frequency

For a low-water flow lower or equal to a given value "a", the ratio between the number of observations made in a series of flows (usually monthly), where the flow remains below this value "a", and the total number of observations in this series (FEtiage = number of observations with Q ? a / total number of observations). For a flood flow rate greater than or equal to a given value "b", the frequency is the ratio between the number of observations made in a series of flows (usually monthly), where the flow exceeds this value "b", and the total number of observations in this series (FCrue = number of observations with Q ? b / total number of observations). These two limit values "a" and "b" are not identical. For the frequency to be reliable, the series must have at least 30 observations.