Modification of standardized precipitation index in different climates of Iran

The most popular metric for drought monitoring is the standardized precipitation index (SPI). It does have some drawbacks, though, such as using gamma probability distribution function as the default distribution and not taking into account seasonal variations. The effectiveness of SPI in monitoring droughts in various Iranian climates was examined in this study, and in order to address its limitations, it was contrasted with the modified SPI (mod-SPI). Then, the performance of the mod-SPI and SPI in drought monitoring was compared with nine meteorological stations throughout Iran during the period 1956–2010. The results showed that the gamma distribution function was not selected in any of the 12 timescales (TS) considered at the studied meteorological stations. Based on the Cohen's kappa index analysis, there is a clear difference between the SPI and mod-SPI in monitoring drought classes, which is more evident at low TS (less than 6 months), but at much higher TS (above TS-9), the two indices almost coincide. The results showed that in the arid and semiarid areas, the SPI and mod-SPI are significantly different, so it seems that the SPI monitors aridity rather than drought. The results imply that using mod-SPI instead of SPI provides more accurate drought monitoring by eliminating the seasonal effects of precipitation data. Considering the low efficiency of SPI in short-term drought monitoring (less than 9 months) in arid and semiarid regions, it is recommended to use mod-SPI instead of SPI in most parts of Iran, especially on the southern coasts of Iran.