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.
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.