The return period (or recurrence interval) is a statistical concept indicating the average time between extreme events of equal or greater magnitude. An event with a 100-year return period has a 1 % probability of occurring in any given year — it does not mean it occurs exactly every 100 years, but that its annual probability is 1/100.
Calculation and application
It is estimated from historical data series (precipitation, river flow, temperature, wind) using extreme value distributions such as Gumbel, GEV, or Log-Pearson III. The reliability of the estimate depends on the series length: to estimate a 500-year return period, records of over 50-100 years are ideally needed.
In engineering, the return period determines infrastructure design: urban drainage is designed for 10-25 year events, bridges for 100-500 years, and dams for 1,000-10,000 years. In Spain, after the catastrophic 2024 Valencia DANA, debate has intensified over whether Mediterranean infrastructure is designed for sufficient return periods.
Climate change complicates this concept: events that historically had a 100-year return period may now be occurring every 20-50 years due to intensification of the hydrological cycle. This forces revision of IDF (Intensity-Duration-Frequency) curves at all stations. See also: heat wave.