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
• 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: