水文學(xué)原理英文版課件

PhysicalHydrologyS.LawrenceDingman1PhysicalHydrologyS.LawrencWhoamI?ZhangXiang68772303-601(office:0101609zhangxiang@2WhoamI?ZhangXiang2TextbookandreferencesTextbook

PhysicalHydrologyReferences

HydrologyforEngineersbyR.K.LinsleyHydrology:principles,analysisanddesignbyH.M.RaghunathHydrology:anintroductiontohydrologicscience

byR.L.Bras水文學(xué)原理(一)胡方榮候宇光水文學(xué)原理(二)于維忠水文學(xué)原理芮孝芳3TextbookandreferencesTextbooTimeArrangeLecture:36classhours

part1and2:4classhourspart4:8classhourspart6:8classhourspart7:6classhourspart8:6classhourspart9:4classhoursLab:6classhoursforcomputer4TimeArrangeLecture:36classTheFinalsAssignments(30%)Checkingonattendance(20%)Examination(50%)5TheFinalsAssignments(30%)5Attention!Prepareyourcoursebeforehandwords,sentences,phasesNoabsenceisallowedDon'tbelateforclassReadasmuchaspossible,writeasmuchaspossible,usetheInternetforhelpasmuchaspossible6Attention!Prepareyourcourse1IntroductiontoHydrologicScienceDEFINITIONANDSCOPEOFHYDROLOGYDEVELOPMENTOFSCENTIFICHYDROLOGYAPPROACHANDSCOPEOFTHISBOOK71IntroductiontoHydrologi1.1DEFFINTIONANDSCOPEOFHYDROLOGY81.1DEFFINTIONANDSCOPEOFHHydrologyisbroadlydefinedasthegeosciencethatdes-cribeandpredictstheoccurrence,circulation,anddistri-butionofthewateroftheearthanditsatmosphere.Theglobalhydrologiccycle

Thelandphaseofthehydrologiccycle99WaterCycle10WaterCycle10111112121313InterdisciplinaryScienceTomanagewaterresourcesandwaterrelatedhazardsEngineeringhydrology,economicsandrelatedsocialscienceforwater-resourcesmanagement14Interdisciplinary141.2DEVELOPMENTOFSCIENTIFICHYDROLOGY5000-6000B.P.Pakistan,China,Egypt…：Canals,levees,dam,well…3800B.P.Egyptians:monitoringofriverflow2400B.PIndia:rainfallmeasurement151.2DEVELOPMENTOFSCIENTIFIC

Theconceptofaglobalhydrologycycledatesfromatleast3000P.B(Nace1974),whenSolomonwroteinEcclesiastes1:7that

Alltheriversrunintothesea;yettheseaisnotfull;untotheplacefromwhencetheriverscome,thithertheyreturnagain.

The18thcenturysawconsiderableadvanceinapplicationsofmathematicstofluidmechanicsandhydraulicsbyPitot,BeroulliChezy,Euler,andothersinEurope.Useofterm“hydrology”inapproximatelyitscurrentmeaningbeganabout1750.

1616Treatisesonvariousaspectsofhydrology,beginningwiththeEnglishmanNathanielBeard-more’sManualofHydrologyin1862,appearedwithincreasingfrequ-encyinthelasthalfofthe19thcentury.

Thehalfofthetwentiethcenturysawgreatprogressinmanyaspectofhydrologyand,withtheformationoftheSectionofScientificHydrologyintheInternationalUnionofGeodesyandGeophysics(IUGG,1992)andHydrologySectionoftheAmericanGeophysicalUnion(1930),thefirstformrecognitionofthescientificstatusofhydrology.1717

Thereare,infactgreatopportunitiesforprogressinphysicalhy-drologyinmanyareas,includingthedeterminationofregionalevapotranspirationrate,themovementofgroundwaterinrockfracture,therelationbetweenhydrologybehavioratdifferentscales,there-lationofhydrologyregimestopastandfutureclimatesandtheinteractionofhydrologyprocessesandland-formdevelopment(Eaglesonetal.1991).

