SCS Type I, IA, II and III Rainfall Distribution Tables (24-hour)

The four distributions

Type IType IAType IIType III Cumulative fraction of the 24-hour depth.

Type	Climate (TR-55 App. B)	Peak 1-hour share	Peak hour	Fallen by 12:00
I	Pacific maritime; wet winters, dry summers	28 %	09:24 – 10:24	68 %
IA	Pacific maritime, the mildest; Washington to northern California	17 %	07:24 – 08:24	66 %
II	Most of the continental US	45 %	11:24 – 12:24	66 %
III	Gulf of Mexico and Atlantic coast; tropical storms	40 %	11:30 – 12:30	50 %

Each type is a mass curve: it starts at 0 at midnight and reaches 1 at 24:00. Types II and III put the peak at the middle of the day and are almost symmetrical about it, so the rainfall of every shorter duration is centred on hour 12. Types I and IA peak earlier, near hours 8 to 10, and are much flatter. The type controls the shape only; the depth comes from your design rainfall for the 24-hour duration at the required return period.

Hourly cumulative ordinates

Fraction of the 24-hour rainfall that has fallen by each hour. The source tables are at 0.1-hour intervals (241 ordinates per type); interpolate linearly between the hours below, or use the design storm generator, which reads the full tables.

Time (h)	Type I	Type IA	Type II	Type III
0	0.000	0.000	0.000	0.000
1	0.017	0.020	0.011	0.010
2	0.035	0.050	0.022	0.020
3	0.054	0.082	0.035	0.031
4	0.076	0.116	0.048	0.043
5	0.100	0.156	0.063	0.057
6	0.125	0.206	0.080	0.072
7	0.156	0.268	0.099	0.090
8	0.194	0.425	0.120	0.114
9	0.254	0.520	0.147	0.146
10	0.515	0.577	0.181	0.189
11	0.623	0.624	0.235	0.250
12	0.684	0.664	0.663	0.500
13	0.732	0.701	0.772	0.750
14	0.770	0.736	0.820	0.811
15	0.802	0.769	0.854	0.854
16	0.832	0.801	0.880	0.886
17	0.860	0.831	0.902	0.909
18	0.886	0.860	0.921	0.928
19	0.910	0.887	0.938	0.943
20	0.932	0.913	0.952	0.957
21	0.952	0.937	0.965	0.969
22	0.970	0.959	0.977	0.981
23	0.986	0.980	0.989	0.991
24	1.000	1.000	1.000	1.000

From the table to a design storm

For a 24-hour design depth P₂₄, the cumulative depth at time t and the depth in the interval from t to t + Δt are:

P (t) = P_{24} F (t) Δ P = P_{24} [F (t + Δ t) - F (t)]

For example, with P₂₄ = 100 mm and the Type II distribution, 66.3 mm has fallen by 12:00, and the most intense hour, 11:24 to 12:24, brings 45.4 mm. Choose the time step from the catchment's response: NEH Part 630 Chapter 16 suggests about 0.133 times the time of concentration, which for small urban catchments means a few minutes.

Use the storm with the TR-55 calculator for the peak flow and runoff volume, or with a unit hydrograph to route it through a catchment.

Where the tables come from

TR-55 (1986) gives the four distributions only as a graph. The tabulated 0.1-hour ordinates are the ones NRCS distributes with WinTR-55 and WinTR-20; NEH Part 630 Chapter 4 refers to those programs' documentation as the listing. The values here match NEH 630 Figure 4-36 (Type II hourly ratios, 12:00 at 0.663, 20:00 at 0.952) to the three decimals printed. NEH 630 Chapter 4 (2019) recommends the NOAA Atlas 14 regional distributions where they exist; the four legacy types remain the default in much of the world and in older US manuals.

Common questions

Which SCS storm type should I use?

In the United States, TR-55 Appendix B maps the types: I and IA for the Pacific maritime climate with wet winters and dry summers (IA the milder, from Washington to northern California), III for the Gulf of Mexico and Atlantic coastal areas where tropical storms bring large 24-hour depths, and II for the rest of the country. Where NOAA Atlas 14 regional distributions are available, NRCS now recommends those instead (NEH Part 630, Chapter 4).

How do I turn the table into a hyetograph?

Multiply the cumulative fraction at each time by the 24-hour design depth to get the cumulative rainfall, then take the difference between successive times to get the depth in each interval. Dividing by the interval gives the intensity. The design storm generator does this at any time step that is a multiple of 6 minutes.

Why is the Type II storm so intense at hour 12?

The Type II distribution nests the most intense rainfall of every duration from 6 minutes up to 24 hours within one storm, centred on 12:00. So the peak hour holds 45 % of the 24-hour depth. That makes one 24-hour storm critical for catchments of every size, but it is not what a real storm looks like.

Can I use the SCS distributions outside the US?

They are widely used, but they were fitted to US rainfall. Many countries publish their own temporal patterns: Australian Rainfall and Runoff (2019) provides regional ensembles, the UK Flood Estimation Handbook gives summer and winter profiles, and South Africa's SCS-SA method has four regional distribution types of its own. Use the local pattern where one exists.

Sources

USDA Soil Conservation Service (1986) Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55), 2nd ed. Appendix B, Figures B-1 and B-2.

USDA Natural Resources Conservation Service (2019) National Engineering Handbook, Part 630 Hydrology, Chapter 4: Storm Rainfall Depth and Distribution. 210-630-H, Amendment 88.

USDA Natural Resources Conservation Service (2009) WinTR-55 Small Watershed Hydrology, version 2.00: rainfall distribution tables Type I, IA, II and III (0.1-hour increment).

Bonnin, G.M. et al. (2006 onwards) NOAA Atlas 14, Precipitation-Frequency Atlas of the United States. NOAA National Weather Service.

The values are the ones Delta Hydro Studio's calculators use. Last reviewed 9 October 2026. Check them against the design manual your project is reviewed under.

SCS 24-hour rainfall distributions