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 during all years that measurements have been made. In these plots, the
percentiles are based on historic daily values calculated for each month. 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).