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Comparing SCS Runoff

Comparing SCS & SBUH Runoff Results

This document will help you identify the exact cause of any differences between HydroCAD runoff and other hydrology programs you may be using. 

bulletThis document deals with the SCS and SBUH runoff procedures.  If you're using the rational method, please see the rational method comparison guide.
bulletIf your runoff results are in good agreement, you may skip this document and compare your pond or reach routing results.
bulletBefore you proceed with this page, be sure you've read the general comparison guidelines.

Note: HydroCAD runoff calculations have been extensively cross-checked against TR-20, and generally match the TR-20 runoff volumes and peaks to within 1%.  This is demonstrated by the "TR-20 Sample Job #1" that is installed with HydroCAD.  See the comments at the top of each summary report for full details on the comparison.  Significant differences will occur only at short Tc values, where the coefficient-based rainfall tables used in HydroCAD will generally produce more accurate (higher) peak runoff.  However, runoff volumes will remain the same under all conditions.  If your goal is to exactly match the TR-20 results, you can download tabular rainfall tables for use with HydroCAD.

1) Start by looking at the runoff volume

Ignore the peak flow for now.  Check the volume as shown at the top of the subcatchment summary.  This will be displayed as a standard volume (e.g. 4.3 acre-feet) and as an equivalent runoff depth (e.g. 1.3 inches.)  If the HydroCAD volume matches your other software to within 1%, you can skip ahead to part 2 and compare the peak runoff.  But if the volume is different, check the following input parameters:

1a) Runoff method. Make sure each program is set to use the same runoff procedure: the SCS Unit Hydrograph method (TR-20), -or- the SBUH method.  If your other software provides a choice of runoff equations, make sure it is set to use the SCS runoff equation(Note that HydroCAD does not provide the TR-55 tabular method, which can deviate from TR-20 results by up to 25% percent.  Use the TR-20 method for these applications.)

1b) Rainfall depth. This is the total rainfall depth for the entire storm. Check the value and the units.  If you're using multiple rainfall events, use just one event for the initial review.

1c) Back-to-back storms. Make sure this feature is disabled by setting the number of storms to "1".  Higher values will generate multiple occurrences of the same storm, with a corresponding increase in rainfall depth and runoff.

1d) Subcatchment area. Make sure the total area of the subcatchment is the same. Check the value and the units.  Be aware of differences in the number of decimal places, especially when using different units (e.g. square-feet vs square miles.)

1e) Curve Number. For the purposes of this test, enter a single CN value.  Programs may use different approaches for weighting of multiple CN values and/or impervious areas, so start by using a single CN value such as 80.

1f) Antecedent Moisture Condition.  Make sure you haven't changed the default Antecedent Moisture Condition of II (2).  This value can be changed on the Rainfall tab of the calculation settings.

1g) Initial abstraction ratio. Make sure you're using the default value of 0.2.  For HydroCAD 7.1 and later, this is specified as the Ia/S ratio on the Advanced tab of the calculation settings.  For earlier versions, the value is fixed at 0.2

1h) Calculation time span. When comparing volumes, the time span must start soon enough to include the earliest runoff, and must extend long enough to include the latest runoff.  For a 24-hour storm, a 0-30 hour span should generally be adequate, depending on the Tc.  Check this by looking at the hydrograph plot to be sure it starts and ends with zero flow.

If all these parameters are the same, the runoff volume should match very closely.  If the volumes are different, check the runoff depth by hand with the SCS runoff equation:

     ( P - 0.2 S )               1000
Q = ----------------   and   S = -------  - 10
       P + 0.8 S                   CN

The runoff depth (Q) is strictly a function of the precipitation depth (P) and the Curve Number (CN). For example, a rainfall depth of 4" and a curve number of 80 will yield a runoff depth of 2.04"

bulletIf the HydroCAD runoff volume doesn't match this equation, there must be a discrepancy in one of the values listed above.
bulletIf your "other" software doesn't match the SCS runoff volume, there must be an input discrepancy, or the software may not be using the SCS runoff equation.
bulletIf the volumes agree, continue with part 2 to compare the runoff peak.


2) Now check the runoff peak

After the runoff volume is matched (see part 1 above), you should check the peak runoff.  If the peaks are significantly different, check the following parameters:

2a) Time-of-concentration.  Are the values the same?  Since there are many ways to calculate the Tc, compare runoff results by entering the Tc value directly.  Start with a mid-range value such as 30 minutes. (Shorter values may reveal subtle differences in rainfall distributions, as described below.)  Check your units, including any conversions between minutes and hours.  Note that some programs (e.g HEC-HMS) use a time lag parameter which is typically defined as Tlag = 0.6 Tc.  Entering the Tc value as the time lag will produce a lower peak flow.

2b) Calculation time step.  For the best match, use the same time increment (dt) in each program.  If either program recommends a smaller dt, reduce the value in both programs to the same value.  Smaller values are generally more accurate.

2c) Rainfall distribution.  Make sure each program is using the same rainfall distribution, such as the SCS Type II storm.  But even if the name is the same, each program may use a different version of the rainfall distribution, a different number of rainfall points, or even a different technique to represent the curve.   These differences will generally be most pronounced with shorter Tc values.  Longer Tc values (30+ minutes) are recommended for an initial comparison.  If there are doubts about the rainfall table, create a custom rainfall table in HydroCAD using your "other" rainfall data.

Note: For maximum accuracy, HydroCAD uses polynomial versions of the SCS rainfall tables, which may produce higher peak flows at small Tc values.  If your goal is to exactly match the TR-20 results, you can download tabular rainfall tables for use with HydroCAD.

2d) Rainfall duration.  The rainfall duration should usually be set to the default value, such as 24-hours for a 24-hour storm.  Other values will cause HydroCAD to rescale the rainfall distribution, which will change the rainfall intensity and the rate of runoff.

2e) Unit hydrograph.  (Used for the SCS method only. Skip this step for the SBUH procedure.)  The UH is typically characterized by a "peak factor", which is 484 for the standard SCS unit hydrograph.  Are the peak factors the same?  If in doubt, create a custom UH table in HydroCAD using the UH data from your "other" program.

2f) How is the peak calculated?  HydroCAD always reports a peak flow by interpolating from the highest three values on the runoff hydrograph, just like TR-20.  Some programs may report the highest discrete value, which will generally be somewhat lower than the interpolated peak.  Details here.

These are the only additional parameters that influence the peak runoff.  If your peak runoff is significantly different, re-check each of these values.  Unfortunately, there is no easy way to verify the peak runoff by hand.  This is a numerical procedure that requires hundreds or thousands of iterations, and is not feasible to perform by hand.


For further information

If you need further details of the any runoff calculations, see the HydroCAD Owner's Manual, which is included with all full programs.  For an illustrated presentation of many of these topics, also see the Stormwater Training Disk.

Once the runoff volume and peak are in agreement, you can proceed to compare your reach or pond routing results.


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