Assessment of runoff characteristics changes of the left bank tributaries of the Upper Dniester under warming conditions

The inflow of water from the left-bank tributaries of the Upper Dniester is an important component of the runoff formation for the whole river. The study of changes in their water content in the context of global warming is quite urgent and agrees with the provisions of the strategic programme for the Dniester Basin for 2021-2035 aimed at ‘climate change mitigation’ and ‘promoting the principles of sustainable water management’. The paper's objective is to carry out a research into the changes in climatic factors and runoff characteristics that occurred on the left bank of the Upper Dniester in the early 21st century. The research was performed on the basis of hydrometeorological data for the period of 1945-2018. The main research methods include the regression analysis and the method of differential integral curves. It is found that, within the area under study, there is an increase in average annual air temperatures against the background of constant or insignificantly increasing amounts of annual precipitation, thus creating unsatisfactory conditions for runoff formation. It is established that, during the months of cold period, there is an increase in air temperature and that since 1989 the frequency of cases, when the temperature crosses the range of positive values, keeps increasing. 1989 is a turning point in the chronological course of average annual air temperatures; a positive phase of long-term fluctuations starts since that year. With regard to annual precipitation fluctuations from the mid-60s of the last century up to 2013, a positive phase was also identified; within this phase there was a short period of insufficient moisture (1981-1996). It was found that the response of runoff characteristics to climate change was not the same. Maximum runoff during the spring flood is the most sensitive to global warming, since such warming has worsened the conditions for accumulation of water reserves in the snow cover. The transition of maximum runoff fluctuations to the low-water phase took place in 1981. The average long-term decrease in the maximum water flow rates during the spring flood for the period of 1950-2018 amounts to -16.9%. Unlike maximum runoff, the characteristics of the annual and minimum runoff changed gradually during the winter and summer low-water periods and the final transition to the low-water phase of fluctuations occurred only in 2009-2011. Certain inertia of changes in the water resources of Podillya rivers under the warming conditions is caused by the subsurface component making a high contribution to the annual runoff formation (60%). Significant portion of the groundwater supply ‘mitigated’ the effects of surface runoff loss during spring floods. The research allowed establishing the occurrence of statistically significant negative trends in the annual and minimum runoff fluctuations since 1998. If continue to preserve, the identified climate change trends will decrease the inflow of water from the Podillya rivers to the main Dniester River