Determination of rainfall temporal pattern in Kohkiloyeh and Boyerahmad province by Pilgrim method

The first step for design flood estimation of a watershed is determination of the designrainfall characteristics. One of rainfall characteristics is its distribution over the rainfallduration. This is known as the temporal pattern of the design rainfall. In the presentstudy, the data of four synoptic stations in Kohkiloyeh and Boyerahmad province havebeen used to determine the temporal pattern of rainfall, using the Pilgrim (ranking)method. Rainfalls of various durations less than 30 hours were classified into 1, 2, 3, 6,9, 12, 18, and 24 hour durations, and then dimensionless temporal pattern were derived.In ranking method, each event is divided into a number of equal periods. Then, eachindividual burst is ranked according to the amount of rainfall in that period. Thepercentages of rain in all periods could be then ranked. The average percentages ofrainfall among all recorded rainfall are estimated. The results show that, short durationrainfalls have maximum intensity in the second quarter, while longer duration rainfalls(12, 18, 24 hours) have maximum intensities in the third and fourth quarters. Thus, asrainfall duration increases, the maximum rainfall intensity moves towards the secondhalf of time duration.