Florida Water Science Center
Tallahassee Region - Duration Plots Based on Mean Daily Flow
Explanation of Duration Graphs
Duration graphs charaterize the state of the hydrology of a stream with respect to time. The plotted percentiles are best explained by an example. Suppose 40 years of daily value flow data exist for a station and the 5-percentile flow is 25 cubic feet per second (cfs) for a single day of the year, say January 3. This means that 5 percent of all flows that occurred on the day January 3 of each of the 40 years of data were equal to or less than 25 cfs. To quantify the state of the hydrology for the current year, the current daily mean flows are compared to these percentiles.
In the duration graphs, the percentiles are smoothed to minimize considerable day-to-day variation. However, the daily minimums and maximums for the period of record are not smoothed.
To interpret the graphs, please consider the following:
1. Although not completely correct in a statistical sense, the non-exceedance of the 5-percentile and the exceedance of the 95-percentile flows may be considered to have a recurrence interval of approximately 20 years.
Likewise, the 10- and 90-percentile flows may be considered to have a recurrence interval of approximately 10 years, and the 25- and 75-percentile flows may be considered to have recurrence intervals of approximately 4 years.
2. In the graphs, it is assumed that "normal flows" lie between the 25- and 75-percentiles, based on the assumption that agriculture, industry, and municipalities can operate normally under high and low flows having a recurrence interval of 4 years.
3. The 7-day minimum flow with a recurrence interval of 10 years is represented on the graph as the 7Q10 line.
4. A line representing 20 percent of the average flow at a station is also included as an indicator of drought severity.
Information Regarding These Data
U.S. Geological Survey
2639 North Monroe St., Suite A-200
Tallahassee, FL. 32303
Phone: (850) 553-3660
FAX: (850) 553-3641