nrcs runoff curve number hydrology - acwi runoff curve number hydrology development, status, and...

NRCSRunoffCurveNumberHydrology Development, Status, and Updates ClaudiaC.Hoe7,P.E.,F.ASCE,

Na=onalHydraulicEngineer,USDA-NRCS

Outline

1.Introduc=on

2.HistoryandDevelopmentoftheCNMethod

3.NeedforUpdates

4.ASCE-NRCSPartnershipforDevelopingUpdatestoNEHPart630

5.SummaryandConclusions

•  WhatistheRunoffCurveNumberMethod?– NRCS’approachtosolvinghydrology(runofffromrainfall)forungagedwatersheds

–  Intendedtobeasimpleproceduretoes2matetotalstormrunofffromtotalstormrainfallasanintegratedlossfunc2onconcept

– Usedtodescribetypicalwatershedresponsefrominfrequentrainfallanywhereinthecountry,forwatershedswiththesamelanduse,soilhydrologicgroup,andsurfacerunoffcondi2ons

Introduc2on

•  WhatistheRunoffCurveNumberMethod?TheCurveNumberRunoffEqua2on:

𝑸= (𝑷−𝟎.𝟐𝑺)↑𝟐 /(𝑷+𝟎.𝟖𝑺)  for 𝑷> 𝑰↓𝒂  𝑸=𝟎for 𝑷≤ 𝑰↓𝒂 

where: Q=runoffvolume(inches) P=rainfall(inches) Ia=ini2alabstrac2onorrainfallbeforerunoffbegins(inches) S=maximumpoten2alreten2on(inches)

=52mesini2alabstrac2on,or𝑰𝒂=𝟎.𝟐𝑺

TheCNisatransforma2onofS:

𝑪𝑵= 𝟏𝟎𝟎𝟎/𝟏𝟎+𝑺 

Introduc2on

Introduc2on

WhatdoyougetfromtheCNMethod?

RunoffVolume

WhatDON’TyougetfromtheCNMethod?

PeakDischarge

Hydrograph

•  UsesoftheCNMethod:– Singleeventmodeling

•  Developedthroughobserva2onofextreme[rainfall]eventsandaQemptedquan2fica2onofrunofffromtheseevents

– Con2nuousmodels•  Adapta2onofCNmethodtocon2nuousmodelsrequirestrea2ngeachdayor2mestepasadiscreteextreme(?)event

– Other

Introduc2on

•  VictorMockus–  FatheroftheCNMethod(alsodevelopedtheSCSstandardunit

hydrograph;watershedlagequa2on;convexmethodoffloodrou2ng)

–  Establishedhowtechnicalexper2seinFederalGov’tcouldbeusedtocarryoutimportantpolicychangesbyprovidinganequitabledistribu2onofFederalfundingtostateswiththemostsignificant[soilerosionandflooding]problemsthroughdevelopmentandpublica2onofthecurvenumberrela2onshipbetweenrainfallandrunoff

–  PrimaryauthorofSec2on4–HydrologyoftheNa2onalEngineeringHandbook(nowNEHPart630,Hydrology)

History&Development

•  Mockus’goal:–  Determineanequa2onforacurveto:

•  describeapaQernwhereacurvedrawnthroughaplotoftotalstormrunoffversustotalstormrainfallformanystormsonawatershedwasconcaveupwardsandshowednorunoffforsmallstorms;and

•  asstormsizeincreasedforthecurvetobecomeasympto2ctoalineparalleltoalineofequality(rainfall=runoff)

•  Anappropriatecurveresultedfromusingarela2onshipamongrainfall,runoff,andreten2on(rainfallnotconvertedintorunoff)

History&Development

•  Whywasitneeded?–  SCSneededaplanninganddesignhydrologythat…

•  Showedlandcondi)on•  Usedsoilsdata•  Couldbeapplieduniformlyacrossthena2ontoequalizecompe22onamongstatesforprojects(PL-566)•  Metthedatalimita)ons(Availableweredailyrainfallsfromafewresearchwatershedsandsomeemergingsoilstechnology)

History&Development

•  Whatwasavailable?–  SOME(notalot)ofdatafromexperimentalwatersheds–  Sliderules–  Graphpaper–  Crea2vity–  Senseofurgency

History&Development

History&Development

Table 1. Research watersheds used in the determination of NEH4 Curve Numbers ------------------------------------------------------------------------------------------------------------ State Location State Location ----------------------------------------------------------------------------------------------------------- Arizona Safford New Mexico Albuquerque Arkansas Bentonville New Mexico Mexican Springs California Santa Paula New York Bath California Watsonville Ohio Coshocton Colorado Colorado Springs Ohio Hamilton Georgia Americus Oklahoma Muskogee Idaho Emmett Oregon Newberg Illinois Edwardsville Texas Garland Maryland Hagerstown Texas Vega Montana Culbertson Texas Waco Nebraska Hastings Virginia Danville New Jersey Freehold Wisconsin Fennimore ---------------------------------------------------------------------------------------------------------- Source: Rallison(1980)

