Tpo24LakeWaterWheredoesthewaterinalakecomefrom,andhowdoeswaterleaveit?Waterentersalakefrominflowingrivers,fromunderwaterseepsandsprings,fromoverlandflowoffthesurroundingland,andfromrainfallingdirectlyonthelakesurface.Waterleavesalakeviaoutflowingrivers,bysoakingintothebedofthelake,andbyevaporation.Somuchisobvious.河水的来源和去向个人收集整理勿做商业用途Thequestionsbecomemorecomplicatedwhenactualvolumesofwaterareconsidered:howmuchwaterentersandleavesbyeachroute?Discoveringtheinputsandoutputsofriversisamatterofmeasuringthedischargesofeveryinflowingandoutflowingstreamandriver.Thenexchangeswiththeatmospherearecalculatedbyfindingthedifferencebetweenthegainsfromrain,asmeasured(ratherroughly)byraingaugesandthelossesbyevaporation,measuredwithmodelsthatcorrectfortheothersourcesofwaterloss.Forthemajorityoflakes,certainlythosesurroundedbyforests,inputfromoverlandflowistoosmalltohaveanoticeableeffect.Changesinlakelevelnotexplainedbyriverflowsplusexchangeswiththeatmospheremustbeduetothenetdifferencebetweenwhatseepsintothelakefromthegroundwaterandwhatleaksintothegroundwater.Notetheword"net":measuringtheactualamountsofgroundwaterseepageintothelakeandoutofthelakeisamuchmorecomplicatedmatterthanmerelyinferringtheirdifference.如何定义流入和流出河流的水量个人收集整理勿做商业用途Onceallthisinformationhasbeengathered,itbecomespossibletojudgewhetheralake’sflowismainlyduetoitssurfaceinputsandoutputsortoitsundergroundinputsandoutputs.Iftheformeraregreater,thelakeisasurface-water-dominatedlake;ifthelatter,itisaseepage-dominatedlake.Occasionally,commonsensetellsyouwhichofthesetwopossibilitiesapplies.Forexample,apondinhillycountrythatmaintainsasteadywaterlevelallthroughadrysummerinspiteofhavingnostreamsflowingintoitmustobviouslybeseepagedominated.Conversely,apondwithastreamflowinginoneendandouttheother,whichdriesupwhenthestreamdriesup,isclearlysurfacewaterdominated.区分“地面水源河”和“地下水源河”个人收集整理勿做商业用途Bywhatevermeans,alakeisconstantlygainingwaterandlosingwater:itswaterdoesnotjustsitthere,or,anyway,notforlong.Thisraisesthematterofalake’sresidencetime.Theresidencetimeistheaveragelengthoftimethatanyparticularmoleculeofwaterremainsinthelake,anditiscalculatedbydividingthevolumeofwaterinthelakebytherateatwhichwaterleavesthelake.Theresidencetimeisanaverage;thetimespentinthelakebyagivenmolecule(ifwecouldfollowitsfate)woulddependontherouteittook:itmightflowthroughaspartofthefastest,mostdirectcurrent,oritmightcircleinabackwaterforanindefinitelylongtime.引入“水源停留时间”的概念个人收集整理勿做商业用途Residencetimesvaryenormously.Theyrangefromafewdaysforsmalllakesuptoseveralhundredyearsforlargeones;LakeTahoe,inCalifornia,hasaresidencetimeof700years.TheresidencetimesfortheGreatLakesofNorthAmerica,namely,LakesSuperior,Michigan,Huron,Erie,andOntario,are,respectively,190,100,22,2.5,and6years.LakeErie’sisthelowest:althoughitsareaislargerthanLakeOntario’s,itsvolumeislessthanone-thirdasgreatbecauseitissoshallow-lessthan20metersonaverage.每条河流的水源停留时间是不同的个人收集整理勿做商业用途Agivenlake’sresidencetimeisbynomeansafixedquantity.Itdependsontherateatwhichwaterentersthelake,andthatdependsontherainfallandtheevaporationrate.Climaticchange(theresultofglobalwarming?)isdramaticallyaffectingtheresidencetimesofsomelakesinnorthwesternOntario,Canada.Intheperiod1970to1986,rainfallintheare...