1818Theabilitytounderstandandmodelhydrologicprocessesatcontinentalandglobalscaleisbe-comeincreasingimpor-tantbecauseoftheneedtopredicttheeffectsoflarge-scalechangesinlandandinclimate.FrompointtolargerInthelandphaseofthehydrologiccycle,itisinterestingthatdetailedfieldstudiestounderstandthemechanismsbywhichwaterentersstreamsbegantoproliferateonlyinthe1960s,pioneeredbyT.Dunneandothers.

Thetemporalandspatialvariabilityofnaturalconditions19192

BasicHydrologicConcepts2.1PHYSICALQUANTITIESANDLAWS2.2HYDROLOGICSYSTEMS2.3THECONSERVATIONEQUATIONS2.4THEWATERSHED(DRAINAGEBASIN)2.5THEREGIONALWATERBALANCE2.6SPATIALVARIABILITY2.7TEMPORALVARIABILITY2.8STORAGE,STORAGEEFFECTS,ANDRESIDENCETIME

202BasicHydrologicConcepts22.1PHYSICALQUNTITESANDLAWSHydrologyisaquantitygeophysicalscience,Inprinciple,thesenumericalvaluesofhydrologicvaluesaredeterminedbyeither1.counting,inwhichcasethequantitytakesonavaluethatisapositiveintegerorzero;or2.Measuring,inwhichcasethequantitytakesonavaluecorrespondingtoapointontherealnumberscalethatistheratioofthemagnitudeofthequantitytothemagnitudeofastandardunitofmeasurement.212.1PHYSICALQUNTITESANDLAWSThebasicrelationsofphysicalhydrologyarederivedformfundamentallawsofclassicphysics,particularlythoselistedinTable2-1.22Thebasicrelationsofphysica2.2HYHDROLOGICSYSTEMSeveralbasichydrologicconceptsarerelatedtothesimplemodelofasystemshowninFigure2-1.TheouterdashedlineinFigure2-1indicatesthatanygroupoflinkedsystemscanbeaggregatedintoalargersystem;thesmallersystemscouldthenbecalledsubsystems.232.2HYHDROLOGICSYSTEM2324242.3THECONSERVATIONEQUATIONSTheamountofaconservativequantityenteringacontrolvolumeduringadefinedtimeperiod,minustheamountofthequantityleavingthevolumeduringthetimeperiod,equalsthechangeintheamountofthequantitystoredinthevolumeduringthetimeperiod.

252.3THECONSERVATIONEQUATIONS(2-2)(2-3)AmountIn–Amountout=ChangeInstorage(2-1),,26(2-2)(2-3)AmountIn–Amount(2-4)(2-5)(2-6),,,27(2-4)(2-5)(2-6),,,27Anotherversionoftheconservationequationcanbedevelopedbydefiningtheinstantaneousratesofinflow,i,andoutflow,q,as(2-7)(2-8)(2-9),,,28AnotherversionoftheconserEquations(2-2),(2-6),and(2-9),arecalledwater-balanceequa-tionwhenappliedtothemassofwatermovingthroughvariousportionsofthehydrologiccycle;controlvolumesintheseappli-cationsrangeinsizefrominfinitesimaltoannualorlonger(Figure1-3).evaporationandsnowmeltenergy-balancetheconservationofmomentumfluidflow29292.4

THEWATERSHED(DRAINAGEBASIN)Watershed

(alsocalled

drainagebasin,

riverbasin,

orcatch-ment),definedastheareathatappearsonthebasisoftopographytocontributeallthewaterthatpass-esthroughagivencrosssectionofastream(Figure2-2).dividedrainagearea2.4.1

