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چکیده
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The Standardized Precipitation Index (SPI) is widely used as a drought meteorological index, to identify drought characteristics including number of events, frequency, duration, and intensity. In the present study the SPI is used to detect drought characteristics in spatial and temporal patterns at various time scales of 1, 3, 6, 9, 12, 18, 24 and 48-months. For this purpose, monthly climatic data from43 synoptic stations during 1969-2019 were analysed from Iran’s various climates (coastal wet, mountain, semi mountain, semi desert, desert and coastal desert). Abrupt and gradual changes in SPI1-48 were detected using the non-parametric Pettitt test and Mann–Kendall (M-K) test separately in the various climates. The M-K results show that a trend was increasingly significant at longer time scales, with the trend of SPI-1 in climates not being significant, while SPI-48 was significant at P less than 0.01 (P<0.01) at almost all stations in all climates. According to Pettitt’s test results, the changing points of SPI vary between climates and start from Sep-92 (SPI1 in semi mountain climates) to Jan-04 (SPI48 in desert climates). These results indicate that abrupt changes in the drought index based on SPI, started first in semi mountain and mountain climates in the 1990s and then progressed towards the semi desert and desert climates in the 2000s. The number of drought events changed from 7 in coastal desert stations (Chabahar) to 52 occurrences in coastal wet stations (Bandar Anzali). Spatial distribution of drought intensity shows that drought severity is greater in coastal wet climates than in desert climates. Drought frequency varied from 15.2 to 16.3 % at timescales ranging from 1 to 48 months. The mean drought duration was from 12.9 months in semi mountain to 15 months in coastal deserts for SPI1-48. Generally, the study demonstrates the need for more research to select proper time scales for drought indices which are related to climate conditions of given areas.
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