ИСТИНА

On the rivers of the central part of the Great Russian Plain, we revealed long phases (lasting 10-15 years or more) of increased/decreased annual and seasonal runoff during the past two centuries. Extreme low water flow was quite often formed on the background of these phases. The frequencies of these low flow conditions were quite different during the above mentioned contrasting phases. To identify the phase boundaries, cumulative deviations curves and criteria for statistical homogeneity (the Student t-test and Mann-Whitney-Pettitt test) were used. The frequencies of specific water flow values were estimated using the empirical curves of the exceedance probability of annual and seasonal water flows based on their long-term time series. In the century-long changes of all rivers considered, two long contrasting phases were revealed. These phases characterize increased and decreased runoff during annual and hydrological seasons. Near simultaneously, a phase of increased runoff was first observed for the freshet season, which was replaced by the phase of decreased runoff in 1971 (on the Volga near Staritsa, the Oka near Kaluga, the Don near Kazanskaya), in 1988 (on Moskva-River near Mozhaysk) and in 1959 (on the Dnieper near Orsha). On the contrary, phases of decreased runoff were first observed for low water seasons. Thereafter, almost synchronously (in 1976-1979), a phase of increased runoff occurred at all these rivers. The character of the runoff phases differs significantly among rivers. The duration of long-term phases of different annual runoff components on these rivers is within the following limits: on the Volga-River from 28 to 86 years, the Dnieper-River from 34 to 99 years, the Moskva-River from 31 to 66 years, the Oka-River from 35 to 96 years, and the Don-River from 37 to 91 years. The differences in the flow (as a percentage of the phase of decreased flow) for contrasting phases are on the Oka-River: 20-72%, Don-River: 19-68%, Moskva-River: 18-144%, Volga-River: 24-81%, and Dnieper-River: 20-79%. The differences in runoff, as a rule, reach the greatest values in the low water seasons of the year, and are the smallest for the annual runoff. Very significant differences were revealed in the frequency of low-water years for low runoff with exceedance probability ≥75% and ≥95%. These differences were observed at all rivers of the region and for both phases of annual and seasonal runoff (i.e., for snow-melt freshet, as well as for summer-autumn and winter low flow seasons). For the summer-autumn and winter seasons on the Oka, Don, and Moskva Rivers during the phases of increased runoff, extreme low water flows were not observed at all. However, in the same seasons but in contrasting phases, there were more significant differences in the frequency of low-water years compared with the snow-melt freshet and annual runoff. The range of changes in the frequency of low-water years, the runoff of which was lower than the runoff of 75 and 95% exceedance probability, is different for the phases of increased and decreased runoff (see attached table).