Page 95 - Soil Degradation, Conservation and Remediation

82 3 Soil Erosion by Water

Soil type Erodibility K value range
Fine-textured; high in clay Low 0.05–0.15
Coarse-textured; sandy Low 0.05–0.20
Medium-textured; loams Moderate 0.25–0.45
High silt content High 0.45–0.65
Although soil organic matter reduces erodibility, extrapolation of the K factor
nomograph beyond an organic matter of 4 % is neither recommended by the NRCS
nor allowed by RUSLE software. Soil structure affects erodibility by infl uencing
detachment and infiltration. Permeability of the soil profile affects K because it

affects runoff (Laflen and Moldenhauer 2003 ).

3.3.9.3 Slope Length and Steepness Factor (LS)
The LS factor represents the combined effects of slope length and steepness relative
to a standard unit plot on the erodibility. Slopes of nonuniform steepness require

dividing the slope into segments. Usually, five segments comprised of slope length

and steepness are sufficient to define a nonuniform slope profile. There are different
equations for calculating LS factor for different slope conditions.
3.3.9.4 Cover and Management Factor ( C )

The C factor represents the effect of plants, soil cover, belowground biomass, and
soil-disturbing activities on soil erosion. Both time-variant (cropping/rotation sce-
nario) and time-invariant (average annual values) modules have been constructed.
The time-variant option is used when plant and/or soil conditions change enough to

significantly affect erosion during the year, during a rotation cycle, or over an extended
period. This option is typically applied to croplands; rangelands where cover changes

significantly during the year such as from grazing, burning, or herbicide application.
The time-invariant option is used where constant conditions can be assumed. The C
factor depends on effective root mass in top 4″ of soil, percent canopy, average fall
height (ft), surface roughness value (index of average micro- elevation: generally ranges
from 0.3 to 1.5), percent ground cover (rock + litter, excluding plant basal cover), and
surface cover function expressed as B value (the relative effectiveness of surface cover
for reducing soil loss). The choice of B value is based on the ratio of rill/interrill erosion
under bare soil conditions. Some typical B values are given below:

Field Conditions ( B Value)

Flat and short slopes, where soil is resistant to erosion by flow, consolidated lands 0.025
(e.g., pasture)
Moderate slopes and slope lengths with moderate disturbance 0.035
Steep and long slopes where soil is highly disturbed and where soil is susceptible 0.045
to erosion by fl ow
Range lands, where runoff tends to be low and affected by cover 0.045
Long-term no-till cropping, especially where no-till significantly reduces runoff 0.050