Statistical estimation of 24-h probable maximum precipitation using correction of Hirschfield coefficient (case study for Chaharmahal and Bakhtyari Province)

The fountain-head of two main rivers of the country such as Karoon and Zayanderood is located in Chaharmahal and Bakhtiari province, and there are many important dams that located along the rivers, it is essential to know the probable maximum precipitation in the fountain-head of these two rivers. Hershfield method is commonly used for computing probable maximum precipitation (PMP) in most parts of the world for which the frequency factor (Km) is considered to be 15 for computing 24-h PMP (PM 24). This value may not be suitable for different stations under different climatic conditions. In 2001 in Malayza Desa and his colleages introduced a new approach to the Hershfield method. Only the maximum value was considered in this approach, and caused a severe decrease in Km which was more rational. In this paper maximum daily rainfall values for 7 stations located in south-west of Iran with record length of 11 to 50 years was adopted to estimate the appropriate Km values. For the first approach, Km was found to be in the range of 2.2 and 4.3 and the PMP24 was in the range of 138.6 and 410.3 mm. This high range is in part influenced by short record length of the stations. The corresponding values of Km for the second approach were 1.8 and 6.2 and for PMP24 was 101.6 and 269.2 mm. The probable maximum precipitation amount is calculated 101.6 millimeter in Shahrekord station, 269.2 millimeter in Kouhrang, 106.4 millimeter in Borojen, 141.4 millimeter in lordegan, 117.5 millimeter in Boldaji, 174.7 millimeter in EmamGheyse and 127.4 millimeter in PolZamankhan station and actual maximum precipitations in these stations are101.6, 269.2, 106.4, 141.4, 11. , 174.7 and 127.4 milimeter respectively.