Precipitation Temporal Distribution

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When using precipitation depth estimates for hydrologic modeling, it is necessary to define the "temporal distribution of the rainfall, which is how the precipitation is distributed over time throughout a storm of a particular duration. For example, a 24-hour rainfall distribution may be defined at a time interval of 0.1 hour by the cumulative distribution of rainfall, starting at the beginning of the storm and ending at the 24-hour rainfall value."

"Rainfall distributions used for design of engineering projects are different from actual storms in several ways. One is that design rainfall distributions used by NRCS are generally 24 hours in duration. Actual storms have any duration from minutes up to days. Another major difference is the distribution of rainfall throughout the duration of the storm. The actual storm has variable rainfall during each increment and could even have increments of very high intensity rainfall, very low intensity rainfall, or both. The design rainfall distribution has an intensity starting low and increasing to a maximum value then gradually reducing in intensity approaching the end of the storm. A third major difference between actual storms and design storms is that even though the actual event may have a 100-year 24-hour total rainfall, the actual storm may include the 10-year 3-hour rainfall and the 5-year 5-minute rainfall (or any combination of other durations and return periods). The design rainfall distribution is developed to have the 100-year 24-hour rainfall, the 100-year 12-hour rainfall, etc., down to the 100-year 5-minute rainfall imbedded in a single storm."

"Little documentation is available that describes the development of the Type II and other legacy rainfall distributions. Study of what is available leads to the conclusion that their use be discontinued in areas covered by NOAA Atlas 14 data. When these legacy rainfall distributions were developed, they were developed using the best data, technology, and engineering judgment available at the time. With current data of improved quantity and quality, geographic information systems, and computer capabilities, a higher standard may be set with respect to developing and using updated rainfall distributions."

Best Practices Resources

National Engineering Handbook: Chapter 4 - Storm Rainfall Depth and Distribution (Natural Resources Conservation Service)

Citations:

Revision ID: 5232
Revision Date: 12/13/2022