Soil Erodibility Factor (K)
The evaluation of the K-index in the Ben Ahmed watershed was based on the 1 / 100.000 soil map, supplemented by data analysis of studied soil (texture, structure, organic matter, permeability). There are 5 soil types within the Ben Ahmed watershed, K values related to the different soil types have been estimated from the USLE Nomograph (Wischmeier and Smith, 1978). By plotting the K values in respect to different soil types, the soil erodibility map of the study area was elaborted (Figure 3). The K factor map shows a maximum value of 0.43 and minimum value of 0.007, with 25.5% of the soil showing very low erodibility (K <0.05), represented by the vertisols and isohumic soils. The moderate values of erodibility ranging between 0.1 and 0.2, were reported in the calcareous regosols, covering 3.8% (20 ha) of the total catchment area, while for clayey regosols, the K value is high, ranging from 0.2 to 0.4. The maximum K value (0.43) was reached for rendzinas. Spatial distribution of K value is shown in Table 4. From this study, it has been found that very low K factor (<0.05) was found in 25.5% of the total area while major portion (59.2%) of the total basin has recorded very high K values (K>0.4).
Table 3 here
Figure 3 here

Evaluation of soil losses

After completing data input procedure and preparation of the appropriate maps as GIS raster layers, they were overlaid using the ”Raster calculator” module of the GIS software ”ESRI ArcGIS ©” in order to estimate of the potential rate of soil erosion. In order to predict the annual average soil loss rate in the Ben Ahmed basin, all parameters of the USLE model are overlaid using multiplicative function of the Raster calculator. The soil erosion map (Figure 4) represents the spatial distribution of potential annual average soil loss rates of the study area showing a maximum value of 0.88 t/ha/yr and minimum of 0 t /ha/yr distributed in the total study area.
Our study revealed that 90.1% of the total study area is characterized by an erosion rate ranging between 0 and 2.5 t/ha/yr, while the moderate values of rate soil loss are ranging between 2.5 and 10 t/ha/yr cover 8.2% of the total catchment and the area with soil loss rate greater than 10 t /ha/yr cover 1.7% of the total area (Table 4).
Table 4 here
Figure 4 here