Investigation of Drought Trend on the Basis of the Best Obtained Drought Index

The drought phenomenon is a primary natural hazard in Iran. The drought can be analyzed using different indices. Therefore, the use of suitable indices will have an essential role in quantitatively investigating this phenomenon. Since precipitation is one of the most critical factors in drought analysis, different drought indices consist of precipitation and have been developed based on this parameter. In this study, the time series of nine precipitation-based drought indices were compared in 30 synoptic stations of Iran, and the superior index was determined during 1960–2014. Then, using the selected drought index, the annual trend of wet and dry periods was calculated by the modified Mann–Kendall test. Among studied indices, the SPI index was selected as the superior one. This index was well fitted by the Normal distribution and had a correlation coefficient of more than 0.92 with precipitation. Also, this index was sensitive to the amount of precipitation, and it could detect drought events corresponding to the minimum precipitation. In other words, the SPI had the best capability to determine extreme drought based on minimum precipitation. The results of drought analysis based on the SPI index showed that at least one drought event happened in the majority of studied years (82%). It reflects drought conditions in the studied area, especially in 2008 and 2010. The trend analysis of wet and dry periods showed the decreasing trend of the SPI index in the most studied stations. The decreasing trend of the mentioned index was significant (at 95% confidence level) in the northwest and west of Iran. The trend line slope values corresponding to the SPI index were negative in most studied stations. Tabriz and Esfahan stations had the maximum value of negative (-0.027) and positive (0.025) trend line slope, respectively.