US SCS Method
    • 23 Oct 2022
    • 8 Minutes to read

    US SCS Method


    Article summary

    Note
    The US SCS unit has been superseded by the Generic Rainfall/Runoff Boundary which incorporates a more detailed interpretation of the US SSCS hydrological methodology. The US SCS boundary is still a valid unit for 1D river networks and this section details how to use it. You can still access the properties window for this unit, but if adding a new US SCS unit, we recommend you use the Generic Rainfall/Runoff Boundary. If you do need to add a new US SCS unit, you will need to highlight the insertion position, right-click on the Network panel, and select Insert > New Unit from the displayed menu.

    Data

    Field in Data Entry FormDescription

    Name in Datafile

    Baseflow

    Baseflow (m3/s)

    BF

    Minimum flow

    Adjusted baseflow (m3/s). Usually zero but can be used to force minimum flows without affecting the main part of the hydrograph shape

    BFADJS

    Catchment Area

    Catchment area (km2)

    carea

    Curve Number

    Curve number (NB this is saved to the data file as a negative number, eg a user-entered CN of 33 is saved as --33.0)

    CN

    Data interval

    Time interval used in rainfall profiles and unit hydrograph time interval (hrs). Recommended to be one fifth of Tp and where stdur/t is equal to an odd integer.

    t

    n/a

    Number of rain profile values

    nrp

    n/a

    Number of unit hydrograph values

    nuh

    Rainfall Depth

    Event rainfall precipitation (mm)

    P

    Percentage Runoff

    Percentage runoff (entered as a positive number, eg 60). An alternative to specifying CN

    PR

    Rainfall

    Observed Rainfall profile (mm) starting at tstart with data interval t over given storm duration (stdur)

    rp

    Runoff flag

    Curve Number - use SCS curve number method to derive runoff, or Specified (PR) to enter a percentage runoff value directly

     

    Storm Profile Flag

    Rainfall profile flag: FSR 50% Summer (rpflag='SUMRP'): use standard FSR 50 % Summer profile; FSR 75% Winter (rpflag = 'WINRP'): use standard FSR 75% Winter profile; or Observed (rpflag=@OBSRP'):use observed rainfall [profile].

    RPFLAG

    Storm Duration

    Storm duration (hrs)

    stdur

    Unit Hydrograph Flag

    Unit hydrograph flag.SCS (uhflag='SCSUH'): use SCS derived unit hydrograph, or Observed (uhflag='OBSUH'): use user-defined unit hydrograph.

    UHFLAG

    Units

    Units of unit hydrograph ordinates: Default is Cumecs/mm (uhunit='MMAREA')

    uhunit

    UH Time

    Unit hydrograph time (hrs)

    t

     Tp Value

    Time to peak (hrs) of t-hour unit hydrograph

    Tp

    Simulation Type

    Flow values to use during simulation. Can be one of:

    'base flow (bfonly')

    'peak flow ('pfonly')

    'full hydrograph' (null:default)

    For example, if "base flow" is selected then the baseflow contribution is used for the boundary flow value for the simulation.

    bfonly

    Hydrograph Scaling Method:Scale

    Hydrograph scaling method 'FULL' (scaling= 'FULL' (default) scales the whole hydrograph; Quick Runoff (scaling="'RUNOFF"):scaling only applies to the quick runoff component of the hydrograph.

    Used in conjunction with SCFLAG and scfact.

    SCALING

    Method

    Hydrograph scaling option. By a factor of scflag='SCALE'[default]) - applies the specified factor; To fit peak of (scflag='PEAK') - fits the hydrograph peak to the specified value.

    SCFLAG

      -----

    If SCFLAG=PEAK then all hydrograph ordinates are scaled (by a constant value) to achieve a peak flow of scfact

    If SCFLAG=SCALE then all hydrograph ordinates are multiplied by scfact (default is 1.0)

    scfact

    Boundary Type Flag

    Boundary mode flag: 'HYDROGRAPH' (the default if blank) denotes a generated hydrograph (i.e. the unit behaves as a QTBDY); 'HYETOGRAPH' indicates that it behaves as a REBDY-type unit, applying the Rainfall profile to a Rainfall-only boundary (the latter must be used in conjunction with a lateral inflow node).

    hymode

    Time Delay

    Optional delay time (hrs), eg if tdelay = 2hrs then the hydrograph will start 2hrs after the start time of the simulation

    tdelay

    n/a

    Elevation (mAD) (not used in calculations)

    z

    Theory and Guidance

    The US SCS Method Hydrological Boundary is a hydrological model for determining runoff from rainfall for a subcatchment using the United States Soil Conservation Service (US SCS) unit hydrograph method. It is used as an upstream boundary condition producing output equivalent to a Flow Time Boundary.

