# How to Develop IDF Curves

The Intensity-Duration-Frequency Curve, or IDF Curve, is used in the fields of meteorology and hydrology to estimate the peak rainfall rate, or intensity, during a storm event. The peak rainfall rate experienced varies with the duration of the storm and the location of the storm on the earth's surface. The probability of a certain rainfall rate occurring in a particular place and time can also be determined. This information is used to construct a curve showing the average rainfall intensity during any given length of storm, for a storm with a particular probability of occurring during a given year.

Determine the location for the IDF curve on a map. The IDF curve information varies by location, and is usually computed down to the county level, but information at a finer level may be available. Also determine the storm probability for which you will be computing the IDF curve. Most data is available for the 50 percent and the 1 percent probabilities, and is computed for other probabilities based on the 50 percent and 1 percent storms.

Be aware that the storm probabilities are typically stated as "Return Frequencies", which are the reciprocal of the probability. For example, the 50 percent probable storm is called the "two-year storm" and the 1 percent probable storm is called the "100-year storm." Other storms typically computed are the five-, 10-, 25-, and 50-year storms.

See the references at the end of the article for maps of storm intensities for different durations and probabilities.

Consult the maps for the storm return period you are interested in. For the majority of the United States, the HYDRO-35 mapping provides sufficient information to compute the IDF curve. Using the HYDRO-35 mapping, then, as an example, find your position on the map.

For each of the five-, 15-, and 60-minute storm durations, determine the precipitation depth. Do this for the two- and 100-year storms. The depth on the map is shown as isopluvial lines, which are labeled with the depth in inches. If your location is between the lines, interpolate the depth to an accuracy of two decimal places.

Compute the precipitation depths for the intermediate return periods of five-, 10-, 25- and 50-year storms using the formulas nine through 12 in the HYDRO-35 publication. Do this for the five-, 15-, and 60-minute storm durations.

Compute the precipitation depth for the 10- and 30-minute durations for each of the return periods. To do this, use the formulas seven and eight as provided in the HYDRO-35 publication. These formulas are generally valid for all parts of the country, so they can be used for other maps besides HYDRO-35.

Plot the information of precipitation depth on the y-axis of the graph paper versus the storm duration in minutes on the x-axis of the graph paper. Connect the plotted points with a smooth curve, beginning at the five-minute point and ending at the 60-minute point. Plot separate curves on the graph paper for each of the return periods you have computed.

You can now use the curves to predict the precipitation depth for any storm duration from five minutes to 60 minutes.