Duration graphs characterize the state of the hydrology of a stream with respect to time. The plotted exceedances are best explained by an example. Suppose 40 years of daily value flow data exist for a station and the 90-percent exceedance flow is 25 cubic feet per second (cfs) for a particular day of the year, say January 3. This means that 90 percent of all flows that occurred on the day January 3 of each of the 40 years of data were equal to or greater than 25 cfs. To quantify the state of the hydrology for the current year, the current daily mean flows are compared to these exceedance flows. The 30-day moving average flow graphs highlight trends or patterns in the data. The flow shown for the most recent day is an average of the previous 30 days.
The 10-, 25-, 75-, and 90-percent exceedances for the duration graphs have been smoothed. The 0- and 100-percent exceedances (maximum and minimum flows) have not been smoothed. All exceedance data shown on the 30-day moving average flow graphs have been smoothed.
To interpret the graphs, please consider the following:
1. Although not completely correct in a statistical sense,
the non-exceedance of the 90-percent and the exceedance of the 10-percent flows may be considered to have a recurrence interval of approximately 10 years.It is assumed that flows between the 90- and 100-percent exceedances occur during very dry hydrologic conditions, as shown by the red (lower) band on the duration plot, because these flows occur at intervals longer than approximately 10 years.
2. Likewise, it is assumed that flows between the 0- and 10-percent exceedances occur during very wet hydrologic conditions, as shown by the dark blue (upper) band on the duration 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 gray (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.
The 7-day minimum flow with a recurrence interval of 10 years (7Q10) is often used in regulation of water quality for the state of New York.
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U.S. Department of the Interior, U.S. Geological Survey
Maintainer: New York District URL: http://ny.water.usgs.gov/projects/duration/duration.plots/explanation.html Last update: 16:08:06 Monday 29 April 2002 Privacy Statement || Disclaimer |
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