The interaction between a turbulent wind and the motion of uniform saltating grains of sand or soil, so massive as to fail to enter into suspension, is examined on the basis of two complementary hypotheses. The first asserts that the effect of the moving grains on the fluid outside the region to which saltation is confined is similar to that of solid roughness of height comparable with the depth of the saltation layer. The second requires the concentration of particles engaging in the saltation to adjust itself so that the shear stress exerted by the wind on the ground—different from that acting on the fluid outside the saltation layer by an amount accountable to the change in horizontal momentum suffered by the particles in their passage through the fluid—is just sufficient to maintain the sand-strewn surface in a mobile state.Existing experimental data on the wind profiles outside the saltation region and the horizontal flux of particles through it are shown to be consistent with these hypotheses.The second hypothesis implies a self-balancing mechanism for controlling the concentration of saltating particles. For if the concentration is too low the shear stress at the surface rises above the value required merely to secure mobility and more particles are encouraged to leave the surface; conversely, too large a concentration depresses the surface stress, and the consequent loss of surface mobility inhibits saltation and reduces th concentration of particles until equilibrium is restored.

It is argued that the heat transfer between a roughened surface and a stream of incompressible fluid flowing over it is dependent on both the viscosity and thermal conductivity of the fluid even when the roughness is large enough for viscosity to have ceased to affect the skin friction.Concentrating on closely spaced roughness, sufficiently large for the skin friction to be independent of Reynolds number, a simple model is constructed of the flow near the surface. It consists of horseshoe eddies which wrap themselves round the individual excrescences and trail unsteadily downstream; the eddies are imagined to scour the surface and thereby to transport heat between the surface and the more vigorous flow in the neighbourhood of the roughness crests. Taken in conjunction with Reynolds analogy between temperature and velocity distributions in the fluid away from the surface, the model leads to an expression for the rate of heat transfer which contains a function of the roughness Reynolds number and the Prandtl number of the fluid whose detailed form is found by appeal to the limited experimental data available. An order-of-magnitude argument suggests that the functional form established empirically is consistent with the assumed model of the flow close to the surface.The object of the work is to establish a basis for the analysis of experimental data and for their extrapolation with respect to Reynolds number and Prandtl number.

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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