Common Rainfall Issues
What rainfall tables are provided with HydroCAD?
The vast majority of 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).
How do I define a custom rainfall table?
For special applications, you can also download additional rainfall tables to your computer. If you need an unusual rainfall table that is not listed, or want to model an observed rainfall event, then you can create a custom rainfall definition.
To create a custom rainfall, open the storm report window (see above), select one of the existing files to use as a template, and click the Edit button. (We suggest the Sample A file for intensity curves and the Sample B file for mass curves.) Before saving your work, be sure to enter a new name from your rainfall, so you don't overwrite the template! For further details see the Rainfall.txt file in your "Rainfall" folder, or select Start|Programs|HydroCAD|Rainfall Info.
How do I determine the correct rainfall depth?
Rainfall depth maps for the central and eastern United States are given in Appendix B of the HydroCAD Owner's Manual, or you may download Appendix B of the TR-55 manual (1.4mb PDF file) Rainfall data for the western states is available at www.wrcc.sage.dri.edu/pcpnfreq.html Consolidated rainfall frequency data for the entire US is under development by NOAA at http://hdsc.nws.noaa.gov/hdsc/pfds/
You may also use other sources (such as TP-25) 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, which is always entered by hand.
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 any number of event definitions within each project. This makes it easy to switch between events, and lets you run multi-event reports and data export.
To define your events, open the Settings|Calculation screen and select the Rainfall tab. Enter the rainfall parameters for your first event. Type the desired name (such as 100-year or water quality
) in the Event Name box and click the Save button. Repeat for each event.You can define any number of events in each project. For readability, keep your event names as short as possible. Do not include the word "event" or "storm". (This will be displayed automatically as required.) Once you have defined the events for a given location, you can use the Import button to load them into other projects.
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, beginning with HydroCAD 4.5, rainfalls can be automatically scaled to any desired duration. However, this should be done only with rainfalls that are specifically intended to be re-scaled, such as the Huff distributions. Most rainfalls are intended for use only at the predefined duration, and rescaling to a different duration will produce an incorrect rainfall distribution. If in doubt, check the applicable regulations for details on the prescribed distribution.
Warning: Many requests to modify the rainfall duration stem from confusion between the SCS method and the Rational Method:
 | The Rational Method routinely involves the use of different durations. 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 "6-hour event," but this is not a proper use of the
Rational Method. Details here. |
| The SCS method is used almost exclusively with 24-hour rainfall distributions. These curves are derived by combining all events of 24-hours duration and less. This means that the overall duration rarely needs to be adjusted, even when studying subcatchments with a much smaller time of concentration. |
When a duration other than 24-hours is called for, the use of Rational Method is often being implied. Further research may be required to determine what method is being specified or assumed. There are only limited situations (such as special rainfalls for Florida and Illinois) where other durations are commonly used. Please read about the rainfall tables supplied with HydroCAD.
Also read about subcatchments, runoff, curve numbers and Time-of-Concentration.