An investigation of drought magnitude trend during 1975-2005 in arid and semi-arid regions of Iran

In addition to drought intensity, drought magnitude (DM) is an important parameter in drought analysis. Therefore, in this study, the drought intensity and DM trends are investigated based on the standardized precipitation index (3, 6, 9, 12, 18 and 24 monthly SPI time series) from 1975 until 2005 in 25 synoptic weather stations located in arid and semi-arid regions of Iran. Although the DM is commonly computed for negative SPI values, for analysis the wet years besides drought conditions, this study has also involved positive SPI values and therefore, each time series was divided into four drought intensity conditions as follows: drought (D; SPI values less than -1), normal near drought (ND; SPI values more than -1 and less than 0), normal near wet (NW, SPI values more than 0 and less than +1), and wet (W; SPI values more than 1). The non-parametric rank-based Mann-Kendall test was used to detect trends of SPI values (drought intensity and DM) for 3, 6, 9, 12, 18 and 24 monthly time scales. Based on the results, a considerable percentage (50 %) of the stations at the level of 0.05 showed a significant trend with regards to drought intensity when compared with the DM trend (5.8 %). Generally, it can be said that the arid and semi-arid areas of Iran had a negative SPI trend and it means the more drought severities. Also, the more frequent significant positive DM trend in comparison to negative ones is a sign of increasing DM during the three past decades.