# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
## 2 Ephemeral Rivers
### The Kuiseb River  
The Kuiseb River is one of twelve major west-flowing ephemeral rivers in Namibia, traversing the central Namib Desert on the southwestern African coastal plain (Figure 1a,b). It is one of the longest ephemeral rivers in Namibia, with a length of 560 km and a drainage basin of 15,500 km\({}^{2}\)[15]. The Kuiseb River begins its course on the high plateau of the Khomas Hochland Mountains (1,500-2,000 m above sea level), cutting through the Great Escarpement [15] before crossing the Namib Desert to reach the Atlantic Ocean at Walvis Bay [3; 16]. The river valley is largely incised into schists and quartzite bedrock, of which the erosion products are the source of an alluvial sand and gravel, which fill in its riverbed. The lower part of the Kuiseb River, approximately 150 km from the coast, below the Kuiseb Canyon, consists of a sandy channel bordered by riparian vegetation before widening at Rooibank, approximately 30 km from the ocean, into a terminal delta [8; 16]. The climate over the Kuiseb catchment is dominated by a steep climatic gradient from the eastern highlands, with 500 mm/year of rainfall, to virtually no rainfall on the coast in  
Figure 1: (**a**) World map and location of Namibia with a red box. (**b**) Locations of the 12 ephemeral rivers in Namibia, with the Kuiseb River highlighted in blue (number 10, from https://namibia. africageoportal.com/ (accessed on 4 April 2022)). (**c**) Sentinel-2 image of the Kuiseb River, (**d**) picture taken by the Gobabeb Research Center at the arrival of the flood mid-January 2021, and (**e**) the flood in the Kuiseb River after 4 days. the West [3; 17]. Along the lower Kuiseb River, there is too little rainfall, ranging from 10 to 50 mm per year, to sustain a river flow [16]. Episodic waterflows in the Kuiseb River are mostly due to rainfall that occurs in the upstream in the upper catchment on the highlands. Rainfalls in the Kuiseb catchment are predominantly due to summer thunderstorms from January to April, with 88% of floods occuring between December and April [15]. These floods have a duration from a few hours to a few days, which is why the Kuiseb River is called an \"ephemeral river\" [18]. The mean duration of Kuiseb floods is 12 days, and a \"big\" flood occurs, on average, every 25 years [15; 18; 19]. Flood events rarely reach the ocean as the water infiltrates, evaporates, or evapotranspiration before arriving in the deltaic zone. In the lower reaches, the riverbed forms the boundary between two distinct ecosystems: gravel plains to the north and the Namib Sand Sea to the south. Close to the ocean, this border is being forced towards the north over time by aeolian dune encroachment (Figure 1c, [20]). Over the last 20 years, there were only three large flood events in the Kuiseb River--in 2011, 2021, and in 2022. Photographs from the Gobabeb Namib Research Institute shows the arrival of a flood event at Gobabeb on 13 January 2021 (Figure 1d), and when it started to \"disappear\" on 17 January 2021 (Figure 1e). Floods in the Kuiseb River are the main source of water to the coastal region of central Namibia, even if the water flow is rare, since flood water infiltration recharges the alluvial aquifers along the river [3; 19]. Numerous alluvial aquifers occur along the Kuiseb River that not only provide essential water needs for humans, vegetation, and wildlife, but also for industry, tourism, and irrigation. These hidden treasures are essential for life during dry periods [21]. However, the recharge of alluvial aquifers will not be the same as it depends on the intensity and duration of a flood and the nature of the alluvial sediment. The alluvium in the bed of the Kuiseb has a thickness of about 15-20 m with high permeability, as indicated by high pumping rates, which suggests rapid recharge of aquifers when flood events occur [21]. Indeed, at Gobabeb for example, recharge of the alluvial aquifer occurs at a rate of 0.5 to 1 cm/h when the water level in the river bed is greater than 15 cm.