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.

GEOGloWS services display symbology based on precalculated return periods for all river reaches. The return periods were calculated using the Gumbell type I distribution and the 80+ year historical data. The return period provides an easy way to recognize in the map or graph how high the flow is relative to normal and historic flow. They are available for 2, 5, 10, 25, 50, and 100 year intervals.