Keywords: Maximum snowmelt-rainfall runoff, long-term trends

The study is devoted to the investigation of long-term spatial and temporal trends of the characteristics of maximum snowmelt-rainfall runoff in the rivers of Ukrainian Polissya (right-bank part) using the hydro-genetic method of statistical analysis. The study of maximum river discharge has significant scientific and practical importance. Understanding the conditions of formation of extreme values of maximum river discharge, their frequency of occurrence, as well as the analysis of long-term trends, is particularly important for engineering design and hydrological forecasting activities. In recent years, floods in rivers caused by snowmelt and precipitation have begun to occur in earlier, almost winter periods of the year, forming the maximum snowmelt-rainfall runoff of the rivers of Ukrainian Polissya. At the same time, in a period of climate change the reduction of snow cover and its melting due to the increase of winter air temperatures has led to a decrease in floods in terms of magnitude and spatial extent, but the probability of catastrophic snow-rain floods has increased. Using the hydro-genetic method of statistical analysis, the study performed a verification of the statistical homogeneity of hydrological time series of maximum snowmelt-rainfall runoff in the rivers of Ukrainian Polissya (maximum water discharges, runoff depths), as well as an investigation of temporal trends, cyclic fluctuations of river runoff, and synchrony in their series. The analysis of the homogeneity of time series of maximum water discharges of snowmelt-rainfall runoff in the rivers of Ukrainian Polissya, based on cumulative curves, revealed that the observation series show a violation of homogeneity, in contrast to the time series of runoff depths. The main causes of this violation of homogeneity in the time series of maximum water discharges of snowmelt-rainfall runoff are the redistribution of runoff under the influence of meteorological conditions in the current climatological period and the anthropogenic regulation of river discharge. Summarizing the results of the analysis of fluctuations in the maximum snowmelt-rainfall runoff of the rivers of Ukrainian Polissya based on the integral curves of maximum water discharges and runoff depths, it can be noted that they are synchronous, and the inflection points of the integral curves almost coincide. At the same time, their overall shape may differ due to varying ratios of deviations, and as a result, the termination of identical hydrological phases according to these curves may not coincide in the multi-year context. Analyzing the chronological course of the time series of runoff values of the maximum snowmelt-rainfall runoff of the rivers of Ukrainian Polissya, it can be noted that all of them exhibit phases of cyclic fluctuations in water abundance, which determine the pronounced trends in the time series of maximum runoff. For the rivers of Ukrainian Polissya, the high-water phase was observed mainly until 1981-1982, while the low-water phase has persisted in the rivers until 2020. This provides the possibility of further application of statistical methods for the determination of the design parameters of maximum snowmelt-rainfall runoff of the rivers of Ukrainian Polissya, based on the available observational data on water runoff.

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