Change in snow flood flow in the southern macro-slope of the Russian Plain in the period 1930-2014

In recent decades, in the southern macro-slope (catchment) of Te Russian Plain, mainly within the basins of the Volga and the Don rivers, the spring flood flow decreased due to both anthropogenic and climatic factors. Of anthropogenic factors, the creation of reservoirs played a signifcant role in changing the water regime of the Volga and the Don. Another important factor, affecting the river runoff, is agricultural activities on flat interfluves and slopes. Compared with the conventional-natural period (for the Volga from 1879, and for the Don from 1876 to 1929) to the date (for the period 1930–2014), the flood flow of the Volga has been decreased by more than 4300 km3, and the Don – by almost 900 km3. Te contribution of anthropogenic factors to this decrease in the Volga basin exceeded 70%, and for the Don it was equal to 45%, while the climatic ones contributed 30 and 55%, respectively. During the period of instrumental observations on the rivers of the region, long-lasting phases of high/low water content with duration from 15–20 to 90 years or longer, caused by the climatic changes, were determined. Tese are a characteristic feature of long-term changes in flood flow (and in a runoff of other seasons) on both medium and large rivers of the southern macro-slope of the Russian Plain. Te period since early 2000s was a start of the decreased flood flow everywhere, i.e. on medium and large rivers. It is most clearly (by tens of percent) manifested in the decrease of the spring surface flow due to the following factors: more frequent thaws, reduction of the depth of soil freezing, increase of infltration, especially in the forest-steppe and steppe zones. Increase in flow of the infltration origin does only partially compensate the decrease of the flow in rivers.