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| Duration Graphs for Selected North Carolina Stations |
Explanation of Duration Graphs
The duration graphs characterize a stream's state of flow conditions with respect to time using percentiles computed for each date of the year. For example, if a gaging station has 40 years of daily flow data and the 10-percentile flow is 25 cubic feet per second (cfs) for a given date of the year, then 10 percent of all flows that occurred that same date in 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 running 7-day average flows are compared to these percentiles. The 10-, 25-, 75-, and 90-percentiles have been smoothed; the 0- and 100-percentiles (minimums and maximum flows, respectively) have not been smoothed.
To interpret the graphs, please consider the following:
- 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 lower band on the graphs, because these flows occur at intervals longer than approximately 10 years. In a similar manner, it is assumed that flows between the 90- and 100-percentile occur during very wet hydrologic conditions, as shown by the upper band on the graphs, because those flows occur at intervals longer than approximately 10 years.
- It is also assumed that flows between the 25- and 75-percentiles occur during normal hydrologic conditions, as shown by the middle band on the graphs. These flows have recurrence intervals less than approximately 4 years; it is assumed that agriculture, industry, and municipalities can operate 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.