•  DevelopmentoftheCurveNumberEqua=on𝑸= (𝑷−𝟎.𝟐𝑺)↑𝟐 /(𝑷+𝟎.𝟖𝑺)  for 𝑷> 𝑰↓𝒂 

𝑸=𝟎for 𝑷≤ 𝑰↓𝒂 where: Q=runoffvolume(inches) P=rainfall(inches) Ia=ini2alabstrac2onorrainfallbeforerunoffbegins(inches) S=maximumpoten2alreten2on(inches)

=52mesini2alabstrac2on,or𝑰𝒂=𝟎.𝟐𝑺

and: 𝑪𝑵= 𝟏𝟎𝟎𝟎/𝟏𝟎+𝑺 

History&Development

𝑭/𝑺 = 𝑸/𝑷 where:F=actualreten2onagerrunoffbegins(inches)

S=maximumpoten2alreten2onagerrunoffbegins,S>F,(inches)Q=actualrunoff(inches)P=actualrainfall,P>Q,(inches)Ia=ini2alabstrac2on(inches)=0

Tosa2sfytheconserva2onofmass:F=P–Q

Subs2tu2ng: 𝑷−𝑸/𝑺 = 𝑸/𝑷  SolvingforQ: 𝑸= 𝑷↑𝟐 /𝑷+𝑺 

History&Development

•  WhenIa≠0,P=P-Iaandequa2onbecomes:

𝑸= (𝑷− 𝑰↓𝒂 )↑𝟐 /(𝑷− 𝑰↓𝒂 )+𝑺 •  Ini2alabstrac2onconsistsof:

–  Intercep2on(canbees2matedfromcoverandsurfacecondi2ons)–  Infiltra2onduringearlypartsofthestorm(highlyvariable–dependentupon

rainfallintensity,soilcrus2ng,soilmoisture)–  Surfacedepressionstorage(canbees2matedfromcoverandsurface

condi2ons)

•  Agermuchdebateanddiscussion(andperhapsajugofmar2nis?),adecisionwasmadetouseanassump2onthatIa = 0.2S,resul2ngintheequa2on:

𝑸= (𝑷−𝟎.𝟐𝑺)↑𝟐 /𝑷+𝟎.𝟖𝑺 (anddon’tforget!ForP < Ia, Q = 0)

History&Development

•  TransformingtheSparameterintosomethingmore“useful”resultsintheCurveNumber:

𝑪𝑵= 𝟏𝟎𝟎𝟎/𝟏𝟎+𝑺 whereCNisdimensionlessandrangesfrom0(noraineverbecomesrunoff)

to100(allrainisalwaysrunoff)

History&Development

History&Development

•  WhatdoesaCNrepresent?–  Landuse–  Hydrologiccovercondi2on–  Soils–  Antecedentrunoffcondi2on

•  NEHPart630,Chapter9,presentstablesofrunoffcurvenumbersforlandusesdeterminedusingrainfall/runoffdataandtherunoffequa2onassumingIa/S=0.2

History&Development

History&Development

•  BUT….WheredidtheCNscomefrom?

History&Development

0.0000

0.8000

1.6000

2.4000

3.2000

4.0000

0.0000 0.8000 1.6000 2.4000 3.2000 4.0000

P(in)

Q(in

)

WS26010 Coshocton, OH (1939-1986)

CN(II) = 84

CN(I) = 68

CN(III) = 93

•  Plotsofrainfallversusrunofffromexperimentalwatershedsdata–  Typicallydailyrainfallvalues(24-hour?)–  ProvidesP(measured),Q(measured),andIa(observed)–  SolveforS–  SolveforCN

History&Development

History&Development

Documenta=on??(Orlackthereof...)

Mostfilesandsuppor2ngdatausedinthedevelopmenthavebeenlost,destroyed,ormisplacedovertheyears

CNmethodneverunderwentacri2calopenreview

Suppor2ngdatawasneverpublished

Whatsurvivesaretradi2on,wriQensummaries,customsofentrenchedusage,ins2tu2onalmemory

•  SomeIssues:–  Naturalvs.OrderedData–  Antecedentrunoffcondi2on?

0.0000

0.8000

1.6000

2.4000

3.2000

4.0000

0.0000 0.8000 1.6000 2.4000 3.2000 4.0000

P(in)

Q(in

)

WS26010 Coshocton, OH (1939-1986)

CN(II) = 84

CN(I) = 68

CN(III) = 93

NeedforUpdates

Coshocton Ohio, Watershed 26031

0

0.5

1

1.5

2

2.5

0 1 2 3 4

P(inches)

Q(in

ches

)

Ordered

P(5_bar)

Q(5_bar)

PQ(5_bar)

N = 1818Number of Year = 34

•  SomeIssues:– Stormcharacteris2cs

•  Intensity?•  Distribu2on?•  Dura2on?

