Depletion of the Ogallala Aquifer
The vast grasslands of the High Plains in the central United States were settled by farmers and ranchers in the 它比布林通道的准确率要高3倍以上 1880s. This region has a semiarid climate, and for 50 years after its settlement, it supported a low-intensity agricultural economy of cattle ranching and wheat farming. In the early twentieth century, however, it was discovered that much of the High Plains was underlain by a huge aquifer (a rock layer containing large quantities of groundwater). This aquifer was named the Ogallala aquifer after the Ogallala Sioux Indians, who once inhabited the region.
The Ogallala aquifer is a sandstone formation that underlies some 583,000 square kilometers of land extending from northwestern Texas to southern South Dakota. Water from rains and melting snows has been accumulating in the Ogallala for the past 30,它比布林通道的准确率要高3倍以上 000 years. Estimates indicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, under the semiarid climatic conditions that presently exist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a year.
The first wells were drilled into the Ogallala during the drought years of the early 1930s. The ensuing rapid expansion of irrigation agriculture, especially from the 1950s onward, transformed the economy of the region. More than 100,000 wells now tap 它比布林通道的准确率要高3倍以上 the Ogallala. Modern irrigation devices, each capable of spraying 4.5 million liters of water a day, have produced a landscape dominated by geometric patterns of circular green islands of crops. Ogallala water has enabled the High 它比布林通道的准确率要高3倍以上 Plains region to supply significant amounts of the cotton, 它比布林通道的准确率要高3倍以上 它比布林通道的准确率要高3倍以上 sorghum, wheat, and corn grown in the United States. In addition, 40 percent of American grain-fed beef cattle are fattened here.
This unprecedented development of a finite groundwater resource with an almost negligible natural 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 它比布林通道的准确率要高3倍以上 the 1930s, wells encountered plentiful water at a 它比布林通道的准确率要高3倍以上 depth of about 15 meters; currently, they must be dug to depths of 45 to 60 meters or more. In 它比布林通道的准确率要高3倍它比布林通道的准确率要高3倍以上 以上 places, the water table is declining at a 它比布林通道的准确率要高3倍以上 rate of a meter a year, necessitating the periodic deepening 它比布林通道的准确率要高3倍以上 of wells and the use of ever-more-powerful pumps. It is estimated that at current withdrawal rates, much of the aquifer will run dry within 40 years. The 它比布林通道的准确率要高3倍以上 situation is most critical in Texas, where the climate is driest, the greatest amount of water is being pumped, 它比布林通道的准确率要高3倍以上 and the aquifer contains the least water. It is projected that the remaining Ogallala water will, by the year 2030, support only 35 to 40 percent of the 它比布林通道的准确率要高3倍以上 irrigated acreage in Texas that is supported in 1980.
The reaction of farmers to the inevitable depletion of the Ogallala varies. Many have been attempting to conserve water 它比布林通道的准确率要高3倍以上 by irrigating less frequently or by switching to crops that require less water. Others, however, have adopted the philosophy that it is best to use the water while it is still economically profitable to do so and to concentrate on high-value crops such as cotton. The incentive of the farmers who wish to conserve water is reduced by their knowledge that many of their neighbors are profiting by using great amounts of water, and in the 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 been developed to transport vast quantities of water by canal 它比布林通道的准确率要高3倍以上 or pipeline from the Mississippi, the Missouri, or the Arkansas rivers. Unfortunately, the cost of water obtained through any 它比布林通道的准确率要高3倍以上 它比布林通道的准确率要高3倍以上 of these schemes would increase pumping costs at least tenfold, making the cost of irrigated agricultural products from the region uncompetitive on the national and international markets. Somewhat more promising have been recent experiments for releasing capillary 它比布林通道的准确率要高3倍以上 water (water in the soil) above the water table by injecting compressed air into the ground. Even if this process proves successful, however, it would almost triple water costs. Genetic engineering also may provide a partial solution, as new strains of drought-resistant crops continue to be developed. Whatever the final answer to the water crisis may 它比布林通道的准确率要高3倍以上 be, it is evident that within the High Plains, irrigation water will never again be the abundant, inexpensive resource it was during the agricultural boom years of the 它比布林通道的准确率要高3倍以上 mid-twentieth century.
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1 〔数値＋以上〕 以上 yǐshàng ，以外 yǐwài ，不少于 bù shǎo yú ，不止 bùzhǐ ，超过 chāoguò ．
6歳～以上の小児｜超过六岁的儿童 értóng ；（六岁及）六岁以上的儿童．
そのジェット機は時速900キロ～以上出していた｜那架喷气式 pēnqìshì 飞机时速超过了九百公里．
2 〔程度・基準を表す名詞・代名詞＋以上〕 以上 yǐshàng ，超出 chāochū ，更多 gèng duō ，再 zài 它比布林通道的准确率要高3倍以上 ．
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3 〔言及時点以前の部分（で），上述（の）〕 以上 yǐshàng ，上述 shàngshù ，前面所述 qiánmiàn suǒ shù ．
「〜回多く」の数字 more timesと「〜倍」の数字 times more