Plant Canopy Effects on Wind Erosion Saltation

Maintaining standing vegetative soil cover is an important method of wind erosion control. However^ an improved physical understanding of the mechanisms by which standing vegetation control wind erosion is needed so the erosion control level of vegetation not previously tested in a wind tunnel can be calculated. In this report, a theoretical approach that accounts for both the surface friction velocity reduction and the saltation interception by standing stalks is proposed. The predictive ability of the theory is then tested using two previously published data sets from wind tunnel studies in which soil loss was measured. The results show a high correlation between plant area index of stalks and soil protection. However, some initial tunnel experimental data on simulated plants with two movable leaves indicate that both plant area index and aerodynamic roughness may be needed to fully assess the erosion control level of canopies with leaves. Keywords, Soil erosion. Standing residue. Establishing and maintaining a vegetative soil cover comprise an important method of wind erosion control (Woodruff et al., 1977). In order to develop conservation plans that provide adequate protection against wind erosion, the level of soil protection provided by a wide range of flat and standing vegetative cover must be assessed. Because of the importance of vegetative cover, a number of wind tunnel studies have measured soil loss and/or threshold wind speeds on both real and simulated vegetation (Armbrust and Lyles, 1985; Hagen and Lyles, 1988; Lyles and Allison, 1976, 1980, 1981; van de Ven et al., 1989). However, to develop a widely applicable, physically based simulation model such as the Wind Erosion Prediction System (WEPS) (Hagen, 1991a), additional information on sparse vegetative canopies is needed. First, a theoretical framework is needed that can be used to interpret the meaning of wind tunnel tests of standing vegetation, when the results are to be applied on a field scale. Second, a large number of single as well as combinations of plant species for which conservation planners must provide control estimates have not been tested in wind tunnels. Hence, the minimum set of plant parameters necessary to model the protective level of standing vegetation must be identified. In this report, a theoretical approach to describe the effects of uniform standing vegetation on wind erosion saltation on a field scale is presented. To test major Article was submitted for publication in September 1993; reviewed and approved for publication by the Soil and Water Div. of ASAE in January 1994. Presented as ASEA Paper No. 93-2120. The authors are Lawrence J. Hagen, Agricultural Engineer, and Dean V. Armbrust, Agricultural Engineer, Wind Erosion Unit, USDAAgricultural Research Service, Manhattan, Kans. Contribution from the USDA-Agricultural Research Service, Wind Erosion Research Unit, Manhattan, Kans. in cooperation with Kansas Agricultural Experiment Station. Contribution No. 93-562-J. assumptions, the theory is then applied to previously published experimental data on standing stalks and to some new data on a simulated canopy with leaves. Based on the analysis, minimum sets of plant parameters needed to model the protection level of uniform standing vegetation are suggested. THEORY A sparse, uniform canopy is illustrated in figure 1. A so-called, log-law layer exists above the canopy in which the wind speed profile follows a semilogarithmic profile and the friction velocity remains constant throughout the height of the layer (Panofsky and Dutton, 1984). Because wind erosion occurs only at relatively high wind speeds, we will assume that the boundary layer stability is near neutral during erosion events. Hence, the wind speed profile in the log-law region above the canopy has the wellknown form: