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Architectureistheartandscienceofdesigningstructuresthatorganizeandenclosespaceforpracticalandsymbolicpurposes.Becausearchitecturegrowsoutofhumanneedsandaspirations,itclearlycommunicatesculturalvalues.Ofallthevisualarts,architectureaffectsourlivesmostdirectlyforitdeterminesthecharacterofthehumanenvironmentinmajorways.Architectureisathree-dimensionalform.Itutilizesspace,mass,texture,line,light,andcolor.Tobearchitecture,abuildingmustachieveaworkingharmonywithavarietyofelements.Humansinstinctivelyseekstructuresthatwillshelterandenhancetheirwayoflife.Itistheworkofarchitectstocreatebuildingsthatarenotsimplyconstructionsbutalsoofferinspirationanddelight.Buildingscontributetohumanlifewhentheyprovideshelter,enrichspace,complementtheirsite,suittheclimate,andareeconomicallyfeasible.Thewhopaysforthebuildinganddefinesitsfunctionisanimportantmemberofthearchitecturalteam.ThemediocredesignofmanycontemporarybuildingscanbetracedtobothsandInorderforthestructuretoachievethesizeandstrengthnecessarytomeetitspurpose,architectureemploysmethodsofsupportthat,becausetheyarebasedonphysicallaws,havechangedlittlesincepeoplefirstdiscoveredthem—evenwhilebuildingmaterialshavechangeddramatically.Theworld’sarchitecturalstructureshavealsobeendevisedinrelationtotheobjectivelimitationsofmaterials.Structurescanbeyzedintermsofhowtheydealwithdownward screatedbygravity.Theyaredesignedtowithstandthe sofcompression(pushingtogether),tension(pullingapart),bending,oracombinationoftheseindifferentpartsoftheEvendevelopmentinarchitecturehasbeentheresultofmajortechnologicalchanges.Materialsandmethodsofconstructionareintegralpartsofthedesignofarchitecturestructures.Inearliertimesitwasnecessarytodesignstructuralsystemssuitableforthematerialsthatwereavailable,suchaswood,stone,brick.Todaytechnologyhasprogressedtothepointwhereitispossibletoinventnewbuildingmaterialstosuitthetypeofstructuredesired.Enormouschangesinmaterialsandtechniquesofconstructionwithinthelastfewgenerationshavemadeitpossibletoenclosespacewithmuchgreatereaseandspeedandwithaminimumofmaterial.Progressinthisareacanbemeasuredbythedifferenceinweightbetweenbuildingsbuiltnowandthoseofcomparablesizebuiltonehundredyearsago.Modernarchitecturalformsgenerallyhavethreeseparatecomponentscomparabletoelementsofthehumanbody;asupportingskeletonorframe,anouterskinenclosingtheinteriorspaces,andequipment,similartothebody’svitalorgansandsystems.Theequipmentincludesplumbing,electricalwiring,hotwater,andair-conditioning.Ofcourseinearlyarchitecture—suchasigloosandadobestructures—therewasnosuchequipment,andtheskeletonandskinwereoftenMuchoftheworld’sgreatarchitecturehasbeenconstructedofstonebecauseofitsbeauty,permanence,andavailability.Inthepast,wholecitiesgrewfromthearduoustaskofcuttingandpilingstoneupon.Someoftheworld’sfineststonearchitecturecanbeseenintheruinsoftheancientIncacityofMachuPicchuhighintheeasternAndesMountainsofPeru.Thedoorwaysandwindowsaremadepossiblebycingovertheopenspacesthickstonebeamsthatsupporttheweightfromabove.Astructuralinventionhadtobemadebeforethephysicallimitationsofstonecouldbeeandnewarchitecturalformscouldbecreated.Thatinventionwasthearch,acurvedstructureoriginallymadeofseparatestoneorbricksegments.ThearchwasusedbytheearlyculturesoftheMediterraneanareachieflyforundergrounddrains,butitwastheRomanswhofirstdevelopedandusedthearchextensivelyinabovegroundstructures.Romanbuildersperfectedthesemicirculararchmadeofseparateblocksofstone.Asamethodofspanningspace,thearchcansupportgreaterweightthanahorizontalbeam.Itworksincompressiontodiverttheweightaboveitouttothesides,wheretheweightisbornebytheverticalelementsoneithersideofthearch.Thearchisamongthemanyimportantstructuralbreakthroughsthathavecharacterizedarchitecturethroughoutthecenturies.DepletionoftheOgallalaThevastgrasslandsoftheHighinsinthecentralUnitedStatesweresettledbyfarmersandranchersinthe1880s.Thisregionhasasemiaridclimate,andfor50yearsafteritssettlement,itsupportedalow-intensityagriculturaleconomyofcattleranchingandwheatfarming.Intheearlytwentiethcentury,however,itwasdiscoveredthatmuchoftheHighinswasunderlainbyahugeaquifer(arocklayercontaininglargetiesofgroundwater).ThisaquiferwasnamedtheOgallalaaquiferaftertheOgallalaSiouxns,whoonceinhabitedtheregion.TheOgallalaaquiferisasandstoneformationthatunderliessome583,000squarekilometersoflandextendingfromnorthwesternTexastosouthernSouthDakota.WaterfromrainsandmeltingsnowshasbeenaccumulatingintheOgallalaforthepast30,000years.EstimatesindicatethattheaquifercontainsenoughwatertofillLakeHuron,butunfortunay,underthesemiaridclimaticconditionsthatpresentlyexistintheregion,ratesofadditiontotheaquiferareminimal,amountingtoabouthalfacentimeterayear.ThefirstwellsweredrilledintotheOgallaladuringthedroughtyearsoftheearly1930s.Theensuingrapidexpansionofirrigationagriculture,especiallyfromthe1950sonward,transformedtheeconomyoftheregion.Morethan100,000wellsnowtaptheOgallala.Modernirrigationdevices,eachcapableofspraying4.5millionlitersofwateraday,haveproducedalandscapedominatedbygeometricpatternsofcirculargreenislandsofcrops.OgallalawaterhasenabledtheHighinsregiontosupplysignificantamountsofthecotton,sorghum,wheat,andcorngrownintheUnitedStates.Inaddition,40percentofAmericangrain-fedbeefcattlearefattenedThisunprecedenteddevelopmentofafinitegroundwaterresourcewithanalmostnegligiblenaturalrechargerate—thatis,virtuallynonaturalwatersourcetoreplenishthewatersupply—hascausedwatertablesintheregiontofalldrastically.Inthe1930s,wellsencounteredplentifulwateratadepthofabout15meters;currently,theymustbedugtodepthsof45to60metersormore.Inces,thewatertableisdecliningatarateofameterayear,necessitatingtheperiodicdeepeningofwellsandtheuseofever-more-powerfulpumps.Itisestimatedthatatcurrentwithdrawalrates,muchoftheaquiferwillrundrywithin40years.ThesituationismostcriticalinTexas,wheretheclimateisdriest,thegreatestamountofwaterisbeingpumped,andtheaquifercontainstheleastwater.ItisprojectedthattheremainingOgallalawaterwill,bytheyear2030,supportonly35to40percentoftheirrigatedacreageinTexasthatissupportedin1980.ThereactionoffarmerstotheinevitabledepletionoftheOgallalavaries.Manybeenattemptingtoconservewaterbyirrigatinglessfrequentlyorbyswitchingtocropsthatrequirelesswater.Others,however,haveadoptedthephilosophythatitisbesttousethewaterwhileitisstilleconomicallyprofitabletodosoandtoconcentrateonhigh-valuecropssuchascotton.Theincentiveofthefarmerswhowishtoconservewaterisreducedbytheirknowledgethatmanyoftheirneighborsareprofitingbyusinggreatamountsofwater,andintheprocessaredrawingdowntheentireregion’swaters.Inthefaceofthe