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WaterResourcesofPakistanEstimatingMissingRainfallDataPrecipitationinHydrologyEvaporationinHydrology

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EngineeringHydrology

Ebooks|HydrologyLectures&Notes

Contents

1. Whatwestudyinhydrology?

2. ScopeofHydrology3. EngineeringHydrology

1. UsesofEngineeringHydrology

4. BranchesofHydrology5. HydrologicalCycle6. Catchment&Basin,

watershed&streamorder

7. WaterBalanceComponents

8. Precipitation1. Mechanismof

Precipitation2. Typesof

Precipitation3. Measurement

9. HydrologicalSoilGroups

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WhatwestudyinHydrology

In hydrology we study Hydrologic cycle, its processes, water balance, precipitation types, estimation ofprecipitation, and analysis of precipitation data. We also study infiltration phenomena, solution of theRichards equation and approximate infiltration models. Methods of measurement of stream flow, stagedischargerelation,unithydrographtheory,TranspositionofHydrograph,Synthesisofhydrographfrombasincharacteristics,streamflowrouting,floodfrequencyanalysisandattenuationoffloodflowsarealsostudiedinHydrology.

Definitionofhydrology:

Thestudyofwaterinallitsforms(rain,snowandwaterontheearthssurface),andfromitsoriginstoallitsdestinationsontheearthiscalledhydrology.

ScopeofHydrology

1. Waterisonethemostvaluablenaturalresourcesessentialforhumanandanimallife,industryandagriculture.

2. ItisalsousedforPowergeneration,navigationandfisheries.3. Tremendousimportanceisgiventothehydrologyallovertheworldinthedevelopmentand

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ConcreteCivilRelatedLinks

Jobs

EngineeringProjectsSuggestus,WhattoAdd

managementofwaterresourcesforirrigation,watersupply,floodcontrol,waterloggingandsalinitycontrol,Hydropowerandnavigation.

EngineeringHydrology

Ituseshydrologicprinciplesinthesolutionofengineeringproblemsarisingfromhumanexploitationofwaterresourcesoftheearth.Theengineeringhydrologist,orwaterresourcesengineer,isinvolvedintheplanning,analysis,design,constructionandoperationofprojectsforthecontrol,utilizationandmanagementofwaterresources.Hydrologiccalculationsareestimatesbecausemostlytheempiricalandapproximatemethodsareusedtodescribevarioushydrologicalprocesses.

UsesofEngineeringHydrology

EngineeringHydrologyHelpsinthefollowingways:

1. Hydrologyisusedtofindoutmaximumprobablefloodatproposedsitese.g.Dams.2. Thevariationofwaterproductionfromcatchmentscanbecalculatedanddescribedbyhydrology.3. Engineeringhydrologyenablesustofindouttherelationshipbetweenacatchmentssurfacewaterand

groundwaterresources4. Theexpectedfloodflowsoveraspillway,atahighwayCulvert,orinanurbanstormdrainagesystem

canbeknownbythisverysubject.5. Ithelpsustoknowtherequiredreservoircapacitytoassureadequatewaterforirrigationormunicipal

watersupplyindroughtscondition.6. Ittellsuswhathydrologichardware(e.g.raingauges,streamgaugesetc)andsoftware(computer

models)areneededforrealtimefloodforecasting

BranchesofHydrology

Hydrologicalcycle

1. Thehydrologiccycledescribesthecontinuousrecirculatingtransportofthewatersoftheearth,linkingatmosphere,landandoceans.

2. Waterevaporatesfromtheoceansurface,drivenbyenergyfromtheSun,andjoinstheatmosphere,movinginlandasclouds.Onceinland,atmosphericconditionsacttocondenseandprecipitatewaterontothelandsurface,where,drivenbygravitationalforces,itreturnstotheoceanthroughriverandstreams.

3. Theprocessisquitecomplex,containingmanysubcycles.4. EngineeringHydrologytakesaquantitativeviewofthehydrologiccycle.5. Thequantificationofthehydrologiccyclewhichisanopensystemcanberepresentedbyamass

balanceequation,whereinputsminusoutputsareequaltothechangeinstorage.6. ItisabasicHydrologicPrincipleorequationthatmaybeappliedeitheronglobalorregionalscale

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IO=S

Thewaterholdingelementsofthehydrologicalcycleare:

1. Atmosphere2. Vegetation3. Snowpacks4. Landsurface5. Soil6. Streams,lakesandrivers7. Aquifers8. Oceans

HydrologicalProcesses

1. Precipitation2. Evaporation3. Transpiration4. Infiltration5. Overlandflow6. SurfaceRunoff7. Groundwateroutflow

WaterBalanceComponentsInflow:

