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.