definition302.4THEWATERSHED(DRAINAGEBA3131Thusthewatershedcanbeviewedasanaturallandscapeunit,integratedbywaterflowingthroughthelandphaseofthehydr-ologiccycleand,althoughpoliticalboundariesdonotgenerallyfollowwatershedboundaries,water-resourceandland-useplanningagenciesrecognizethateffectivemanagementofwaterqualityandqualityrequireawatershedperspective.Thelocationofthestreamcrosssectionthatdefinesthewater-shedisdeterminedbythepurposeoftheanalysis.3232Theconventionalmanualmethodofwatershedde-lineationrequiresatopographicmap(orstereo-scopicallyviewedaerialphotographs).Increasingly,topographicinformationisbecomingavailableintheformofdigitalelevationmodels(DEMs).Thisautomatedapproachtowatersheddelineationallowstheconcomitantrapidextractionofmuchhydrologicallyusefulinformationonwatershedcharacteristics(suchasthedistri-butionofelevationandslop)thatpreviouslycouldbeobtainedonlybyverytediousmanualmethods.2.4.2Delineation

332.4.2Delineation332.5THEREGIONALWATERBALANCETheregionalwaterbalanceistheapplicationofthewater-balanceequationtoawatershed(ortoanylandarea,suchasastateorcontinent).342.5THEREGIONALWATERBALANCE2.5.1TheWater-BalanceEquation352.5.1TheWater-BalanceEquati,(2-10)Ifweaveragethesequantitiesoverareasonablylongtimeperiod(say,manyyears)inwhichtherearenosignificantclimatictrendsorgeologicalchangesandnoanthropogenicinputs,orstoragemodifications,wecanusuallyassumethatnetchangeinstoragewillbeeffectivelyzeroandwritethewaterbalanceas(2-11),36,(2-10)Ifweaveragethesequarunoff,RO;hydrologicproduction;(2-12)(2-13),,37runoff,RO;(2-12)(2-13),,37,(2-14)Whenweassumethatisnegligibleandwritethewater-balanceequationas,(2-15)38,(2-14)WhenweassumethatisBothprecipitationandevaportranspirationcanbeconsideredtobeexternallyimposedclimatic‘boundaryconditions’.This,fromEquation(2-15),runoffisaresidualordifferencebetweentwoclimatically-determinedquantities.2.5.2EstimationofRegionalEvaportranspirationPerhapsthemostcommonformofhydrologicanalysisistheestimationofthelong-termaveragevalueofregionalevapor-transpirationviathewater-balanceequation.39BothprecipitationandevaportItisusuallyassumedthatground-waterflowseitherarenegligibleorcanceloutandthatisnegligible,sothatequation(2-15)becomes(2-16),Modelerror,whichreferstotheomissionofpotentiallysignifi-canttermformtheequation,andmeasurement

inthequantitiesand,whichisunavoidable.40ItisusuallyassumedthatgroGround-waterFlowsStreamsdraininglargerwatershedstendtoreceivethesubsur-faceoutflowsoftheirsmallerconstituentwatershed,sotheimportanceofground-waterout-flowgenerallydecreaseasoneconsiderslargerandlargerwatershed.4141StorageChangeHydrologistsattempttominimizeitsvalueby(1)us-inglongmeasurementperiodsanda(2)selectingthetimeofbeginningandendofthemeasurementperiodsuchthatstoragevaluesarelikelytobenearlyequal.4242MeasurementErrorAccuracyofRegionalPrecipitationValuesindividualgagesarealaveragesInregionsofthehighrelieforwithfeworpoorlydistributedgages,orforshortermeasurementperi-ods,theuncertainlycanbeconsiderablylarger.43MeasurementError43AccuracyofStreamflowValues

Winter(1981)estimatedthatthemeasurementun-certainlyforlong-termaveragevaluesofstreamflowatagagingstationisontheorderof.(Theac-curacyofsuchmeasurementsisdiscussedfurtherinSectionF.2.4)whereisestimatedforlocationsothercarefullymaintainedgagingstations,theuncertaintycanbemuchgreater.4444Potentialmeasurementerrorsareusuallyassumedtobedistri-butedsymmetricallyaboutthetruevalue(equalchanceofunder-orover-estimation)andtofollowthebell-shapednormaldistributiondescribeinAppendixC:thefutureameasuredvalueisfromthetruevalue(i.e.,thelargeristheerror),thesmallisprobabilitythatitwilloccur(Figure2-4).Thespread,orvariation,ofthepotentialmeasuredvaluesaboutthetruevalueisexpressedasthestandarddeviationofthepotentialerrors.4545Thestandarddeviationsoftheerrorsduetomeasurementofthequantitiesarerelatedas,(2-17)“Iam100.p%surethatthetruevalueofprecipitationiswithinofthemeasuredvalue.”(2-18a)46,(2-17)“Iam100.p%surethat

istheestimateofaverageprecipitationandistherelativeuncertaintyintheestimate(e.g,ifthemeasurementuncertaintyisstatedtobe10%,=0.1).Theabsoluteuncer-taintyinis.

