## 6 Conclusions  
Using the example of the Mississippi River Basin, this study shows the potential and limitations of satellite altimetry in the monitoring of basin-wide surface water storage changes. Based on a newly developed automated target detection method, the number and size of all lakes larger than about 0.1 km\({}^{2}\) in the MRB can be determined. The merging of this information with past and current satellite altimeter constellations reveals an overview of the possibilities of remote sensing technique to monitor lake storage changes from space.  
With the developed target detection approach, about 5700 lakes and reservoirs can be detected in the MRB. This is in accordance with the information of GLWD, even if a larger total surface area is documented. This number reduces significantly when only water bodies with permanent water are taken into account.  
The study reveals that in the MRB, with the current altimeter configuration consisting of Jason, Sentinel-3, Cryosat-2, and Saral-DP, only about 20% of water bodies larger than 0.1 km\({}^{2}\) can be captured currently by satellite altimetry. This number increases to about 80% for lakes larger than 10 km\({}^{2}\), even though reliable water level time series are not determinable for all of these. The main limitation is the measurement technique, which requires the satellites to directly overpass the water bodies for reliable measurements. When analyzing the larger water bodies available in the global hydrology model WGHM, the situation improves: from these larger lakes, more than 90% are captured by satellite altimetry, and 98% of the related storage changes can be monitored. However, for long-term studies that rely on past altimeter configurations, the situation is significantly worse: until 2016, nearly 25% of storage changes (i.e., more than 40 km\({}^{3}\)) are missed by the satellite tracks.  
This study is limited to the MRB and not directly transferable to other basins with different lake densities and characteristics. However, since the region is quite large, with significant variations in the number of lakes per 10\({}^{6}\) km\({}^{2}\)[33], transferability to a global scale might be valid.  
Significant improvements are expected from the new SWOT mission, which will cover the entire MRB without any data gap due to the orbit configuration of the mission. Moreover, smaller lakes will be reliably measured due to the higher spatial resolution and increased height precision. However, challenges will remain; in particular, the consistent combination of SWOT with classical altimetry data and the deviation of volume changes without good knowledge of the lakes' bathymetry. Besides that, after 2021, the greatest limitation of satellite altimetry will be the temporal resolution of time series and the risk of missing extreme short-periodic events.  
[PERSON] conceptualized the research work and wrote the major part of the manuscript; [PERSON] developed the automated target detection method and prepared a draft version of the paper. [PERSON], [PERSON] and [PERSON] performed the data processing. C.N. provided the lake volume changes from WGHM. [PERSON], [PERSON] and [PERSON] contributed to the writing of the manuscript and helped with the discussion of the applied methods and results. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the German Research Foundation DFG grant number DE 2174/10-1.  
The authors would like to acknowledge the European Commission's Joint Research Centre for allowing access to the Global Surface Water Explorer GSWE ([[https://global-surface-water.appspot.com/](https://global-surface-water.appspot.com/)]([https://global-surface-water.appspot.com/](https://global-surface-water.appspot.com/))). The support of the German Research Foundation (DFG) within the framework of the Research Unit GlobalCDA (FOR2630) is gratefully acknowledged. We thank four anonymous reviewers for their valuable comments that helped to improve the manuscript.  
The authors declare no conflict of interest.