What is an intensity-duration-frequency (IDF) curve?
In short: An intensity-duration-frequency (IDF) curve relates three quantities: rainfall intensity, storm duration, and event frequency, usually expressed as a return period. It helps engineers estimate design rainfall for drainage, culverts, storm sewers, detention systems, and flood-risk studies.
How to read an IDF curve
Each line normally represents one return period, such as 2, 10, 50, or 100 years. The horizontal axis is rainfall duration and the vertical axis is intensity. Select a duration and read upward to the required return-period line to obtain the fitted design intensity. Short-duration rainfall is usually more intense than long-duration rainfall at the same return period.
How an IDF curve is calculated
The analysis first forms a maximum rainfall series for each duration. A probability distribution or empirical plotting-position method is then applied to estimate the rainfall quantile associated with each return period. Because these values are statistical estimates rather than guaranteed future events, distribution choice, record length, missing data, and extrapolation all affect uncertainty.
See the IDF Curve Tool tutorial for the workflow and the IDF formulas for the aggregation, frequency-analysis, confidence-band, and equation-fitting methods.
Use the IDF Curve Tool to calculate and export rainfall IDF curves.