Hydrological Drought Index Correction Based on Determining the Most Appropriate Probability Distribution

Streamflow Drought Index (SDI) is based on the assumption that the flow data series follows from Gamma distribution. In this study, the efficiency of 65 statistical distributions was evaluated through the use of monthly and annual flow data series of 49 hydrometric stations. The results show that on the annual scale, Gamma distribution is not the superior distribution. On a monthly scale, this distribution was found only in 1% of all cases examined as the superior distribution. This is while the Wakeby distribution was introduced on an annual basis in about 35 percent of the cases, and on a monthly scale, up to 43 percent of the scenarios were introduced as a top-ranked distribution. Based on the SDI index calculated based on Gamma distribution and Wakeby distribution, in the case of the P-Value of the Gamma distribution is less than 0.3, it is possible to move the drought classes using Wakeby distribution. These conditions were observed in 30% of the examined cases. The highest and the lowest displacements of the hydrological drought classes due to the use of superior distribution were in December and May, respectively. It was found that the highest frequency of displacement of the drought classes resulted from the use of superior distribution occurs in the normal class. Accordingly, it is recommended that the Wakeby distribution be replaced by the gamma distribution in the SDI calculations.