Design Rainfall — NOAA Atlas 14 / IDF Reference
Where design storm depths come from, which precipitation-frequency source covers your state, and how to move between depth, duration, and intensity without the classic mistakes.
The Workflow
| Step | What to do |
|---|---|
| 1 | Open the PFDS (hdsc.nws.noaa.gov/pfds), click the site or enter lat/lon. |
| 2 | Read the depth-duration-frequency (DDF) table — depths by duration (5 min–60 day) and ARI (1–1000 yr), with 90% bounds. |
| 3 | Rational Method: read depth at D = tc, convert i = P/D. NRCS methods: take the 24-hr depth and shape it with the applicable storm distribution. |
| 4 | Record station/grid, volume, and retrieval date in the drainage report — estimates change when volumes are updated. |
Coverage by Source
| Region | Source |
|---|---|
| Ohio Valley & mid-Atlantic/upper Southeast (NC, SC, VA, WV, TN, KY, OH, IN, IL, PA, NJ, MD, DE, DC) | Atlas 14 Vol. 2 (2004) |
| Deep Southeast (AL, AR, FL, GA, LA, MS) | Atlas 14 Vol. 9 (2013) |
| Midwest (CO, IA, KS, MI, MN, MO, ND, NE, OK, SD, WI) | Atlas 14 Vol. 8 (2013) |
| Northeast (New England, NY) | Atlas 14 Vol. 10 (2015) |
| Texas | Atlas 14 Vol. 11 (2018) |
| Semiarid Southwest (AZ, NV, NM, UT), California, Alaska, islands | Atlas 14 Vols. 1, 6, 7, 3–5 |
| WA, OR, ID, MT, WY | NOAA Atlas 2 (1973) + state studies |
NOAA Atlas 15 (in development) will supersede all of the above with nationwide, climate-informed estimates — watch the governing agency for adoption.
Depth ↔ Intensity
| Duration | Example depth P | Intensity i |
|---|---|---|
| 5 min (0.083 hr) | 0.45 in | 5.4 in/hr |
| 15 min (0.25 hr) | 0.90 in | 3.6 in/hr |
| 1 hr | 1.8 in | 1.8 in/hr |
| 24 hr | 4.8 in | 0.2 in/hr |
Illustrative depths for a single hypothetical return period, to show the duration effect — pull the actual values for your site from PFDS. The 24-hr average intensity is useless for Rational Method work; the short-duration cell is where storm sewers live.
Sources: NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Vols. 1–11, NWS Hydrometeorological Design Studies Center; PFDS at hdsc.nws.noaa.gov. NRCS National Engineering Handbook Part 630 Ch. 4 (storm distributions). NOAA Atlas 2 (1973) for the Northwest gap.
Worked example: picking the right intensity
Rational Method, 3.0 ac, C = 0.60, tc = 15 min, using the illustrative depths in the table above
The depths in this example are the illustrative ones from the table, not real data for any site. Pull your own from PFDS. The method carries over unchanged. Depth-duration curves are close to straight on log-log axes, so interpolate in logs if tc falls between tabulated durations. Some agencies require a minimum tc, often 5 or 10 minutes. Use the larger of the computed and the minimum value.
Common mistakes
- Mixing partial-duration and annual-maximum series. PFDS publishes both. Most design criteria assume the partial-duration series for frequent storms, and the two differ noticeably below the 10-year event. Use the series your manual specifies.
- Reading the upper 90% bound as the design value. The bounds express uncertainty in the estimate. The design value is the middle number unless a regulation says otherwise.
- Using a point depth over a large watershed. Atlas 14 values are point estimates. Over large drainage areas, an areal reduction factor applies (HEC-HMS and NRCS methods describe it). For small urban sites the point value is used as-is.
- Leaving the volume and retrieval date out of the report. PFDS values are revised as new volumes are published, and a reviewer needs to be able to reproduce the number.
When this card doesn’t apply
Where a state or local agency publishes its own design rainfall, that source governs, even if it differs from current PFDS. Some states still require older IDF curves, and some require future-climate multipliers on top of Atlas 14. Probable Maximum Precipitation for dam spillway design comes from HMR studies or state-specific PMP studies, not from Atlas 14. Once Atlas 15 is published and adopted, it replaces the volumes listed here.
Related cheat sheets and tools
Pair with the SCS storm distributions card, the tc methods card, and the composite curve number tool for the full NRCS chain.