عنوان مقاله [English]
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 (PMP24). This value may not be suitable for different stations under different climatic conditions. In 1986 the world meteorological organization standardized the Hershfield method and introduced the record length and the maximum value observed for the correction of the Km. In 2001 in Malazia Desa et al. 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 46 raingage stations located in north-east of Iran with record length of 13 to 48 years was adopted to estimate the appropriate Km values. For the first approach, Km was found to be varied in the range of 16.7 and 19.5 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 was 1.8 and 6.2 and for PMP24 was 62.7 and 207.7 mm. Numerous data in the literature supported that the ratio of PMP24 to maximum of p24, as a criteria independent of climatic conditions, was consistent for the second approach and was in harmony with multi-station method that uses all of the data at a time. Based on the literature, it was shown that ratio of statistical PMP24 to synoptic PMP24 is closer to 1 for the second approach. Finally, the map of PMP24 isochrones was prepared for the second approach which was assumed to be a sound estimation of PMP24 for the north-east ofIran. This map may be in use for runoff control of rivers which are drained either to other countries or to the interior deserts.