Return Periods

What is a return period?

A return period in hydrology is an estimate of how frequent or rare an event is. Usually return periods are recurrance intervals for floods. They do not indicate if floods occur. It is a statistical measurement, ideally based on historical data over an extended period of time. In GEOGloWS Version 2, return periods are based on a historical simulation since 1940. It is important to note that return periods are just estimates. They cannot predict the exact timing of hydrologic events but help understand forecast and retrospective data by providing context for the significance of a high or low flow on a simulation. They can be used to better understand river data so decision makers can appreciate the severity of their river data.