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Variation in computing the Length Factor in the Universal Soil Loss Equation Ernest W. Tollner

Abstract

The universal soil loss equation, A = R*K*L*S*C*P, estimates average annual soil loss A based on rainfall (R), soil factor (K), length factor (L), slope (S), effective cover factor C, and a practice factor P. In teaching the use of the relationship, students can find values of R on maps, soil factor in tables based on a soil classification readily available in soil surveys, slope S may be estimated from the topographic map of the site., the cover factor C may be ascertained based on a specified crop rotation and the erosion handbook and the practice factor P may be determined from the erosion handbook for specified conservation practices such as terracing, contouring, etc. One source of variation in estimates is to arrive at meaningful slope length L. The erosion handbook suggests that one look for the length of the steepest segments of slope in the typical field of variable slope but leaves to judgment the actual quantification. The paper will report on the variation in techniques that students and professionals learn and use when asked to implement this procedure. A new approach for making a quantitative estimate of the length factor will be presented and compared with research data.

Introduction

The universal soil loss equation, A = R*K*L*S*C*P, estimates average annual soil loss A based on rainfall (R), soil factor (K), length factor (L), slope (S), effective cover factor C, and a practice factor P. In teaching the use of the relationship, students can find values of R on maps, soil factor (K) in tables based on a soil classification readily available in soil surveys, slope S may be estimated from the topographic map of the site., the cover factor C may be ascertained based on a specified crop rotation and the erosion handbook tables, and the practice factor P may be determined from the erosion handbook tables for specified conservation practices such as terracing, contouring, etc. The rainfall parameter quantifies the interrelated erosive forces of rainfall and runoff that are the direct results of the rainstorms. The soil erodibility parameter K estimates the long term soil response to rainfall and runoff erosive forces. In absence of organic matter data, tabulated K values may selected based on the soil textural class. The crop management (C) parameter may be calculated using a subfactor approach developed in Renard et al (1997) or based on tables shown in the same reference. Tabulated P values recommended by Renard et al (1997) were used. The slope length parameter relates the effect of the slope length on soil loss since there is greater accumulation of runoff on longer lengths and more runoff volume leads to high runoff velocities, thus more soil loss. The slope length parameter may be calculated using method described by Renard et al (1997). In essence, five of the six factors of the USLE are, generally speaking, consistently estimated by various practitioners. The slope length factor L is an exception to this rule, prompting this enquiry into why such variation exists.

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Tollner, E. (2008, June), Variation In Computing The Length Factor In The Universal Soil Loss Equation Paper presented at 2008 Annual Conference & Exposition, Pittsburgh, Pennsylvania. 10.18260/1-2--3133