
LS Factor
L
is the slope length factor, representing the effect of slope length on
erosion. It is the ratio of soil loss from the field slope length to that
from a 72.6foot (22.1meter) length on the same soil type and gradient.
Slope length is the distance from the origin of overland flow along its
flow path to the location of either concentrated flow or deposition.
Fortunately, computed soil loss values are not especially sensitive to
slope length and differences in slope length of + or – 10% are not
important on most slopes, especially flat landscapes.
Slope lengths are best determined by visiting the site, pacing out flow
paths, and making measurements directly on the ground. Obtain L by
measuring perpendicular to the contour from the point of origin of
overland flow to where deposition begins or runoff enters a well defined
channel. Contour maps having intervals greater then 2ft should be used
cautiously, if at all, to determine slope lengths. Slope length values are
generally too long when contour maps are used to choose slope length.
Slope lengths usually do not exceed 400 ft. Slope lengths longer than 1000
ft should not be used in RUSLE because the reliability of RUSLE at these
long slope lengths is questionable, and flow becomes concentrated on most
landscapes before such long slope lengths.
Determining where slope lengths end can be difficult in Michigan's glacial
topography. Generally it is necessary to measure several slopes in a field
with variable soils. The main areas of deposition that end RUSLE slope
length are at the base of concave slopes. If no signs of deposition are
present, the user will have to visualize where deposition occurs. The
slopeending depositional area on a concave slope is usually below where
the slope begins to flatten.
As a rule of thumb, if no signs of deposition are present on a concave
slope, assume that deposition begins at the location where the steepness
is 1/2 of the average steepness of the concave area.
Another difficulty is determining if a channel is a concentrated flow
channel that ends a RUSLE slope length. Channels that collect the flow
from numerous rills are generally considered to be slope ending
concentrated flow channels.
S
is the slope steepness. Represents the effect of slope steepness on
erosion. Soil loss increases more rapidly with slope steepness than it
does with slope length. It is the ratio of soil loss from the field
gradient to that from a 9 percent slope under otherwise identical
conditions. The relation of soil loss to gradient is influenced by density
of vegetative cover and soil particle size.
L factor and S factor are usually considered together. LS factors = the
slope length factor L computes the effect of slope length on erosion and
the slope steepness factor S computes the effect of slope steepness on
erosion. Values of both L and S equal 1 for the unit plot conditions of
72.6 ft. length and 9 percent steepness. Values of L and S are relative
and represent how erodible the particular slope length and steepness is
relative to the 72.6 ft long, 9% steep unit plot. Thus some values of L
and S are less than 1 and some values are greater than 1. Stripcropping or
contouring does not affect the LS value.
Click
here to check LS values for Cropland
Click
here to check LS values for Construction Sites
From Technical
Guide to RUSLE use in Michigan, NRCSUSDA State Office of Michigan.
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