US SCS Method
- 23 Oct 2022
- 8 Minutes to read
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US SCS Method
- Updated on 23 Oct 2022
- 8 Minutes to read
- Print
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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The US SCS unit has been superseded by the Generic Rainfall/Runoff Boundary which incorporates a more detailed interpretation of the US SSCS hydrological methodology.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 Form | Description | 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:
- Soil characteristics as described above.
- Soil surveys (if available).
- 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
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')
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