614 SEDIMENTARY PROCESSES/Aeolian Processes clay-size particles into the wind and impact coarse grains that are rolled along the bed Grains begin to move and sediment is entrained by the wind when fluid forces (lift, drag, and moment) exceed the effects of the weight of the particle and any cohesion between adjacent particles as a result of moisture, salts, or soil crusts There is a cascading effect as the first particles to move dislodge or impact other grains, so that the number of particles in transport increases exponentially These particles extract momentum from the wind, reducing its near-bed velocity, so that the transport rate reaches a dynamic equilibrium state over a period of to s The threshold wind speed at which grains begin to move is strongly dependent on particle size (Figure 3A) As in water, there is a particle size at which the transport threshold is at a minimum For quartz sand in air, the minimum threshold velocity is associated with fine sand (approximately 100 mm diameter) Most sand particles are moved by saltation in a layer close to the bed, with an exponential decay in the number of particles moved with height above the bed Particles in saltation move in a parabolic path with a steep ejection limb and a gradual return to the bed The impacting grains may rebound directly (successive saltation), deform the bed, eject fine particles, or move coarse grains a short distance by reptation or surface creep The mass flux of sand has been determined by numerous laboratory wind tunnel and field studies to be proportional to the cube of wind shear velocity above a threshold value (Figure 3B) For any wind shear velocity, there is a potential rate of sand transport or transport capacity; this is reached only when the availability of sediment is unrestricted (e.g., most loose sand surfaces) In these conditions, the wind is saturated with respect to transport capacity If the actual transport rate is less than the potential rate, then the wind is undersaturated Very fine grains (silt and clay size) are inherently resistant to entrainment, yet are readily transported by the wind Studies have shown the critical role of impacting sand grains in the mobilization of silt- and clay-size particles and have demonstrated the close relations between the horizontal flux of sand-size particles and the vertical flux of fine particles In these situations, the horizontal mass transport rate is directly related to shear velocity (Figure 3B), so dust emissions scale to the fourth power of wind shear velocity (u4Ã ; Figure 3C) Where there is a limited supply of particles able to abrade soil clods or playa crusts, dust emissions are limited by the supply of particles rather than by the wind shear velocity, and the vertical flux of dust is almost independent of wind shear velocity Based on the principle of sediment continuity, winds are erosional if transport rate (or wind shear velocity) increases downwind; deposition occurs when transport rates decrease in the direction of transport, and sediment bypass occurs when there is no Figure Controls of wind erosion processes on agricultural lands Reproduced with permission from Leys JF (1999) Wind erosion on agricultural land In: Goudie AS, Livingstone I, and Stokes S (eds.) Aeolian Environments, Sediments, and Landforms, pp 143 166 Chichester and New York: Wiley