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Wheredoesthewaterinalakecomefrom,andhowdoeswaterleaveit?Waterentersalakefrominflowingrivers,fromunderwaterseepsandsprings,fromoverlandflowoffthesurroundingland,andfromrainfallingdirectlyonthelakesurface.Waterleavesalakeviaoutflowingrivers,bysoakingintothebedofthelake,andbyevaporation.Somuchisobvious.Thequestions emorecomplicatedwhenactualvolumesofwaterareconsidered:howmuchwaterentersandleavesbyeachroute?Discoveringtheinputsandoutputsofriversisamatterofmeasuringthedischargesofeveryinflowingandoutflowingstreamandriver.Thenexchangeswiththeatmospherearecalculatedbyfindingthedifferencebetweenthegainsfromrain,asmeasured(ratherroughly)byraingauges,andthelossesbyevaporation,measuredwithmodelsthatcorrectfortheothersourcesofwaterloss.Forthemajorityoflakes,certainlythosesurroundedbyforests,inputfromoverlandflowistoosmalltohaveanoticeableeffect.Changesinlakelevelnotexplainedbyriverflowsplusexchangeswiththeatmospheremustbeduetothenetdifferencebetweenwhatseepsintothelakefromthegroundwaterandwhatleaksintothegroundwater.Notetheword"net":measuringtheactualamountsofgroundwaterseepageintothelakeandoutofthelakeisamuchmorecomplicatedmatterthanmerelyinferringtheirdifference.Onceallthisinformationhasbeengathered,it espossibletojudgewhetheralake’sflowismainlyduetoitssurfaceinputsandoutputsortoitsundergroundinputsandoutputs.Iftheformeraregreater,thelakeisasurface-water-dominatedlake;ifthelatter,itisaseedominatedlake.Occasionally,commonsenselsyouwhichofthesetwopossibilitiesapplies.Forexample,apondinhillycountrythatmaintainsasteadywaterlevelallthroughadrysummerinspiteofhavingnostreamsflowingintoitmustobviouslybeseepagedominated.Conversely,apondwithastreamflowinginoneendandouttheother,whichdriesupwhenthestreamdriesup,isclearlysurfacewaterdominated.Bywhatevermeans,alakeisconstantlygainingwaterandlosingwater:itswaterdoesnotjustsitthere,or,anyway,notforlong.Thisraisesthematterofalake’sresidencetime.Theresidencetimeistheaveragelengthoftimethatanyparticularmoleculeofwaterremainsinthelake,anditiscalculatedbydividingthevolumeofwaterinthelakebytherateatwhichwaterleavesthelake.Theresidencetimeisanaverage;thetimespentinthelakebyagivenmolecule(ifwecouldfollowitsfate)woulddependontherouteittook:itmightflowthroughaspartofthefastest,mostdirectcurrent,oritmightcircleinabackwaterforanindefiniylongtime.Residencetimesvaryenormously.Theyrangefromafewdaysforsmalllakesupseveralhundredyearsforlargeones;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.Agivenlake’sresidencetimeisbynomeansafixedty.Itdependsontherateatwhichwaterentersthelake,andthatdependsontherainfallandtheevaporationrate.Climaticchange(theresultofglobalwarming?)isdramaticallyaffectingtheresidencetimesofsomelakesinnorthwesternOntario,Canada.Intheperiod1970to1986,rainfallintheareadecreasedfrom1,000millimetersto650millimetersperannum,whileabove-averagetemperaturesspeededuptheevapotranspirationrate(therateatwhichwaterislosttotheatmospherethroughevaporationandtheprocessesofplantlife).Theresulthasbeenthattheresidencetimeofoneofthelakesincreasedfrom5to18yearsduringthestudyperiod.Theslowingdownofwaterrenewalleadstoachainoffurtherconsequences;itcausesdissolvedchemicalsto eincreasinglyconcentrated,andthis,inturn,hasamarkedeffectonalllivingthingsinthelake.

