Storms with durations of 6 hours or less tend to be associated more often with first-quartile distributions, and those lasting more than 6 hours and less than or equal to 12 hours are most commonly the second-quartile type. Storms having durations longer than 12 and less than or equal to 24 hours most commonly follow the third-quartile distribution. Storms with a duration longer than 24 hours are most frequently associated with the fourth-quartile distributions. However, a particular storm from any duration may be associated with any of the four quartile types.

We recommend the use of the most common quartile for the design storms. A design storm with a duration less than or equal to 6 hours should be a first-quartile type storm. The second quartile type design storms should be used for durations longer than 6 and up to 12 hours. For storms longer than 12 hours in duration and less than or equal to 24 hours, we recommend the use of the third-quartile time distribution. Finally, design storms longer in duration than 24 hours should be modeled using the fourth quartile type.

Related Topics:

• Rainfall Time-Distribution Information
• Watershed Area
• Data Sources
• Data Format
• Bulletin 70/71 Data
• Circular 173 Data