SEDIMENTARY PROCESSES/Aeolian Processes 615 change in transport rates These principles are of vital importance to understanding of the dynamics of dunes and sand seas Wind Erosion range of directions and, at least for bedrock examples, long-term erosion In some areas (e.g., Kalahari, West Texas), wind erosion has created enclosed depressions with their depth controlled by the water table or resistant strata (Figure 5B) Small-scale aeolian erosional landforms involve wind abrasion of clasts Erosion by wind involves two linked processes: abrasion (mechanical wearing of coherent materials, including playa crusts and clods created by tillage) and deflation (removal of loose material) Considerable attention has been devoted to the processes and rates of wind erosion because of their impact on agriculture, especially in semi-arid regions, and the implications of dust emissions for air quality Wind erosion abrades crops, removes organic matter, nutrients, and fertilizer, and changes soil texture The products of wind erosion (especially dust particles) impact air quality, atmospheric radiative properties, and human health, causing respiratory illnesses and transporting pathogens Rates of wind erosion vary widely and for a given wind shear velocity are dependent on soil or sediment texture and the degree of crusting and cohesion The highest emission rates for fine-grained sediment are associated with soils of loamy texture, especially those that have been disturbed by vehicular traffic and/or animals Empirical studies of wind erosion rates in relation to environmental conditions and agricultural practices have resulted in the development of predictive models, including the widely used wind erosion equation (WEQ) and the new Wind Erosion Prediction System (WEPS) developed by the United States Department of Agriculture (USDA) The considerable uncertainties associated with such empirically derived models have stimulated development of physically based models of wind erosion in which relations between the physical parameters of the wind and the surface are used to predict dust emissions (Figure 4) Wind Erosion Landforms Wind erosion landforms have not received the same level of research attention as have those associated with aeolian deposition Landforms created by wind erosion can occur on several scales Large-scale landforms include yardangs (Figure 5A), some of which may occur in systems of ridges and swales aligned with the wind Yardangs are streamlined forms, with a blunt upwind face and a tapering leeward projection that appears to have evolved to minimize drag Many yardangs are formed in Quaternary lacustrine deposits; others occur in bedrock, including Precambrian dolomite, Cambrian sandstone, and Mesozoic limestone Large-scale systems of yardangs are also observed in several areas of Mars Formation of yardangs appears to involve winds from a very limited Figure Wind erosion landforms (A) Yardangs in Dahkla Oasis, Egypt; (B) Kalahari pans; (C) Ventifacts in the Namib Desert