湖里的水從哪里來,又流向哪里去呢?湖中的水來自于河流的水,水的滲透以具體含量是多少?發(fā)現(xiàn)河流水量的流進(jìn)和流出量是測量河水進(jìn)出容量的法.中水平面的變化不能被河水和大氣水量變化的凈差量所解釋是因?yàn)樗臐B入和滲出.注意一下“凈”這個(gè)詞:測量真正的水滲入和滲出量比僅僅推斷它還是由水進(jìn)出量決定的就變得可能了.如果前者大,那么湖泊就是一個(gè)表面水決定的湖,如果是后者,那么它就是一個(gè)滲透水決定的湖.有時(shí)候會(huì)告訴你這兩種可能性哪一種是對的.比如說一個(gè)多山地區(qū)的在整個(gè)干燥的夏天都保持塘.相反,一個(gè)有河流流進(jìn)和流出,河水干枯的時(shí)候就干枯,那么這就是一HuronErie,Ontario190,100,22,2.56年。Erie湖是最低的，盡管不到20米。響著西北部的Ontario湖群中的一些湖泊的停留時(shí)間。在1970到1986這1000650毫升，而同時(shí)平均溫度的Ofallthephysiologicaldifferencesinhumansleepcomparedwithwakefulnessthathavebeendiscoveredinthelastdecade,changesinrespiratorycontrolaremostdramatic.Notonlyaretheredifferencesinthelevelofthefunctioningofrespiratorysystems,thereareevenchangesinhowtheyfunction.Movementsoftheribcageforbreathingarereducedduringsleep,makingthecontractionsofthediaphragmmoreimportant.Yetbecauseofthephysicsoflyingdown,thestomachappliesweightagainstthediaphragmandmakesitmoredifficultforthediaphragmtodoitsjob.However,therearemanyotherchangesthataffectrespirationwhenDuringwakefulness,breathingiscontrolledbytwointeractingsystems.Thefirstisanautomatic,metabolicsystemwhosecontroliscenteredinthebrainstem.Itsubconsciouslyadjustsbreathingrateanddepthinordertoregulatethelevelsofcarbondioxide(CO2)andoxygen(O2),andtheacid-baseratiointheblood.Thesecondsystemisthevoluntary,behavioralsystem.Itscontrolcenterisbasedintheforebrain,anditregulatesbreathingforuseinspeech,singing,sighing,andsoon.Itiscapableofignoringoroverridingtheautomatic,metabolicsystemandproducesanirregularpatternofbreathing.DuringNREM(thephaseofsleepinwhichthereisnorapideyemovement) esdeeperandmoreregular,butthereisalsoadecreaseinthebreathingrate,resultinginlessairbeingexchangedoverall.ThisoccursbecauseduringNREMsleeptheautomatic,metabolicsystemhasexclusivecontroloverbreathingandthebodyuseslessoxygenandproduceslesscarbondioxide.Also,duringsleeptheautomaticmetabolicsystemislessresponsivetocarbondioxidelevelsandoxygenlevelsintheblood.Twothingsresultfromthesechangesinbreathingcontrolthatoccurduringsleep.First,theremaybeabriefcessationorreductionofbreathingwhenfallingasleepasthesleeperwaxesandwanesbetweensleepandwakefulnessandtheirdifferingcontrolmechanisms.Second,oncesleepisfullyobtained,thereisanincreaseofcarbondioxideandadecreaseofoxygeninthebloodthatpersistsduringNREM.Butthatisnotallthatchanges.Duringallphasesofsleep,severalchangesintheairpassageshavebeenobserved.Ittakestwiceasmuchefforttobreatheduringsleepbecauseofgreatertoairflowintheairwaysandchangesintheefficiencyofthemusclesusedforbreathing.Someofthemusclesthathelpkeeptheupperairwayopenwhenbreathingtendto emorerelaxedduringsleep,especiallyduringREM(thephaseofsleepinwhichthereisrapideyemovement).Withoutthismuscularaction,inhalingislikesuckingairoutofaballoon—thenarrowpassagestendtocollapse.Alsothereisaregularcycleofchangeinbetweenthetwosidesofthenose.Ifsomethingblocksthe"good"side,suchascongestionfromallergiesoracold,thenincreasesdramatically.Coupledwiththesefactorsisthelossofthecomplexinteractionsamongthemusclesthatcanchangetherouteofairflowfromnosetomouth.Otherrespiratoryregulatingmechanismsapparentlyceasefunctioningduringsleep.Forexample,duringwakefulness,thereisanimmediate,automatic,adaptiveincreaseinbreathingeffortwheninhalingismademoredifficult(suchasbreathingthrougharestrictivefacemask).ThisreflexiveadjustmentistotallyabsentduringNREMsleep.Onlyafterseveralinadequatebreathsundersuchconditions,resultingintheconsiderableelevationofcarbondioxideandreductionofoxygenintheblood,isbreathingeffortadjusted.Finally,thecoughingreflexinreactiontoirritantsintheairwayproducesnotacoughduringsleepbutacessationofbreathing.Iftheirritationissevereenough,asleewillarouse,cleartheairway,thenresumebreathingandlikelyreturntosleep.AdditionalbreathingchangesoccurduringREMsleepthatareevenmoredramaticthanthechangesthatoccurduringNREM.TheamountofairexchangedisevenlowerinREMthanNREMbecause,althoughbreathingismorerapidinREM,itisalsomoreirregular,withbriefepisodesofshallowbreathingorabsenceofbreathing.Inaddition,breathingduringREMdependsmuchmoreontheactionofthediaphragmandmuchlessonribcageaction.