1. Precipitation2. Importdefinedaswaterchanneledintoagivenarea.3. Groundwaterinflowfromadjoiningareas.

Outflow:

1. Surfacerunoffoutflow2. Exportdefinedaswaterchanneledoutofthesamearea.3. Evaporation4. Transpiration

ChangeinStorage:

Thisoccursaschangein:

1. Groundwater2. Soilmoisture3. Surfacereservoirwateranddepressionstorage4. DetentionStorage

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GlobalWaterBalanceIntheatmosphere:

Precipitation(P)=Evapotranspiration(ET)100+385=61+424

Onland:

P=Evapotranspiration(ET)+Surfacerunoff(R)+Groundwateroutflow100=61+38+1

Overoceansandseas:

Oceanprecipitation+Surfacerunoff+Groundwateroutflow=Evaporation(E)385+38+1=424

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HydrologicalSystems

Ahydrologicsystemisasastructureorvolumeinspace,surroundedbyaboundary,thatacceptswaterandotherinputs,operatesontheminternally,andproducesthemasoutputs.Thestructure(forsurfaceorsubsurfaceflow)orvolumeinspace(foratmosphericmoistureflow)isthetotalityoftheflowpathsthroughwhichthewatermaypassfromthepointitentersthesystemtothepointitleaves.Theboundaryisacontinuoussurfacedefinedinthreedimensionsenclosingthevolumeorstructure.Aworkingmediumentersthesystemasinput,interactswiththestructureandothermedia,andleavesasoutput.Physical,chemicalandbiologicalprocessesoperateontheworkingmediawithinthesystemthemostcommonworkingmediainvolvedinhydrologicanalysisarewater,airandheatenergy.Theglobalhydrologiccyclecanberepresentedasasystemcontainingthreesubsystems:theatmosphericwatersystem,thesurfacewatersystem,andthesubsurfacewatersystem.

CatchmentandBasin1. ACatchmentisaportionoftheearthssurfacethatcollectsrunoffandconcentratesitatits

furthestdownstreampoint,referredtoasthecatchmentoutlet.2. Therunoffconcentratedbyacatchmentflowseitherintoalargercatchmentorintotheocean.3. Theplacewhereastreamentersalargerstreamorbodyofwaterisreferredtoasthemouth.4. Thetermswatershedandbasinarecommonlyusedtorefertocatchments.Generally,

watershedisusedtodescribeasmallcatchment(streamwatershed),whereasbasinisreservedforlargecatchments(riverbasins).

WatershedandStreamorder1. Thewatershedorbasinisdefinedbythesurroundingtopography,theperimeterofwhichis

calledadivide.Itisthehighestelevationsurroundingthewatershed.Allofthewaterthatfallsontheinsideofthedividedhasthepotentialtobeshedintothestreamsofthebasinencompassedbythedivide.Waterfallingoutsideofthedivideisshedtoanotherbasin.

2. Thewaterflowinginstreamsiscalledstreamflow

Hortonsuggestedaclassificationofstreamorderasameasureoftheamountofbranchingwithinabasin.Afirstorderstreamisasmall,unbranchedtributary.Asecondorderstreamhasonlyfirstordertributaries.Athirdorderstreamhasonlyfirstandsecondordertributariesandsoon.Whenachanneloflowerorderjoinsachannelofhigherorder,thechanneldownstreamretainsthehigherofthetwoorders.

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WaterBalanceProblem

Inagivenyear,acatchmentwithanareaof2500km2received1.3mofprecipitation.Theaveragerateofflowmeasuredinariverdrainingthecatchmentwas30m3s1.

1. Howmuchtotalriverrunoffoccurredintheyear(inm3)?2. Whatistherunoffcoefficient?3. Howmuchwaterislostduetothecombinedeffectsofevaporation,transpiration,andinfiltration?

(Expressedinm).

Solution

Totalrunoffvolume

=numberofsecondsinayearaverageflowrate=3153600030=9.4608108m3

Runoffcoefficient

=Runoffvolume/precipitationvolume=(9.4608108)/(1.32500106)=0.29(29%)Thewaterbalanceequationcanbearrangedtoproduce:ET+F=PRS

Where:P=(1.32500106)=3.25109m3

R=9.4608108m3(fromTotalrunoffvolume)S=0(i.e.nochangeinstorage)

So,ET+F=3.251099.4608108=2.30392109m3=(2.30392109)/(2500106)=0.92m

PrecipitationWhatisPrecipitation?Defined:

Waterfallinginsolidorliquidforme.g.rain,snow,andhail.

