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Rainfall Duration

Rainfall Duration


Most rainfall distributions are developed for a specific rainfall duration.  Due to the historic practice of reading rain gauges on a daily basis, most rainfall distributions use a 24-hour duration.  Although shorter durations are sometimes used for special purposes (such as water quality events), most studies are performed with a 24-hour nested synthetic storm.  Because these storms contain intensity data for all events up to 24-hours, there is generally no need to use a shorter storm duration for smaller watersheds.

While most of the rainfall distributions used with HydroCAD are 24-hour duration, HydroCAD supports the use of any storm duration, from short water-quality events up to multi-day historic events.  HydroCAD also supports dimensionless storms, such as the Huff distributions, which are scaled to a specific duration as required for each project.

Setting the Rainfall Duration

For most rainfall distributions the duration is set automatically and should not be changed.  As soon as you select a rainfall distribution (Storm Type on the rainfall screen at right) the Storm Duration is automatically set to the predefined default value for that storm.

For dimensionless rainfalls (such as the Huff distributions) the Duration Mode is automatically set to "Scale", and the appropriate Storm Duration must must be entered by hand.

Changing the Rainfall Duration

For applications that require a different rainfall distribution, you can specify a new duration by setting the "Duration Mode" to "Trim" or "Scale".

Trim can be used to reduce the rainfall duration by "zooming in" to the desired duration and trimming away the portion of the storm that lies outside this time period.  The resulting storm is centered within the original time span.

For example, if you have a 24-hour storm with the peak intensity at 12 hours, and you trim to 12 hours you will get a 12-hour storm with the peak at 6 hours.  The portions of the original mass curve before 6 hours and after 18 hours are "trimmed" and not used, while the 6-18 hour portion is retained and becomes the new 12-hour distribution, as shown in the illustration at the right.

Most nested synthetic rainfall distributions are candidates for trimming, in that the peak rainfall intensity is preserved in the resulting reduced-duration event.

Scale can be used to increase or decrease the rainfall duration by rescaling the entire mass curve.  This procedure is most commonly used with the dimensionless Huff distributions, or with the "Constant Intensity" storm that is pre-installed with HydroCAD.

Important: Storms with a preset rainfall duration generally should not be scaled to a different duration, since the scaling operation will alter the peak intensity in direct proportion to the change in duration!

For example, if you have a 24-hour storm with the peak intensity at 12 hours, and you scale to 12 hours you will get a 12-hour storm with the peak at 6 hours.  The entire mass curve is retained, but the 24 hour time scale is compressed to 12 hours, increasing the peak intensity two-fold!


The "Trim" option was introduced in the HydroCAD 10.1 update.  This makes it possible to trim storms to a shorter duration, eliminating the need for multiple versions of a given distribution, such as the Type II 6, 12, and 24 hour storms that have been preinstalled with HydroCAD.  But the primary benefit is the ability to generate reduced-duration versions of any rainfall distribution, without having to maintain a separate rainfall definition for each duration.

Since previous versions of HydroCAD did not have the "Trim" capability, changing the rainfall duration would always result in the storm being rescaled.  These projects will continue to produce the exact same results when opened in the new version, which will automatically use the "Scale" option.  However, a new Hint 35 will be generated whenever a fixed duration storm is rescaled, indicating that you should review the rainfall settings and consider using the new "Trim" option instead of rescaling.

Project files using the "Scale" setting are backwards compatible with earlier versions.  Project files using the "Trim" option are not backwards compatible, since this capability is not present in earlier versions.


The "Trim" procedure is further described in Example 4-5 of the National Engineering Handbook, Part 630, Hydrology, Chapter 4, revised March 2017.

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