睡眠中的呼吸得的話就會(huì)有一個(gè)立即自動(dòng)適應(yīng)性的呼吸增強(qiáng)（比如戴上面具呼吸時(shí)。但律，包括一些間斷的呼吸暫停。另外，REM時(shí)期的呼吸取決于橫膈膜而不IntheMesaVerdeareaoftheancientNorthAmericanSouthwest,livingpatternschangedinthethirteenthcentury,withlargenumbersofpeoplemovingintolargecommunaldwellingscalledpueblos,oftenconstructedattheedgesofcanyons,especiallyonthesidesofcliffs.Abandoningsmallextended-familyhouseholdstomoveintotheselargepuebloswithdozensifnothundredsofotherpeoplewasprobablytraumatic.Fewoftheculturaltraditionsandrulesthattodayallowustodealwithdensepopulationsexistedforthesepeopleaccustomedtohouseholdautonomyandtheabilitytomovearoundthelandscapealmostatwill.Andbesidestheawkwardnessofhavingtosharewallswithneighbors,livinginaggregatedpueblosintroducedotherproblems.Forpeopleincliffdwellings,haulingwater,wood,andfoodtotheirhomeswasamajorchore.Thestressonlocalresources,especiallyinthefirewoodneededfordailycookingandwarmth,wasparticularlyintense,andconditionsinaggregatedpuebloswerenotveryhygienic.Givenallthedisadvantagesoflivinginaggregatedtowns,whydidpeopleinthethirteenthcenturymoveintothesecloselypackedquarters?Fortransitionsofsuchsuddenness,archaeologistsconsidereitherpullfactors(benefitsthatdrewfamiliestogether)orpushfactors(someexternalthreatorcrisisthatforcedpeopletoaggregate).Inthiscase,pushexplanationsdominate.Populationgrowthisconsideredaparticularlyinfluentialpush.Afterseveralgenerationsofpopulationgrowth,peoplepackedthelandscapeindensitiessohighthatcommunalpueblosmayhavebeenanecessary e.AroundSandCanyon,forexample,populationsgrewfrom5-12peoplepersquarekilometerinthetenthcenturytoasmanyas30-50bythe1200s.Asdensitiesincreased,domesticarchitecturebecamelarger,culminatingincrowdedpueblos.Somescholarsexpandonthisideabyemphasizingacorrespondingneedforarablelandtofeedgrowingnumbersofpeople:constructionofsmalldams,reservoirs,terraces,andfieldhousesindicatesthatfarmerswereintensifyingtheireffortsduringthe1200s.Competitionforgoodfarmlandmayalsohavepromptedpeopletobondtogethertoassertrightsoverthebestfields.AnotherimportantpushwastheonsetoftheLittleIceAge,aclimaticphenomenonthatledtocoolertemperaturesintheNorthernHemisphere.AlthoughtheheightoftheLittleIceAgewasstillaroundthecorner,someevidencesuggeststhattemperatureswerefallingduringthethirteenthcentury.Theenvironmentalchangesassociatedwiththistransitionarenotfullyunderstood,butpeoplelivingclosesttotheSanJuanMountains,tothenortheastofMesaVerde,wereaffectedfirst.Growingfoodattheseelevationsisalwaysdifficultbecauseoftheshortgrowingseason.AstheLittleIceAgeprogressed,farmersprobablymovedtheirfieldstolowerelevations,infringingonthelandsofotherfarmersandpushingpeopletogether,thuscontributingtotheaggregations.ArchaeologistsidentifyacorrespondingshiftinpopulationstowardthesouthandwesttowardMesaVerdeandawayfromhigherInthefaceofallthesepushes,peopleintheMesaVerdeareahadyetanotherreasontomoveintocommunalvillages:theneedforgreatercooperation.SharingandcooperationwerealmostcertainlypartofearlyPuebloanlife,evenforpeoplelivinginlar