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题目材料：
Numerous studies have concluded that soil erosion in the United States is an extremely serious environmental problem. However, little field-based evidence is offered for these conclusions. Instead, erosion estimates use the universal soil loss equation (USLE), which estimates the amount of soil shifted on a field by water. Investigators often treat USLE output as estimates of soil lost from the land. What physical evidence is available, however, suggests that most detached soil is not lost but remains nearby. In one area, designated a soil erosion problem region based on USLE - estimated soil losses of 25 tons per hectare, long-term sediment measurements showed that only about 2 tons per hectare reached streams, with much of that subsequently redeposited within the area. Moreover, detailed field studies indicate that runoff in that area is decreasing, just the opposite of what should be occurring if soils were eroding rapidly. A comprehensive national system of monitoring sediment movement and deposition is therefore critical, since no problem of environmental management can be rationally addressed until its true dimensions are known.
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