Presentation of precipitation data, IDF curve

Ways of presenting precipitation data

Drawing an IDF curve

1. Collect long-term rainfall records from an autographic (recording) gauge.
2. For each year, find the maximum rainfall depth for durations such as 5, 10, 15, 30, 60, 120, 360 and 1440 min (annual maximum series).
3. For each duration, fit a probability distribution (Gumbel, log-Pearson III) and get the depth for chosen return periods (2, 5, 10, 25, 50, 100 years).
4. Convert depth to intensity: $i = P/D$.
5. Plot intensity (y, usually log scale) against duration (x, log scale) with one curve for each return period.
6. If needed, fit $i = \dfrac{K T^x}{(D+a)^n}$.

 i
  |\
  | \\   100-yr
  |   \\\  25-yr
  |      \\\ 5-yr
  +---------------- D

The curve is read for a design duration (usually the time of concentration) and chosen return period to get the design intensity.