    The US SCS unit hydrograph method is a well established method for determining a flow hydrograph based on a unit hydrograph approach which changes rainfall to runoff through the convolution procedure.

    The flow-time curve produced by this unit can be checked prior to a full computational run using the Calculated Hydrograph tab or running a Boundary mode simulation. A summary of the data is output as an ASCII file with the extension '.zzb' and the hydrograph is tabulated in an ASCII file with the extension 'zzh'. If the boundary run extends over the length of the event then the resultant hydrograph can be graphed using the Time Series option. You can generate a datafile with only one node (which is a Hydrological Boundary) and run Boundary Mode in order to generate the hydrograph.

    The main requirements for input data are rainfall, calculated time to peak for the selected unit hydrograph and a Runoff Curve Number, CN, which is determined from a set of tables which are reproduced below. The choice of Curve Number depends on an assessment of the dominant hydrological soil group, the type of land use and antecedent soil moisture conditions.

    The US SCS Boundary is most usually applied to applications outside the United Kingdom. For the United Kingdom the Revitalised Flood Hydrograph (ReFH), Flood Estimation Handbook (FEH) and UK Flood Studies Report (FSR) Methods are the standard approaches.

    Derivation of Data

    The US SCS Boundary requires you to input the Runoff Curve Number (or alternatively the percentage runoff) and the time to peak of the unit hydrograph. The derivation of Curve Number and time to peak is summarised below and you are advised to refer to a published description of the SCS method for a full explanation (for example, see Bureau of Reclamation (1987) or Chow V.T. et al (1988)).

    Selection of Runoff Curve Number (CN)

    To determine the Curve Number it is necessary to know the hydrologic soil group. These are classified as follows:

    • Group A : deep sand, deep loss, aggregated silts
    • Group B : shallow loess, sandy loam
    • Group C : clay loams, shallow sandy loam, soils low in organic content and soils usually high in clay
    • Group D : soils that swell significantly when wet, heavy plastic clays, and certain saline soils.

    The SCS soil group can be identified by:

    1. Soil characteristics as described above.
    2. Soil surveys (if available).
    3. Minimum infiltration rates as shown below.

    Soil Group

    Minimum Infiltration Rate (mm/hr)

    A

    7.5 to 11.5

    B

    3.8 to 7.5

    C

    1.3 to 3.8

    D

    0 to 1.3

    The following tables present CN values for the different land uses, treatment, and hydrologic condition.

    (a) Residential

     

     

     

    Average lot size

    Average Percent Impervious

     

    CN for Hydrologic Soil Groups

     

     

     

     A B C D

    1/8 acre or less

    65

     

    77 85 90 92

    1/4 acre

    38

     

    61 75 83 87

    1/3 acre

    30

     

    57 72 81 86

    1/2 acre

    25

     

    54 70 80 85

    1 acre

    20

     

    51 68 79 84

    (b) Paved Parking Lots, Roofs, Driveways, etc.

     

     

    98 989898

    (c) Streets and Roads

     

     

     

    Paved with curbs and storm sewers

     

     

    98 989898

    Gravel

     

     

    76 85 89 91

    Dirt

     

     

    72 82 87 89

    (d) Commercial/Business Areas (85 % Impervious)

     

     

    89 92 94 95

    (e) Industrial Districts (72 % Impervious)

     

     

    81 88 91 93

    (f) Open spaces, Lawns, Parks, Golf Courses, Cemeteries etc.

     

     

     

    Good condition: grass cover on > 75 % of area

     

     

    39 61 74 80

    Poor condition: grass cover on 50 to 75 % of area

     

     

    49 69 79 84

    (g) Farmland

     

     

     

    Land Use

    Treatment

    Hydrologic Condition

    CN for Hydrologic Soil Groups

     

     

     

     A B C D

    Fallow

    Straight row

    -

    77 86 91 94

    Row crops

    Straight row

    Poor

    72 81 88 91

    Row crops

    Straight row

    Good

    67 78 85 89

    Row crops

    Contoured

     Poor

    70 79 84 89

    Row crops

    Contoured

    Good

    65 75 82 86

    Row crops

    Contoured & terraced

    Poor

    66 74 80 82

    Row crops

    Contoured & terraced

    Good

    62 71 78 81

    Small grain

    Straight row

    Poor

    65 76 84 88

    Small grain

    Straight row

    Good

    63 75 83 87

    Small grain

    Contoured

    Poor

    63 74 82 85

    Small grain

    Contoured

    Good

    61 73 81 84

    Small grain

    Contoured & terraced

    Poor

    61 72 79 82

    Small grain

    Contoured & terraced

    Good

    59 70 78 81

    Close-seeded legumes or rotation meadow

    Straight row

    Poor

    66 77 85 89

     

