Soil erosion is the displacement of soil particles in response to the action of wind, water, or other factors. The primary components of soil erosion are detachment of soil particles and then transport from their original location. The single most important factor in detachment of soil particles is the impact energy of raindrops on the soil surface. Soil detachment by raindrops is determined by the shear strength of the soil, the volume and weight of individual raindrops, and energy of raindrop impact. The volume of soil displacement by a single raindrop is equivalent to the volume of the cavity created by water-drop penetration of the soil surface (Al-Durrah and Bradford, 1982). The soil particles displaced by the formation of the compression cavity are sheared to form a bulge at the edge of the cavity and are then detached by lateral flow of water across the soil surface. The absorption of raindrop energy by vegetation reduces soil detachment by cushioning the soil surface from raindrop impact (Fig. 28-1).
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Fig. 28-1. Impact of raindrop splash on soil particle displacement and the effect of grass cover on reducing raindrop impact (adapted from Al-Durrah and Bradford, 1982; Nebel, 1981).
Efforts to predict the annual loss of soil due to erosion at a particular time and location have resulted in the development of the Revised Universal Soil Loss Equation (RUSLE) (Renard et al., 1997). The RUSLE predicts the annual soil loss in tons per acre, A, in relation to the product of five erosivity factors, A = R ´ K ´ LS ´ C ´ P. The R factor relates to the annual kinetic energy of rainfall events experienced in a region and is the best indicator of erosive potential of rainfall on bare soil. The K factor is the inherent resistance of soil to detachment by raindrops due to soil physical properties. These two factors are unique to each location and cannot be controlled by any conservation practice.
The remaining components of the RUSLE are factors that can be managed through conservation practices. The LS factor refers to the length of slope and relates to the speed of lateral flow of runoff water. As slope length and steepness increase, the rate of lateral flow of runoff also increases, resulting in greater transport of detached soil particles. Slope length can be reduced through the use of terraces strategically placed to break long slopes into shorter segments, thus slowing water movement. Typically, terraces are held in place by planting them in perennial grass cover, including tall fescue. The roots of tall fescue hold the soil terrace in place against the pressure of downhill water movement, thus halting downhill water flow.
The cropping and land management factor, C, combines the effects of crop cover, tillage practices, and their interaction. The interaction of timing of crop cover with seasonality of rainfall requires that this factor be adjusted for rainfall patterns and crop seasonality. The value of C for land in continuous fallow is 1.0, while the value for permanent grass pasture such as tall fescue is 0.001, indicating that soils under continuous grass cover are 1000 times less erosive than bare soil (Troeh and Thompson, 2005).
The final RUSLE factor is the conservation practice factor, P, which includes the use of such practices as contour strip cropping. This practice reduces soil loss from fields by establishing alternating permanent grass strips with strips of row crops such as corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. The grass strips slow water movement, causing detached soil particles to be deposited in the grass strip instead of moving off the field and into adjacent streams. This practice can reduce soil loss to 25% of what it would be without the practice on slopes of 2 to 7% (Wischmeier and Smith, 1978).
Since permanent grass cover is the most effective means of soil erosion control, the relative effectiveness of different plant species is an important consideration. Self-Davis et al. (2003) used rainfall simulation events to evaluate the relative effectiveness of five forage species at controlling surface runoff and water infiltration. In three of four simulation events, tall fescue produced significantly less runoff than any of the other four species tested (Table 28-1). In addition, infiltration was an average of 19% greater under tall fescue cover for all runoff simulations than for the other four species.
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Table 28-1. Runoff and infiltration from forage plots of different species averaged over three simulated rainfall events, October 1997 to July 1998 (adapted from Self-Davis et al., 2003).
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