|Revised Universal Soil Loss Equation 2 - RUSLE2 Definitions|
RUSLE2 uses a specific set of definitions, partly because the disciplines involved in soil erosion have not developed a common set of definitions. Observance of RUSLE2 definitions is required to obtain proper erosion estimates from RUSLE2 and to make the proper interpretation of those estimates.
The basic computational unit in RUSLE2 is an overland-flowpath. The overland-flow path used in RUSLE2 is the path that runoff follows from the origin of overland flow to the point where it enters a concentrated flow area, defined as a channel. The topographic information entered into RUSLE2 by the user for a specific site describes the slope steepness along this path.
Basic RUSLE2 Erosion Variables
This sediment yield is for a site only if the RUSLE2 flow path happens to end at the site boundary.
Detachment is the separation of soil particles from the soil mass. Net detachment adds sediment to the sediment load and causes sediment load to increase in a downslope direction. Deposition is the transfer of sediment from the sediment load back to the soil mass. Local deposition is the deposition of sediment very near to the point where the sediment was detached. Deposition of sediment eroded from soil clods in nearby depressions formed by the clods is an example of local deposition. Remote depositionis the deposition of sediment far from its point of origin such as deposition in a terrace channel or on the toe of a concave slope.
Main RUSLE2 Outputs
RUSLE2 displays the four output values of: soil loss from the eroding portion of the slope, detachment for the entire overland flow path, conservation planning soil loss, and sediment delivery (yield). Soil losshas a specific meaning. Soil loss is the net loss of sediment from the eroding portion of the overland-flow path. This value is used in conservation planningto select cover-management and support practices to control soil loss to a value less than soil loss tolerance or some other conservation planning criteria. Detachment is the total sediment production for the overland flow path length represented in a RUSLE2 computation. Sediment delivery (yield) is the amount of sediment leaving the flow path represented in a RUSLE2 computation. Total deposition for the overland-flow path, which is not displayed, is the differences between total detachment (sediment production) and sediment yield. Conservation planning soil loss gives partial credit to remote deposition depending on where the deposition occurs along the overland-flow path. RUSLE2 gives very little credit as “soil saved” for deposition that occurs near the end of the overland-flow path. Conservation planning soil loss is generally less than total detachment (sediment production) and greater than sediment yield. Full credit is taken for local deposition as soil saved.
However, RUSLE2 can also be applied to only the eroding portion of the slope to make USLE compatible soil loss estimates. In this application, the USLE slope length can be used in RUSLE2 as the distance from the origin of overland flow to the point where deposition begins on concave slopes or to a concentrated flow channel.
The recommended RUSLE2 approach is to represent the entire overland-flow path from origin of the overland flow to a concentrated flow channel. RUSLE2 automatically determines where deposition occurs as a part of its computations. RUSLE2 computes and displays the soil loss on the eroding portion of the overland-flow path, the deposition on the depositional portion of the overland-flow path, and sediment yield from the overland-flow path without any additional consideration. RUSLE2 uses the hillslope profile description entered by the user to make the appropriate computations. This application illustrates the increased power of RUSLE2 over the USLE.
RUSLE2 also computes the deposition that occurs in diversions and terrace channels that end an overland flow path. However, RUSLE2 does not compute the erosion that occurs in these channels if they are on a steep grade. RUSLE2 also computes deposition in small sediment basin type impoundments.
RUSLE2 computes the erosion rate (or deposition) for individual slope segments. These erosion (or deposition) values represent net sediment production (or deposition) within each segment. The ability to compute net sediment production along a slope is a very powerful RUSLE2 feature, especially for convex shaped slopes. Erosion rate at the end of a convex slope can be much greater than the average erosion rate for the entire slope.