# 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)
At the end of every year during the monitoring period (2015-2018), the annual soil erosion was evaluated at selected measurements points. The erosive rainfall events that characterize the different erosion periods are significant in 2017 and 2018. Therefore, the dates related to the above measurement periods (for average and maximum soil loss) were used to identify the exceptional rainfall events that occurred during these periods. The spatial distribution of erosive rainfall together with topography soil texture and organic matter were analyzed for 25 sub-catchments. Since there were climatic (spatial rainfall distribution), physical (topography, slope), soil characteristics (texture, organic matter) and geographical conditions, the catchments were divided into three groups. The first group is located in the catchment upstream, represented by 10 cisterns (9 Majel and 1 Fesguia) and characterized by a mean annual rainfall of about 200 mm, and the mean slope exceeds 15%. The second group is located in the middle of the Koutine catchment and is represented by eight cisterns (6 Majel and 2 Fesguia), characterized by a moderate slope (4%) and a mean annual rainfall of 183 mm. The last group contains seven cisterns (4 Majel and 3 Fesguia), and the mean slope is 3% and the annual mean rainfall is 168 mm. The mean annual soil loss was 3.8 t/ha in the upstream and 1.3 t/ha in the middle and downstream regions. Figure 10 emphasizes the relationship between soil loss and slope. Results showed that the permissible limit of 12 t/ha/yr [22] was achieved in the upstream under the slope of 22%. In the downstream area, values stretch only between 50 kg/ha and 227 kg/ha (Figure 10). Figure 11 represents the relationship between specific events and maximum soil loss during a wet year (2017). Soil erosion loss was more important for specific events with high precipitation values (over 40 mm). The minimum soil erosion was 0.2 t/ha for events with a depth between 20 and 38 mm. The maximum (3.7 t/ha) was recorded for a rainfall event with an intensity depth of only 64.6 mm. Thus, erosion does not depend only on precipitation value but also on its intensity and soil properties. The event with a rain depth of 173 mm that happens at the beginning of the dry season results in less soil loss than a rainfall event of 64.6 mm that occurred in the wet season (Figure 11), reflecting the effects of antecedent conditions on soil-erosion delivery. Figure 10: Average annual soil erosion variation upstream (**a**) and downstream (**b**) of Koutine catchment parts as a function of the slope. At the end of this work, and to verify the accuracy of the model results, we made a comparison between values of erosion predicted by the RUSLE model and observed results (Figure 12). The RUSLE model could reasonably reproduce the measured soil losses (Figure 12), reflected by the square of the correlation coefficient (R\({}^{2}\)) of 0.81. Our results showed lower soil loss in the arid region of Tunisia (average of about 1 t/ha/yr). Other studies revealed  
Figure 11: Relationship between specific events and maximum soil loss during the wet year (2017). The rainfall intensity of each erosive event is given together with the bare plot of soil loss. Figure 12: Observed and predicted annual soil loss (t/ha/yr) on selected sites. higher soil erosion in the semi-arid compared to the arid regions (such as [15; 42]). In the arid region, the topography and the erosivity are the determinant factors for soil-erosion delivery. While in the semi-arid region, the interaction between rainfall and land use becomes an important factor in either accelerating or attenuating the soil detachment during the erosive events [62]. Comparing the spatially distributed RUSLE model simulation and observed values contributes to a better understanding of the mechanisms and processes leading to catchment soil loss.