Regional analysis of trend and change point in seasonal series of SPEI drought index in Iran

Climate change and global warming are the greatest challenges that humanities have ever faced. Temperature variations, quantity and pattern of precipitation are the important potential impacts of climate change which may affect water resources and drought conditions. Mann-Kendall trend test and Pttitt homogeneous test were respectively used to detect trend variation and abrupt changes in the time series.The SPEI drought index which is based on precipitation and temperature data, is not only designed in a way to consider both precipitation and potential evapotranspiration (PET) but also has the capacity to include the effects of temperature variability on drought assessment. Therefore, the objet of this study is to investigate the trend and the detection of change points in seasonal series of SPEI drought index in Iran, and for this purpose the non-parametric Mann-Kendall and Pettitt statistical tests has been used. In the first step, the SPEI data, calculated based on monthly rainfall and evapotranspiration is extracted from those parts of the SPEI world database covering Iran, and are about 624 points of the 0.5° geographic network were separated from the entire data and stored in the Excel format. Then, the 50 years seasonal times series of SPEI data during the period of 1969-2018 were made for different seasons of the year. Finally, the nonparametric Mann-Kendall trend and Pettitt change point detection tests were performed on the data, and the results were analyzed. The results of using the Pettitt test in 624 points from Iran showed that in between the series of SPEI drought index in the spring, summer, autumn, and winter, 11, 31, 8, and 15 percent of points had significant change point, respectively, and their spatial location are the decreasing change points in the eastern and north-west in spring, northernwest, northern east, middle part and southern east in winter. Meanwhile in summer and autumn seasons the increasing and decreasing change points are in dispersed points from the southwest, and the south and northeast and the increasing change points in the middle parts of Iran, and parts of the south and north of the country, respectively. Also, the results of using Mann-kendall test showed a significant decreasing trend in 28 percent of points for spring, a significant increasing and decreasing trend in 5 and 25 percent of points for the summer season, a significant decreasing trend in 18 percent of points for the autumn, and also the existence of significant increasing and decreasing trends is in 1 and 15 percent of points, respectively in winter. The spatial location of these points for the four studied seasons was in the east, northwest and south of the country, half east, southwest and scattered parts from the south of the country, the central parts of the country, as well as the northeast and northwest, respectively. Result of using Mann-Kendall and Pettitt statistical tests indicated that climate change has occurred in parts of the country, varies from one season to another in such a way that in east and northwest parts of the country in spring and in north-west and south-east parts of the country in winter, more dry conditions are visible but in autumn, more humid condition were observed in central parts of the country. According to the fact that most of the country's precipitation is in winter and spring, this drying trend will cause a hazard in reducing water resources. The results also indicate that in most of the cases that a change-point exist in the drought index, a trend is also observed in data series based on Mann-Kendall trend test.