Discussion
The GIS analysis was carried out for USLE to estimate annual soil loss on a pixel-by-pixel basis and the spatial distribution of the soil erosion in the study area. This treatment makes it possible to subdivide the study area (545 ha) into 4 distinct units.
Due to insufficient information on climatic data (R-factor), and the absence of anti-erosion practices (P-factor) in the study area, only the values of erosive potential were drawn. The other factors (K, LS, C) of the equation are described as a digital map, and the three digital layers of the equation are overlaid to obtain the spatial distribution of the erosive potential in the basin (Figure 4).
The assessed average annual soil loss rate of Ben Ahmed watershed was grouped into different classes based on the minimum and maximum values and the spatial distribution of each class is presented in Figure 4. The grouping of different soil erosion severity zones was carried out by considering the field conditions. The results presented in table 4 show that about 94.2% (504 ha) of the study area is classified as low potential erosion risk (0-5t/ha/yr), while the rest of the area is under moderate to high erosion risk. In terms of actual soil erosion risk, the study area has 4.1% moderate (5-10 t/ha/yr), and 1.7% high (>10 t/ha/yr) erosion risk levels. The spatial pattern of classified soil erosion risk zones indicates that the areas with low erosion risk are located in the agriculture land and represent 74.5% presented by the class C of 0.1, while the area with high erosion risk is in bare land and built-up, presented by the class C of 1. The spatial pattern of annual averages soil erosion risk map shows high spatial correlation with C factor, and it indicates the role played by cover in controlling soil losses in study area. The lands in our area are almost flat, with low LS values ranging from 0 to 4.5.
The high value of K factor are found in rendzinas (>0.4), and the moderate value ranging between 0.10 and 0.4 4 are found in calcareous and clayey regosol, while the vertisols and isohumic soil show low values of K factor (<0.05), similar to results found by Lahlaoi et al. (2015) at the Oued El Maleh watershed.
The average soil erosion rate recorded in our study area ranges from 0 to 88 t/ha/yr with a mean of 1.1 t/ha/yr and a standard deviation of 3.7t/ha/yr, which remains spatially variant and lower than the rate found by Ouallali et al. (2016), in the Wadi Arbaa Ayacha watershed (Western Rif, Morocco) with a mean of 8t/ha/yr. At the Nakhla watershed level, 65 t/ha/year was estimated by Tribak et al. (2012), 37 t/ha/year at the Khemis basin (Khali Issa et al., 2014) and 224 t/ha/year at the Oum Er-Rbia watershed (Yjjou et al., 2014).