A percentile is a value on a scale of one hundred that indicates the percent of
a distribution that is equal to or below it. For example, on the map of daily
streamflow conditions a river discharge at the 90th percentile is equal to or
greater than 90 percent of the discharge values recorded on this day of the year
during all years that measurements have been made. In general,
a percentile greater than 75 is considered above normal
a percentile between 25 and 75 is considered normal
a percentile less than 25 is considered below normal
In
some hydrological studies, particularly those related to floods, a variation of
the percentile known as the "percent exceedance" is used. It is simply
obtained by subtracting the percentile scale value from 100 percent.
For example, a discharge at the 75th percentile is the same as a discharge at
the 25th percent exceedance (100-75=25).
U.S. Department of the Interior, U.S. Geological Survey, Albuquerque, NM, USA
URL: http://nm.water.usgs.gov/drought/percentile.htm
Contact: webmaster@nm.water.usgs.gov
Last modification: 7-Jan-2004