(2-18b),47(2-18b),47Giventhatpotentialmeasurementerrorsfollowthenormaldistribution,wecanfindfromthepropertiesofthatdistribution,summarizedinTableC-5,thatthereisa95%probabilitythatanobservationwillbewithin1.96standarddeviationsofthecentral(true)value.(2-19),48Giventhatpotentialmeasureme(2-20),(2-21a),,(2-21b)49(2-20),(2-21a),,(2-21b)49(2-22),,(2-23)Assumetherelativemeasurementerrorsforprecipitationandstreamfloware=0.1and=0.05.50(2-22),,(2-23)Assumetherelat2.6

SPATIALVARIABILITY

However,precipitationgagesareusuallyunevenlydistributedoveranygivenregion,andthepointvaluesarethereforeanun-representativesampleofthetrueprecipitationfield,Becauseofthis,andbecauseoftheimportanceofaccuratelyquantifyingvariablessuchasprecipitation,basicstatisticalconceptshavebeenincorporatedintospecialtechniqueforcharacterizingandaccountingforspatialvariability.512.6

SPATIALVARIABILITY512.7

TEMPORALVARIABILITYTheinputs,storagesandoutputsinFigurearealltime-distributedvariables—quantitiesthatcanvarywithtime.Inparticular,thestreamflowrateatagivenlocationishighlyvariableintime.Fromthehumanviewpoint,thelong-termaveragestreamflowrate,,ishighlysignificant:itrepresentsthemaximumrateatwhichwaterispotentiallyavailableforhumanuseandmanagement,andisthereforeameasureoftheultimatewaterresourcesofawatershedorregion.522.7

TEMPORALVARIABILITY52Streamflowvariabilityisdirectlyrelatedtotheseasonalandinterannualvariabilityofrunoff(andhenceoftheclimateofprecipitationandevapo-transpiration)andinverselytotheamountofsto-rageinthewatershed.Humancanincreasewateravailabilitybybuildingstoragereservoirs,asdis-cussedinSection2.8and10.2.5.Humancanalsoattempttoincreasethrough“rain-making”(Sec-tion4.4.5)andtodecreasebymodifyingvegeta-tion(Section7.6.4and10.2.5).Thebasicapproachforconstructingandanalyzingsampleoftine-distributedvariables.5353Eachvalueofwhichisassociatedwithaparticulartimeinasequencetimes,.Suchasequenceiscalledatimeseries.Sometime-seriesvariablesareobtainedbycounting—forexample,thenumberofdayswithmorethan1mmrainineachyearataparticularlocation.Suchvariablesareinherentlydiscrete.Continuoustimetrace:theytakeonvaluesateveryinstantintime.

2.7.1TimeSeries542.7.1TimeSeries545555EXAMPLE2-2Table2-2lists,andFigure2-6plots,threetimeseriesdevelopedfromthecontinuousstreamflowrecordobtainatthestreamgagingstationoperatedbytheU.S.GeologicalSurveyontheOysterRiverinDurham,NH.Inallthreeplots,=1yr,andtheordinateisastreamflowrate,ordis-charge,,However,thediscretizationofthecontinuousrecordwasdonedifferentlyforeachseries:Se-riesaistheaveragestreamflowfortheyear,seriesbisthehigh-estinstantaneousflowratesfortheyear,seriescisthelowestoftheflowratesfoundbyaveragingoverseven-consecutive-dayperiodswithineachyear.(Thesedataarealsointhespreadsheetfiletable2-2,xlsonthediskaccompanyingthistext.)Notethatthelinesconnectingthetime-seriesvaluesineachgraphdonotrepresentatimetracetheyserveonlytoconnectthepointvaluetoprovideavisualtoprovideimpressionofthenatureoftheseries.56EXAMPLE2-2565757Timeseriesareusuallytreatedasmoreorlessrepresentativesample

