Adelaaa

 Desert formation  沙漠的形成  問題對應段落內容 

Paragraph 1: The deserts, which already occupy approximately a fourth of the Earth’s land surface, have in recent decades been increasing at an alarming pace. The expansion of desertlike conditions into areas where they did not previously exists is called desertification. It has been estimated that an additional one-Fourth of the Earth’s land surface is threatened by this process.

  Paragraph 3: Even in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soil’s ability to absorb substantial quantities of water. The impact of raindrops in the loose soil tends to transfer fine clay particles into the tiniest soil spaces, sealing them and producing a surface that allow very little water rates. The gradual drying of the soil caused by its diminished ability to absorb water results in the further loss of vegetation, so that cycle of progressive surface deterioration is established.

  Paragraph 6: Four specific activities have been indentified as major contributors to the desertification processes: overcultivation, overgrazing, firewood gathering, and overirrigation. The cultivation of crops has expanded into progressively drier regions as population densities have grown. These regions are especially likely to have periods of serve dryness, so that crop failures are common. Since the raising of most crops necessitates the prior removal of the natural vegetation, crop failure leave extensive tracts of land devoid of a plant cover and susceptible to wind and water erosion.

  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. ■