    Straight row

    Good

    58 72 81 85

     

    Contoured

    Poor

    64 75 83 85

     

    Contoured

    Good

    55 69 78 83

     

    Contoured & terraced

    Poor

    63 73 80 83

     

    Contoured & terraced

    Good

    51 67 76 80

    Pasture or Range

    Un-contoured

    Un-contoured

    68 79 86 89

     

    Un-contoured

    Fair

    49 69 79 84

     

    Un-contoured

    Good

    39 61 74 80

     

    Contoured

    Poor

    47 67 74 80

     

    Contoured

    Fair

    25 59 75 83

     

    Contoured

    Good

     6 35 70 79

    Meadow

     

    Good

    30 58 71 78

    Woods or Forest

     

    Poor

    45 66 77 83

     

     

    Fair

    36 60 73 79

     

     

    Good

    25 55 70 77

    Farmsteads

     

    -

    59 74 82 86

    To accommodate antecedent soil moisture there are three conditions to consider:

    Condition I

    Soils are dry but not to wilting point; satisfactory cultivation has taken place.

    Condition II

    Average conditions.

    Condition III

    Heavy rainfall, or light rainfall and low temperatures have occurred within the last 5 days; saturated soil.

    The following table gives seasonal rainfall limits for the three antecedent moisture conditions (AMC):

     

    Total 5-day Antecedent Rainfall (mm)

     

    AMC

    Dormant Season

    Growing Season

    I

    <13

    <35

    II

    13 to 28

    35 to 53

    III

    >28

    >53

    The following table can be used to adjust the CN from the average conditions referenced in the above CN tables to Antecedent Moisture Conditions I and III.

    CN for AMC II

    Corresponding CN for

     

     

    AMCI

    AMCIII

    100

    100

    100

    95

    87

    98

    90

    78

    96

    85

    70

    94

    80

    63

    91

    75

    57

    88

    70

    51

    85

    65

    45

    82

    60

    40

    78

    55

    35

    74

    50

    31

    70

    10

    4

    22

    5

    2

    13

    0

    0

    0

    Calculation of Time to Peak (Tp)

    The time to peak (Tp) for a given catchment is entered by the user and can be estimated by the following method. First calculate the time of concentration (Tc). A range of equations for determining Tc are documented in the literature and the following references describe many of these:

    The time to peak of the t-hour unit hydrograph (Tp) can be estimated using the standard SCS equation:

                                         
    where
                = time duration (hrs) of excess rainfall.

    The standardised SCS unit hydrograph exhibits the following characteristics:

    • the time base is approximately 5 times the time to peak
    • some 38% of the discharge volume occurs prior to peak flows
    • the inflection point on the recession limb occurs at 1.7 times the time to peak

    General

    It is important that the time interval, t, of each catchment is chosen carefully according to the following recommendation:

                                

    The value obtained from this equation can be rounded off to a convenient value representing a multiple of the time interval used in autographic measurements. In addition, the number of time intervals over the storm duration should equal an odd integer.

    The rainfall profile can either be entered directly or you can specify the summer or winter rainfall profiles derived for the Flood Studies Report (1975) .

    The lowest hydrograph flow is set to the minimum of Baseflow and Adjusted Baseflow .

    Datafile Format

    Line 1 - Keyword SCSBDY'

    Line 2 - Label

    Line 3 - z

    Line 4 - tdelay, t

    Line 5 - carea, stdur

    Line 6 - P

    Line 7 - CN (if -ve) or PR (if +ve)

    Line 8 - Tp

    Line 9 - BFADJS, BF

    Line 10 - UHFLAG

    Line 11 - 0 (if UHFLAG = SCSUH')

    Line 11 - nuh (if UHFLAG = OBSUH')

    Line 12 to Line 11+nuh - uh (if UHFLAG = OBSUH')

    Line 12+nuh - RPFLAG

    Line 13+nuh - 0 (if RPFLAG = WINRP')

    Line 13+nuh - 0 (if RPFLAG = SUMRP')

    Line 13+nuh - nrp (if RPFLAG = OBSRP')

    Line 14+nuh to Line 13+nuh+nrp - rp (if RPFLAG = OBSRP')

    RiverNodesimagesusscsmethoddata.gif


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