The probable maximum precipitation (PMP) for many stations in Iran and other places using Hershfield formula is routinely estimated as the mean plus 15 standard deviations processed from one-day yearly maximum rainfall values. However, the value of 15 may not be suitable for all stations with different climatic specifications. In this paper, yearly maximum one day rainfalls data of 20-36 years for 30 stations in the Atrak watershed region in northeast of Iran were analyzed in an attempt to estimate PMP for one day duration based on an appropriate frequency factor. Based on the actual maximum daily rainfall data of these stations, the highest value of this frequency factors was found to be 9.63 for one day duration. This frequency factor was subsequently used to estimate one day PMP values. Using these PMP estimates, a generalized pattern for the spatial distributions of one day PMP was demonstrated. It was found that one day PMP over Atrak watershed ranges from 97 to 295 mm and the mean ratio of PMP to the highest observed one day rainfall was about 2.51. The PMP maps are considered important tools to determine reliable and consistent estimates for any location in Atrak watershed for large hydraulic structure designs.

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