Investigation of spatiotemporal trend of the bivariate meteorological drought index, SPEI, in Iran

Declining condition of water resources cannot be shown by single-variable drought indices because of changes in precipitation and temperature due to climate change. Accordingly, multivariate drought indices are considered such as the Standardized Precipitation-Evapotranspiration Index (SPEI). In order to study the trend of changes in the severity of meteorological drought in the geographical zone of Iran, the SPEI global network data was used over a 30-year statistical period, and trend detection test and Sen's slope estimator were performed on their seasonal series. Then, their results were mapped in the GIS and the results showed that the highest drought trend is in the winter between seasons, and more than 94 percent of the country's area shows a declining slope in the drought index time series. The trend of this slope in the west and north-east of the country is statistically significant. Also, the eastern half of the country shows a drought trend in the summer, but a better situation prevailing in Iran in spring and autumn, and a small percentage of the country's area indicates a significant drought trend. So that in the spring and autumn, the slope of the trend line in 85 and 91 percent of the country is less than and equal to 2 percent, respectively. Generally speaking, the result of the Standardized Precipitation-Evapotranspiration Index showed that climatic changes are occurred in Iran and the country is going towards more dry periods in future years.