Аннотация:The long periods/phases (with a duration of more than 10-15
years) of increased and decreased water flow of rivers in the Volga river
basin were identified. The annual and seasonal water flow (over snow-melt
flood period, as well as summer–autumn and winter low-water seasons) of
six representative rivers over observation periods more than a century in
duration, starting from the 1870s–1890s up to 2016, was used. In addition
to this, periods with the average runoff close to its normal value were also
observed. The boundaries of contrast phases were determined using
normalized cumulative deviation curves in combination with Student’s test
of the statistical homogeneity of the data series. The duration of the phases
varies from 10 to 96 years. The phases of lower runoff were generally
longer than those of higher runoff (this is especially typical of the winter
and summer–autumn low-water season). The identified contrast phases
show a statistically significant difference between the annual and seasonal
runoff. The analysis of the data series of alteration of phases with increased
and decreased water flow in hydrological seasons of the year allowed the
authors to identify three major types of their long-term dynamics within
the Volga river basin.