A flow duration analysis graph, as shown here, is helpful in determining how often a certain discharge occurs. This type of information may be helpful in estimating the size of a retention basin based on the runoff from an area.
For this location, the graph shows that flows of six units or greater happen 50 percent of the time. For a higher discharge, flows of 100 units or greater happen only about two percent of the time.
As mentioned before, this analysis, while useful to the hydrologist, is not a forecasting tool.