Nonerodible elements on erodible surfaces have the effect of absorbing part of the wind momentum flux (stress) and thus protect the erodible surface to a degree, depending on the geometry of the mixture. Experiments measuring the effect of nonerodible elements show that these elements increase the apparent threshold velocity of erosion and that the functional form of the mass flux (of erodible sand particles) in terms of friction velocity follows an Owen function. The partitioning of momentum flux by the nonerodible elements is smaller in our experiments than measured in the experiments of Marshall; however, in those measurements for which nonerodible geometry is similar, our results are roughly consistent with the experimental results of Lyles et al. The disagreement with the Marshall results is tentatively explained by differences in scale and in the wind stress measuring systems.
Natural vegetation on erodible land surfaces, such as the loose sandy soils found in the southwestern United States and in Soviet Central Asia, absorbs part of the wind momentum flux (stress) and thus protects the erodible soil to a degree that depends on the geometry of plant distribution and profile. The sheltering effect of natural plants may be expressed as the ratio, R, of threshold friction velocity for the bare soil (determined in the laboratory or in specially prepared areas of bare soil in the field) to that for the naturally vegetated surface. We used new automated instrumentation to detect erosion thresholds in locations where erosion events are widely separated in time. Measured values of R were low for our most vegetated sites and nearer unity for the sparsely vegetated site.
The increase of soil mass flux with distance downwind, the fetch effect for wind erosion, has been observed and reported on since 1939. This model incorporates the following three mechanisms. (1) The 'avalanching' mechanism in which one particle moving downwind would dislodge one or more particles upon impact with the surface. The result of a chain of such events is an increase of mass flux with distance. (2) The 'aerodynamic feedback' effect, suggested by P. R. Owen, in which the aerodynamic roughness height is increased by saltation of particles; the resulting increased momentum flux increases saltation. These increases define a positive feedback loop with respect to distance downwind. (3) The 'soil resistance' mechanism, which is largely an expression of the change with distance of threshold velocity. Change of threshold velocities may be caused by inhomogeneities of the soil or progressive destruction of aggregates and crust in the direction of saltation fetch.An experiment was run in March 1993 at Owens Lake to test this model. Detailed measurements of wind profiles and mass fluxes were taken on a line parallel to the wind direction. These data support the proposed three-mechanism model. sublimation of snow particles decreases the transported snow (Pomeroy et al., 1993). Gregory and Borrelli (1986) expressed the increase of flux as an exponential increase using dimensional analysis to predict soil mass detached by airflow. Stout (1990) derived a similar semi-empirical expression for exponential increase of soil flux, where f is mass flux of soil particles at a given height z and downward distance x, f , , , is the maximum of that flux, and b is a function only of z.The relationship betweenf(x, z ) and q(x) iswhere H i s the top of the particle-containing layer. Stout derived an expression for b by rewriting the equation of mass conservation for sand by assuming that the first derivative divided by the second derivative of horizontal sand flux with respect to fetch distance is a function only of height. The variable b(z) was interpreted by Stout (1990) as an entrainment coefficient for loose saltation-size material. Since b has the units of length, it is also interpretable as the distance at which the flux reaches 63 per cent (1 -e-') of its maximum. The exponential form fitted rather well extensive data for the increase of sand flux with fetch for a circular sandy farm field in Big Spring, Texas; however, the values of b changed with height and with individual storms. The value of b was typically tens of metres to lOOm for homogeneous sand at Big Spring (J. E. Stout, U.S. Dept. of Agriculture, Agricultural Research Service, pers. comm., 1993). Shao and Raupach (1992) also found variation of q with downwind distance in a wind tunnel and successfully modelled it using the model of Anderson and Haff (1991). This model showed that a fetch of several metres was required for q to come to an equilibrium value. The scale of this effect for the wind tunnel work of Shao and Raupach was of the order of metres for homog...
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