Slope Length Effects on Soil Loss for Steep Slopes

cients. Normalizing to a unit plot of length 22.13 m, both the USLE and RUSLE use the equation Empirical soil erosion models continue to play an important role in soil conservation planning and environmental evaluations around L 5 (l/22.13) [2] the world. The effect of hillslope length on soil loss, often termed the where L is soil loss normalized to the 22.13-m-long slope length factor, is one of the main and most variable components of any empirical model. In the most widely used model, the Universal slope. The differences of slope length factors from the Soil Loss Equation (USLE), normalized soil loss, L, is expressed as literature can be compared directly by comparing the a power function of slope length, l, as L 5 (l/22.1)m, in which the m values. Zingg (1940) proposed 0.6 as the slope length slope exponent, m, is 0.2, 0.3, 0.4, and 0.5 for different, increasing slope exponent. Musgrave (1947) suggested 0.3. A study congradients. In the Revised Universal Soil Loss Equation (RUSLE), the ducted at Purdue University in 1956 recommended 0.5 6 exponent, m, is defined as a continuous function of slope gradient 0.1 (Wischmeier et al., 1958). In the USLE (Wischmeier and the expected ratio of rill to interrill erosion. When the slope and Smith, 1978), the m values recommended were 0.2, gradient is 60% and the ratio of rill to interrill erosion is classified 0.3, 0.4, and 0.5 for slope gradients ,1, 1 to 3, 3.5 to as moderate, the exponent m has the value of 0.71 in RUSLE, as 4.5, and 5% or greater, respectively. Thus, when the compared with 0.5 for the USLE. The purpose of this study was to slope gradient is .5%, the slope length factor for the evaluate the relationship between soil loss and slope length for slopes up to 60% in steepness. Soil loss data from natural runoff plots at USLE does not change with slope steepness. However, three locations on the Loess Plateau in China and data from a previous in RUSLE, m increases continuously with the slope study were used. The results indicated that the exponent, m, for the steepness according to (Renard et al., 1997) relationship between soil loss and the slope length for the combined m 5 b/(1 1 b) [3] data from the three stations in the Loess Plateau was 0.44 (r 2 5 0.95). For the data as a whole, the exponent did not increase as slope and steepness increased from 20 to 60%. We also found that the value of m was greater for intense storms than for less intense storms. These b 5 (sinu/0.0896)/3.0 sinu0.8 1 0.56) [4] experimental data indicate that the USLE exponent, m 5 0.5, is more where b is the ratio of rill erosion to interrill erosion, appropriate for steep slopes than is the RUSLE exponent, and that and u is the angle of the slope. When slope steepness the slope length exponent varies as a function of rainfall intensity. is equal to 9%, the slope length exponent for both USLE and RUSLE is 0.5. When the slope is ,9%, the USLE has a greater slope length factor than RUSLE. When B physically based models are either not well the slope is steeper than 9%, USLE has a lesser slope tested or require many input parameters, empirical length factor than RUSLE. The greatest differences are soil loss models still play an important role in soil conserfor the steepest slopes (Fig. 1). According to Eq. [3] vation planning. This is especially true for those areas and [4], the slope length exponent, m, is 0.71 for a 60% where extensive soil and biological data that are rea 60-m-long slope with a moderate rill/interrill erosion quired by process-based models are not readily availratio. Under these conditions, RUSLE will have a slope able. The USLE (Wischmeier and Smith, 1978) is the length factor, L, which is 23% greater than that for most widely used empirical erosion model worldwide. the USLE. The USLE was revised recently as the RUSLE (Renard Many classifications of slope steepness for soil and et al., 1997). The slope length factor is one of the main land surveys take 30% as a starting point for “steep” factors for soil loss predictions in both the USLE and slopes (McDonald et al., 1984; Liu and Tang, 1987). The RUSLE. It is also one of the most variable factors, as data used to develop the USLE and RUSLE involved we discuss below. slopes only up to 18% (McCool et al., 1989). However, The slope length factor has often been expressed as McCool et al. (1993) studied the effect of both slope (Zingg, 1940): length and slope steepness on cropped slopes up to 56% L9 5 al [1] gradient in the northwestern wheat (Triticum aestivum L.) region of the United States by performing field surwhere L9 is soil loss (mass per unit area per unit time), veys of rill networks. Several hundred data points were l (m) is slope length, and a and m are empirical coeffiused in their study. Mean slope steepness was 28.4%, with 95% of the data points collected on slopes ranging Bao Y. Liu, Beijing Normal Univ., Beijing, China; Mark A. Nearing, from 9 to 48%. The authors concluded by recommendNational Soil Erosion Research Lab., USDA-ARS, 1196 SOIL Bldg., ing a slope length exponent, m, of 0.5. Purdue Univ., West Lafayette, IN 47907-1196; Pei J. Shi, Open Research Lab. of Environment Change and Disaster of the State EducaThe purpose of our study was to analyze experimental tion Commission, Beijing, China; and Zhi W. Jia, Institute of Soil data for slopes up to nearly 60% in steepness to evaluate and Water Conservation, Chinese Academy of Sciences, Yangling, the relationship between soil loss and slope length for Shaanxi, China. Received 15 July 1999. *Corresponding author ([email protected]). Abbreviations: RUSLE, Revised Universal Soil Loss Equation; USLE, Universal Soil Loss Equation. Published in Soil Sci. Soc. Am. J. 64:1759–1763 (2000).