Universal Soil Loss Equation
The Universal Soil Loss Equation (USLE) is a widely used mathematical model that describes soil erosion processes. Erosion models play critical roles in soil and water resource conservation and nonpoint source pollution assessments, including: sediment load assessment and inventory, conservation planning and design for sediment control, and for the advancement of scientific understanding. The USLE or one of its derivatives are main models used by United States government agencies to measure water erosion. The USLE was developed in the U.S., based on soil erosion data collected beginning in the 1930s by the U.S. Department of Agriculture  (USDA) Soil Conservation Service (now the USDA Natural Resources Conservation Service). The model has been used for decades for purposes of conservation planning both in the United States where it originated and around the world, and has been used to help implement the United States' multibillion-dollar conservation program. The Revised Universal Soil Loss Equation (RUSLE) and the Modified Universal Soil Loss Equation (MUSLE) continue to be used for similar purposes. Overview of erosion models. The two primary types of erosion models are process-based models and empirically based models. Process-based (physically based) models mathematically describe the erosion processes  of detachment, transport, and deposition and through the solutions of the equations describing those processes provide estimates of soil loss and sediment yields from specified land surface areas. Erosion science is not sufficiently advanced for there to exist completely process-based models which do not include empirical aspects. The primary indicator, perhaps, for differentiating process-based from other types of erosion models is the use of the sediment continuity equation discussed below. Empirical models relate management and environmental factors directly to soil loss and/or sedimentary yields through statistical relationships. Lane et al. provided a detailed discussion regarding the nature of  process-based and empirical erosion models, as well as a discussion of what they termed conceptual models, which lie somewhere between the process-based and purely empirical models. Current research effort involving erosion modeling is weighted toward the development of process-based erosion models. On the other hand, the standard model for most erosion assessment and conservation planning is the empirically based USLE, and there continues to be active research and development of USLE-based erosion prediction technology. Description of USLE. The USLE was developed from erosion plot and rainfall simulator experiments. The USLE is composed of six factors to  predict the long-term average annual soil loss (A). The equation includes the rainfall erosivity factor (R), the soil erodibility factor (K), the topographic factors (L and S) and the cropping management factors (C and P). The equation takes the simple product form: The USLE has another concept of experimental importance, the unit plot concept. The unit plot is defined as the standard plot condition to determine the soil's erodibility. These conditions are when the LS factor = 1 (slope = 9% and length = 22.1 m (72.6 ft) where the plot is fallow and tillage is up and  down slope and no conservation practices are applied (CP=1). In this state: A simpler method to predict K was presented by Wischmeier et al. which includes the particle size of the soil, organic matter content, soil structure and profile permeability. The soil erodibility factor K can be approximated from a nomograph if this information is known. The LS factors can easily be determined from a slope effect chart by knowing the length and gradient of the slope. The cropping management factor (C) and conservation practices factor (P) are more difficult to obtain and must be determined empirically from plot data . They are described in soil loss ratios (C or P with / C or P without).