Determination of suitable probability distribution function in among frequency distribution models in evaluating occurrences for any phenomena is very important. The gradient ratio is the one of the best important parameter for evaluation of envelope clogging. To conduct this research, data of each replicate was divided into four hourly periodic lengths of 24, 48, 72, and 96. Then, with analysis of data in time series according to HYFA software output and goodness fit of Relative Residual Square Mean, suitable frequency distribution function for each envelope was selected. Based on relative frequency, the best fitted distribution was found to be Three Parameter Log Normal distribution (Moment method) and Two Parameter Log Normal distribution (maximum likelihood method) for synthetic envelope of PP450, Normal distribution (Moment method and maximum likelihood method) for synthetic envelope of PP700, Three Parameter Log Normal distribution (Moment method) and Two Parameter Log Normal distribution (maximum likelihood method) for synthetic envelope of PP900 and Pearson type III distribution (Moment method) and Normal distribution maximum likelihood method) for envelope of gravel)
