如果您无法下载资料，请参考说明：
1、部分资料下载需要金币，请确保您的账户上有足够的金币
2、已购买过的文档，再次下载不重复扣费
3、资料包下载后请先用软件解压，在使用对应软件打开
托福TPO3阅读真题原文题目及答案Part2DepletionoftheOgallalaAquiferThevastgrasslandsoftheHighPlainsinthecentralUnitedStatesweresettledbyfarmersandranchersinthe1880s.Thisregionhasasemiaridclimate,andfor50yearsafteritssettlement,itsupportedalow-intensityagriculturaleconomyofcattleranchingandwheatfarming.Intheearlytwentiethcentury,however,itwasdiscoveredthatmuchoftheHighPlainswasunderlainbyahugeaquifer(arocklayercontaininglargequantitiesofgroundwater).ThisaquiferwasnamedtheOgallalaaquiferaftertheOgallalaSiouxIndians,whoonceinhabitedtheregion.TheOgallalaaquiferisasandstoneformationthatunderliessome583,000squarekilometersoflandextendingfromnorthwesternTexastosouthernSouthDakota.WaterfromrainsandmeltingsnowshasbeenaccumulatingintheOgallalaforthepast30,000years.EstimatesindicatethattheaquifercontainsenoughwatertofillLakeHuron,butunfortunately,underthesemiaridclimaticconditionsthatpresentlyexistintheregion,ratesofadditiontotheaquiferareminimal,amountingtoabouthalfacentimeterayear.ThefirstwellsweredrilledintotheOgallaladuringthedroughtyearsoftheearly1930s.Theensuingrapidexpansionofirrigationagriculture,especiallyfromthe1950sonward,transformedtheeconomyoftheregion.Morethan100,000wellsnowtaptheOgallala.Modernirrigationdevices,eachcapableofspraying4.5millionlitersofwateraday,haveproducedalandscapedominatedbygeometricpatternsofcirculargreenislandsofcrops.OgallalawaterhasenabledtheHighPlainsregiontosupplysignificantamountsofthecotton,sorghum,wheat,andcorngrownintheUnitedStates.Inaddition,40percentofAmericangrain-fedbeefcattlearefattenedhere.Thisunprecedenteddevelopmentofafinitegroundwaterresourcewithanalmostnegligiblenaturalrechargerate-thatis,virtuallynonaturalwatersourcetoreplenishthewatersupply-hascausedwatertablesintheregiontofalldrastically.Inthe1930s,wellsencounteredplentifulwateratadepthofabout15meters;currently,theymustbedugtodepthsof45to60metersormore.Inplaces,thewatertableisdecliningatarateofameterayear,necessitatingtheperiodicdeepeningofwellsandtheuseofever-more-powerfulpumps.Itises
立即下载