ofthelong-termbehaviorofthevariableanddescribedandcomparedonthebasisoftheirstatisticalattributes.Forexample,thetemporalvariabilityofatimeseriescanbecharac-terizedinabsolutetermsbyitsinterquartilerangeorbyitsstandarddeviation,andinrelativetermsbytherationofitsinterquartilerangtoitsmedianorbyitscoefficientofvariation.5858Itis

importanttonotethattimeseriesdevelopedfromasinglecontinuoustimetracebychoosingdifferentdiscretizingschemes(asinExample2-2)ordifferentvaluewillingeneralhaveverydifferentstatisticalcharacteristics.59592.8

STORAGE,STORAGEEFFECTS,ANDRESIDENCETIMEThecontrolvolumesinFigures1-1and1-2representstorageinthehydrologiccycle,andtheentirewater-shedorregionwithinthedashedboundariescanalsobethoughtofasastoragereservoir.2.8.1Storage602.8

STORAGE,STORAGEEFFECTS,AN(2-25).naturewatershedslinearreservoir,(2-26)61(2-25).naturewatershedslinearWhereEquation(2-25)applies,storagehastwoeffectsonoutflowtimeseries:Itdecreasestherelativevariabilityoftheoutflowsrelativetotheinflows.Itincreasesthepersistence.2.8.2StorageEffects622.8.2StorageEffects62Residencetime,oraveragetransittime,isauniversalrelativemeasureofthestorageeffectofareservoir.Itisequaltotheaveragelengthoftimethata“parcel”ofwaterspendsinthereservoir.2.8.3ResidenceTime632.8.3ResidenceTime636464Residencetimecanbecalculatedbydividingtheaveragemass(orvolume)ofthesubstanceofinte-restinthereservoir,,bytheaveragerateofoutflow,orinflow,ofthatsub-stance.,(2-27)65,(2-27)65PhysicalHydrologyS.LawrenceDingman66PhysicalHydrologyS.LawrencWhoamI?ZhangXiang68772303-601(office:0101609zhangxiang@67WhoamI?ZhangXiang2TextbookandreferencesTextbook

PhysicalHydrologyReferences

HydrologyforEngineersbyR.K.LinsleyHydrology:principles,analysisanddesignbyH.M.RaghunathHydrology:anintroductiontohydrologicscience

byR.L.Bras水文學(xué)原理(一)胡方榮候宇光水文學(xué)原理(二)于維忠水文學(xué)原理芮孝芳68TextbookandreferencesTextbooTimeArrangeLecture:36classhours

part1and2:4classhourspart4:8classhourspart6:8classhourspart7:6classhourspart8:6classhourspart9:4classhoursLab:6classhoursforcomputer69TimeArrangeLecture:36classTheFinalsAssignments(30%)Checkingonattendance(20%)Examination(50%)70TheFinalsAssignments(30%)5Attention!Prepareyourcoursebeforehandwords,sentences,phasesNoabsenceisallowedDon'tbelateforclassReadasmuchaspossible,writeasmuchaspossible,usetheInternetforhelpasmuchaspossible71Attention!Prepareyourcourse1IntroductiontoHydrologicScienceDEFINITIONANDSCOPEOFHYDROLOGYDEVELOPMENTOFSCENTIFICHYDROLOGYAPPROACHANDSCOPEOFTHISBOOK721IntroductiontoHydrologi1.1DEFFINTIONANDSCOPEOFHYDROLOGY731.1DEFFINTIONANDSCOPEOFHHydrologyisbroadlydefinedasthegeosciencethatdes-cribeandpredictstheoccurrence,circulation,anddistri-butionofthewateroftheearthanditsatmosphere.Theglobalhydrologiccycle

