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Duration Plots 


Water-Level-Duration Plot description


Water-Level-duration plots depict specific water-level-statistic values selected from water-level-duration curves. The water-level-duration curve is a cumulative frequency curve that shows the percent of values that specified water levels were equaled or less than (percentile), or the percent of values that specified water-levels were equaled or exceeded (percent exceedance). Percent exceedance is 100 minus the percentile. Example: the 75th percent exceedance (the value that 75 percent of the water levels are equal to or greater than) is the 25th percentile (the value that 25 percent of the water-levels are equal to or less than). While the water-level-duration curve does not show the chronological sequence of water levels, it is an important means of representing water-level characteristics of an aquifer throughout the range of water levels on a single curve.

The water-level-duration plots presented here represent the water-level-duration curves of daily water-level data grouped by months. All the daily water-level values for each month are arranged according to magnitude and the percentile for each daily water-level value is computed. The value for a specific percentile is determined by interpolation between the computed values. From the data, the 5th, 10th, 25th, 50th, 75th, 90th and 95th percentiles are determined and plotted. Colors are used to separate above normal (blue), normal (green), and below normal (brown) conditions. The maximum and minimum daily values for each month are selected from the data set. The instantaneous minimum is determined from historic flow records.



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