Variability and trend of changes in the severity-area of drought and wet in Iran

This study aims to investigate the variability and trend of changes in area severity of wet and drought on annual, seasonal, and monthly scales in Iran. For this purpose, the ZSI index, Mann-Kendall trend test, and the monthly precipitation data (ERA5) of the European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 1979-2021 were used. The results showed that, in annual wet-drought intensities, there are three distinct periods; 1980s with almost equal wet-drought area 1990s wet dominate, and the first two decades of the 21st century, which began in 2000-1999 rainy year with more than 97% of the country (70% very severe drought area) under drought conditions where the most areas of drought observed. The pattern of variability of wet-drought intensity area in winter and spring is almost similar to the annual whereas it is different in autumn from the annual pattern. However wet area increases. at the end of the period. In this season in November, the area of drought intensities, especially (moderate and severe) has been decreasing and in contrast, the area of wet intensities has been increasing. But, the trend of wet-drought intensity was reversed in winter and spring, especially in January and to some extent in March. The intensity of the index also has a trend when the increase in the intensity of drought classes (increasing the intensity of the negative index) is more pronounced. Overall, in all three scales, the incidence of consecutive weak and moderate droughts and very severe and severe wet with the low sequence was higher. Increasing the intensity and area of wet in autumn may indicate a change in the precipitation regime. In contrast, the increase in droughts, especially high-intensity droughts in the two main rainy seasons of Iran is a warning for serious water resources management in the warm period.