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