Patterns of runoff fluctuations of the Dniester River (Ukraine) under conditions of climate change at the beginning of the 21st century

Abstract

The relevance of the chosen topic is associated with the impact of climate change on fluctuations of water content in Ukrainian rivers. The consequences of warming depend on physical and geographical conditions and vary significantly across different natural zones. The behaviour of catchment areas affected by climate change varies depending on landscape (mountainous or flat terrain) and latitude. This results in instability of boundaries of catchment areas with annual runoff synchronous and out-of-phase fluctuations. The study aims at establishing the patterns of annual runoff fluctuations in the Dniester basin with relation to fluctuations in the maximum runoff of rainfalls and spring floods. The spring floods are the ones that determine a significant part of annual runoff and depend on the impact of warming during the winter season. The study's main method is the method of constructing difference integral curves of annual and maximum rainfalls and maximum spring runoff and their analysis. There are 7 groups (zones) within the Dniester River catchment area identified by the type of difference integral curves of annual runoff. Fluctuations in annual, maximum rain and spring runoff were compared using the curves averaged within each group. The degree of synchronicity of fluctuations was estimated via correlation coefficients between the ordinates of difference integral curves. It was found that the main river oscillations retain their cyclicity along its entire length, even below the Dniester HPP. The last transition to a low-water phase occurred in 2010-2011. It was discovered that the main river's cyclical nature mostly results from fluctuations in the runoff of mountainous Carpathian tributaries that form the Dniester's runoff formation zone. This part of the catchment area is characterized by a high amount of snow and rainfalls feeding the river with the role of rainfalls having a predominant character. In the upper reaches of Carpathian rivers, where the impact of warming is not yet sufficiently pronounced at high altitudes, spring floods play a significant role in runoff formation. In the upper reaches of Carpathian rivers there is an additional zone where fluctuations in annual runoff are caused by both rainfalls and spring floods. The boundaries of this zone (2nd district) may change as the effects of warming increase high in the mountains. On the left-bank tributaries, the runoff fluctuations in Verkhnepodilskyi and Serednepodilskyi districts are similar to those of the main river, however, there is a shift in the dates of the last transition to a low-water phase. The study indicates the likelihood of karst and artificial reservoirs influence. It also indicates a significant difference in the nature of runoff fluctuations of the upper left-bank tributaries (Strvyazh, Vereshchytsia, Shcherek) and lower left-bank tributaries. They are affected by other atmospheric processes and climatic factors associated with such processes.