工程水文学-BasicHydrology-HYDR-310-Lecture4

BasicHydrology(HYDR310)(Topic2),Precipitationprocesses,measurements,andvariability(PartII),Dr.Xixi(Jack)Wang,P.E.,PrecipitationVariations,Precipitationcanbeinformsof:liquiddroplets(e.g.,rainfall)icecrystals(e.g.,snow,hail,sleet)mixture(e.g.,icerain)Precipitationisusuallyrecordedastheequivalentwaterdepth,EWD(in,mm)Forrainfall,itsdepthisEWDForicecrystalsormixture,theyneedtobemeltedandrecordedasEWDGeographicvariation:global,regional,andbasinwideTemporalvariation:annual,seasonal,monthly,daily,hourly,minutesForagivenlocationandtemporalscale,precipitationvarieswithaltitude,PrecipitationVariations:Geographic,HighestinthetropicalregionsoneithersideoftheequatorDecreasetowardthePolesduetothedecreasingtemperatureDecreasewiththedistancefromtheoceansMountainscancausedramaticdifferencesovershortdistances,PrecipitationVariations:Geographic,PrecipitationVariations:Geographic,/printable/precipitation.html,PrecipitationVariations:Temporal,FromyeartoyearWithinayear,fromseasontoseasonThetemporalvariationscale,called“duration”canbeassmallasminutesInhydrology,thecommonlyused“durations”are:meanannualmaximum1yearmaximum1monthmaximum2or3daysmaximum24hoursmaximum12hoursmaximum30minutesothers,PrecipitationVariations:Temporal,Hyteograph:aplotshowingtheprecipitationwithrespecttotimeatthedurationinterval,PrecipitationVariations:Temporal,Summarystatistics:reflectthetemporaltrendandpatternofprecipitation,Forspecificstudies,wemaybeonlyinterestedinsomeofthesestatistics.Thecomputationcouldbeveryintensive!,PrecipitationVariations:Temporal,Fortheworld-recordedextremehighprecipitations,weareonlyinterestedinthemaximumsatvariousdurationintervals,Measurementofprecipitation,Precipitationisconventionallymeasuredat“gauges,”whicharespecificpointsacrossanareaThegaugerainfallisrecordedatpre-definedtimeintervals,suchasdailyandhourlyRecently,Radar(e.g.,DopplerradarinU.S.)cancoverlargeareaswithhighspatialandtemporalresolutionsTheraingaugedataareusuallymoreaccuratethanthecorrespondradardataThedatafromthesetwosourcescanbeconjunctivelyused,Measurementofprecipitation:NonreordingGauge,NWSstandardgage:8-indiameterrainreceptorAnobserverreadsandemptiesperiodically(usuallydaily),Consideroverflow,Measurementofprecipitation:RecordingGauge,AutomaticallyrecordsprecipitationdepthaccumulationsandcorrespondingtimesDatarecordedbypaperchartsordataloggerUseatippingbucket,weighingmechanism,orfloatdevice,Measurementofprecipitation:RecordingGauge,Tippingbucketgauge:funnelingthecollectedraintoasmallbucketthattiltsandemptieseachtimeitfills,Theprecipitationdepthdeterminedbasedonthenumberofbuckettilts,Measurementofprecipitation:RecordingGauge,Weighingprecipitationgauge:continuouslyrecordtheweightoftheaccumulatedprecipitation,Measurementofprecipitation:RecordingGauge,Float-typeprecipitationgauge:operatebycatchingrainfallinatubecontainingafloat,theriseofwhichisrecordedovertime,Measurementofprecipitation:DopplerRadar,Basedonthetimebetweenemissionoftheradarpulseandreceiptofitsecho,Availableat:/,Arealprecipitation:WatershedScale,Simplearithmeticaverage:theaverageofdepthsforallgaugeslocatedwithinawatershed,ThearealprecipitationistheaverageofthedepthsatgaugesA,B,andC.,Arealprecipitation:WatershedScale,Thiessenpolygonarea-weightedaverage:,B,57mm,Arealprecipitation:WatershedScale,Thiessenpolygon:ConnecttheadjacentgaugestoformatriangularnetworkDrawperpendicularbisectorlinestotheedgesofthetrianglesThepolygons,formedbytheintersectpointsamongthebisectorlinesandbetweenthemandthewatershedboundary,areusedtocomputeai,A,C,D,E,45mm,69mm,66mm,78mm,Arealprecipitation:WatershedScale,Theissenpolygons,Arealprecipitation:WatershedScale,Isohyetalmethod:IsohyetsarecontoursofequalprecipitationIsohyetsaredrawnonamapbasedonthedepthsatgaugesAnarea-weightedaverageiscomputedbasedonweighingprecipitationinproportiontotheareasbetweenisohyets,Arealprecipitation:WatershedScale,Gridapproach:ConstructtheisohyetsUsingalinearinterpolationmethodtoestimatethevaluesforthegridswithinthewatershedComputethesummationofthevaluesofthegridsCountthenumberofgridsenclosedbythewatershedboundaryThewatershedaverageprecipitationiscomputedastheratioofthesummationtothenumberofgridsForthisexample,theresultis1.90,ApplicabletoNEXRADdata,OtherClimaticParameters,Win