Hydrosphere's Exceedances program is a Windows program that calculates an exceedance frequency table for streamflow or other data. Exceedances operates directly against any Hydrodata USGS Daily Values or NCDC Summary of the Day disc, or against a user-supplied text file of data values. The Windows interface allows specification of the station, parameter and period of record for analysis.
The results include an exceedance table, the maximum, minimum and average values across the analysis period and a count of zero values contained in the period. The results can be saved to a file, copied via the clipboard to other programs, or graphed directly within the Exceedances program.
User-specified parameters include:
The program can be run interactively, entering stations from the application, or in a batch mode where numerous runs may be specified in a file that is read into the application.
The exceedance values are calculated using the Weibul formula to determine the plotting position and then locating (interpolating if necessary) the
data value at that position
P = M / (N + 1)
where N is the number of values and P is the position of the value in the list of values ordered by decreasing value.
We do not report absolute 0 and 100 percentile rankings. Instead, we compute the 1 / (N + 1) for the lowest percentile, and N / (N + 1) for the highest. The value assigned to the lowest percentile is the minimum value in the dataset. The value assigned to the highest percentile is the maximum value in the dataset.
Summary statistics such as the maximum value and the date on which it occurred, the minimum value and the date on which it occurred, and the average value are reported, as well as a count of values included in the analysis, a count of missing values that are excluded from the analysis, and the count of zero value encountered, which may or may not be included, depending on the specification for handling of zeros when the analysis was run.
Download the User's Manual as a Help File (88,094 bytes)
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