Specifically, we will use the cumulative distribution function (CDF) of the fitted distribution to calculate the annual exceedance probability (AEP), or the probability that the event is equaled or exceeded in any single year. For example, considering a 200,000 cfs level, the exceedance probability is calculated in the following way:

**Exceedance_P = P{X≥200} = 1 – P{X<200} = 1 – F(200) = 1 – 0.975 = 0.025** *Note: F(200) is the CDF at 200.*

To obtain the return period (also known as the recurrence interval) of the event, we should calculate the reciprocal of the exceedance probability:

**Return_Period = 1 / Exceedance_P = 1 / 0.025 = 40 years.**

The interpretation is that in a very long series, the 40-year flood value would be exceeded every 40 years on the average. For example, about twenty-five 40-year floods can be expected during a 1000 year period (on the average).

Source: http://www.mathwave.com/applications/flood_frequency.html