Extreme rainfall (and flood/drought) events are typically of concern in engineering hydrology
Magnitude of an extreme event is inversely related to its frequency of occurrence.
Frequency analysis of historic data relates the magnitude of events to their frequency of occurrence using theory of probability and statistics (mean, standard deviation, coefficient of variation, coefficient of skewness, return period)
Return period (T) of an event is the average time (recurrence interval) between events greater than or equal to a particular magnitude.
For example, 25 year return period storm occurs on average once every 25 years and has a probability of 1/25 of occurring in any one year.
To estimate return period from flood/drought/rainfall records
Select annual maximum/minimum of a particular duration from historical record to form annual maximum/minimum series.
Rank annual maximum/minimum from largest to smallest (or smallest to largest if interested in drought)
Calculating Probabilities of Occurence
What is probability T-year return period will occur once in N years?