Features of Extremely Low Water Years on the Rivers of the Russian Plain during the Period of the Modern Global Warming

Extreme low water on rivers (in years in which the runoff was equal to or less than 75 and 90–95% of the probability exceedance) in the river basins of the Russian Plain (Volga, Don, Dnieper, Western Dvina and Northern Dvina) in the 19th–21st centuries was studied. It is shown that the proportion of the total number of years in which low water was observed in one or several seasons of the year or the whole year varied on the rivers considered in relation to the duration of the entire observation period in the range from 46 to 57%. In the 1930s, low water years occurred in only one of the basins considered, and in the 1950s in several (from two to five) basins in the same year. At the end of the 19th century, in the 1920s and 1930s, and in the 1970s, there were seven years when low water occurred simultaneously in different seasons and in the whole year in all the rivers considered, and in nine cases in four basins. Modern global warming leads to a significant decrease in the number of low water years in the winter and summer-autumn low flow seasons and the year as a whole on all the rivers considered. While the number of years with low flood runoff on the Don, on the Northern and Western Dvina has increased significantly, and on the Volga and Dnieper it has changed little. During the long-term phases of increased/decreased annual and seasonal runoff on the rivers of the central part of the Russian Plain, cardinal differences in the frequency of low water years were revealed. In the years when the most extreme droughts were observed on the territory of all river basins considered, low water was observed only in 1921 and 1972.