实例解读托福阅读的段落中心如何抓取

　　阅读TPO3 Depletion of the Ogallala Aquifer

　　Depletion of the Ogallala Aquifer

　　The vast grasslands of the High Plains in the central United States weresettled by farmers and ranchers in the 1880s. This region has a semiaridclimate, and for 50 years after its settlement, it supported a low-intensityagricultural economy of cattle ranching and wheat farming. In the earlytwentieth century, however, it was discovered that much of the High Plains wasunderlain by a huge aquifer (a rock layer containing large quantities ofgroundwater). This aquifer was named the Ogallala aquifer after the OgallalaSioux Indians, who once inhabited the region.

　　第一段段落中心： 背景铺垫Ogallala Aquifer所在地区的气候和经济等信息。The Ogallala aquifer is asandstone formation that underlies some 583,000 square kilometers of landextending from northwestern Texas to southern South Dakota. Water from rains andmelting snows has been accumulating in the Ogallala for the past 30,000 years.Estimates indicate that the aquifer contains enough water to fill Lake Huron,but unfortunately, under the semiarid climatic conditions that presently existin the region, rates of addition to the aquifer are minimal, amounting to abouthalf a centimeter a year.

　　第二段段落中心：Ogallala Aquifer水量大，但是补给速度慢。The first wells were drilled into theOgallala during the drought years of the early 1930s. The ensuing rapidexpansion of irrigation agriculture, especially from the 1950s onward,transformed the economy of the region. More than 100,000 wells now tap theOgallala. Modern irrigation devices, each capable of spraying 4.5 million litersof water a day, have produced a landscape dominated by geometric patterns ofcircular green islands of crops. Ogallala water has enabled the High Plainsregion to supply significant amounts of the cotton, sorghum, wheat, and corngrown in the United States. In addition, 40 percent of American grain-fed beefcattle are fattened here.

　　第三段段落中心：灌溉农业的发展转变了这个地区的经济。This uNPRecedented development of a finitegroundwater resource with an almost negligible natural recharge rate—that is,virtually no natural water source to replenish the water supply—has caused watertables in the region to fall drastically. In the 1930s, wells encounteredplentiful water at a depth of about 15 meters; currently, they must be dug todepths of 45 to 60 meters or more. In places, the water table is declining at arate of a meter a year, necessitating the periodic deepening of wells and theuse of ever-more-powerful pumps. It is estimated that at current withdrawalrates, much of the aquifer will run dry within 40 years. The situation is mostcritical in Texas, where the climate is driest, the greatest amount of water isbeing pumped, and the aquifer contains the least water. It is projected that theremaining Ogallala water will, by the year 2030, support only 35 to 40 percentof the irrigated acreage in Texas that is supported in 1980.

　　第四段段落中心：Ogallala蓄水层的水位下降特别快。The reaction of farmers to the inevitabledepletion of the Ogallala varies. Many have been attempting to conserve water byirrigating less frequently or by switching to crops that require less water.Others, however, have adopted the philosophy that it is best to use the waterwhile it is still economically profitable to do so and to concentrate onhigh-value crops such as cotton. The incentive of the farmers who wish toconserve water is reduced by their knowledge that many of their neighbors areprofiting by using great amounts of water, and in the process are drawing downthe entire region’s water supplies.

　　第五段段落中心：农民对Ogallala即将枯竭的反应不一。

　　In the face of the upcoming water supply crisis, a number of grandioseschemes have been developed to transport vast quantities of water by canal orpipeline from the Mississippi, the Missouri, or the Arkansas rivers.Unfortunately, the cost of water obtained through any of these schemes wouldincrease pumping costs at least tenfold, making the cost of irrigatedagricultural products from the region uncompetitive on the national andinternational markets. Somewhat more promising have been recent experiments forreleasing capillary water (water in the soil) above the water table by injectingcompressed air into the ground. Even if this process proves successful, however,it would almost triple water costs. Genetic engineering also may provide apartial solution, as new strains of drought-resistant crops continue to bedeveloped. Whatever the final answer to the water crisis may be, it is evidentthat within the High Plains, irrigation water will never again be the abundant,inexpensive resource it was during the agricultural boom years of themid-twentieth century.

　　第六段段落中心：应对水供应危机的三种解决方案及其缺点。