all. In the past, with a combination of satellites from the Jason and Envisat family (past configuration), about 76% of the water volume variations issued by WGHM were detectable. In comparison, the current configuration of Jason, Sentinel-3, Cryosat-2 and Saral-DP meets the requirement of monitoring 98% of the water volume of lakes and reservoirs in the MRB (since 2018). When using only Jason, this number is much smaller, at about 50%. All numbers can be found in Table 3.  
Although WGHM represents only about half of the available water surface area in the MRB (14,671 of 29,101 km\({}^{2}\) defined by the automated target detection), about 300 km\({}^{2}\) (2% of the WGHM total area) still cannot be mapped by satellite altimetry due to the orbit configuration of the missions. The averaged surface area of WGHM targets is 116 km\({}^{2}\) and the mean size of mapped water bodies is 124 km\({}^{2}\). The percentage of missed water volume changes is in the same order of magnitude: about 2% of WGHM storage changes cannot be monitored by current satellite altimetry. Thus, it is reasonable to say that the current altimetry configuration is able to provide almost the same information to that available from global hydrological models. However, the temporal resolution differs: while WGHM provides monthly values, the altimetry resolution can yield values from a few days to 10 or 35 days, or even fewer, depending on the missions involved (see Sections 2.2 and 5.4). However, concerning long-term storage changes, approximately 25% are missed by the satellites, even reaching 50% when only Jason missions are used. These numbers improve when only considering larger lakes (see Figure 5).