Statistical estimation of the maximum possible 24-hour rainfall in the Bakhtegan catchment area

The greatest depth of precipitation possible for a given duration is called probable maximum precipitation denoted by PMP. Probable maximum precipitation is generally evaluated using two meteorological and statistical methods. In this research, the Hershfield's statistical methods and conventional torque method were used to evaluate probable maximum precipitation of Bakhtegan Watershed. In Hershfield's equation, the frequency factor was considered to be constant at 15. Since this factor might not be consistent to all the stations in different climatic conditions, then, in this study, the frequency factor in Bakhtegan Watershed was obtained based on the maximum precipitation of 24h, at 35 meteorological stations in three statistical Hershfield's methods І , Hershfield's ІІ, Hershfield's ІІ with eliminated outlier data. Afterwards, the probable maximum precipitation of 24h in Watershed's stations were analyzed and calculated for all three methods. In this study, the conventional torque method was also used for 17 stations having more than 30 years of precipitation statistics. In this method, after performing data homogeneity test (Mann-Kendall) and removing outlier data, maximum precipitation amount was performed for normal distribution, two-parameter lognormal, three-parameter lognormal, Pearson type-III, Log-Pearson type-III and Gumbel was using SMADA software; and the best frequency distribution was obtained using RMSE test for each station. Using RRMSE test for all stations, it is found that Gumbel distribution was the most appropriate one; by the use of Gumbel distribution, 24-h PMP with various return period is obtained for each station. In addition, for each approaches, a maximum 24-h PMP watershed precipitation was plotted to show the spatial distribution.