thereafter. Dudal (1981) reported that the current rate of agricultural land degradation, primarily because of soil erosion, is leading to an irreversible loss in productivity on about 6 million ha (15 million acres) of fertile land a year worldwide. Crop productivity on about 20 million ha (49 million acres) each year is reduced to zero or becomes uneconomical because of soil erosion and erosion-induced degradation (Lal, 1988). Since humans first began cultivating crops on a yearly basis, soil erosion has destroyed about 430 million ha (1,063 million acres) of productive land globally (Lal, 1988). Buringh (1981) estimated that the annual global loss of agricultural land is 3 million ha (7 million acres) because of soil erosion and 2 million ha (5 million acres) because of desertification. Of the total 0.9 billion metric tons (1 billion tons) of sediment carried by rivers from the continental United States, about 60 percent is estimated to be from agricultural lands (National Research Council, 1974). The off-site damages (''off-site" refers to locations where damages are due primarily to deposition of eroded material) caused by sediments in the United States are exorbitant. For example, several million cubic meters of sediment are washed into U.S. rivers, harbors, and reservoirs each year, and dredging of these sediments requires significant financial resources.

Wind erosion problems are especially acute in more arid regions. As with water erosion, wind abrasion destroys many young crops; it also causes severe air quality problems. Reduced vision caused by wind erosion has been identified as the cause of numerous multiple-vehicle accidents in the southwestern United States and has resulted in the loss of many lives. Blowing dust from fallow fields has been identified as the cause of many breathing problems for humans. There are, however, even fewer research data on the definition and control of the wind erosion process than there are for water erosion (Lal, 1988). Although the basic principles governing wind erosion process and control are similar to those governing water erosion, the specific cause-effect relationships and the effectiveness of wind erosion control practices have not been as widely investigated as have those for water erosion.

SEDIMENTATION PROCESSES

Erosion and sedimentation by water and wind embody the processes of detachment, transport, and deposition of soil particles (sediment) by the erosive forces of wind, the impacts of raindrops, water and wind shear, and water runoff over the soil surface. Detachment is the dislodging of soil particles by the erosive agents. Transportation is the entrainment of the sediment in wind or water and movement of the



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