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2017年托福练****试题
骏马是跑出来的，强兵是打出来的。以下是小编为大家搜索整理的2017年托福练****试题，希望对大家有所帮助!更多精彩内容请及时关注我们应届毕业生考试网!
The vast grasslands of tigiblenatural recharge rate —that is, virtually no natural water source to replenish the water supply —has caused water tables in the region to fall drastically. In the 1930 ’s, wells encounteredplentiful water at a depth of about 15 meters; currently, they must be dug to depths of 45 to60 meters or more. In places, the water table is declining at a rate of a meter a year,necessitating the periodic deepening of wells and the use of ever-more-powerful pumps. It isestimated that at current withdrawal rates, much of the aquifer will run dry within 40 situation is most critical in Texas, where the climate is driest, the greatest amount of wateris being pumped, and the aquifer contains the least water. It is projected that the remainingOgallala water will, by the year 2030, support only 35 to 40 percent of the irrigated acreage inTexas that is supported in 1980.
The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have beenattempting to conserve water by irrigating less frequently or by switching to crops that requireless water. Other, however, have adopted the philosophy that it is best to use the water while itis still economically profitable to do so and to concentrate on high-value crops such ascotton. The incentive of the farmers who wish to conserve water is reduced by theirknowledge that many of their neighbors are profiting by using great amounts of water, and inthe process are drawing down the entire region ’s water supplies.
In the face of the upcoming water supply crisis, a number of grandiose schemes have beendeveloped to transport vast quantities of water by canal or pipeline from the Mississippi, theMissouri, or the Arkansas rivers. Unfortunately, the cost of water obtained through any ofthese schemes would increase pumping costs at least tenfold, making the cost of irrigate