DrainageCalculators

Rainfall Distribution Calculator

Generate SCS/NRCS 24-hour rainfall distributions (Type I, IA, II, III) for design storms. Creates hyetographs showing rainfall intensity over time per TR-55 methodology.

What This Solves

Generates SCS/NRCS 24-hour rainfall distributions (Type I, IA, II, III) that describe how rainfall intensity varies over the duration of a design storm.

Best Used When

  • You need a temporal rainfall distribution (hyetograph) as input to hydrograph calculations
  • You are developing design storm inputs for SCS Unit Hydrograph or hydrograph convolution analysis
  • You want to compare rainfall intensity patterns across different SCS storm types for your region

Do NOT Use When

Key Assumptions

  • The 24-hour storm duration is used with SCS dimensionless cumulative distributions
  • Total rainfall depth is distributed according to the selected SCS type (I, IA, II, or III)
  • The distributions are based on NRCS TR-55 and NEH Part 630 tabulated values
  • Rainfall is spatially uniform across the watershed during the storm

Input Quality Notes

Select the correct SCS storm type for your geographic region (Type II covers most of the eastern US). Total 24-hour rainfall depth should come from NOAA Atlas 14 for the desired return period.

Input Parameters

Storm Parameters

24-hour design storm characteristics

in

Design storm precipitation depth

SCS rainfall distribution pattern

Most of the continental United States. Characterized by high-intensity, short-duration storms.

Peak at 12 hours (50% of storm duration)

Output Settings

Time step and extraction options

hours

Output interval (0.1 = 6 minutes)

hours

Extract portion starting at this hour

hours

Extract portion ending at this hour

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

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PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.