Accurate rainfall data is the foundation of every drainage design. The design rainfall intensity or depth determines how large your pipes, channels, and detention facilities need to be. Using the wrong data can lead to undersized systems that flood or oversized systems that waste money.
NOAA Atlas 14 — Primary US Source
NOAA Atlas 14 is the current authoritative source for precipitation frequency estimates in the United States. It provides point-specific rainfall data for durations from 5 minutes to 60 days and return periods from 1 year to 1,000 years.
Access: NOAA Precipitation Frequency Data Server (PFDS)
hdsc.nws.noaa.gov/pfds/
Enter coordinates, an address, or click on the interactive map to get precipitation frequency estimates for any US location. Results include 90% confidence intervals.
Types of Rainfall Data
Precipitation Frequency Estimates
The total depth of rainfall for a given duration and return period. For example, the "10-year, 24-hour" precipitation is the 24-hour rainfall depth that has a 10% chance of being exceeded in any given year.
| Term | Definition |
|---|---|
| Return Period | Average time between events of equal or greater magnitude (e.g., 10-year, 100-year) |
| Annual Exceedance Probability (AEP) | Probability of exceedance in any given year. AEP = 1 / Return Period |
| Duration | Time period over which rainfall is accumulated (5 min to 60 days) |
| Intensity | Rainfall rate = Depth / Duration (typically in/hr) |
IDF Curves
Intensity-Duration-Frequency curves plot rainfall intensity vs. duration for multiple return periods. These are derived from precipitation frequency estimates and are the most common format used with the Rational Method.
See our IDF Curves reference page for details on reading and using IDF data.
Temporal Distributions
For hydrograph-based methods (SCS, unit hydrograph), you need to know not just the total rainfall but how it's distributed over time. Common distributions include:
- NRCS Type I: Pacific maritime climate (Pacific Northwest coast)
- NRCS Type IA: Pacific maritime interior
- NRCS Type II: Most of the continental US (thunderstorm-dominated)
- NRCS Type III: Gulf Coast and Atlantic coastal areas
Return Period and AEP Conversion
| Return Period (years) | AEP (%) | Common Use |
|---|---|---|
| 1 | 100% | Water quality / first flush |
| 2 | 50% | Minor drainage, channel forming flow |
| 5 | 20% | Minor storm drainage |
| 10 | 10% | Storm sewers, minor conveyance |
| 25 | 4% | Major storm drains, culverts |
| 50 | 2% | Major bridges, critical infrastructure |
| 100 | 1% | FEMA floodplains, building drainage |
| 500 | 0.2% | FEMA 500-year floodplain, dam safety |
Data Sources by Region
| Region | Primary Source | Atlas 14 Volume |
|---|---|---|
| Eastern US (East of 105°W) | NOAA Atlas 14 | Volumes 1–3, 9–10 |
| Central US | NOAA Atlas 14 | Volumes 4, 8 |
| Western US | NOAA Atlas 14 | Volumes 5–7, 11 |
| Pacific Islands | NOAA Atlas 14 | Volume 5 |
| Alaska | TP-47 (being updated) | N/A |
| Canada | IDF_CC Tool (Environment Canada) | N/A |
Important Design Notes
- • Climate change: Historical rainfall records may not represent future conditions. Some agencies now require climate-adjusted rainfall data or additional factors of safety.
- • Local requirements: Always verify which data source and edition your local reviewing agency requires. Some jurisdictions have not yet adopted Atlas 14 and still reference TP-40 or older data.
- • Areal reduction: Atlas 14 provides point estimates. For large drainage areas (> 10 sq mi), apply an areal reduction factor to account for the spatial variability of rainfall.
- • Partial duration vs. annual maximum: Atlas 14 uses partial duration series for return periods ≤ 10 years and annual maximum series for longer return periods. This is standard practice and produces slightly higher values for frequent events.
Primary Sources
- NOAA (2004–2022). Atlas 14: Precipitation-Frequency Atlas of the United States, Volumes 1–11. National Weather Service, National Oceanic and Atmospheric Administration.
- Hershfield, D.M. (1961). Rainfall Frequency Atlas of the United States (Technical Paper No. 40). U.S. Weather Bureau.
- NRCS (1986). Urban Hydrology for Small Watersheds (Technical Release 55). U.S. Department of Agriculture.
- FHWA HEC-22 (2009). Urban Drainage Design Manual, 3rd Edition.