PhysicalHydrologyS.LawrenceDingman1WhoamI?ZhangXiang68772303-601(office:0101609zhangxiang@whu.edu.cn2TextbookandreferencesTextbook

PhysicalHydrologyReferences

HydrologyforEngineersbyR.K.Linsley

Hydrology:principles,analysisanddesignbyH.M.Raghunath

Hydrology:anintroductiontohydrologicscience

byR.L.Bras

水文學原理(一)胡方榮候宇光水文學原理(二)于維忠水文學原理

芮孝芳3TimeArrangeLecture:36classhours

part1and2:4classhourspart4:8classhourspart6:8classhourspart7:6classhourspart8:6classhourspart9:4classhoursLab:6classhoursforcomputer4TheFinalsAssignments(30%)Checkingonattendance(20%)Examination(50%)5Attention!Prepareyourcoursebeforehandwords,sentences,phasesNoabsenceisallowedDon'tbelateforclassReadasmuchaspossible,writeasmuchaspossible,usetheInternetforhelpasmuchaspossible61IntroductiontoHydrologicScienceDEFINITIONANDSCOPEOFHYDROLOGYDEVELOPMENTOFSCENTIFICHYDROLOGYAPPROACHANDSCOPEOFTHISBOOK71.1DEFFINTIONANDSCOPEOFHYDROLOGY8Hydrologyisbroadlydefinedasthegeosciencethatdes-cribeandpredictstheoccurrence,circulation,anddistri-butionofthewateroftheearthanditsatmosphere.Theglobalhydrologiccycle

Thelandphaseofthehydrologiccycle9WaterCycle10111213InterdisciplinaryScienceTomanagewaterresourcesandwaterrelatedhazardsEngineeringhydrology,economicsandrelatedsocialscienceforwater-resourcesmanagement141.2DEVELOPMENTOFSCIENTIFICHYDROLOGY5000-6000B.P.Pakistan,China,Egypt…：Canals,levees,dam,well…3800B.P.Egyptians:monitoringofriverflow2400B.PIndia:rainfallmeasurement15

Theconceptofaglobalhydrologycycledatesfromatleast3000P.B(Nace1974),whenSolomonwroteinEcclesiastes1:7that

Alltheriversrunintothesea;yettheseaisnotfull;untotheplacefromwhencetheriverscome,thithertheyreturnagain.

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

16Treatisesonvariousaspectsofhydrology,beginningwiththeEnglishmanNathanielBeard-more’sManualofHydrologyin1862,appearedwithincreasingfrequ-encyinthelasthalfofthe19thcentury.

Thehalfofthetwentiethcenturysawgreatprogressinmanyaspectofhydrologyand,withtheformationoftheSectionofScientificHydrologyintheInternationalUnionofGeodesyandGeophysics(IUGG,1992)andHydrologySectionoftheAmericanGeophysicalUnion(1930),thefirstformrecognitionofthescientificstatusofhydrology.17

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

18Theabilitytounderstandandmodelhydrologicprocessesatcontinentalandglobalscaleisbe-comeincreasingimpor-tantbecauseoftheneedtopredicttheeffectsoflarge-scalechangesinlandandinclimate.FrompointtolargerInthelandphaseofthehydrologiccycle,itisinterestingthatdetailedfieldstudiestounderstandthemechanismsbywhichwaterentersstreamsbegantoproliferateonlyinthe1960s,pioneeredbyT.Dunneandothers.

