Evaluation of Common Statistical Distribution Functions and the Calculation Methods of Their Parameters in Order to Estimate Probability of Hydrological Drought Events (Case Study: West of Urmia Lake Basin)

In most cases, it is observed that the statistical parameters values are not coincident with the moments of observed data moments. In the present study the data of water deficit in dry spells were extracted for the nine rivers located in the west basins of Lake Urmia from the daily flow rates data for sixty-day durations and these calculated data were used to the frequency analysis. In order to estimate the hydrological drought probability in the watersheds of the mentioned rivers, 12 common statistical probability distributions were tested. Using the Chi-square and the coefficient of determination tests between the observed and modeled data, the Log-Pearson type 3 was selected as the most suitable model among the used ones. In order to estimate the parameters of the selected distribution, the results of the methods namely maximum likelihood, ordinary moments, sundry average method and applied moment method based on the logarithms of the observations, were compared. The results showed that the accuracy of the sundry average method was more than those of the other used methods. The average value of the root mean square error statistic for the mentioned method was 0.855 m3/s.