# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
###### Abstract  
Ephemeral rivers are characterized by brief episodic flood events, which recharge subterranean alluvial aquifers that sustain humans, riparian vegetation, and wildlife in the hyper-arid Namib Desert. Yet we only have a poor understanding of the dynamics and feedback mechanisms in these hydrological systems as arid and semi-arid zones are typically poorly equipped with reliable in situ monitoring stations to provide necessary information. The main objective of our study is to show the potential of satellite data to monitor the dynamics of ephemeral rivers, such as the Kuiseb located in Namibia, since remotesensing offers the advantage of adapted spatial and temporal resolutions. For this study, multi-spectral imagery (Sentinel-2), Synthetic Aperture Radar (SAR, Sentinel-1), and SAR interferometry (Sentinel-1) data were used to produce Normalized Difference Vegetation Index (NDVI) maps, backscattering maps (as \(\sigma_{0}\)), and interferograms, respectively. These parameters provide information on the hydrologic and vegetation dynamics of the river. Strong variations in NDVI, \(\sigma_{0}\), and interferograms are observed during March-April 2017 and June-July 2018 in a tributary of the Kuiseb in the central Namib Desert. In those years, rain events caused the reactivation of the tributary. However, during a major flood in 2021, when no rain occured, no variations in NDVI were detected in this tributary, unlike the \(\sigma_{0}\) and interferogram anomalies after the flood. Thus, these variations cannot be explained by rains, which were non-existent during this period, but seem to be linked to the dynamics of the aquifer of the Kuiseb River, wherein floods recharge the alluvial aquifers and the rising water table levels produce a signal that is measurable by satellite radar sensors. All these results present a preliminary work that might be used by water resource managers to automate the processing and methods used to create an ephemeral river monitoring tool. fluid aquifer hydrology; groundwater monitoring; multispectral imagery; radar backscattering; interferometry 202214314202220221431420222143142022214314202221431420222143142022214314202  
of these rivers usually takes place as flood events after rain episodes in their catchments. Such floods recharge groundwater aquifers by seepage of water into the alluvial substrate of the riverbed and on floodplains [3; 8], which are a major source of freshwater in arid areas. Understanding floods and the associated hydrological dynamics is therefore crucial for determining sustainable yields in global water resources. Moreover, in the context of climate change, arid and semi-arid zones are likely to increase globally, and therefore the number of ephemeral rivers will also increase. Improved monitoring and understanding of the dynamics of ephemeral rivers and associated groundwater resources is therefore an essential component of adaptation to climate change. Until recently, the monitoring of ephemeral rivers and the associated groundwater aquifers was mainly through in situ hydrological and geohydrological stations [3] to gather data and model their dynamics [9]. However, a major limitation for in situ data from arid zones is the low number of available monitoring stations and data gaps from inoperable stations due to the high cost of installation and maintenance of such stations. Adequate spatial coverage, which was adapted to the study of an ephemeral river system as a whole, and continuous data to identify key hydrological fluxes, are rarely available. Models, for their part, represent a complementary tool, but they require reliable in situ data for their calibration and validation. Remote sensing from Earth observation satellites offers a low-cost opportunity to monitor ephemeral rivers and their associated groundwater resources, allows access to data over extended time periods, and can cover the specific spatial resolution that may be required.