ИСТИНА

The long-term changes in river water flow of the Arctic rivers, as well as those in other regions, are characterized by contrasting periods or phases, according to the terminology adopted in Russia of their increased and decreased values with various durations. A large amount of long-term variability exists: some long-lasting periods can be distinguished by a duration of 10–15 years, while others last many decades. These contrasting phases are characterized by specific, relatively steady water regimes. This article is devoted to the study of long-lasting periods (phases) of increased and decreased water flow of the Lena River at Kyusyur. The long-term series of the Lena River daily water discharges near the village of Kyusyur have been naturalized after the creation of the Vilyuysky reservoir. The Kalinin-Milyukov flow routing method [1] conceptualizes a relation between the inflow and outflow of a river section as a linear function of water stored within the reach. The parameters of the influence function were determined on the basis of data on average daily water discharges in the Kyusyur and Tabaga on the Lena River, as well as in the Okhotsk Ferry on the Aldan River, for which there were observational data covering periods during which there was no noticeable anthropogenic impact on the flow of the mentioned rivers. This made it possible for the entire observation period (1936-2019) to determine the shift points between the long-term phases of increased/decreased annual and seasonal water flow and investigate their features. The quasi-synchronous nature of the sequence of phase changes of reduced and increased values of annual and seasonal flow was revealed. It is common for rivers within the basins of the Lena and Yenisei rivers. The duration of the contrasting phases varied in a wide range from 10 years (the phase of decreased snow melt flood flow) to 58 years (the phase of decreased annual water flow). At the same time, both for annual and seasonal flow, the phases of decreased flow were longer. At the same time, the most noticeable difference in the flow of contrast phases was observed in the winter and summer-autumn seasons, when it was 48 and 30%, respectively. The smallest difference is typical for snow-melt flood flow and annual flow (11-12%). References [1] G.P. Kalinin, P.I. Milyukov. Approximate calculation of unsteady motion of water masses. Proc. of the Central Institute of Forecasts. Issue 66. Leningrad, 72 p. (1958) (In Russian).