# A SAR-Based Index for Landscape Changes in African Savannas
## 4 Discussion
### Landscape Changes  
One main point to report is that the landscape in the study area did not underlie large-scale changes, neither positively nor negatively. As shown in Figure 6, there in fact are certain hot spots of accumulated change over the investigated period, however, these did not add up to a gradual overall increase or decrease of landscape quality for larger areas. Addressing anthropogenic impact on the investigated landscape, neither of the two main settlements of camp Djabal and the city of Goz Beida, nor the plains within the Hadjer Arkop massif, had noticeable impact on their surrounding landscapes. These findings were expected in terms of no reported growth of these settlements during the investigated period [113; 114; 115]. It is still notable that this landscape did not negatively respond to the direct presence of human activities as arid savannas are often observed to be susceptible towards anthropogenic pressure [116]. One reason for the described stability within this area is the high amount of bare soil as a consequence of the relatively intensive agricultural use. Accordingly, their NR values were already quite low and only expansion of human settlements--or the transition from soil areas to bare rock--would have caused negative NRD\({}_{\text{w}}\) developments. In addition to that, no source of land degradation or desertification, as it might be indicated by transitions from vegetation to bare soil or soil to rock, could be observed among the bare rock areas of the Hadjer Arkop massif. One region which was prone to many changes during the investigated period is Wadi Aouada, reaching from the West to the North of the study area. As it is the only area with vital vegetation of higher orders during the whole year, it is particularly prone to changes in external circumstances. A direct comparison of the NR of 2007 with 2017 (Figure 7) does not depict a clear image on which parts of the wadi underlay significant positive or negative developments. It can, however, be stated  
Figure 8: Frequency distribution of the variability of grid cells within the study area compared to their NRD\({}_{\text{w}}\) index for the time between 2007 and 2017. Note the logarithmic scaling of the \(y\)-axis. that the core area of the wadi did not change in a critical degree--for example, due to erosion or lesser subsurface drainage. Looking at the annual changes in Figure 6 indicates that there are some years where the wadi is subject to larger changes, notably 2008-2009 (positive) and 2010-2011 (negative). These observations could be linked to climatic variations. However, comparison to records of temperature and precipitation provided by the Climatic Research Unit (CRU) of University of East Anglia (UEA) [117] does not confirm these trends. As Figure 9 shows, the contrary could have been expected as a considerable decrease in precipitation occurred between 2008 and 2009, which in turn shows the most visible positive development within Wadi Aouada. So it can be concluded that climatic variations are most likely not responsible for the smaller changes within the wadi. We expect them to result from overall variation in the vegetation signature within the SAR data. Another mentionable point is the striking accumulation of negative \(\mathrm{NRD_{w}}\) values in for 2011-2015 in the North of the Hadjer Arkop massif near the center of the study area. However, the fact that this area is again strikingly positive for the subsequent image pair from 2015-2016 indicates that this anomaly is rather the outcome of a wrong classification. The low variability values in this area (Figure 7) furthermore support this indication and lead to the conclusion that this area is in fact stable regarding LULC and the assessed changes are wrong. Nevertheless, many larger changes for the period from 2011-2015 are realistic results obtained by the observed increment of 4 years instead of one. But this is one example of how investigating variabilities along a time-series can help to detect errors and check the results for plausibility.