Bolbot H. Climate change influence on the intra-annual water runoff distribution of the Siverskyi Donets River Basin within Ukraine

The thesis consists of studying the intra-annual distribution of the Siverskyi Donets Basin rivers runoff under the influence of present climate change. The relevance of the research is determined by the fact that the Siverskyi Donets River Basin is the main source of Eastern Ukraine water supply. Therefore, an important task is to assess the climate change influence on the basin's water resources. Currently, the relevance of the research has increased due to the warfare within the researched region. The main results of the research can be included as a part of the Don River Basin (Siverskyi Donets sub-basin) management plan. The aim of the thesis is to determine the spatial-temporal regularities of the average annual water runoff and to study the present intra-annual distribution of the Siverskyi Donets Basin rivers runoff (within Ukraine) in the conditions of present climate change. The object of the research is the main climatic and water runoff characteristics of the Siverskyi Donets Basin rivers within Ukraine. The subject of the study is the present spatiotemporal regularities of the average, maximum, and minimum annual water runoff, its intra-annual distribution, and the main climatic runoff formation conditions within the Ukrainian part of the Siverskyi Donets River Basin. An analytical review of previous research on the subject of the thesis was conducted. Based on this, in the thesis, the influence of climate change on river runoff is evaluated by a comparative analysis of the results obtained using a number of methods. The hydrometeorological characteristics of the Siverskyi Donets River Basin observations data used in the research. In order, to identify the present climate change influence on water runoff characteristics were chosen two thirty-year periods: the period of the climatological normal - 1961 - 1990 and the present period - 1991 - 2020. The homogeneity of the exiting database was verified using hydrological-genetic and statistical methods. The Siverskyi Donets River Basin runoff formation conditions were evaluated. According to the conditions of the runoff formation in the basin, it was distinguished two parts - the left-bank and the right-bank. Long-term fluctuations of the average annual, maximum and minimum runoff within the Ukrainian part of the Siverskyi Donets Basin were evaluated. Differences in the runoff characteristics of the left-bank and the right-bank rivers of the Siverskyi Donets were revealed. It was established that the present period is characterized by a tendency to decrease the runoff characteristics. Based on the stochastic approach, the long-term variability regularities of the average annual water runoff of the Siverskyi Donets Basin rivers have been established. Water cycles and phases of water runoff (wet and low periods) are highlighted with reference to specific years. Using the identified stochastic regularities, which are based on historical data, it was received the Siverskyi Donets Basin rivers runoff predictive estimates for the nearest future. Under the influence of present climate change, it was revealed a redistribution of hydrometeorological characteristics during the year. The intra-annual redistribution of precipitation is reflected in the river runoff fluctuations. The changes in the Siverskyi Donets Basin catchments’ water balance components in the present period (1991-2020) compared to the period of the climatological normal (1961-2020) were determined. The intra-annual runoff distribution of the Ukrainian part Siverskyi Donets Basin rivers was analyzed for the representative periods. Certain changes were revealed in the present (1991-2020) period. Certain differences can be traced between the left and right tributaries of the Siverskyi Donets River. The most pronounced changes were found for the intra-annual runoff distribution of the highly-low year. It was established that at the present period (1991-2020), the intra-annual distribution of the flow of the Siverskyi Donets Basin rivers has undergone significant changes. In general, for the basin, the spring flood runoff has decreased (decrease in the range of 17-56%) and the summer-autumn and winter low periods runoff has increased compared to the previous period of the climatological normal (1961-1990). The share of the low months runoff significantly increased (increase in the range of 2-29% - winter low period and 13-35% - summer-autumn low period). There was a leveling of the intra-annual runoff distribution.