The thesis is focused on the water regime transformation of the Horyn basin’s rivers under the influence of modern climate change. The relevance of research was due to the fact that the issues of a comprehensive assessment of the rivers’ water regime in the context of the spatial and temporal climate change impact were not sufficiently studied in Ukraine. Identification of the scale and trends of climate change and their impact on the water regime transformation is an important task in the working out of river basin management plans, flood risk management plans, construction engineering surveys, river channel regulation, environmental impact assessment, strategic and environmental assessment, etc
Determining quantitative indicators of the long-term, intra-annual, and seasonal dynamics of water resources is one of the fundamental management tasks requiring a comprehensive approach to their solution. The purpose of the thesis is to determine the spatial and temporal features of the water regime and to study the existing effects of climate change on the Horyn River basin territory. The subject of the thesis is water regime elements changes, water balance elements dynamics, river feeding, spatial and temporal patterns of the Horyn River basin rivers water runoff.
The study of identifying the impact of climate change on water resources methodology showed the need to use a complex approach. These are using hydrological and genetic methods, the water balance method and the rivers' feeding types determining method, based on which subsequent calculations were made. To a lesser extent, mathematical statistics methods were used. The research results were illustrated using the functionality of geographic information systems.
The assessment of moisture redistribution over time was performed for two climatic periods 1961–1990 (the climatological standard normal) and 1991–2020 (“modern climatic period”). Using Thyssen polygons, the values of the meteorological elements, measured at meteorological stations were adjusted to the corresponding territories of the studied river catchments, which are closed by hydrological gauges.
The Horyn river basin's river runoff formation conditions were assessed. In general terms, there is a tendency of air temperature increasing, precipitation redistribution within the annual context, and reducing of snow reserves, which leads to a change in the quantitative characteristics of the area moisture exchange
The analysis of the average daily air temperature and precipitation has revealed the impact of climate change on water runoff in the Horyn basin in the modern period. Since the late 90s of the last century, an increase in average annual air temperature has been recorded at all hydrological gauges in the study basin, and since the 2010s, there has been a clear decrease in precipitation in the basin. This was due to the increase in air temperature in winter, frequent thaws, and, accordingly, a decrease in snow reserves, which play a major role in the basin’s rivers feeding and form the majority of the annual runoff during the spring flood. On the other hand, this has led to increase in the winter low water runoff. The impact of climatic factors on runoff in summer and autumn has also led to decrease in the water discharge of these seasons in the modern period.
Based on the climatic water balance, it was found that in the modern period, the Horyn River basin moisture regime is close to the transition from excessive to unstable. On average, the amount of moisture depletion in the Horyn River basin has increased, while its opposite value, i.e., moisture accumulation, has decreased.
Changes in the individual shares of the river feeding types were estimated for the studied basin. In particular, the following were revealed: a decrease in the relative share of snow and rainfall feeding in the modern period by 8,4 % and 1,6 %, respectively; and an increase in the share of underground and soil feeding by 5,7 % and 4,3 %, respectively.
An increase in the average annual air temperature by 1,3 ºС in the modern climate period led to the following: a decrease in the number of days with negative temperatures and, as a result, a decrease in snow accumulation in the territory of the basin; insignificant soil freezing or its absence, which in accordance contributes to free atmospheric water infiltration (even in winter) and replenishment of underground water reserves. As a result, the share of the surface feeding decreased. An increase in the share of underground feeding is associated with a decrease in the river's water level, i.e. an increase in the hydraulic gradient.
Obtained results (with further development) can be applied to several issues. Such as rational use and integrated management of water resources, hydropower development, agriculture, and improvement of methodological approaches to determining the river's feeding types and assessing water balances of river basins, etc.
