Second Quarterly Progress Report September 2004
North Fork Kaskaskia River Watershed
Limno-Tech, Inc. Page 7
Universal Soil Loss Equation
The Universal Soil Loss Equation (USLE), and variations of the USLE, are the most
widely used methods for predicting soil loss. When applied properly, the USLE can be
used as a means to estimate loads of sediment and sediment-associated pollutants for
TMDLs. The USLE is empirical, meaning that it was developed from statistical
regression analyses of a large database of runoff and soil loss data from numerous
watersheds. It does not describe specific erosion processes. The USLE was designed to
predict long-term average annual soil erosion for combinations of crop systems and
management practices with specified soil types, rainfall patterns, and topography.
Required model inputs to the USLE consist of:
• Rainfall erosivity index factor
• Soil-erodibility factor
• Slope length factor reflecting local topography
• Cropping-management factor
• Conservation practice factor
Most of the required inputs for application of the USLE are tabulated by county Natural
Resources Conservation Service (NRCS) offices.
There are also variants to the USLE: the Revised USLE (RUSLE) and the Modified
USLE (MUSLE). The RUSLE is a computerized update of the USLE incorporating new
data and making some improvements. The basic USLE equation is retained, but the
technology for evaluating the factor values has been altered and new data introduced to
evaluate the terms for specific conditions. The MUSLE is a modification of USLE, with
the rainfall energy factor of the USLE replaced with a runoff energy factor. MUSLE
allows for estimation of soil erosion on an event-specific basis.
While the USLE was originally designed to consider soil/sediment loading only, it is also
commonly used to define loads from pollutants that are tightly bound to soils. In these
situations, the USLE is used to define the sediment load, with the result multiplied by a
pollutant concentration factor (mass of pollutant per mass of soil) to define pollutant load.
The USLE is among the simplest of the available models for estimating sediment and
sediment-associated loads. It requires the least amount of input data for its application
and consequently does not ensure a high level of accuracy. It is well suited for screening-level
calculations, but is less suited for detailed applications. This is because it is an
empirical model that does not explicitly represent site-specific physical processes.
Furthermore, the annual average time scale of the USLE is poorly suited for model
calibration purposes, as field data are rarely available to define erosion on an annual
average basis. In addition, the USLE considers erosion only, and does not explicitly
consider the amount of sediment that is delivered to stream locations of interest. It is best
used in situations where data are available to define annual loading rates, which allows
for site-specific determination of the fraction of eroded sediment that is delivered to the
surface water.
