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1、7.1TheHydrologicCycleThehydrologiccycleisthetransferofwaterfromtheoceanstotheatmospheretothelandandbacktotheoceans.Theprocessesinvolvedincludeevaporationofwaterfromtheoceans;precipitationonland;evaporationfromland;andrunofffromstreams,rivers,andsubsu
2、rfacegroundwater.Thehydrologiccycleisdrivenbysolarenergy,whichevaporateswaterfromoceans,freshwaterbodies,soils,andvegetion.Ofthetotal1.3billionkmwateronEarth,about97%isinoceans,andabout2%isinglaciersandicecaps.Therestisinfreshwateronlandandintheatmos
3、phere.AlthoughitrepresentsonlyasmallfractionofthewateronEarth,thewateronlandisimportantinmovingchemicals,sculpturinglandscape,weatheringrocks,transportingsediments,andprovidingourwaterresources.Thewaterintheatmosphere—only0.001%ofthetotalonEarth—cycl
4、esquicklytoproducerainandrunoffforourwaterresources.Especiallyimportantfromanenvironmentalperspectiveisthatratesoftransferonlandaresmallrelativetowhat’shappeningintheocean.Forexample,mostofthewaterthatevaporatesfromtheoceanfallsagainasprecipitationin
5、totheocean.Onland,mostofthewaterthatfallsasprecipitationcomesfromevaporationofwaterfromland.Thismeansthatregionalland-usechanges,suchasthebuildingoflargedamsandreservoirs,canchangetheamountofwaterevaporatedintotheatmosphereandchangethelocationandamou
6、ntofprecipitationonland—waterwedependontoraiseourcropsandsupplywaterforoururbanenvironments.Furthermore,aswepaveoverlargeareasoflandincities,stormwaterrunsoffquickerandingreatervolume,therebyincreasingfloodhazards.Bringingwaterintosemi-aridcitiesbypu
7、mpinggroundwaterortransportingwaterfromdistantmountainsthroughaqueductsmayincreaseevaporation,therebyincreasinghumidityandprecipitationinaregion.Approximately60%ofwaterthatfallsbyprecipitationonlandeachyearevaporatestotheatmosphere.Asmallercomponent(
8、about40%)returnstotheoceansurfaceandsubsurfacerunoff.Thissmallannualtransferofwatersuppliesresourcesforriversandurbanandagriculturallands.Unfortunately,distributionofwateronlandisfarfromuniform.Ashumanpopulationincreases,watershortageswillbecomemoref
