Page 94 - Soil Degradation, Conservation and Remediation

3.3 Types of Water Erosion 81

K – the soil erodibility factor. K is a measure of the susceptibility of soil particles to
detachment and transport by rainfall and runoff. Texture is the principal factor
affecting K , but structure, organic matter, and permeability also contribute.
LS – the slope length and steepness factor. The LS factor represents a ratio of soil
loss under given conditions. The steeper and longer the slope is, the higher is the
risk for erosion. This is a very important factor in the overall erosion rate.
C – the crop management factor. It is used to determine the relative effectiveness of
soil and crop management systems in terms of preventing soil loss. The C factor
is a ratio comparing the soil loss from land under a specific crop and manage-

ment system to the corresponding loss from continuously fallow and tilled land.
The crop grown, type and timing of tillage, the use of winter cover, and the appli-
cation of solid manure will all impact on the C factor.
P – the support practices factor. The P factor compares the soil losses from up- and
downslope farming to losses that result from practices such as cross-slope
cultivation, contour farming, and strip cropping.
In RUSLE, the factors have been updated with recent information, and new
factor relationships have been derived based on modern erosion theory and data.
Major changes to the USLE incorporated into RUSLE include:
R factor: new and improved isoerodent maps and erodibility index distributions for
some areas
K factor: time-variant soil erodibility which reflects freeze – thaw in some geo-

graphic areas
LS factor: new equations to account for slope length and steepness
C factor: additional subfactors for evaluating the cover and management factor for
cropland and rangeland
P factor: new conservation practice values for cropland and rangeland

3.3.9.1 Rainfall Erosivity Factor ( R )

Generally, local variations in rainfall erosivity (+5 %) can be represented with a

single R value. R values can be calculated for specific locations from rainfall inten-
sity data. However, this is a very time- and labor-intensive process requiring erod-
ibility index calculations for each storm event greater than 0.5 in. for each rain
gauge over a period of years. “Equivalent” R values have been developed for many
areas in the USA. The effect of the distribution of R over the year is incorporated
into the model in the computation of the C factor.

3.3.9.2 Soil Erodibility Factor ( K )

The K factor represents both susceptibility of soil to erosion and the amount and rate
of runoff. Soil texture, organic matter, structure, and permeability determine the
erodibility of a particular soil. K values for various soil types are given below.