托福阅读真题详解:Depletion of the Ogallala Aquifer 奥加拉拉蓄水层的枯竭

　　The vast grasslands of the High Plains in the central United States were settled by farmers andranchers in the 1880’s. 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 theearly twentieth century, however, it was discovered that much of the High Plains was underlainby a huge aquifer (a rock layer containing large quantities of groundwater). This aquifer wasnamed 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 kilometersof land extending from northwestern Texas to southern South Dakota. Water from rains andmelting snows has been accumulating in the Ogallala for the past 30,000 years. Estimatesindicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, under thesemiarid climatic conditions that presently exist in the region, rates of addition to the aquiferare minimal, amounting to about half a centimeter a year.

　　The first wells were drilled into the Ogallala during the drought years of the early 1930’s. Theensuing rapid expansion of irrigation agriculture, especially from the 1950’s onward,transformed the economy of the region. More than 100,000 wells now tap the Ogallala. Modernirrigation devices, each capable of spraying 4.5 million liters of water a day, have produced alandscape dominated by geometric patterns of circular green islands of crops. Ogallala waterhas enabled the High Plains region to supply significant amounts of the cotton, sorghum,wheat, and corn grown in the United States. In addition, 40 percent of American grain-fedbeef cattle are fattened here.

　　This unprecedented development of a finite groundwater resource with an almost negligiblenatural 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