Investigation of Methods for Determining the Regional Skewness Coefficient of the Maximum Discharge in East Azarbaijan province

To determine the flow rate with different return periods, it is necessary to use the skewness coefficient with acceptable precision. Estimation of population skewness in different regions is improved by different methods such as weighted skewness and generalized skewness. The main objective of the present research is to determine the coefficients of skewness using different methods and present the best method for determining the mentioned coefficients in East Azerbaijan, Iran. East Azerbaijan has three different hydrologic regions. The north of the province is a part of Aras catchment area, the center of the province is a part of Urmia lake watershed and a part of the south of the province is a part of Sefidrood catchment area. In this research, different generalized skewness coefficients methods were used. In the present study, the generalized skewness coefficients were estimated using four methods including generalized skewness coefficient without considering hydrological regions, generalized skewness coefficient with accounting hydrological regions, unbiased skewness coefficient and weighted skewness coefficient for three hydrological regions and 62 observed stations during studied period. Based on the obtained results and on the basis of the analysis of RMSE values for different methods, the method of weighting skewness coefficient can be suggested as the selected method in East Azerbaijan with an average value of 0.104. Also, the results of calculating the Nash-Shutcliffe and the square of the mean squares of relative error coefficients showed that the weighting method is the best method for calculating the skewness coefficient.