Choose Design Storm and Return Periods

Hydrology Studio offers automatic multiple return period processing. In other words, it computes your model for all “Active” return periods at the same time. There’s no need to rerun your model for each event.

You simply activate the return periods you need to use. Each hydrograph you create will then be calculated for each activated frequency using the corresponding precipitation values entered at this screen. You only need to do this once.

Like the IDF curves, you can save the values as an event file for loading at a later time. Similarly, these files are automatically saved when you exit the program and reloaded upon its start. These files also become part of all project files. Thus it is not necessary, for example, to email an associate, the .hys project file and the associated event file. Similar to the IDF file, it’s already embedded in the .hys project file.

Built-in Design Storms

Hydrology Studio has a full library of built-in design storms, including 10 that you can customize to suite your local criteria, from which you can pick and choose at any time. Their names are listed on the Precipitation Manager screen. Their full detailed hyetographs can be viewed from the “Design Storms” tab.

Storm NameDuration (hrs)Description
Type I, IA, II, & III24Dimensionless distributions developed by SCS using Weather Bureau’s Rainfall Frequency Atlases for different geographic regions.
Type II Fla Mod24Type II modified for parts of Florida.
SCS Standard6Dimensionless distribution developed by SCS for a shorter, 6-hr duration.
Synthetic, Nested, IDF-based1, 2, 3, 6
12 & 24
Dimensionless distribution is automatically developed using current IDF curves and symmetrically arranged rainfall depths.
Huff, Bulletin 71. All QuartilesVaries with QuartileDeveloped from Time Distributions of Heavy Rainstorms in Illinois by Floyd A. Huff. Used primarily in the Midwest.
CustomUser-definedA user-specified dimensionless storm. Up to ten unique storms can be entered and saved for future use.
Huff Indiana. Indianapolis, Evansville, Ft Wayne, S Bend, Quartiles 1, 2 & 30.5, 1, 2, 3, 6, 12, 24Huff curve dimensionless ordinates, Indiana LTAP 2015
NJDEP Water Quality Storm2New Jersey 2-hour, 1.25- inch water quality distribution
NRCS MSE324NOAA Atlas 14, Vols 7 and 8
NRCS MSE424NOAA Atlas 14, Vols 7 and 8

My Storm Isn’t Listed

If you don’t see a design storm that you wish to use from the library, you can enter it in (or import the data) as a Custom Distribution. See Custom Design Storms for more information.

To begin, click the [Rainfall] button on the Main Window Ribbon Toolbar to open the Rainfall IDF Wizard.

Select the “Precipitation” tab on the Ribbon Toolbar. The Precipitation Manager screen should look like the following:

Hydrology Studio Design Storms


This screen basically needs to know three things:

  1. Which return periods you want your project(s) to use.
  2. Which storm distribution you want to use.
  3. Rainfall precipitation amounts associated with Items 1 & 2.

1. Choose Your Return Periods

This is where you set the return periods you wish to always use. The top row of the table lists the available frequencies. Just below that are check boxes used to activate those frequencies. Simply check those you wish to always use. The screen below indicates the 2, 10 and 100 year events are active. The remaining inactive frequencies will not be used and will appear grayed-out throughout the software.

If you wondering why the return periods you want to use are not showing up in your results, revisit this screen and turn On / Off your desired frequencies.

2. Select a Design Storm Distribution

Click on the row header arrow corresponding to the desired design storm. This drops-down the list of available durations. For example, clicking on NRCS Storms displays the following:

To activate a distribution, simply click its corresponding check-box under the “Active” column. To use the Type II, for example, check the box in the Active column next to Type II as shown above. Remember, only one storm can be active at a time.

3. Enter Rainfall Precipitation Amounts

The last step in setting up your Precipitation & Events is specifying the rainfall amounts local to your area. These are then applied to the selected distribution when you create runoff hydrographs. (Note that this step is not required when using IDF-Based storms. Their values are computed and entered automatically.)

There are two ways to accomplish this step:

  1. Enter the rainfall amounts manually
  2. Import from NOAA’s National Weather Service “Precipitation Frequency Data Server”

To enter manually, simply type in the rainfall amounts associated with the selected design storm and frequencies. When finished, click the [Apply] button.

A sampling of SCS Type II precipitation amounts is shown in the screen above.

Save your rainfall by clicking the [Save] button and specifying a name for your file. A “.pcp’ extension will applied. This file will automatically open each time you launch Hydrology Studio. You can change this file any time afterwards.


Simply close this screen and return back to the main window. There’s nothing else you need to do. From this point forward, each runoff hydrograph you create will automatically use this rainfall data, return periods and selected design storm. There’s no need to re-enter this data.

You can, of course, return to this screen at any time to edit and choose other design storms as well as activate other return periods. Click the [Open] or [Save] buttons to open or save an existing .pcp file.


Was this article helpful?

Related Articles