Oklahoma Climatological Survey

Determining Return Periods
for Rainfall

It is useful to develop some rules-of-thumb as a first-guess as to how much rain falling in a given time period will produce stream flooding. For these estimates, hydrologists use Return Periods. Simply stated, a return period is an estimate of how long it will be between rainfall events of a given magnitude. For example, the return period for a 3-hour rainfall total of 4.25 inches in Oklahoma City is 25 years. This means that, on the average, a 4.25 inch rainfall over three hours occurs in Oklahoma City once every 25 years.

It is important to remember that this is based on statistics - there is no guarantee that once you have had this much rain it will not happen again for another 25 years. If you had 100 years of records, there should be four such rainfall events (an average once every 25 years). These events could be in consecutive years or even multiple events in a single year! There is also no guarantee that a nearby location will not experience such an event.

A general rule-of-thumb for stream flooding is that a 3-hour rainfall total exceeding the 10-year return period is likely to cause flooding. Whether or not a stream floods from the given rainfall amount (or less) is dependent upon a number of factors other than rainfall amount. Antecedent soil moisture, prior stream stage, whether tributaries are already near flooding, urbanization, or basin characteristics can affect the likelihood of flooding.


The image above depicts a map of rainfall for the 10-year, 3-hour return period for the southern U.S. The contour lines show how much rain is expected for points through which the lines pass. To estimate this threshold at a point in between the lines, you must interpolate. To do this:

1. Locate the contour lines on either side of your location. The rainfall amounts are given at each end of the line.

2. Estimate how far you are between the contours from the lower contour line. For example, Oklahoma City is about 4/5 (.8) of the way from the 3 to the 3.5 inch contour.

3. Calculate the difference in rainfall total between the two lines. For example, the contours on either side of Oklahoma City are 3 and 3.5 inches, so the difference is 3.5 - 3.0 = 0.5 inch.

4. Multiply the rainfall difference by the distance you calculated for your location. For Oklahoma City, this would be 0.5 inch x 0.8 = 0.4 inch (note: this number should be rounded to the tenth of an inch because the maps are only an approximation)

5. Add the result to the lower contour's rainfall total. For this example, you would add 0.4 inch to the 3 inch contour, which yields 3.4 inches.

Therefore, a 3-hour rainfall total of 3.4 inches can be expected in Oklahoma City once every ten years. Rainfall totals of this amount or higher are likely to cause stream flooding.


Find the 10-year 3-hour rainfall total for your area of responsibility using the steps in the example above.




  1. How is it possible to have an event with a 50-year return period occur three times in one year?


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