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UsesofPrecipitationDataRunoffestimationanalysisGroundwaterrechargeanalysisWaterbalancestudiesofcatchmentsFloodanalysisfordesignofhydraulicstructuresRealtimefloodforecastinglowflowstudies

MechanismProducingPrecipitation

Threemechanismsareneededforformationofprecipitation.1.LiftingandCoolingLiftingofairmasstohigheraltitudescausescoolingofair.2.Condensationconversionofwatervaporintoliquiddroplets.3.DropletFormationGrowthofdropletsisrequirediftheliquidwaterpresentinacloudistoreachgroundagainsttheliftingmechanismofair.

TypesofPrecipitation

Dependinguponthewayinwhichtheairisliftedandcooledsoastocauseprecipitation,wehavethreetypesofprecipitation,asgivenbelow:

CyclonicPrecipitationConvectivePrecipitationOrographicPrecipitation

CyclonicPrecipitation:

Cyclonicprecipitationiscausedbyliftingofanairmassduetothepressuredifference.Cyclonicprecipitationmaybeeitherfrontalornonfrontalcyclonicprecipitation.

Frontalprecipitation:

Itresultsfromtheliftingofwarmandmoistairononesideofafrontalsurfaceovercolder,denserairontheotherside.Afrontmaybewarmfrontorcoldfrontdependinguponwhetherthereisactiveorpassiveaccentofwarmairmassovercoldairmass.

Nonfrontalprecipitation:

Iflowpressureoccursinanarea(calledcyclone),airwillflowhorizontallyfromthesurroundingarea(highpressure),causingtheairinthelowpressureareatolift.Whentheliftedwarmaircoolsdownathigherattitude,nonfrontalcyclonicprecipitationwilloccur.

Inthecaseofacoldfront,acolder,denserairmassliftsthewarm,moistairaheadofit.Astheairrises,itcoolsanditsmoisturecondensestoproducecloudsandprecipitation.Duetothesteepslopeofacoldfront,forcefulrisingmotionisoftenproduced,leadingtothedevelopmentofshowersandoccasionallyseverethunderstorms.

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Inthecaseofawarmfront,thewarm,lessdenseairrisesupandoverthecolderairaheadofthefront.Again,theaircoolsasitrisesanditsmoisturecondensestoproducecloudsandprecipitation.Warmfrontshaveagentlerslopeandgenerallymovemoreslowlythancoldfronts,sotherisingmotionalongwarmfrontsismuchmoregradual.Precipitationthatdevelopsinadvanceofasurfacewarmfrontistypicallysteadyandmorewidespreadthanprecipitationassociatedwithacoldfront.Warmfrontprecipitationisgenerallylighttomoderate.

ConvectivePrecipitation

Convectiveprecipitationiscausedbynaturalrisingofwarmer,lighterairincolder,densersurroundings.Generally,thiskindofprecipitationoccursintropics,whereonahotday,thegroundsurfacegetsheatedunequally,causingthewarmerairtoliftupasthecolderaircomestotakeitsplace.Theverticalaircurrentsdeveloptremendousvelocities.Convectiveprecipitationoccursintheformofshowersofhighintensityandshortduration.

OrographicPrecipitation

Orographicprecipitationiscausedbyairmasseswhichstrikesomenaturaltopographicbarrierslikemountains,andcannotmoveforwardandhenceriseup,causingcondensationandprecipitation.AlltheprecipitationwehaveinHimalayanregionisbecauseofthisnature.Itisrichinmoisturebecauseoftheirlongtraveloveroceans.

DefinitionsofsomeusefulTechnicalterms

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Depth:depthofrainfallatapointoroveranarea(mm)

Duration:theperiodoftimeduringwhichrainfell(hours)

Intensity:Depthofrainfallperunittimei.e.depth/duration(mm/hr)

Timedistribution:Rainfallhyetographsareplotsofrainfalldepthorintensityasafunctionoftime.Cumulativerainfallhyetographsarealsocalledrainfallmasscurve.

Isohyets(contoursofconstantrainfall)canbedrawntodevelopisohyetalmapsofrainfalldepth.

NormalAnnualPrecipitation(meanof30yearsannualppt)

MeasurementofPrecipitationPointDataGaugesNonrecordingraingauges

Theyareknownasnonrecordingbecausetheydonotrecordtherainbutcollecttherain.

Recordingraingauges

Theseareusedtodetermineratesofrainfalloverperiodsoftime.Threetypesincommonuseare:

1. WeighingType2. TippingBucketType3. 3.FloatType

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