Investigation of long term drought trend in semi-arid, arid and hyper-arid regions of the world

The lack of sufficient fresh water resources, climate change and lengthy and severe droughts are of the major today’s world challenges particularly in arid and semi-arid regions. In the current conditions, especially in the case of droughts and in order to water resources planning, it is essential to analyze the changes and trends in drought particularly in hyper-arid, arid and semi-arid regions of the globe. In this study, Mann-Kendall statistics were applied to determine the significant and non-significant positive or negative trends in long term drought time series (9, 12, 18 and 24 monthly time scales) in moderate to severe and extreme classes of droughts derived based on the Standardized Precipitation Index (SPI). Results indicated the both significant positive and negative trends can be found in these regions. By increasing the time scales of droughts time series, the percentage of significant upward or downward trend grow. For example, in the case of decreasing trend of SPI (increasing in the drought), the percentage of 17.8 for 9 monthly SPI increased to 21.9, 28.48 and 31.71 for the 12, 18 and 24 monthly SPIs, respectively. The increasing in long severe and extreme classes of drought can be found in Middle East and west of America. The upward trends of drought particularly in Middle East region which is suffered by expensive tensions among the communities can intensify current critical situations. Therefore, more attention should be paid to this subject particularly in regional and international scales.