Thetemporalandspatialvariabilityofnaturalconditions192

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

202.1PHYSICALQUNTITESANDLAWSHydrologyisaquantitygeophysicalscience,Inprinciple,thesenumericalvaluesofhydrologicvaluesaredeterminedbyeither1.counting,inwhichcasethequantitytakesonavaluethatisapositiveintegerorzero;or2.Measuring,inwhichcasethequantitytakesonavaluecorrespondingtoapointontherealnumberscalethatistheratioofthemagnitudeofthequantitytothemagnitudeofastandardunitofmeasurement.21Thebasicrelationsofphysicalhydrologyarederivedformfundamentallawsofclassicphysics,particularlythoselistedinTable2-1.222.2HYHDROLOGICSYSTEMSeveralbasichydrologicconceptsarerelatedtothesimplemodelofasystemshowninFigure2-1.TheouterdashedlineinFigure2-1indicatesthatanygroupoflinkedsystemscanbeaggregatedintoalargersystem;thesmallersystemscouldthenbecalledsubsystems.23242.3THECONSERVATIONEQUATIONSTheamountofaconservativequantityenteringacontrolvolumeduringadefinedtimeperiod,minustheamountofthequantityleavingthevolumeduringthetimeperiod,equalsthechangeintheamountofthequantitystoredinthevolumeduringthetimeperiod.

25(2-2)(2-3)AmountIn–Amountout=ChangeInstorage(2-1),,26(2-4)(2-5)(2-6),,,27Anotherversionoftheconservationequationcanbedevelopedbydefiningtheinstantaneousratesofinflow,i,andoutflow,q,as(2-7)(2-8)(2-9),,,28Equations(2-2),(2-6),and(2-9),arecalledwater-balanceequa-tionwhenappliedtothemassofwatermovingthroughvariousportionsofthehydrologiccycle;controlvolumesintheseappli-cationsrangeinsizefrominfinitesimaltoannualorlonger(Figure1-3).evaporationandsnowmeltenergy-balancetheconservationofmomentumfluidflow292.4

THEWATERSHED(DRAINAGEBASIN)Watershed

(alsocalled

drainagebasin,

riverbasin,

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

definition3031Thusthewatershedcanbeviewedasanaturallandscapeunit,integratedbywaterflowingthroughthelandphaseofthehydr-ologiccycleand,althoughpoliticalboundariesdonotgenerallyfollowwatershedboundaries,water-resourceandland-useplanningagenciesrecognizethateffectivemanagementofwaterqualityandqualityrequireawatershedperspective.Thelocationofthestreamcrosssectionthatdefinesthewater-shedisdeterminedbythepurposeoftheanalysis.32Theconventionalmanualmethodofwatershedde-lineationrequiresatopographicmap(orstereo-scopicallyviewedaerialphotographs).Increasingly,topographicinformationisbecomingavailableintheformofdigitalelevationmodels(DEMs).Thisautomatedapproachtowatersheddelineationallowstheconcomitantrapidextractionofmuchhydrologicallyusefulinformationonwatershedcharacteristics(suchasthedistri-butionofelevationandslop)thatpreviouslycouldbeobtainedonlybyverytediousmanualmethods.2.4.2Delineation

332.5THEREGIONALWATERBALANCETheregionalwaterbalanceistheapplicationofthewater-balanceequationtoawatershed(ortoanylandarea,suchasastateorcontinent).342.5.1TheWater-BalanceEquation35,(2-10)Ifweaveragethesequantitiesoverareasonablylongtimeperiod(say,manyyears)inwhichtherearenosignificantclimatictrendsorgeologicalchangesandnoanthropogenicinputs,orstoragemodifications,wecanusuallyassumethatnetchangeinstoragewillbeeffectivelyzeroandwritethewaterbalanceas(2-11),36runoff,RO;hydrologicproduction;(2-12)(2-13),,37,(2-14)Whenweassumethatisnegligibleandwritethewater-balanceequationas,(2-15)38Bothprecipitationandevaportranspirationcanbeconsideredtobeexternallyimposedclimatic‘boundaryconditions’.This,fromEquation(2-15),runoffisaresidualordifferencebetweentwoclimatically-determinedquantities.2.5.2EstimationofRegionalEvaportranspirationPerhapsthemostcommonformofhydrologicanalysisistheestimationofthelong-termaveragevalueofregionalevapor-transpirationviathewater-balanceequation.39Itisusuallyassumedthatground-waterflowseitherarenegligibleorcanceloutandthatisnegligible,sothatequation(2-15)becomes(2-16),Modelerror,whichreferstotheomissionofpotentiallysignifi-canttermformtheequation,andmeasurement

