A descriptor of flood magnitude. If there is a 20% chance (that is a one-in-five chance) of flow exceeding 100 m3/s occurring in any one year, then 100 m3/s is termed a 1 in 5 year Annual Exceedence Probability flow. One in five year is not the interval between floods, instead it is the chance of the flood exceeding this flow in any given year. So 1 in 5 year floods can happen two years in a row or twice in any given year. It is based on past data, so the accuracy is unknown for existing and future climates.