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HydroCAD Stormwater Modeling - Since 1986


Common Rainfall Issues

This page pertains to rainfall distributions used with the SCS/NRCS or SBUH runoff procedure, often referred to as mass curves.  If you are using the Rational Method, you will need an IDF file instead of a mass curve.

What is a "rainfall event"?

A rainfall event typically refers to a specific rainfall depth distributed in time according to a specific temporal rainfall distribution.  In most cases each event is referred to by it's return period, such as the "25-year event".  However, an event definition can also be used to define other rainfall conditions, such as a "Water Quality" event or "Historic Rainfall".

How do I define my rainfall events?

Rather than having to re-enter the rainfall depth for each return period (10-year, 25-year, etc.) event, you can create unlimited rainfall event definitions within each project.  This makes it easy to switch between events, and lets you run multi-event reports and data export.  Details here.

What rainfall distributions are provided with HydroCAD?

Many projects can be modeled with one of the standard rainfall distributions that are automatically installed with HydroCAD.  To explore the rainfall tables on your computer, start HydroCAD and select View|Storm. This will open a storm report window, as shown below.  Use the check-boxes at the right to display the desired rainfall(s).

Instant rainfall distribution reports

Are additional rainfall tables available?

If one of the preinstalled rainfalls doesn't meet your needs, check the online rainfall library for additional distributions.  If you need an unusual rainfall table that isn't listed, or want to model an observed rainfall event, then you can create a custom rainfall definition, as described below.

Can I use local rainfall data instead of one of the standard distributions?

Yes.  Although most jurisdictions continue to use one of the standard rainfall distributions discussed above, you can also use precipitation frequency data (from NOAA, NRCC, or other sources) to generate mass curves for your specific location.

How do I define a custom rainfall table?

To create a custom rainfall by hand, open the storm report window (see above), select one of the existing files to use as a template, and click the Edit button.  Before saving your work, be sure to enter a new name from your rainfall, so you don't overwrite the template!  Details here.

How do I determine the correct rainfall depth?

With HydroCAD 9.1 and later, the rainfall depth for each event can be automatically set for your project location, as described above.  Rainfall depth information is also available from these sources:

bulletTraditional rainfall depth maps for the central and eastern United States are provided in Appendix B of the HydroCAD Owner's Manual.  This information is also available in Appendix B of the TR-55 manual (1.4mb PDF file)
bulletElectronic rainfall data for New York and New England is available from NRCC at www.precip.net
bulletElectronic rainfall data for most other states is available from NOAA at http://hdsc.nws.noaa.gov/hdsc/pfds/.
bulletRainfall data for the western states is available at http://www.wrcc.dri.edu/pcpnfreq.html
bulletRainfall documents for all states are available at http://www.nws.noaa.gov/oh/hdsc/currentpf.htm 

You may also use other sources (such as TP-25 or TP-40) to determine the total rainfall depth.  In general, the use of updated rainfall data does not require changes to the built-in rainfall tables.  The only value you need to change is the total (24-hour) depth for each event.

How do I use the new Atlas 14 rainfall data?

There are several options for using the latest Atlas 14 data as discussed here.

How do I set the return period?

Although the return period is used to determine the rainfall depth for a given location, the return period itself is not directly used in the runoff calculations or entered into HydroCAD.  However, you can define rainfall events which are usually named according to the corresponding return period.  For example, you could define a "25-year" event which specifies a 3.5 inch rainfall depth.

What is the "P2" Precipitation?

P2 is the 2-year 24-hour precipitation depth.  It is used when calculating the time-of-concentration with the TR-55 Sheet Flow method.  In general, P2 will not be the same as the project's current rainfall depth.  P2 is always the 2-year depth and remains constant regardless of the current depth as set on the calculation settings screen.  See the TR-55 manual for further details.

Can I use a 24-Hour storm, even when my Tc is much smaller?

Yes. The 24-Hour storms are derived by combining all events of 24-hours duration and less. This allows one curve to be used with most watersheds, even if they have a much smaller time of concentration.

Can I change the duration of the storm distribution?

Yes.  Rainfalls can be automatically scaled to any desired duration.  However, this changes the peak intensity, and should be done only with rainfalls that are specifically intended to be re-scaled, such as the Huff distributions.  Most rainfalls (including the SCS 24-hour Type I, IA, II and III) are intended for use only at the predefined duration, and rescaling to a different duration will change the peak intensity and produce incorrect results.  If in doubt, check the applicable regulations for details on the prescribed distribution.  Some agencies provide a special distribution for low-depth events, or require a 6 or 12 hour version of an SCS rainfall.  Many of these specialized rainfalls are available on the download page.

Note that some requests for different durations stem from confusion between the SCS method and the Rational Method.  The Rational Method routinely involves the use of different durations. In fact, the duration must typically be adjusted to a critical value that maximizes runoff. This can be time consuming and imprecise. Some localities attempt to simplify this process by specifying a fixed duration, such as a "3-hour event," but this is not a proper use of the Rational Method.  Details here.

Why are the tabulated rainfall depths different than the original values?

Many rainfall definitions are based on tables of depth vs. time values.  The depth at intermediate times is commonly determined by linear interpolation.  However, this procedure produces "steps" in the intensity curve which may also be visible in the resulting runoff hydrographs, especially at small Tc values.

To produce a continuous (non-stepped) intensity curve, HydroCAD automatically applies a "smoothing" operation to all rainfalls that employ tabular depth values.  This results in a slight delay in the depth curve.  However, peak intensity and runoff are usually not effected, and the visible steps are eliminated.

bulletIf you prefer to have an exact match to the original depth values, you can disable Rainfall Smoothing on the Advanced tab of the Calculation Settings screen.  (This option was added in HydroCAD 9.1)
bulletFor earlier versions of HydroCAD, you can add the statement "Smoothing=False" to the end of the applicable rainfall definition.  Note that this will change the results for all projects that utilize this rainfall definition, so the previous option is generally preferred.  (When making this change, add "Steps" to the end of the rainfall file name to distinguish it from the original file.)


Also read about subcatchments, runoff, curve numbers and Time-of-Concentration.

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