South Atlantic Water Science Center - North Carolina Office
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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:
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.