The most widely used method for predicting soil loss from overland areas
is the Universal Soil Loss Equation (USLE) (Novotny and Chesters 1981;
Overcash and Davidson 1980; Mitchell and Bubenzer 1980). Overcash
and Davidson (1980) note that the USLE "is and will continue to be for
the foreseeable future, the best equation for estimating long term, average-annual
and monthly soil loss." The USLE was developed to predict average-annual
soil loss from sheet, rill, and interrill erosion. Sheet erosion
refers to sediment movement from small natural areas having little topographic
relief (Eagleson 1970). Rill erosion refers to concentrated soil
movement due to channelized flow (Meyer 1974). Interrill erosion
refers to uniform soil movement in the remaining areas between rills; interrill
erosion results primarily from raindrop impact (Meyer 1974).
Wischmeier (1976) notes that the USLE may be used to predict average-annual
soil loss from a field-sized plot with specified land use conditions (Mitchell
and Bubenzer 1980). The assumptions associated with the USLE are
as follows (Goldman el al. 1986; Novotny and Chesters 1981; Foster 1976;
Onstad and Foster 1975):
The USLE is an empirically derived algorithm and does not mathematically represent the actual erosion process.
The USLE was developed to estimate long-term, average-annual, or seasonal
soil loss. Unusual rainfall seasons, especially higher than normal
rainfall and atypically heavy storms, may produce more sediment than estimated.
The USLE estimates soil loss on upland areas only; it does not estimate
sediment deposition. Sediment deposition generally occurs at the bottom
of a slope (i.e., change in grade) where the slope becomes milder.
The USLE estimates sheet, rill, and interrill erosion and does not estimate
channel or gully erosion. Gully erosion, caused by concentrated flows
of water, is not accounted for by the equation and yet can produce large
volumes of eroded soil.
The USLE was developed originally to address soil loss from field-sized
plots, although with proper care, watersheds can be addressed.
Because the USLE only estimates the volume of sediment loss (i.e., the
volume of soil detached and transported some distance), it can be used
to estimate sediment transport capacity at a site.
Because the USLE represents an empirically derived expression, consistently
accurate estimates of soil loss are fortuitous at best. (a)
The USLE does not estimate soil loss from single storm events unless a
modified form of the original equation is used.
The general form of the USLE, as expressed in metric units, is as follows (Goldman et al. 1986):
where
Aloss is the average-annual soil loss (t ha-1 yr-1)
Rfact is the rainfall erosivity factor (100 m) (tha-1) (cm hr-1)
Kfact is the soil erodibility factor (t ha-1 yr-1 per unit Rfact)
Sfact is the slope length and steepness factor (unitless)
Cfact is the vegetative cover factor (unitless)
Pfact is the erosion control practice factor (unitless).
The five factors are multiplied together to produce an estimate of the
soil eroded from the site in an average year. Goldman et al. (1986)
note that the USLE is most effective when evaluation of site characteristics
is done over areas no larger than 40 ha (100 acre). To produce the
most representative results, the soil loss analysis at the waste site should
be conducted over areas that have similar conditions.
Generally, the waste sites are
small enough such that these criteria are approximately met. At sites where
these conditions cannot be approximated, the site can be subdivided into
smaller areas. The five factors constituting the USLE are discussed in
more detail as follows.
(a)
Although
the USLE is best used to evaluate the relative effectiveness of different
land use patterns and practices, this does not diminish its utility for
predicting soil loss, especially as it relates to the source-term release
module methodology.
