## 3 Results
### Impact of Land Use and Land Cover Change and SWC Techniques on Soil Loss
#### 3.4.3 Spatial Variability of the Net Sediment Yield  
To analyze the spatial variability of NSL, siltation data measured at the outlets of the 46 hill lakes and small dams were used. This observation network is related to the observation network set up by the General Directorate for Agricultural Land Management and Conservation in the 1990s. The sediment yield was generated at the outlet for all the observed sub-watersheds (Figure 14). These areas predominantly align with regions characterized by intensive cultivation and steep slopes. The majority of the sediment reaching the main channel is indeed generated from cropland and rangeland areas. The western section of the catchment experienced the most substantial rainfall and had the steepest slopes, resulting in high rates of soil loss due to intensive cultivation. Similarly, certain portions of the eastern catchment also contributed significant sediment yields for the same reasons.  
In the southeastern regions of the catchment, particularly in proximity to the Houareb Dam, elevated sediment yield is anticipated. Although certain regions within these areas feature relatively gentle slopes and low mean annual rainfall, the substantial sediment yield can be attributed to insufficient land management practices and extensive overgrazing. Figure 15 illustrates the relationship between the NSL values given by the RUSLE-SDR models and the erosion values observed at the outlets of the sub-catchments, which were measured by bathymetry. This figure shows a very good correlation (R\({}^{2}\) = 0.8) between the simulated values and the observed values. This result demonstrates the reliability of the RUSLE-SDR models in reproducing the eros  
Figure 14: Sub-watershed NSL maps.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **1980** & **1990** & **2000** & **2010** & **2020** \\ \hline NSL & 9.65 & 8.86 & 8.17 & 8.57 & 8.95 \\ (t/ha/year) & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Evolution of NSL rate for the period between 1980 and 2020.  
ent sub-catchments. Figure 15 highlights significant variability in sediment yield across individual sub-catchments as indicated by the predicted outcomes.  
The significant variations observed are an outcome of the varied land use practices, the broad spectrum of land slopes, and the proximity to channels within each of the sub-catchments. Sub-catchments where forests dominate as the primary land cover exhibit minimal soil erosion and sediment yield; however, it is worth noting that certain sub-catchments among them still have relatively high sediment delivery ratios.  
The results derived from Figure 15 suggest that depending solely on the mean sediment yield at the sub-catchment outlets may not adequately encompass the full extent of the sediment yield that eventually reaches those outlets. Consequently, this approach falls short in explaining the spatial distribution of sediment across the sub-catchments. Specifically, the results suggest that a considerable portion of sediment in areas with gentle slopes on hillslopes tends to deposit before reaching the mainstream outlet.