35 S : Code of Soil Structure
P : Code of Soil Permeability
3.6.3. Slope Length and Slope Steepness Factor LS
Slope length L is defined as horizontal distance from the origin of overland flow to the point where the slope gradient decreases considerably, so that
the deposition begins, or where runoff becomes concentrated in a defined channel Weischmeier and Smith 1978. Surface runoff will usually concentrate in less
than 400 ft, which is the recommended practical slope-length limit in RUSLE. The LS factor is the most difficult one to derive in GIS, because the length
aspect is not direct. The digital elevation model DEM of the study area was used in generating the LS factor. Because of the difficulties commonly experienced in
generating the LS factor, two methods were used. In the first method, the slope steepness S and length L factor layers were generated separately and later
overlaid to get the RUSLE slope length factor layer Mongkolsawat et al., 1994. In the second method, an ArcView based technique was used. The LS
factor can be estimated from the DEM. The technique described for computing LS requires a flow accumulation theme. Flow accumulation can be computed from a
DEM using the hydrologic extension or other watershed delineation techniques. Flow accumulation is used to estimate slope length. First we will compute slope
steepness using the DEM. Before beginning the analysis, check the Analysis Properties to make sure the extent and gird cell size are acceptable. The grid cell
size should be set to the grid cell size of the DEM for this analysis. •
Make the DEM active, select the Surface pull down menu as shown below and select the Derive Slope option.
36 •
Use the map calculator to create new themes of the flow accumulation classification as 0 and 1.
• Use map calculator to compute the LS factor as shown below. Note that
the slope must be converted to radians from degrees by multiplying the slope by 3.14 pi and dividing by 180.
The technique for estimating the RUSLE’s LS factor that will be used here was proposed by Moore and Burch 1986a and 1986b. They derived an equation
for estimating LS based on flow accumulation and slope steepness. The equation is:
LS = Flow Accumulation Cell Size22.130.4 sin slope0.08961.3
Fig 3.5: The process of manipulate LS factor from DEM
3.6.4. Cover Management Factor C
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