Predicting Soil Loss through Erosion: RUSLE

Several mathematical models exist for calculating the potential for soil loss from a parcel of land under various conditions. The most commonly used model to estimate potential soil loss is the Revised Universal Soil Loss Equation (RUSLE), the 1990’s successor to the Universal Soil Loss Equation (USLE), developed in the 1970’s by the USDA Natural Resource Conservation Service (NRCS). This model includes several variables in the calculation:

• Rainfall erosivity factor (R): a metric based on the total amount, intensity, and seasonal distribution of the rainfall of the region;
• Soil erodibility factor (K): based on the texture, infiltration capacity, organic matter content, and soil structure;
• Topographic factor (LS): a measure of both the length and steepness of slopes;
• Cover and management factor (C): the type of land cover and management on a parcel of land;
• Support practice factor (P): this value is set to 1.0 unless erosion control measures such as conservation tillage, contour cultivation, or terracing are implemented.

The values are calculated for each of these factors and multiplied together to estimate annual soil loss in tons (A) from a parcel of land:

A = R x K x L x S x C x P

An example of a calculation using RUSLE can be accessed here.

Management Actions Influence Soil and Water Cycling

Soil function and erosion are affected in many ways by human modifications to the ecosystem. For example, construction of buildings, parking lots, and roads creates impervious surfaces, from which water and pollutants directly run off. This can have subsequent impacts on water quality and quantity. No infiltration can occur on impervious surfaces. Water is often channeled from impervious surfaces into a drainage basin or channel of some sort, concentrating the dissolved pollutants. Surface drainage and water cycling are described in more detail in the Streams module of this website.

Altering the vegetation cover of a parcel of land also impacts infiltration rates. Removing native plants in lieu of a golf course can decrease infiltration and increase runoff. The removal of vegetative cover at construction sites exposes soil and can lead to erosion by wind or water.

Land uses which result in soil compaction also reduce infiltration and can increase runoff. Wheel traffic is a major source of soil compaction, the effects of which can be seen for decades. Soil compaction is still evident along sections of wagon trails used by settlers in the late 1800’s.

Soils perform very important functions in our ecosystems. An understanding of these functions can lead to better watershed management decisions. The next section addresses ways in which soil knowledge is relevant to watershed conservation, protection, and restoration.

Right: Image courtesy of: Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).