Comparison of Two Procedures for Calculation of the Standardized Precipitation Index (SPI) in Dry and Wet Climates of Iran

One of the most important and widely used indices for drought monitoring، especially in our country، is the Standardized Precipitation Index (SPI). This index is the basis for drought definition and monitoring in most of studies. In this study، two procedures were used for the calculation of SPI. In the first procedures، with the hypothesis that the precipitation data follow the gamma distribution and in the second، using the optimum distribution، the SPI series in three time scales of 3، 6 and 12 months were calculated. In the next step، the t-student test was applied to consider significance differences in the results of two procedures. In three time scales، on average، the frequency of significant difference in the results of two procedures was maximum in normal class and thereafter، across the wet classes، extreme wet class and mild wet class and across the drought classes، mild drought class and moderate drought class، had the most frequency of significant differences. The results of the study showed that the calculation of the SPI based on gamma distribution give rise to the wrong diagnosis of the SPI classes. Then، calculation of SPI on the basis of the optimum distributions instead of blindly using of gamma distribution is proposed، in order to more accurate drought monitoring for the suitable management of drought.