《新托福突破口TPO24》PAGE\*MERGEFORMAT25ReadingLakeWaterWheredoesthewaterinalakecomefrom,andhowdoeswaterleaveit?Waterentersalakefrominflowingrivers,fromunderwaterseepsandsprings,fromoverlandflowoffthesurroundingland,andfromrainfallingdirectlyonthelakesurface.Waterleavesalakeviaoutflowingrivers,bysoakingintothebedofthelake,andbyevaporation.Somuchisobvious.Thequestionsbecomemorecomplicatedwhenactualvolumesofwaterareconsidered:howmuchwatereiStersandleavesbyeachroute?Discoveringtheinputsandoutputsofriversisamatterofmeasuringthedischargesofeveryinflowingandoutflowingstreamandriver.Thenexchangeswiththeatmospherearecalculatedbyfindingthedifferencebetweenthegainsfromrain,asmeasured(ratherroughly)byraingauges,andthelossesbyevaporation,measuredwithmodelsthatcorrectfortheothersourcesofwaterloss.Forthemajorityoflakes,certainlythosesurroundedbyforests,inputfromoverlandflowistoosmalltohaveanoticeableeffect.Changesinlakelevelnotexplainedbyriverflowsplusexchangeswiththeatmospheremustbeduetothenetdifferencebetweenwhatseepsintothelakefromthegroundwaterandwhatleaksintothegroundwater.Notetheword"net":measuringtheactualamountsofgroundwaterseepageintothelakeandoutofthelakeisamuchmorecomplicatedmatterthanmerelyinferringtheirdifference.Onceallthisinformationhasbeengathered,itbecomespossibletojudgewhetheralake’sflowismainlyduetoitssurfaceinputsandoutputsortoitsundergroundinputsandoutputs.[■]Iftheformeraregreater,thelakeisasurface-water-dominatedlake;ifthelatter,itisaseepage-dominatedlake.[■]Occasionally,commonsensetellsyouwhichofthesetwopossibilitiesapplies.[■]Forexample,apondinhillycountrythatmaintainsasteadywaterlevelallthroughadrysummerinspiteofhavingnostreamsflowingintoitmustobviouslybeseepagedominated.Conversely,apondwithastreamflowinginoneendandouttheother,whichdriesupwhenthestreamdriesup,isclearlysurfacewaterdominated.[■]Bywhatevermeans,alakeisconstantlygainingwaterandlosingwater:itswaterdoesnotjustsitthere,or,anyway,notforlong.Thisraisesthematterofalake’sresidencetime.Theresidence