Thelandphaseofthehydrologiccycle749WaterCycle75WaterCycle10761177127813InterdisciplinaryScienceTomanagewaterresourcesandwaterrelatedhazardsEngineeringhydrology,economicsandrelatedsocialscienceforwater-resourcesmanagement79Interdisciplinary141.2DEVELOPMENTOFSCIENTIFICHYDROLOGY5000-6000B.P.Pakistan,China,Egypt…：Canals,levees,dam,well…3800B.P.Egyptians:monitoringofriverflow2400B.PIndia:rainfallmeasurement801.2DEVELOPMENTOFSCIENTIFIC

Theconceptofaglobalhydrologycycledatesfromatleast3000P.B(Nace1974),whenSolomonwroteinEcclesiastes1:7that

Alltheriversrunintothesea;yettheseaisnotfull;untotheplacefromwhencetheriverscome,thithertheyreturnagain.

The18thcenturysawconsiderableadvanceinapplicationsofmathematicstofluidmechanicsandhydraulicsbyPitot,BeroulliChezy,Euler,andothersinEurope.Useofterm“hydrology”inapproximatelyitscurrentmeaningbeganabout1750.

8116Treatisesonvariousaspectsofhydrology,beginningwiththeEnglishmanNathanielBeard-more’sManualofHydrologyin1862,appearedwithincreasingfrequ-encyinthelasthalfofthe19thcentury.

Thehalfofthetwentiethcenturysawgreatprogressinmanyaspectofhydrologyand,withtheformationoftheSectionofScientificHydrologyintheInternationalUnionofGeodesyandGeophysics(IUGG,1992)andHydrologySectionoftheAmericanGeophysicalUnion(1930),thefirstformrecognitionofthescientificstatusofhydrology.8217

Thereare,infactgreatopportunitiesforprogressinphysicalhy-drologyinmanyareas,includingthedeterminationofregionalevapotranspirationrate,themovementofgroundwaterinrockfracture,therelationbetweenhydrologybehavioratdifferentscales,there-lationofhydrologyregimestopastandfutureclimatesandtheinteractionofhydrologyprocessesandland-formdevelopment(Eaglesonetal.1991).

8318Theabilitytounderstandandmodelhydrologicprocessesatcontinentalandglobalscaleisbe-comeincreasingimpor-tantbecauseoftheneedtopredicttheeffectsoflarge-scalechangesinlandandinclimate.FrompointtolargerInthelandphaseofthehydrologiccycle,itisinterestingthatdetailedfieldstudiestounderstandthemechanismsbywhichwaterentersstreamsbegantoproliferateonlyinthe1960s,pioneeredbyT.Dunneandothers.

Thetemporalandspatialvariabilityofnaturalconditions84192

BasicHydrologicConcepts2.1PHYSICALQUANTITIESANDLAWS2.2HYDROLOGICSYSTEMS2.3THECONSERVATIONEQUATIONS2.4THEWATERSHED(DRAINAGEBASIN)2.5THEREGIONALWATERBALANCE2.6SPATIALVARIABILITY2.7TEMPORALVARIABILITY2.8STORAGE,STORAGEEFFECTS,ANDRESIDENCETIME

852BasicHydrologicConcepts22.1PHYSICALQUNTITESANDLAWSHydrologyisaquantitygeophysicalscience,Inprinciple,thesenumericalvaluesofhydrologicvaluesaredeterminedbyeither1.counting,inwhichcasethequantitytakesonavaluethatisapositiveintegerorzero;or2.Measuring,inwhichcasethequantitytakesonavaluecorrespondingtoapointontherealnumberscalethatistheratioofthemagnitudeofthequantitytothemagnitudeofastandardunitofmeasurement.862.1PHYSICALQUNTITESANDLAWSThebasicrelationsofphysicalhydrologyarederivedformfundamentallawsofclassicphysics,particularlythoselistedinTable2-1.87Thebasicrelationsofphysica2.2HYHDROLOGICSYSTEMSeveralbasichydrologicconceptsarerelatedtothesimplemodelofasystemshowninFigure2-1.TheouterdashedlineinFigure2-1indicatesthatanygroupoflinkedsystemscanbeaggregatedintoalargersystem;thesmallersystemscouldthenbecalledsubsystems.882.2HYHDROLOGICSYSTEM2389242.3THECONSERVATIONEQUATIONSTheamountofaconservativequantityenteringacontrolvolumeduringadefinedtimeperiod,minustheamountofthequantityleavingthevolumeduringthetimeperiod,equalsthechangeintheamountofthequantitystoredinthevolumeduringthetimeperiod.

