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
- You only need a single peak rainfall intensity value for the Rational Method — Use Rational Method Calculator
- You need the full runoff hydrograph, not just the rainfall pattern — Use SCS Unit Hydrograph Calculator
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.
Generate an SCS/NRCS 24-hour design-storm hyetograph. Enter a total 24-hour rainfall depth and pick a distribution type (I, IA, II or III) to get the cumulative depth, incremental depth and rainfall intensity at every time step — the input most runoff and detention models need.
How it works
Each SCS distribution is a table of cumulative rainfall fractions (P/P24) versus time, taken from TR-55 Appendix B, Table B-1. The cumulative depth at any time is the tabulated fraction multiplied by the total 24-hour rainfall:
P(t) = f(t) × P24 where f(t) = P(t) / P24
Between the tabulated hours the calculator uses linear interpolation on the fraction. The incremental depth over a step is the difference of consecutive cumulative depths, and the rainfall intensity for that step is the increment divided by the time step:
ΔP = P(t) − P(t − Δt) i = ΔP / Δt
- P24 — total 24-hour rainfall depth (in or mm)
- P(t) — cumulative rainfall depth at time t
- f(t) = P(t) / P24 — dimensionless cumulative fraction from Table B-1
- Δt — output time step in hours (0.1 hr = 6 minutes)
- i — average rainfall intensity over the step (in/hr or mm/hr)
The peak intensity is the largest i across all steps; for Type II and Type III it falls at hour 12, the midpoint of the storm.
SCS cumulative fractions (TR-55, Table B-1)
Cumulative fraction of the 24-hour rainfall, P(t)/P24, at selected hours for each distribution type. Multiply a fraction by your total depth to get the cumulative rainfall at that hour. Note how Type II and III concentrate their rise around hour 12.
| Time (hr) | Type I | Type IA | Type II | Type III |
|---|---|---|---|---|
| 2 | 0.035 | 0.050 | 0.022 | 0.020 |
| 6 | 0.125 | 0.206 | 0.080 | 0.072 |
| 9 | 0.254 | 0.520 | 0.147 | 0.148 |
| 11.5 | 0.654 | 0.645 | 0.283 | 0.298 |
| 12 | 0.682 | 0.664 | 0.663 | 0.500 |
| 15 | 0.801 | 0.769 | 0.854 | 0.849 |
| 18 | 0.882 | 0.860 | 0.921 | 0.922 |
| 24 | 1.000 | 1.000 | 1.000 | 1.000 |
Values from TR-55 (1986), Appendix B, Table B-1. The calculator interpolates the full table at your chosen time step; only selected hours are shown here.
Choosing a distribution type
Type I
Pacific maritime climate with wet winters and dry summers — Hawaii, coastal Alaska and coastal California north of San Luis Obispo. Peak at hour 10.
Type IA
The least intense distribution, for the Pacific maritime climate of coastal Oregon and Washington. This calculator’s reference data also assigns Type IA to coastal California south of San Luis Obispo and inland California. Peak at hour 8.
Type II
Most of the continental United States. High-intensity, short-duration storms with a sharp peak at hour 12. The default for most inland design.
Type III
Gulf of Mexico and Atlantic coastal areas (southern Florida, the coast from Texas to Virginia) where tropical storms are common. Peak at hour 12, broader than Type II.
Worked example
A 6.0 in, 24-hour Type II design storm:
- Cumulative at hour 12 = 0.663 × 6.0 = 3.978 in
- Cumulative at hour 6 = 0.080 × 6.0 = 0.48 in (only ~8% has fallen in the first quarter)
- Peak half-hour (hour 11.5 to 12.0) = (0.663 − 0.283) × 6.0 = 2.28 in in 0.5 hr
- Average intensity over the peak 30 minutes ≈ 2.28 in / 0.5 hr = 4.56 in/hr
Note that peak intensity is time-step dependent. The 4.56 in/hr above is the rainfall averaged over the steepest half hour. The calculator resolves a finer interval: at its default 0.1 hr (6-minute) time step the steepest increment is the 11.9-to-12.0 hr rise of 0.7344 in, so the result card reports a peak intensity of about 7.3 in/hr (0.7344 in / 0.1 hr) for this same storm. A shorter time step resolves a sharper peak; a longer one smooths it toward the half-hour average.
Matches the TR-55 Table B-1 verification value of 3.978 in cumulative at hour 12 for a 6.0 in Type II storm.
Frequently asked questions
What is an SCS 24-hour rainfall distribution?
It is a standardized dimensionless curve, published by the SCS/NRCS in TR-55 (Appendix B), that describes how a 24-hour design storm’s rainfall accumulates over time. The curve gives the cumulative fraction of total rainfall (P/P24) at each hour, from 0.0 at hour 0 to 1.0 at hour 24. Multiplying that fraction by your total 24-hour depth produces a hyetograph — a time series of rainfall used as input to runoff and detention models.
Which distribution type should I use — I, IA, II, III?
Type II covers most of the continental United States and is the default. Type IA is the least intense distribution; it applies to the Pacific maritime climate of coastal Oregon and Washington, and this calculator’s reference data also assigns it to coastal California south of San Luis Obispo and inland California. Type I applies to Hawaii, coastal Alaska and coastal California north of San Luis Obispo. Type III applies to the Gulf of Mexico and Atlantic coastal areas (e.g., Florida and the coast from Texas to Virginia), where tropical systems produce larger 24-hour totals. Always confirm the locally adopted distribution with your reviewing agency, since many states now publish NOAA Atlas 14 based distributions.
Why does the Type II curve jump so sharply around hour 12?
The standard distributions front-load the most intense rainfall near the middle of the storm. For Type II the cumulative fraction rises from 0.283 at hour 11.5 to 0.663 at hour 12.0 — roughly 38% of the entire day’s rain falls in that half hour. This concentrated peak is intentional: it produces a conservative peak discharge for drainage and detention design, but it is steeper than a real storm, which is why short-duration analysis should use IDF curves instead.
Does the rainfall distribution change with return period?
No. The shape of the SCS curve is the same for a 2-year storm as for a 100-year storm — only the total 24-hour depth (P24) changes. You obtain that depth from a rainfall-frequency source such as NOAA Atlas 14 for your location and return period, then apply the dimensionless distribution to spread it over the 24 hours.
Standards & related tools
SCS Unit Hydrograph
Turn a rainfall hyetograph into a runoff hydrograph.
Time of Concentration
Estimate watershed response time for hydrograph timing.
Hydrograph Convolution
Combine rainfall increments with a unit hydrograph response.
Methodology: NRCS TR-55 (1986), Appendix B; NRCS National Engineering Handbook, Part 630; rainfall depths from NOAA Atlas 14.
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Last verified: February 2026