Hydrology Studio has pre-set frequencies or return periods that it uses for the IDF Curves and the Design Storms. Those are specifically 1, 2, 3, 5, 10, 25, 50 and 100-year. You have the option to select alternates to the 25-year, 50-year and the 100-year events. For example, you may wish to use 15 or 20-year frequency instead of the default 25-year or the 500-year instead of the default 100-year.
It is recommended that you customize the return periods prior to creating or importing new IDF curves. Note that these are simply labels and changing them will not affect the underlying data.
How to Modify the Return Period Labels
Open the Rainfall Wizard by clicking the Rainfall button on the Rainfall tab on the main window.
Then click the [Settings] button while viewing the Rainfall Wizard main screen. This button is available on the IDF Curves, Precipitation and Design Storms tabs.
The following screen appears:
Now select alternates to the 25-year, 50-year and 100-year events. The 25-year label can only be changed to 15 or 20; the 50-year to 100-year and the 100-year label can be changed to 200 or 500. The remaining return periods are fixed and cannot be edited.
Once you have selected the desired storm frequencies, click [Ok]. The input and output tables will now include these frequencies. For example, the new frequencies shown below include the 15 and 500-year events.
It should be noted that you cannot make changes to the custom frequencies, 25-year, 50-year and 100-year, when setting up your new IDF curves using Hydro-35 or NOAA Atlas 2. This method is limited to the default frequencies. Any changes made to the return periods will revert back to the defaults.