Spatial and temporal correlation between severe droughts and extreme low flow on rivers in the Russian Plain

Extreme low flow on rivers (when water flow was less than or equal to the value of 75 and 90% exceedance probability), observed in years with severe meteorological and agricultural droughts in the Russian Plain (1972, 1975, 1981, 1995, 1998, 1999, and 2010) were studied. In 1972 and 1975, the exceptional low flow was recorded in the largest area, including the basins of the largest rivers (the Volga, Don, and Dnieper) and even the outlet, as well as in their tributaries. One of the heaviest droughts (2010) accompanied the abnormally low flow only in the Lower Volga and its tributaries in the eastern part of its basin. In other years with severe meteorological and agricultural droughts, their occurrence areas in the Russian Plain were much less. In 1975, extreme low flow was recorded in the rivers of the Volga, Don, and Dnieper basins in the main hydrological seasons (during summer – autumn low-water season and snow-melt flood), as well as all the year round, including their outlet (in their lower reaches in the general case and in the middle reaches in the case of the Dnieper). However, in these seasons in 1972, extreme low flow was recorded in rivers in the basins of the Don and Dnieper (while in the Volga, it took place only during summer – autumn low-water season), and in 2010, it formed only in the Volga basin. Abnormally low flow in years with severe droughts were recorded mostly in rivers of the forest – steppe and steppe zones, as well as the southern part of the forest zone.