# Inland Water Level Monitoring from Satellite Observations:
## 5 Conclusions  
In this scoping literature review, following PRISMA guidelines, a comprehensive analysis of spaceborne missions for inland water level monitoring is provided for the last five years. Using five electronic literature databases, a double screening process found 48 scientific publications to meet the eligibility criteria. This review summarizes and disseminates the research findings, and presents both major results and limitations of satellite observations for inland water surface heights monitoring. Among the satellite missions that provide inland water level time series, it is shown that altimetry is the dominant technique. There are also a few alternative techniques that can measure water surface heights with quite good coverage, such as Gravimetry and GNSS-R. Sentinel-3, as the first altimetry satellite operated both in OLTC and SAR mode, is the dominant mission for monitoring inland water levels, achieving the most accurate results. Only the Sentinel-6 mission's data outperforms the Sentinel-3 mission, capturing water level changes with higher accuracy and in higher temporal resolution. However, Sentinel-6 altimetry data has not been widely used for the inland water level monitoring, since the satellite was launched in November 2020. Except for the Sentinel-3/-6 missions, CryoSat-2 and Jason-3 advanced radar altimetry missions show high capability in acquiring accurate inland water level time series. Also, the ICESat-2 laser altimetry mission has exhibited a high growth trend thanks to its higher sampling density than radar altimetry missions. Furthermore, the FF-SAR method is able to retrieve inland water levels of narrow rivers of smaller than 300 m in width, with precision between 4 and 11 cm. The dual-frequency (Ku-Ka) SAR radar altimeter of CRISTAL future mission, will have the capability for FF-SAR processing to further enhance the along-track sampling resolution. Thus, in the upcoming years, more research for the evaluation of the performance of missions is expected over narrow rivers and small lakes. Apart from the measurement mode of each mission, the quality of water level determination depends on the retracking algorithm, as well as the shape and size of inland water bodies. Novel or improved retrackers are presented in 11 publications to provide more accurate inland water level measurements than the conventional retrackers of each mission. However, despite the advances of state-of-the-art altimeters and advanced algorithms, there is still much to be learned regarding the monitoring of inland water levels by satellite observations. In future missions, the enhancement of spatial resolution and along-track sampling resolution should be also accompanied by an enhancement in temporal resolution to meet the data requirements for applications such as flood monitoring and warning systems. Advancements in future missions such as CRISTAL, SMASH, Sentinel-3C/-D/-NG Topo, and Sentinel-6 NG missions with higher spatial and temporal resolution will play a major role in inland water level monitoring and the derivation of river discharge, and therefore contribute to a better understanding of the global water cycle. In the future, it is expected that more research focused on inland water level monitoring and river discharge derivation, related to new advances in altimetry missions, such as the Sentinel-6 and SWOT missions, will be conducted. Finally, advanced machine and deep learning techniques have not yet been tested in the field of altimetry. The collaboration of altimetry, in-situ, and various proxy data with machine learning techniques can be a promising solution to calibrate spaceborne water level measurements and river discharge derivation algorithms, and potentially augment their spatial and temporal resolution. Conceptualization, S.K., V.T. and G.T. ; methodology, S.K. ; software, S.K. ; validation, S.K., V.T. and G.T. ; formal analysis, S.K. ; investigation, S.K. and V.T. ; resources, A.A.;writing--original draft preparation, S.K. ; writing--review and editing, S.K., V.T. and G.T.