Knowledge of the cyclicity features in the fluctuations of river runoff, duration and nature of the low-water and high-water period interchange in one or other river basins, and especially their prediction, provides invaluable assistance in the planning and sound management of water resources, improving the operational efficiency of hydropower, reclamation and other water facilities. Currently, the interest in the study of long-term cyclical fluctuations in river runoff, as well as patterns of fluctuations of its underlying factor, has highly increased due to their use in long-term forecasts. Time series of annual water runoff for basins of Tisza, Dniester and Prut rivers were estimated with the use of mathematical tools, methodological framework of which is based on a statistical means of summarizing, systemisation of the input data, evaluation methods of time random sets of runoff characteristics, methods of analysis of the time-series variability and manifestation of their structure.

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The purpose of the study is to estimate the effective precipitation during the formation of high floods in 1998 and 2001 on the rivers of the Tisza basin within Ukraine. The flood of November 3-05, 1998 was rainy, the flood of March 3-05, 2001 – snow-rain. The maxima of the studied floods exceeded the average values of the peaks for many years on different rivers by 2.0-3.5 times. To determine the probabilities of exceeding the maximums of floods in 1998 and 2001, series lasting 60-70 years were formed for 18 rivers within the study basin. In 1998, on the rivers of the Tisza basin, the probabilities of annual exceeding the maximum costs are in the range of 1.4-10%. The flood in 2001 did not cover the entire territory of the Tisza basin. In the upper reaches of the Tisza, highs in some cases even exceeded the 1998 highs. In the Latorica and Uzh River basins, 2001 highs were not so rare, with probabilities exceeding 10-35% each year. To estimate the effective precipitation, the runoff coefficients during the 1998 and 2001 floods were defined as the ratio of the water runoff layer of the flood itself (without baseline) to the precipitation layer that fell during the floods in the river basin. For the November 1998 flood, only liquid rainfall was taken into account, and for the March 2001 flood, the amount of rainfall was supplemented by water reserves in the snow, which were within the catchments at the beginning of the flood. The percentage of effective precipitation from total rainfall in November 1998 varies from 35 to 82 %, and during the formation of thaw-rain flood in March 2001 – from 44 to 60 %. The runoff coefficients of the 1998 and 2001 floods depend on the hydrographic characteristics of the studied rivers and their basins. The greater the average weighted slope of the river and the average height of the catchment and the smaller its area, the greater the amount of effective precipitation. The correlations of runoff coefficients from the hydrographic characteristics of the studied rivers and their basins for rain floods in November 1998 are closer than for the snow-rain in March 2001.

The drainage basin of the Upper Tisa is considered to be one of the most flood hazardous areas of the Ukrainian Carpathians. Climate changes in the past decade made a significant impact on the course of hydrological processes, including streamflow. The studies of the processes at global and regional levels are of great importance for grounding of flood protection operations as well as arrangement of measures for adaptation to climate change. The study considers approaches to evaluation of changes of hydrologic balance elements of the investigated drainage basin in the period of the 20 th century. A retrospective analysis of the changes is provided in order to find out relationship between the streamflow and the climate aspects which influence it. A statistical analysis of a series of long-term observations was performed in order to test their steadiness and homogeneity. Inter-row connections were determined between changes in precipitation regime, air temperature of the surface and streamflow in the modern period (1991-2012) compared to the period of the climatic norm (1961-1990). The closest interconnection between mentioned variables is observed for the warm period. For the cold period such dependencies are satisfactory. Consequently, an equation was derived for conversion of the expected influence of climate change on the streamflow. Results of the regional model REMO-ESNAM5 were used to perform forecast evaluation of hydrological consequences of climate change. The projection of changes of climatic aspects for the drainage basin of the Upper Tisa for the time period 2021-2050 as well as А1B sustainable development scenario was created. The evaluation of possible changes of average, seasonal and annual streamflow in the basin is presented up to the middle of the 21 st century. Annual redistribution is clearly registered. Forecast evaluations were performed of the maximum streamflow changes, floods frequency and their recurrence throughout the year.

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