Drought is one of the most complex phenomena in the world, so proper management is very important in monitoring and reducing its damage. For this purpose, Standard Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI) and Reconnaissance Drought Index (RDI) indices were used to analyze the intensity and frequency of drought in the coastal wet, mountain, semi mountain, semi desert, desert and coastal desert climates of Iran in four seasons, separately: autumn, winter, spring and summer. 43 synoptic stations with a common statistical period of 50 years (1969-2019) were selected. The results showed that the trend of drought in winter and summer is increasing in all studied climates. The comparison of the results in the trend analysis of the drought showed the same trend, but the SPEI index compared to the other indicators showed a quicker response to changes in drier climates. The highest correlation (0.80-0.99) between SPI-RDI and SPEI-RDI indices in coastal desert, mountain and semi mountain climates and the lowest correlation (0.34) between SPI-SPEI and SPEI-RDI indices in semi desert, desert and coastal desert climates were obtained. SPI-RDI variations showed similar values in colder climates. The SPEI is based on precipitation and temperature data, and it has the advantage of combining multi-scalar character with the capacity to include the effects of temperature variability in the drought assessment. Thus, SPEI is recommended as a suitable index for studying and identifying the effect of climate change on drought conditions.
