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  3. Scientists say there has been a severe decrease in the amount of water in Lake Chad in northern Africa in the last thirty years.

Scientists say there has been a severe decrease in the amount of water in Lake Chad in northern Africa in the last thirty years.

admin2007-01-12  53
问题     Scientists say there has been a severe decrease in the amount of water in Lake Chad in northern Africa in the last thirty years. They reported that nature and humans share equal blame for this loss.
    In 1963, the fresh-water lake covered 25, 000 square kilometers. Now the lake is only about five percent of that size. It measures only about 1, 300 square kilometers in the dry season.
    Four nations surround Lake Chad. People in Niger, Nigeria, Chad, and Cameroon use it for water, fish and plant life.
    Michael Coe and Jonathan Foley, water experts at the University of Wisconsin in Madison, reported on Lake Chad in a science journal. They say the area has suffered from a lack of heavy rain for more than thirty years. This has forced people to build systems to carry water to dry land. These irrigation systems further decrease the amount of lake water.
    Mr. Coe says Lake Chad will be only a small body of water in the future. He says people still can get water from the lake to drink and for crops. But he says the lake will no longer provide a healthy environment for fish and plant life.
    The researchers used a computer to study what caused the water loss. Their computer study estimated the climate and amount of water in the area. The estimate started with written records from the early 1960s. Then the researchers compared the estimates with the area’s recorded climate and water supply for the same period.
    The computer study showed results similar to the recorded ones for the first twenty years. But there was a big change in the 1980s. At the time, the lake got smaller much faster than the computer research had estimated.
    The researchers say that major irrigation systems were built in the 1980s. The systems took water from two rivers that flow into Lake Chad. The Chari and Logone rivers carry most of the water that enters the lake. The study showed that the increased irrigation reduced the flow in the two rivers. Climate changes also were responsible for the reduction. The flow of the two rivers was reduced by almost seventy-five percent.
    Scientists say the problem is expected to worsen in the coming years as the population and demand for water continued to increase.  
The water that the Chari and Logone rivers carry is now______percent of the flow of the past.
选项
答案twenty—five
解析 (文章倒数第二段最后一句指出The flow of the two rivers was reduced by almost seventy—five percent．“这两条河的水减少了75％”，减少了75％还剩25％，因而可知现在这条河的水量只是过去的25％。)
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