NRCS / SCS 24-Hour Storm Distributions
The synthetic 24-hour rainfall distributions used to turn a design-storm depth into a hydrograph (TR-20, TR-55, HEC-HMS, HydroCAD). Each is a dimensionless mass curve — fraction of total 24-hour rainfall vs. time — chosen by region.
The Four Distributions
| Type | Climate / character | Peak intensity |
|---|---|---|
| IA | Pacific maritime, mild frontal storms (least intense) | Lowest |
| I | Pacific maritime, slightly more intense than IA | Low |
| III | Gulf / Atlantic coastal, tropical & hurricane systems | High |
| II | Continental US, intense short-duration convective burst | Highest |
All four place the peak rainfall intensity near the 12-hour (midpoint) mark; they differ in how sharply the rain is concentrated around that peak. Type II has the steepest central burst, Type IA the flattest.
Geographic Applicability
| Type | Where it applies |
|---|---|
| IA | Coastal Pacific Northwest (coastal OR, WA), northern coastal CA |
| I | California, western OR/WA (inland of IA zone), Hawaii, Alaska |
| II | Most of the continental US — the default for the interior and East outside the coastal strip |
| III | Gulf Coast and Atlantic coastal strip: Florida, coastal TX, LA, MS, AL, GA, and the Carolinas to the Delmarva |
How It Is Used
Pick the 24-hour depth from NOAA Atlas 14 for the design return period (e.g., 25-yr or 100-yr), select the regional distribution, and run it through the NRCS unit hydrograph in TR-20 / HEC-HMS / HydroCAD. The distribution shape controls peak discharge: the same 24-hour depth on a Type II curve yields a higher peak than on a Type IA curve for the same watershed.
Sources: USDA NRCS, TR-55, Urban Hydrology for Small Watersheds (1986), Appendix B; NRCS NEH Part 630, Chapter 4; NOAA Atlas 14 (hdsc.nws.noaa.gov). Regional distribution adoption varies by state.
Worked example: what the distribution does and doesn’t change
50-ac watershed, CN = 80, 24-hr design depth P = 5.0 in
The distribution changes the peak flow, not the runoff volume. That matters for what you are designing. Pipe and culvert sizing depends on the peak, so the distribution is critical there. A detention pond depends on both: its storage is set mostly by volume, but its outlet is set by the peak it has to reduce. A volume-based water-quality or retention requirement depends on volume, so the distribution barely matters there.
Common mistakes
- Using Type II in a Type III region, or the reverse. The two give different peaks and different hydrograph shapes, so swapping them changes both pipe sizes and pond storage, and not always in the conservative direction. Use the type mapped for the site, or the regional curve that replaced it.
- Applying a 24-hour distribution to a shorter storm. These curves are built for a 24-hour total. A 6-hour design storm needs its own distribution, and the 24-hour curve truncated at 6 hours is not one.
- Using an old 24-hour depth from a manual with a current Atlas 14 regional curve. The curve and the depth should come from the same generation of data.
- Changing the computation time step without checking the peak. A coarse time step, such as 30 minutes, can clip the sharp center of a Type II storm and understate the peak noticeably.
When this card doesn’t apply
Where the state or NRCS has adopted Atlas 14-based regional distributions, they replace the Type curves. Very large watersheds, snowmelt regions and long-duration flood studies use other methods. Urban drainage criteria in some cities specify their own design hyetographs, such as Chicago or alternating-block storms, and those govern within their jurisdiction.
Related cheat sheets and tools
Pair the distribution with a curve number and the NRCS runoff tool, and get the time of concentration for the hydrograph timing — see the Tc methods card. For full 24-hour storm simulation with hydrograph routing, ponds, and BMPs, see HydroComplete.