The phases of long-lasting (more than 10-15 years) increased and decreased water flow, water temperature and heat flux values in the Northern Dvina River and the Pechora River were studied for the observation period from the 1930s to 2020. To distinguish between different phases, statistical homogeneity tests and normalized cumulative deviation curves were used. Generally, the identified phases displayed statistically significant differences between average values of the measured characteristics. During contrasting phases, the general pattern of water temperature during the warm season, water runoff and heat flux in the Northern Dvina and Pechora River Basins differed considerably. The number of the identified phases varied between the studied rivers and ranged from two to four contrasting phases in the Northern Dvina River exceeded those of the Pechora River. Consequently, the duration of the phases also varied quite significantly. The difference in mean values of the hydrological characteristics during the contrasting phases in the Northern Dvina River exceeded those of the Pechora River. The longest phases of increased and decreased heat flux nearly coincide with contrasting periods of water runoff and water temperature. The phases of simultaneous increased or decreased values of all hydrological characteristics were associated with corresponding periods of increased or decreased air temperature (on average for a year and for the open water period) and annual precipitation values. Those long-lasting phases of simultaneously increased or decreased values of river flow, heat flux, and water temperature were associated with changes of the global thermal regime, regional cryosphere features, and long-term periods of intensification or weakening of the atmospheric circulation over the North Atlantic, characterized by variability in macrocirculation indices such as the North Atlantic Oscillation (NAO) and Scandinavian circulation pattern (SCAND).
The long-term data sets of the annual and seasonal water flows and the major ions fluxes of the Northern Dvina River at the village of Ust-Pinega and Pechora River at the village of Ust-Tsilma were analyzed. Long-term phases of increased and decreased values of water runoff were identified, the duration of which ranged from 9 to 50 years, and their main characteristics were determined. The sequence and boundaries of the change in the contrasting phases of the annual water flow, the snowmelt flood flow and the summer-autumn flow on the Northern Dvina River and the annual water flow and snowmelt flood flow on the Pechora River practically coincided. The difference between the average annual and seasonal water flow in the phases of its increased and decreased values ranged from 11 to 41%, and the ion fluxes were in the range of 5–36%. The ion flux of contrast phases, as a rule, differs less than the water flow. This is due to the inverse nature of the dependence of the concentration of ions on the daily water discharges, because of which a kind of negative feedback is formed, stabilizing the intensity of chemical denudation in the wa- tersheds of these rivers and the flux of the main ions into the seas, even with noticeable fluctuations in water flow. Keywords: the Northern Dvina River, the Pechora River, water flow, ion flux, long-term phases of changes, cumulative deviation curves.
Changes of annual and seasonal flow of Volga, Don, Yenisei and Lena are estimated by two methods. One of them is to restore the natural flow of recent decades, during which there were observed its significant anthropogenic changes. For these purposes, we used data on the runoff of rivers (tributaries of the considered rivers and their upper parts), the water regime of which is relatively weakly changed by the anthropogenic impact. Another uses the data of water management statistics. Subsequently, the data on the naturalized river flow were compared with the anthropogenic-modified runoff for this and for the previous base period, when anthropogenic impact can be neglected. It is shown that climatic and anthropogenic factors perform either unidirectionally, increasing or decreasing the flow, or in opposite directions. Moreover, the impact of anthropogenic factors, mainly of reservoirs and water consumption, is of the same scale as the influence of climatic factors, while in many cases exceeds it.
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