– Seasonaldata(hydrologiccovercondi2onduringwhat2meofyear)

– Landuse– Slopes– Drainagearea

NeedforUpdates

•  SomeIssues:–  Ini2alabstrac2on–isIa=0.2Stherightvalue?– Watershedresponse–Mockus’workassumed(andsought)auniformresponse•  Concaveupward•  Constantlyincreasing

ornonega2vechangeinslope

NeedforUpdates

•  SomeIssues:–  Inreality,therearemul2plewatershedresponses

•  Asympto2c

NeedforUpdates

6065707580859095100

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

CN

P (in)

Coshocton, Ohio#26004 pasture

1939-1986

CNORD CNO CNfit CN inf

•  SomeIssues:–  Inreality,therearemul2plewatershedresponses

•  Complacent

NeedforUpdates

404550556065707580859095100

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

CN

P (in)

Coshocton, Ohio#26013 soy beans

1985

CNORD CNO CN fit CN inf

•  SomeIssues:–  Inreality,therearemul2plewatershedresponses

•  Violent

NeedforUpdates

70

75

80

85

90

95

100

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

CN

P (in)

Hastings, Nebraska#44012 sorghum

1959-1965

CNORD CNO CN fit CN inf

•  SomeIssues:– Regionalissues(isAMCIIinOhioreallythesameasAMCIIin

Arizona?)

– DoestheCNMethodreally“fit”thecomplacentandviolentwatershedresponses?

– Whenshouldothermethodsofanalyzinghydrologybeused?

NeedforUpdates

•  SomeIssues:– Whatabout….

•  Forestedwatersheds?•  Smallstormsandwaterqualityplanning?•  Localcalibra2onsforCN?•  Newlanduses/type?•  Effectsoffire?•  NonCNwatersheds?•  Urbaniza2onissues?•  LIDissues?•  Karst?

NeedforUpdates

•  ASCE“CurveNumberHydrology–StateofthePrac2ce”

NeedforUpdates

•  Coopera=veAgreement(signedAugust2015)toupdateNRCS’NEHPart630,Chapters8,9,10,and12– Chapter8:LandUseandTreatmentClasses– Chapter9:HydrologicSoil-CoverComplexes– Chapter10:Es2ma2onofDirectRunofffromStormRainfall

– Chapter12:HydrologicEffectsofLandUseandTreatment

ASCE–NRCSPartnership

•  WhyASCE?– AuthorsoftheASCEmonograph,“CurveNumberHydrology–StateofthePrac2ce”serveontheASCE-WMTC-CNTaskCommiQee

•  Whoelseisinvolved?– Universi2es– ASABE– State/LocalGovernmentrepresenta2on– Privateindividuals

ASCE–NRCSPartnership

•  TimeLine– ASCEtodeliverdragsofsubjectchapterstoNRCSbySeptember30,2017

– NRCSwillreviewinternallyandmakeadecisionaboutimplementa2on

– RequestforreviewwillalsobesolicitedoutsideofNRCSthroughASCE

ASCE–NRCSPartnership

•  Loveitorhateit,it’sheretostay.•  Isit“science”?– Ques2onable

•  Doesit“work”?Doesitapproximatereality?– Forthemostpart

•  Wheredoesthatleaveustoday?– NRCSworkingtoincorporatethelatestfindings

Summary/Conclusions

Claudia.Hoe7@wdc.usda.gov

Contact

•  DonWoodward,P.E.,F.ASCE,NRCSNa-onalHydraulicEngineer(re-red),MemberASCECNTaskCommiBee

•  RichardH.(Pete)Hawkins,PhD,P.E.,F.ASCE,F.EWRI,ProfessorEmeritus,SchoolofNaturalResourcesandEnvironment,WatershedResourcesandEcohydrology,DepartmentofAgriculturalandBioSystemsEngineering,DepartmentofHydrologyandWaterResources,UniversityofArizona,TucsonAZ,ChairASCECNTaskCommiBee

•  TimJ.Ward,Ph.D.,P.E.,F.ASCE,F.EWRI,M.NSPE,DeanandProfessor,SchoolofEngineering,ManhaBanCollege,Riverdale,NY,MemberASCECNTaskCommiBee

SpecialThanks

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Ques=ons

•  USDA–NaturalResourcesConserva2onService,Na2onalEngineeringHandbook(NEH),Part630,Chapters8,9,10,and12(availablehQp://direc2ves.sc.egov.usda.gov/viewDirec2ve.aspx?hid=21422)

•  ASCE,“CurveNumberHydrology–StateofthePrac2ce”,PreparedbytheASCE/EWRIRunoffCurveNumberHydrologyTaskCommiQee,2009

•  NotesonVictorMockus,providedbyDonWoodward•  SlidesfromASCE-EWRI,2015WatershedManagementSymposium,

“CurveNumberRainfall-Runoff:ProfessionalApplica2onWorkshop”,providedbyRichardHawkins,DonWoodward,andTimWard

References: