Depth, Duration and Recurrence Intervals
The intensity of a rain event is a simple ratio of the amount of rain, or depth, to the amount of time during which it fell, or duration. It can be brief (half an inch in 5 minutes) or extended (10 inches in the past month).
When we hear of a “100 year rain event,” we are talking about the recurrence frequency, or how often on average a rain event of this intensity is likely to happen. Bear in mind that this is a measure of statistical likelihood. It does not imply that weather keeps to any sort of schedule. Neither does it prevent “100-year” events from occurring multiple times in a single year!
Mesonet Precipitation Recurrence Tool
The Mesonet’s Precipitation Intensity tool relates depth, duration and recurrence frequency in an easy to use interface.
Numbers on the map and the side panel are the recurrence frequency, or average number of years between events of this intensity for the chosen duration. Use the Change Map selector or swipe right/left on mobile to see results for a different duration. We track events as short as 5 minutes and as long as 60 days.
The Table adds the observed time and depth of the event.
The Chart tab displays the relationship between depth, duration and recurrence frequency for the selected station.
How Severe?
Underlying conditions matter: a 10-year rain falling on dry, warm soil will cause less flooding than the same rain falling on frozen or saturated soil. However, here are some general guidelines:
- Urban stormwater systems are generally designed for 1-10 year events.
- Highways and transportation infrastructure are designed for 10 to 25 year events.
- Dams and levees are designed for greater than 100 year events. [1]
We provide recurrence frequency for multiple durations to suit a broad range of needs.
- Short duration heavy rainfall risks erosion, runoff and localized flooding.
- Longer duration sustained rains may overwhelm ponds and lagoons and saturate the soil.
Calculating the Recurrence Frequency
The Mesonet uses depth-duration-recurrence relationships computed by NOAA for each Mesonet station. These are based on historical precipitation data collected from a variety of sources [1] and are available in table form at https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html . We use these tables to classify each rain event in terms of its recurrence frequency, then display the maximal event. This means that if a 2-year rain and a 10-year rain occur in close succession, our tool will display the 10-year event.
The time frame over which we calculate these events depends on the duration.
- For 60 minutes or less, we display the maximal event of the past 24 hours
- For 6 and 24 hour events, use the past 7 days
- For 7 day events, the past 60 days
- For 30 and 60 day events, the past 365 days
Considerations
- Our rain gauges measure liquid precipitation. Freezing conditions can affect when and how much precipitation is observed.
References and Further Reading
[1] Average Recurrence Interval of Event Precipitation in Real-time http://www.appliedweatherassociates.com/uploads/1/3/8/1/13810758/average_recurrence_interval_of_event_precipitation_in_real-time-asdso_annual_conference__2010.pdf
Classifying the Recurrence Interval (Frequency) of Actual Storm Events using the National Weather Service Precipitation-Frequency Data Server (USDA) https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=43446.wba
Contact
Questions? Problems? Please let us know at [email protected]