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The percentages of clay, silt, sand and organic matter were determinate for each major soiltype, and using the USDA texture triangle (Brown, 2003), the texture of each major soil typewas defined. To obtain the K factor for soil we use the chart correlating the textures and thestandard K-factor (Stone and Hillborn, 2002).Fig. 4: Soil map at 1/200 000 scaleFig. 5: Spatial distribution of K factorWith these results, the spatial distribution of the K factor was computed under a GIS at 10meters resolution (Fig. 5). We can see that most of the study area has a K factor between0.013 and 0.03 t.ha.h/ha.MJ.mm. On the areas of utramafic rocks (mining massifs), the Kfactor is relatively low. These, consisting of a hard iron crust, have a sensitivity to erosionlow. Conversely, the Central Range has a sensitivity to erosion with a high K value. This isexplained by the high percentage of sand in the fersiallitic soil who compose it, making themfragile. The coastal plain as a whole presents a medium to high sensitivity to erosionespecially in the river system in the presence of alluvial material with a high K value.III.3/ Slope length and steepness: LS-factorIn the USLE model, the effect of topography on erosion is accounted for by the slope length(L) and the slope steepness (S). The L factor is defined as the distance from the source ofrunoff to the point where either deposition begins or runoff enters a well defined channel thatmay be part of a drainage network. S factor reflects the influence of slope steepness onerosion (Wischmeier and Smith, 1978). The longer the slope length the greater the amount ofcumulative runoff. Also the steeper the slope of the land the higher the velocities of therunoff which contribute to erosion. But soil loss increases more rapidly with slope steepnessthan it does with slope length (McCool et al., 1987). The common equation used forcalculating LS factor provide by the USDA Agricultural Handbook (Wischmeier and Smith1978):m⎛ λ ⎞2LS = ⎜ ⎟ × (65.41sin θ + 4.56sinθ+ 0.065) where λ is the slope length in meters, θ⎝ 22.13⎠is the slope angle in degrees, and m is a slope angle contingent variable ranging from 0.01 to0.56 (McCool et al. 1987).For computing the LS factor, after testing different methods as Mitasova et al.(1998), wechoose to use an algorithm developed by Van Remortel et al. (2001), based on the equation ofRenard et al. (1997). Using an AML (Arc Macro Language) script under ArcInfo and aDigital Elevation Model (DEM) at 10 meters resolution, λ and θ were computed directly6
under the GIS software. The LS factor ranges from 0 to 104 with an average of 6.4 (Fig. 6).Half of the values are under 5 and correspond mostly to plain zones. The high values (superiorto 20) correspond to the crests of the mountains, more sensible to erosion. But it representsonly 7% of the area.Fig.6: Spatial distribution of LS FactorIII.4/ Cover management: C-factorThe C-factor measures the effects of all interrelated cover and management variables (Renardet al., 1997). C factor is measured as the ratio of soil loss from land cropped under specificconditions to the corresponding loss from tilled land under continuous fallow conditions. Bydefinition, C equals 1 under standard fallow conditions. As surface cover is added to the soil,the C factor value approaches zero. A C factor of 0.15 means that 15% of the amount oferosion will occur compared to continuous fallow conditions (Kesley and Johnson, 2003).The C factor was derived using remote sensing techniques. Thus, the land cover layer wasimplemented from the results of the supervised classification method (produced by DTSI)based on a SPOT 3 satellite image taken in 1996 with a 20 meters resolution (Fig. 7). Thesedata seem old but the land cover has not really changed these ten last years because the humanpressure is low in this region. The plain is covered by an extensive grass and dry niaoulissavannah over low hills and flat areas. Large areas of ultramafic rocks correspond tomountainous areas with vivid red lateritic soils covered by an endemic vegetation bush andforests of unique plants species. Along the coastal zone mangroves are present. Thedetermination of factor C depends on the coverage of the soil surface by vegetation. Frombibliographic data, we assign a coefficient C between 0 and 1 for each type of vegetation onthe study area. (Table 3). The distribution of factor C is quite heterogeneous (Fig. 8). Baresoils most erodible, are mainly located on the coast and the mountains degraded by miningactivity. The areas of dense vegetation located mainly in the hills of the “Central Chain areless susceptible to erosion.Table 3: Land cover types and C-FactorVegetation typeC FactorForest 0.001Savanna 0.04Mining scrubland 0.25Swamp 0.28Brush 0.72Bare land 17
Page 1 and 2: ASSESSMENT OF SOIL EROSION USING US
Page 3 and 4: human activities chain that depend
Page 5: Fig.2: Localisation of the weather
Page 9 and 10: Fig. 9: Potential soil loss erosion

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