# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
## 4 Results and Discussion  
### Spatial Variations of Vegetation and Water Spectral Indexes  
NDVI and NDMI were calculated from 123 relevant Sentinel-2 images between June 2015 and January 2022. A total of 18 images were acquired during the austral summer (December-January-February), 26 during autumn (March-April-May), 49 during winter (June-July-August), and 30 during spring (September-October-November).  
Figure 3 depicts the average of the spectral indexes calculated over the time-series. Figure 3a,b shows the temporal average for the NDVI and the associated standard deviation, while Figure 3c,d provides the average for the NDMI and its associated standard deviation. Figure 3a indicates high NDVI values in the study area, particularly in the downstream, deltaic part of the lower Kuiseb River; in the center, near Klipneus; and in the upstream part, near Gobabeb. These three areas are delineated by black boxes in Figure 3a. The standard deviation (Figure 3b) also shows high values for these areas, indicating strong seasonal and interannual vegetation responses.  
The averaged values of the NDMI (Figure 3c) are very different to the NDVI (Figure 3a), with high values north of the Kuiseb River on the gravel plains, and low values in the Namib Sand Sea to the south. Higher soil moisture in small channels and ephemeral tributaries of the gravel plains drainage to the Kuiseb River are clearly highlighted by higher average NDMI values, which are absent in the sand dunes. Even though low NDMI values south of the Kuiseb River--which correspond to the Aeolian dunes of the Namib Sand Sea--are not unexpected, a pronounced gradient of diminishing NDMI values from west to east indicate the effect of advective fog from the ocean on surface moisture in  
Figure 3: (**a**) Temporal averaged NDVI from Sentinel-2 images with names of major locations, (**b**) associated standard deviation **a**, (**c**) averaged NDMI from Sentinel-2 images, and (**d**) associated standard deviation. Black boxes in (**a**) show sub-areas selected for the future analysis. Main cities and villages are represented in black.  
windblown sand. Notably, Figure 3d indicates the high variations in soil moisture within the Kuiseb River at similar locations than the boxed areas delineated in Figure 3a.  
These maps guided us to select and define three sub-areas with high NDVI and NDMI variations. On Figure 4a-c, we present NDVI maps for the box in the center of the lower Kuiseb study area (see Figure 3a). At Klipneus, a tributary with negative NDVI values appears on 5 May 2018 (Figure 4b, black box and red cross) and is no longer detectable on the next date (Figure 4c). Negative NDVI values indicate the presence of water or moisture in the Klipneus tributary that were only briefly present during May 2018. Meteorological data from nearby weather stations indicate a rain event at the time, thus the negative values are therefore linked to the presence of water on the surface.  
Similar NDVI variations can also be observed in NDVI maps (Figure 4d-f) for the box located further upstream, near Gobabeb (see Figure 3a). On 11 March 2018 (Figure 4e), the NDVI index along the main course of the Kuiseb River increases, decreasing again by 5 May 2018 (Figure 4f). However, compared to Figure 4a-c, vegetation growth could not be linked to the rainfall or flood event during 9-12 March 2018; vegetation growth needs approximately one month after the rain or flood. The best explanation for that NDVI signal downstream of Gobabeb is microbial activity from flood debris (which occured before March 2018).