# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
## 5 Conclusions  
The Kuiseb River is a major ephemeral river in Namibia with episodic flood events that typically only last a few days. Until recently, studying the dynamics of such ephemeral rivers was difficult, mainly due to inappropriate monitoring tools as in situ hydrological and geohydrological stations are rare in episodically-flowing rivers of arid and semi-arid areas. However, the development of remotesensing data over the last two decades has offered new tools for the study of hydrological phenomena in ephemeral rivers. The main objective of our study was to show the potential of using Earth Observation Satellite remote-sensing data to monitor the dynamics of the Kuiseb River, using an innovative approach based on time-series of data from multiple sensors on different platforms provided by the Sentinel missions.  
First, Sentinel-2 data were used to produce NDVI and NDMI maps in order to detect changes and define sub-areas for closer study. Local rainfall and flood events in 2018 and 2021 impacted these two spectral indexes, with an offset shift of one month between them, which corresponds to the time for vegetation to grow. Then, Sentinel-1 data were used to produce \(\sigma_{0}\), which showed variations during local rainfall events in 2018, but also following a big flood event in 2021. Finally, coherence maps were produced for 2018 and 2021, which showed a decrease in the signal from a shallow tributary in 2018 and 2021. These observations can be explained by a rise in groundwater levels after the Kuiseb River floods of 2021. This study shows the utility of combining multi-sensor satellite data to better understand the actual hydrological dynamics of episodically flowing drainage systems, such as the Kuiseb River, following rainfall and flood events--and particularly their effects on groundwater levels and local aquifers.  
This study was a preliminary work to show the feasibility of Sentinel data to better understand the dynamics of the ephemeral Kuiseb River by establishing a link between underground and surface dynamics. The different methods deployed during this work could be used in the future by institutions, for example, to automate all of these methods  
Figure 9: **(a)** Google Earth image showing the tributary outlined in red, floodplains outlined in green, and the direction of the water flow in the Kuiseb River during a flood event. The white line is a transect across the Kuiseb River and its tributary at Klipneus. **(b)** Idealised topographic section along a transect (white line in a) to illustrate likely hydrological consequences during and **(c)** after a flood event in the main river channel and the tributary that leads to a reduction in radar image coherence.  
to create a temporal monitoring tool from Sentinel-1 and Sentinel-2 data for different ephemeral rivers.  
All the results produced during this study can be used by water resource managers in desert areas. The different maps from Sentinel-1 and Sentinel-2 can be used to locate groundwater risings, which are useful for the local population, but also animals. In addition, this study will allow them to better understand the link between the flow of surface and underground water, thereby monitoring the ephemeral river.  
Conceptualization, C.N. and P.P.; methodology, C.N. and P.P.; writing--original draft preparation, C.N. and P.P.; writing--review and editing, P.P., S.L., [PERSON] and K.S. [PERSON] was responsible for Living Planet Fellowship supervision of [PERSON]. E.M. helped to provide meteorological data and hydrological data. All authors have read and agreed to the published version of the manuscript.  
This research was funded by European Space Agency in the framework of a Living Planet Fellowship.  
The authors declare no conflict of interest.