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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 10-percentile flow is 25 cubic feet per second (cfs) for a particular day of the year, say January 3. This means that 10 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.

The 10-, 25-, 75-, and 90-percentiles have been smoothed for all stations with more than 20 years of record. For stations with less than 20 years of record, there is no smoothing and only the 0-, 25-, 75-, and 100-percentiles were used. The 0- and 100-percentiles (minimums and maximum flows) have not been smoothed for any stations.

To interpret the graphs, please consider the following:

1. Although not completely correct in a statistical sense, the non-exceedance of the 10-percentile and the exceedance of the 90-percentile flows may be considered to have a recurrence interval of approximately 10 years.

It is assumed that flows between the 0- and 10-percentiles occur during very dry hydrologic conditions, as shown by the yellow (lower) band on the plot, because these flows occur at intervals longer than approximately 10 years.

2. Likewise, it is assumed that flows between the 90- and 100-percentile occur during very wet hydrologic conditions, as shown by the blue (upper) band on the plot, because those flows occur at intervals longer than approximately 10 years.

3. It is assumed that flows between the 25- and 75-percentiles occur during normal hydrologic conditions, as shown by the green (middle) band on the plot. These flows have recurrence intervals less than approximately 4 years. It is assumed that agriculture, industry, and municipalities can operator normally under high and low flows having a recurrence interval less than approximately 4 years.

Duration graphs for unregulated streams more accurately depict the severity of a drought or wet period for natural conditions than duration graphs for regulated streams.

For regulated flows, durations graphs are constructed for both daily mean flows and 7-day running averages of flows. Percentiles computed for regulated flows using daily mean flows may be distorted because of the disparity between weekday flows and weekend flows. Therefore, the duration graphs of 7-day running average flows more accurately characterize the low-flow hydrology of a regulated stream with respect to time. Be advised that rough statistical analyses indicated that some changes did occur in the regulation pattern over the period of data used to compute percentiles for many of the regulated stations.

The 7-day minimum flow with a recurrence interval of 10 years (7Q10) is used in regulation of water quality for the state of Alabama.