902.3THECONSERVATIONEQUATIONS(2-2)(2-3)AmountIn–Amountout=ChangeInstorage(2-1),,91(2-2)(2-3)AmountIn–Amount(2-4)(2-5)(2-6),,,92(2-4)(2-5)(2-6),,,27Anotherversionoftheconservationequationcanbedevelopedbydefiningtheinstantaneousratesofinflow,i,andoutflow,q,as(2-7)(2-8)(2-9),,,93AnotherversionoftheconserEquations(2-2),(2-6),and(2-9),arecalledwater-balanceequa-tionwhenappliedtothemassofwatermovingthroughvariousportionsofthehydrologiccycle;controlvolumesintheseappli-cationsrangeinsizefrominfinitesimaltoannualorlonger(Figure1-3).evaporationandsnowmeltenergy-balancetheconservationofmomentumfluidflow94292.4

THEWATERSHED(DRAINAGEBASIN)Watershed

(alsocalled

drainagebasin,

riverbasin,

orcatch-ment),definedastheareathatappearsonthebasisoftopographytocontributeallthewaterthatpass-esthroughagivencrosssectionofastream(Figure2-2).dividedrainagearea2.4.1

definition952.4THEWATERSHED(DRAINAGEBA9631Thusthewatershedcanbeviewedasanaturallandscapeunit,integratedbywaterflowingthroughthelandphaseofthehydr-ologiccycleand,althoughpoliticalboundariesdonotgenerallyfollowwatershedboundaries,water-resourceandland-useplanningagenciesrecognizethateffectivemanagementofwaterqualityandqualityrequireawatershedperspective.Thelocationofthestreamcrosssectionthatdefinesthewater-shedisdeterminedbythepurposeoftheanalysis.9732Theconventionalmanualmethodofwatershedde-lineationrequiresatopographicmap(orstereo-scopicallyviewedaerialphotographs).Increasingly,topographicinformationisbecomingavailableintheformofdigitalelevationmodels(DEMs).Thisautomatedapproachtowatersheddelineationallowstheconcomitantrapidextractionofmuchhydrologicallyusefulinformationonwatershedcharacteristics(suchasthedistri-butionofelevationandslop)thatpreviouslycouldbeobtainedonlybyverytediousmanualmethods.2.4.2Delineation

982.4.2Delineation332.5THEREGIONALWATERBALANCETheregionalwaterbalanceistheapplicationofthewater-balanceequationtoawatershed(ortoanylandarea,suchasastateorcontinent).992.5THEREGIONALWATERBALANCE2.5.1TheWater-BalanceEquation1002.5.1TheWater-BalanceEquati,(2-10)Ifweaveragethesequantitiesoverareasonablylongtimeperiod(say,manyyears)inwhichtherearenosignificantclimatictrendsorgeologicalchangesandnoanthropogenicinputs,orstoragemodifications,wecanusuallyassumethatnetchangeinstoragewillbeeffectivelyzeroandwritethewaterbalanceas(2-11),101,(2-10)Ifweaveragethesequarunoff,RO;hydrologicproduction;(2-12)(2-13),,102runoff,RO;(2-12)(2-13),,37,(2-14)Whenweassumethatisnegligibleandwritethewater-balanceequationas,(2-15)103,(2-14)WhenweassumethatisBothprecipitationandevaportranspirationcanbeconsideredtobeexternallyimposedclimatic‘boundaryconditions’.This,fromEquation(2-15),runoffisaresidualordifferencebetweentwoclimatically-determinedquantities.2.5.2Estimation