inthequantitiesand,whichisunavoidable.40Ground-waterFlowsStreamsdraininglargerwatershedstendtoreceivethesubsur-faceoutflowsoftheirsmallerconstituentwatershed,sotheimportanceofground-waterout-flowgenerallydecreaseasoneconsiderslargerandlargerwatershed.41StorageChangeHydrologistsattempttominimizeitsvalueby(1)us-inglongmeasurementperiodsanda(2)selectingthetimeofbeginningandendofthemeasurementperiodsuchthatstoragevaluesarelikelytobenearlyequal.42MeasurementErrorAccuracyofRegionalPrecipitationValuesindividualgagesarealaveragesInregionsofthehighrelieforwithfeworpoorlydistributedgages,orforshortermeasurementperi-ods,theuncertainlycanbeconsiderablylarger.43AccuracyofStreamflowValues

Winter(1981)estimatedthatthemeasurementun-certainlyforlong-termaveragevaluesofstreamflowatagagingstationisontheorderof.(Theac-curacyofsuchmeasurementsisdiscussedfurtherinSectionF.2.4)whereisestimatedforlocationsothercarefullymaintainedgagingstations,theuncertaintycanbemuchgreater.44Potentialmeasurementerrorsareusuallyassumedtobedistri-butedsymmetricallyaboutthetruevalue(equalchanceofunder-orover-estimation)andtofollowthebell-shapednormaldistributiondescribeinAppendixC:thefutureameasuredvalueisfromthetruevalue(i.e.,thelargeristheerror),thesmallisprobabilitythatitwilloccur(Figure2-4).Thespread,orvariation,ofthepotentialmeasuredvaluesaboutthetruevalueisexpressedasthestandarddeviationofthepotentialerrors.45Thestandarddeviationsoftheerrorsduetomeasurementofthequantitiesarerelatedas,(2-17)“Iam100.p%surethatthetruevalueofprecipitationiswithinofthemeasuredvalue.”(2-18a)46

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

(2-18b),47Giventhatpotentialmeasurementerrorsfollowthenormaldistribution,wecanfindfromthepropertiesofthatdistribution,summarizedinTableC-5,thatthereisa95%probabilitythatanobservationwillbewithin1.96standarddeviationsofthecentral(true)value.(2-19),48(2-20),(2-21a),,(2-21b)49(2-22),,(2-23)Assumetherelativemeasurementerrorsforprecipitationandstreamfloware=0.1and=0.05.502.6

SPATIALVARIABILITY

However,precipitationgagesareusuallyunevenlydistributedoveranygivenregion,andthepointvaluesarethereforeanun-representativesampleofthetrueprecipitationfield,Becauseofthis,andbecauseoftheimportanceofaccuratelyquantifyingvariablessuchasprecipitation,basicstatisticalconceptshavebeenincorporatedintospecialtechniqueforcharacterizingandaccountingforspatialvariability.512.7

TEMPORALVARIABILITYTheinputs,storagesandoutputsinFigurearealltime-distributedvariables—quantitiesthatcanvarywithtime.Inparticular,thestreamflowrateatagivenlocationishighlyvariableintime.Fromthehumanviewpoint,thelong-termaveragestreamflowrate,,ishighlysignificant:itrepresentsthemaximumrateatwhichwaterispotentiallyavailableforhumanuseandmanagement,andisthereforeameasureoftheultimatewaterresourcesofawatershedorregion.52Streamflowvariabilityisdirectlyrelatedtotheseasonalandinterannualvariabilityofrunoff(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.53Eachvalueofwhichisassociatedwithaparticulartimeinasequencetimes,.Suchasequenceiscalledatimeseries.Sometime-seriesvariablesareobtainedbycounting—forexample,thenumberofdayswithmorethan1mmrainineachyearataparticularlocation.Suchvariablesareinherentlydiscrete.Continuoustimetrace:theytakeonvaluesateveryinstantintime.

2.7.1TimeSeries5455EXAMPLE2-2Table2-2lists,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.5657Timeseriesareusuallytreatedasmoreorlessrepresentativesample

ofthelong-termbehaviorof