# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
## 4 Results and Discussion
### Temporal Variations of NDVI, NDMI and \(\sigma_{0}\)  
After computing spectral indexes (NDVI, NDMI) for the 123 images of Sentinel-2, a time-series of the radar backscattering coefficient (notated as \(\sigma_{0}\)) for the same detailed areas were calculated from 149 Sentinel-1B scenes. Figure 5 shows time-series graphs of \(\sigma_{0}\) in the VV polarization and spectral indexes at three locations in the study area: the Kuiseb delta downstream (Figure 5a,d,g), Gobabeb in the upstream part (Figure 5c,f,i), and at a tributary at Klipneus in the central part (Figure 5b,e,h). Graphs of rainfall at Gobabeb (Figure 5j,k,l)  
Figure 4: **(a–c)** NDVI maps in the central part (Klipneus) of the lower Kuiseb River acquired at three different dates (11 March 2018, 5 May 2018, and 20 May 2018) and (**d–f**) NDVI maps further upstream near Gobabeb acquired on 15 th January 2018, 11 th March 2018, and 5 th May 2018. The black box in 4b represents the Klipneus tributary that were studied further. and Claratal in the upper catchment of the Kuiseb River (Figure 5m,n,o) were repeated for each column of graphs. In the downstream deltaic area of the Kuiseb (Figure 5a), the amplitude of \(\sigma_{0}\) shows important variations (around 7 dB) in 2018 and 2021. NDVI and NDMI spectral indexes (Figure 5d,g) depict similar temporal variations in 2018 and 2021. However, the peak of NDMI always precedes the NDVI peak. This delay corresponds to the time needed for vegetation growth after a moisture event. Not surprisingly, these variations are correlated with rainfall (Figure 5j,m), such as in 2018 and 2021. A series of pronounced flood events during January 2021 caused strong variations in NDMI, NDVI, and \(\sigma_{0}\). A tributary located at Klipneus in the center of the study site (Figure 5b) shows a considerable decrease of \(\sigma_{0}\) (around 7 dB) in 2018, with an associated decrease in NDVI but an increase in NDMI (Figure 5e,h). The low NDVI values in April 2018 cannot be explained by in situ meteorological data from Gobabeb and Claratal (Figure 5k,n), as those sites recorded earlier rainfall events during March-April 2018--although the 2018 NDMI peak (Figure 5h) and rapid increase in NDVI during early 2018 (Figure 5e) are associated with  
Figure 5: (**a**) Time-series of \(\sigma_{0}\) (in VV polarization, in dB) at Gobabeb, (**b**) in the Klipneus tributary and (**c**) in the delta downstream. (**d**) Time-series of NDVI spectral index at Gobabeb, (**e**) in the Klipneus tributary and (**f**) in the delta downstream. (**g**) NDMI spectral index at Gobabeb, (**h**) in the Klipneus tributary and (**i**) in the delta downstream. (**j–l**) are time-series of rainfall (mm) at Gobabeb and (**m–o**) are time-series of rainfall (mm) at Claratal in the upper catchment of the Kuiseb River. the rainfall recorded at Gobabeb. During the flood of January 2021, a small decrease of \(\sigma_{\rm o}\) was observed (Figure 5b), but variations were also visible in the NDMI and NDVI spectral indexes. As expected, the moisture index NDMI peak occurs before the vegetation index NDVI peak, with a shift of about 3-4 weeks (see 2018 for example), probably because even a slight increase in water supply results in rapid growth of vegetation in desert environments. Upstream, at Gobabeb, variations in \(\sigma_{0}\) (Figure 5c) correspond to seasonal variations with the highest peaks during March-April. Increases in NDMI were followed by higher NDVI (Figure 5f,i) within--approximately--a month after the event. In 2021, a high peak in all parameters was linked to the January 2021 floods and rain in the upper catchment, such as on Claratal (Figure 5o). These results highlight the fact that even though vegetation and moisture variations are quite heterogenous along the lower Kuiseb River, these variations seem to be mainly linked to rainfall events. However, variations in \(\sigma_{0}\) throughout the study site do not seem to be related to moisture variations.