Topographic factor (LS)
Topographic factor represents the influence of slope length and slope
steepness on erosion process. LS is calculated using the formula
mentioned in the methodology section, using an ASTER type digital
terrain model with a spatial resolution of 30 m. LS values obtained in
the Ben Ahmed watershed range from 0 to 4.25 (Figure 1).
Figure 1 here
Cropping management factor
(C)
Currently, due to the variety of land cover patterns with spatial and
temporal variations, satellite remote sensing data sets are used for the
assessment of C –factor (Karydas et al., 2009). Based on our land cover
mapping, the results showed that the C factor ranged from 0 to 1. The
land use-land cover of the Ben Ahmed watershed was classified with three
land cover classes, namely, agriculture land which represent 74,5%
(C=0,1), bare land representing 22,4% (C=1) and built-up (C= 0)
representing 3,1%. These land use-land cover features are represented
in Table 1 and Figure 2.
Table 1 here
Figure 2 here
Rainfall-runoff erosivity (R)
factor
Several studies (Jain et al., 2001; Dabral et al., 2008) revealed that
the soil erosion rate at the watershed level is more sensitive to
rainfall. In Ben Ahmed watershed, the mean annual rainfall is 548.83 mm.
It is calculated by reporting the average annual precipitation values
measured between 1969 and 2002 in the surrounding catchment area. The
R-value of Ben Ahmed watershed obtained from annual and monthly
precipitation averages is 75.67 (MJ * mm / ha * hour) (Table 2).
Table 2 here
Support practice (P)
factor
The most conservative practices of soils are contour crops, alternating
strips or terraces, reforestation in banquettes. According to the
Wischmeier equation, P is equal to 1 if none of these practices exist in
watersheds, which is the case of our study area.