# Soil Erosion Estimates in Arid Region: A Case Study of the Koutine Catchment, Southeastern Tunisia
## 4 Results and Discussion
### Simulated Annual Soil Loss (t/ha)  
The simulated annual soil loss results of the RUSLE erosion model in the Koutine catchment vary between 0 and 70.9 (t/ha/yr) with an average annual rate of 1 t/ha/yr (Figure 9) and total losses of 26,487 t/yr. The distribution under RUSLE classes of soil loss shows a predominance of loss lower than 0.5 t/ha/yr, which concerns the major part of the Koutine catchment (70%). Then comes the class 1-5 t/ha/yr, which characterizes 25% of the catchment and corresponds approximately to the piedmonts of the reliefs and the moderate slopes. Rates above 5 t/ha/yr represent only 5% of the study site. This means that 95% of the land in the catchment has less than 5 t/ha/yr (Figure 9). \begin{table}
\begin{tabular}{c c} \hline
**Vegetation Cover** & _C Factor_ \\ \hline Bare soil, fallow & 1.0 \\ Fruit trees & 0.9 \\ Cereal rotation & 0.4 \\ Ordinary pasture & 0.15 \\ \hline \end{tabular}
\end{table} 
Table 2: Land cover management values for different \(C\) factor classes [30]. \begin{table}
\begin{tabular}{c c c} \hline
**Slope Classes** & **Contouring** & **Terracing** \\ \hline
0.0–7.0 & 0.55 & 0.12 \\
7.0–11.3 & 0.60 & 0.14 \\
11.3–17.6 & 0.80 & 0.14 \\
17.6–26.8 & 0.90 & 0.16 \\ \textgreater{}26.8 & 1.00 & 0.18 \\ \hline \end{tabular}
\end{table} 
Table 3: \(P\) factor values for different slope classes in Koutine catchment [51]. Figure 9: Simulated annual soil loss distribution in Koutine catchment. In order to validate simulated results, in situ soil loss measurements were quantified based on observation of deposited sediment in the sediment settlement basin of cisterns mentioned above. Table 4 shows a summary of cisterns characteristics and mean annual soil loss during the monitoring period. Measurement campaigns started in 2015 and continued for a 4-year period. The number of observations for each cistern stretches between 5 and 13 depending on the characteristic of the storm event and the importance of the erosive event. It is worth mentioning that the deposited sediment was not monitored for all storm events. The measurements from storm events characterized by very short duration and negligible eroded sediment were omitted during the study. Therefore, the number of observations depended on cisterns location and storm events; a total of 240 measurements were obtained (Table 4). The annual soil loss in the Koutine catchment during the monitoring period stretches between 0.01 and 12.5 t/ha/yr, slightly exceeding the permissible limit of 12 t/ha/yr [22]. [PERSON] et al. [57] also recorded similar results during an observed period of 5 years (1972-1977) with a minimum erosion of 0.3 t/ha/yr and a maximum of 12.3 t/ha/yr. A maximum of 7.3 t/ha per event was observed after a specific erosive event occurred in the upstream part of the Koutine catchment. Simulation results obtained by [PERSON] et al. [61] using modified Sediment Transport Model (2D-version) indicate that values stretch between 1 and 5 t/ha/yr. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sampling** & **Code** & **Sub-Catchment** & **Slope (\%)** & **Observation Years** & **Number of** & **Mean Soil Loss** \\
**Number** & & **Area (m\({}^{2}\))** & **Slope (\%)** & **Observation Years** & **Storm Events** & **(t/ha/yr)** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 4: Cisterns characteristics and yearly soil loss during the sampling periods (2015–2018). The fi and mi referred to Fesguia and Majel, respectively. Six erosive events (put in bold) are detailed in Figure 10 below. The maximum soil loss rate was observed in the two successive relatively wet hydrological years (2016/2017 and 2017/2018) for specific cisterns. However, for some cisterns, there were only data for single- or two-measurement campaigns due to spatial rainfall distribution. This was the case for cisterns f3, m9, m13, m15 and m16, for which the maximum solid deposits in sediment settlement basins could not be determined.