Journal of Watershed Management Research

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nadi M, Shiukhy Soqanloo S. (2020). Comparison of SPI and SPImod in Drought Monitoring of Several Climatic Samples of Iran. jwmr. 11(21), 108-118. doi:10.52547/jwmr.11.21.108
URL: http://jwmr.sanru.ac.ir/article-1-993-en.html

   Drought study and forecasting requires accurate monitoring of this climate risk. Standard precipitation index (SPI) is the most common index in drought monitoring. But there are some shortcomings, such as the lack of seasonal changes, the problem of self-affiliation in the high time series, and the consideration of the gamma's distribution function for all-time series. In this research, to resolve the bugs of SPI, it was tried to first select the appropriate distribution of precipitation from 8 different distributions. Secondly, the modified standard precipitation index (SPI_mod) was used to resolve the seasonal variation problem. The SPImod and SPI indexes were compared in five stations from different parts of Iran with different temperature and humidity conditions during the 55-year period (1956-2010). The results of the most suitable probability distribution function of precipitation data showed that the gamma function was not one of the selected functions in any of the cases and that the General-pareto, Weibull, and Generalized extreme value functions were the most suitable fitted distribution functions at the stations under study. Comparison of SPI and SPI_mod correlation coefficient showed that increasing the time series, increases the correlation coefficients. In the wet areas, the two indexes are more similar to each other. Based on the findings, there was a significant difference between SPI and SPI_mod at all stations, which was less significant at time series than one, three, and six months at a much higher level than in the time series Over the last 12 months, the two indexes almost matched each other. The results also showed that the difference between these two indexes was more pronounced in dryer stations. SPI seems to monitor dryness, especially in dry areas rather than droughts, so the use of SPI_mod instead of SPI provides more precise results due to the elimination of seasonal effects of precipitation.