When using Technical Release 55 (TR-55), rainfall data should be obtained from one of the SCS (NRCS) 24-hour synthetic rainfall distributions. Because these distributions are synthetic, or hypothetical, they do not represent an actual storm event. Instead, these distributions were derived by applying statistical techniques to observed rainfall records.

The highest peak discharges from small watersheds in the United States are usually caused by intense, brief rainfalls that may occur as distinct events or as part of a longer storm. These intense rainstorms do not usually extended over a large area and intensities vary greatly. It is for these reasons that synthetic rainfall distributions are used instead of observed data. These distributions include maximum rainfall intensities for the selected design frequency arranged in a sequence that is critical for producing peak runoff. This document will describe the synthetic rainfall distributions developed by the Soil Conservation Service (SCS) for use in TR-55. The Soil Conservation Service is now known as the Natural Resources Conservation Service (NRCS).

What are the four SCS (NRCS) synthetic 24-hour rainfall distributions?

Because the intensity of rainfall varies considerably during a storm as well as between different geographic regions, the SCS (NRCS) has developed four synthetic 24-hour rainfall distributions (I, IA, II and III) based upon duration-frequency data and local storm data recorded by the National Weather Service (NWS). A plot of the four distributions is shown below.

For which geographic area is each SCS (NRCS) synthetic 24-hour rainfall distribution applicable?

Types I and IA represent the Pacific maritime climate with wet winters and dry summers. Type III represents the Gulf of Mexico and Atlantic costal areas where tropical storms bring large 24-hour rainfall amounts. Type II represents the rest of the country. Type IA is the least intense while type II is the most intense short duration rainfall. The figure below shows the geographic boundaries of the different rainfall distributions.

Where can I find the SCS (NRCS) synthetic 24-hour rainfall distributions?

StormNET contains an advanced Rainfall Designer that can quickly determine the SCS (NRCS) rainfall distribution for any location in the United States based upon a user specified return period. An electronic document containing the SCS (NRCS) synthetic rainfall distribution maps can be downloaded here.

Why are the rainfall distributions for a 24 hour period?

The SCS (NRCS) rainfall distributions are all based upon a storm period of 24 hours. The 24 hour period was selected based upon the drainage areas for which the SCS (NRCS) usually provides assistance. The 24 hour storm, while longer than that needed to determine peaks for small drainage areas, is appropriate for determining TR-55 based runoff values.

Are the SCS (NRCS) synthetic rainfall distributions the same as IDF curves?

Intensity-duration-frequency (IDF) curves are graphical representations of the probability that a given average rainfall intensity will occur. They are commonly used when determining the rainfall rate for the rational method, modified rational method or the DeKalb rational method. The SCS (NRCS) synthetic rainfall distributions were developed for use with SCS (NRCS) methods, and they are not the same as IDF curves.

What type of rainfall data do I use in TR-20?

Rainfall distributions for use in TR-20 can be either an actual storm or a dimensionless synthetic storm. Tables containing the SCS (NRCS) 24-hour synthetic rainfall distributions are included in TR-20.

What is TR-55?

Technical Release 55 (TR-55) is one of the most commonly used design methods in the United States for the management of storm water runoff in urban settings. Originally published in 1975 by the Soil Conservation Service (SCS), TR-55 presents procedures for the calculation of storm runoff volume, peak rate of discharge, hydrographs and detention pond storage volumes for small watersheds. The Natural Resources Conservation Service (NRCS), formally known as the SCS, published a revised version of TR-55 in 1986. The methods in TR-55 are also described in the hydrology section of the National Engineering Handbook which was also compiled by the SCS (NRCS).

What is TR-20?

Technical Release 20 (TR-20) is an event based model for simulating the movement of surface water through a watershed.
TR-20 can be used to analyze current watershed conditions as well as to examine the impact of proposed changes within the watershed. The original theory and verification of the methods used in TR-20 were developed in the 1940’s and 1950’s by the Soil Conservation Service (SCS) in cooperation with the Agricultural Research Service (ARS). Computer code for TR-20 was developed in the 1960’s on mainframes, and the first PC version of the model was released in the 1980’s for MS-DOS. Recently, the Natural Resources Conservation Service (NRCS), formally known as the SCS, released a Windows version of the model called WinTR-20.

What are some sources of additional information?

The original TR-55 documentation and the MS-DOS version can be found on the NRCS TR-55 webpage. The WinTR-55 program, documentation and tutorials can be downloaded from the NRCS WinTR-55 webpage. Links to all NRCS hydraulics and hydrologic software can be found here.