# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
## 4 Results and Discussion
### Variations at the Klipneus Kuiseb River Tributary  
We focused on a tributary at Klipneus (outlined in red in Figure 7a) to examine the hydrological dynamics in the central part of our study area (Figures 4b and 5b,e,h). The time-series of the backscatter coefficient \(\sigma_{0}\) (Figure 7b) and the NDVI maps acquired in May 2018 (Figure 7c) and February 2021 (Figure 7d) show two negative peaks in 2018 and 2021 that are likely related to rain events (see part 4.2). The tributary is visible in multispectral imagery from 2018, but not in 2021, when an exceptional flood occurred in the Kuiseb River. We were interested in explaining these differences between the two years. Information on the properties of the near subsurface can be obtained from the backscattering coefficient \(\sigma_{0}\), since the C-band penetration depth for Sentinel-1, with a frequency of 5.4 GHz, is in the order of approximately 20 cm for dry soils. In comparison, the MultiSpectral Imagery sensor on Sentinel-2 only allows for measurements from the surface. We also obtained additional information from the phase of the Sentinel-1 radar signal, which is more sensitive to moisture variations. Meteorological records from in situ stations in the Kuiseb River catchment recorded rain in 2017 and 2018 in the lower Kuiseb (Figure 5k), including at Klipneus (_pers. obs._ Marais), which explains the variations in Sentinel-1 amplitude and Sentinel-2 NDVI vegetation growth for those years. However, in 2021, after a large flood caused by heavy rains upstream in the catchment of the Kuiseb River (Figure 5n), no change in the NDVI was visible at the tributary location, yet there was a visible decrease in the \(\sigma_{0}\) amplitude (Figure 7b). Radar interferograms at the tributary location were produced for 2018 and 2021 to try to understand the origin of the variations in the radar backscattering coefficient (Figure 7b). Interferograms mapped coherences in radar returns between two acquisition dates for 14 and 26 March 2018 (Figure 8a), 26 March and 7 April (Figure 8b), 25 May and 6 June (Figure 8c), and 18 June and 30 June (Figure 8d). Figure 8a (14 and 26 March 2018) corresponds to the state before the << disturbance >>, with Figure 8b,c during the << disturbance >>, and Figure 8d after the << disturbance >>. << Disturbance >> is defined by a strong decrease in coherence. It is observed over the entire area in Figure 8b,c, but especially for the tributary (indicated by a black square in Figure 8b). This decrease in coherence can be explained by 5 days of rain that occurred between March and June 2018. Interferograms were also produced for 2021, when a major flood occurred during January in the main channel of the Kuiseb River (Figure 8e-h). As before, the coherence maps correspond to different states before the flood (Figure 8e, 4 and 20 December 2020), during the flood (Figure 8f, 9 and 21 January 2021), and after the flood (Figure 8g, 22 March and 3 April 2021, and Figure 8h, 5 and 17 November 2021). Unlike 2018, there is no overall decrease in coherence over the entire area, but a localized decrease is visible in the tributary (indicated by a black square in Figure 8f). This decrease in coherence seems to be associated with the flood event in the Kuiseb river channel during January 2021. Figure 6: Surface backscattering model outputs with (**a**) fixed \(\sigma\), Lcorr in dry/humid conditions; (**b**) different \(\sigma\), Lcorr in dry/humid conditions; and (**c**) \(\sigma\), Lcorr values in the black boxes allow us to reproduce \(\sigma_{0}\) variations observed in the Sentinel-1 time-series from the lower Kuiseb River. Hydroological activity in the Klipneus tributary seems to be linked to local rainfall, such as the 2018 event, which also affected a wider area as an interferogram << disturbance >> (Figure 8b), or as a more localized << disturbance >> that is linked to water flow in the main channel of the Kuiseb (Figure 8f).