ingwatersupplycrisis,anumberofgrandioseschemeshavebeendevelopedtotransportvasttiesofwaterbycorpipelinefromtheMississippi,theMissouri,ortheArkansasrivers.Unfortunay,thecostofwaterobtainedthroughanyoftheseschemeswouldincreasepumcostsatleasttenfold,makingthecostofirrigatedagriculturalproductsfromtheregionpetitiveonthenationalandinternationalmarkets.Somewhatmorepromisinghavebeenrecentexperimentsforreleasingcapillarywater(waterinthesoil)abovethewatertablebyinjectingcompressedairintotheground.Evenifthisprocessprovessuccessful,however,itwouldalmosttriplewatercosts.Geneticengineeringalsomayprovideapartialsolution,asnewstrainsofdrought-resistantcropscontinuetobedeveloped.Whateverthefinalanswertothewatercrisismaybe,itisevidentthatwithintheHighins,irrigationwaterwillneveragainbetheabundant,inexpensiveresourceitwasduringtheagriculturalboomyearsofthemid-twentieth

世纪80年代，在中部大平原的广阔草原上着农民和农场主们。这里有着半干旱的气候，在人们50年后，它支撑了一个以畜牧业和小麦种植为主的低密度农业经济。然而，在20世纪初，人们发现大平原的大部下面是巨大的蓄水层（水的岩层。这个蓄水层因曾经在这里过的奥拉苏族印第安人而得名，被称作奥拉蓄水层。583000平方公里。雨水和融雪自30000年前便开始在奥拉蓄积。据估计，气候条件下，奥拉蓄水层的蓄水能力极低，每年仅半厘米左右。经在奥拉地区共开凿了100000多口井。日喷水量达到升的现代灌考虑到几乎没有补充率（实质上没有自然水资源进行补充，这种有限水资40年内耗尽。这种现象在气候最干旱的德克萨斯州尤为严重。大量的水被从2030年，德克萨斯州余下的奥拉含水只能支持1980年灌溉面积的30%到40%。西西比河、密苏里河或者色河的水通过运河或管道运到需要用水的地方。不幸的是，通过以上任何式获得水资源都会将抽水的成本提高十倍以上，进 然，大平原地区灌溉水资源再也不会像20世纪中期农业繁荣时期的那样充ntco unitiesassebletheselvesflexibly,andtheirparticularstructuredependsonthespecifichistoryofthearea.Ecologistsusetheterm“succession”torefertothechangesthathappeninntcommunitiesandecosystemsoverThefirstcommunityinasuccessioniscalledapioneercommunity,whilelong-livedcommunityattheendofsuccessioniscalledaclimaxcommunity.Pioneerandsuccessionalntcommunitiesaresaidtochangeoverperiodsfrom1to500years.Thesechanges—inntnumbersandthemixofspecies—arecumulative.Climaxcommunitiesthemselveschangebutoverperiodsoftimegreaterthanabout500years.Anecologistwhostudiesapondtodaymaywellfinditrelativelyunchangedinyear’stime.Individualfishmaybereced,butthenumberoffishwilltendtobethesamefromoneyeartothenext.Wecansaythatthepropertiesofanecosystemaremorestablethantheindividualorganismsthatcomposetheecosystem.Atonetime,ecologistsbelievedthatspeciesdiversitymadeecosystemsstable.Theybelievedthatthegreaterthediversitythemorestabletheecosystem.Supportforthisideacamefromtheobservationthatlong-lastingclimaxcommunitiesusuallyhavemorecomplexfoodwebsandmorespeciesdiversitythanpioneercommunities.Ecologistsconcludedthattheapparentstabilityofclimaxecosystemsdependedontheircomplexity.Totakeanextremeexample,farmlandsdominatedbyasinglecroparesounstablethatoneyearofbadweatherortheinvasionofasinglepestcandestroytheentirecrop.Incontrast,acomplexclimaxcommunity,suchasa ,willtolerateconsiderabledamagefromweathertopests.Thequestionofecosystemstabilityiscomplicated,however.Thefirstproblemisthatecologistsdonotallagreewhat“stability”means.Stabilitycanbedefinedassimplylackofchange.Inthatcase,theclimaxcommunitywouldbeconsideredthemoststable,since,bydefinition,itchangestheleastovertime.Alternatively,stabilitycanbedefinedasthespeedwithwhichanecosystemreturnstoaparticularformfollowingamajordisturbance,suchasafire.Thiskindofstabilityisalsocalledresilience.Inthatcase,climaxcommunitieswouldbethemostfragileandtheleaststable,sincetheycanrequirehundredsofyearstoreturntotheclimaxstate.Eventhekindofstabilitydefinedassimplelackofchangeisnotalwaysassociated umdiversity.Atleastintemperatezones, umdiversityisoftenfoundinmid-successionalstages,notintheclimaxcommunity.Oncearedwoodmatures,forexample,thekindsofspeciesandthenumberofindividualsgrowingonthefloorarereduced.Ingeneral,diversity,byitself,doesnotensurestability.Mathematicalmodelsofecosystemslikewises