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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 72.6-foot (22.1-meter) 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 2-ft 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 slope-ending 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.