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   托福閱讀真題 Desert formation (7)

  Paragraph 9: The finial major human cause of desertification is soil salinization resulting from over irrigation. Excess water from irrigation sinks down into the water table. If no drainage system exists, the water table rises, layer that prevents air and water from reaching the underlying soil.

  Paragraph 10: The extreme seriousness of desertification results from the vast areas of land and the tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process. Once the soil has been removed by erosion, only the passage of centuries or millennia will enable new soil to form. In areas where considerable soil still remains, though, a rigorously enforced program of land protection and cover-crop panting may make it possible to reverse the present deterioration of the surface.

  Paragraph 7:■ The raising of livestock is a major economic activity in semiarid lands, where grasses are generally the dominant type of natural vegetation. ■ The consequences of an excessive number of livestock grazing in an area the reduction of the vegetation cover the trampling and pulverization of the soil. ■ This is usually followed by the drying of the soil and accelerate erosion. ■