Probabilistic characteristics of river ice regimes in the Prypiat basin within Ukraine
Authors:   Ukrainian Hydrometeorological Institute, Afteniuk Oleksandr , Gorbachova Liudmyla 
Published: December 2023

Hydrometeorological support of different branches economics is closely related to the involvement of various hydrological information, in particular, the ice regime characteristics of rivers and reservoirs. The most complete and thorough generalization of observation materials of the ice regime characteristics of the water bodies of Ukraine was carried out in the 60s and 70s of the 20th century. In the future, such studies were carried out for individual river basins and for different observation periods. In addition, in most of these studies, the probabilistic characteristics of the river ice regimes were not determined. At the same time, the probabilistic characteristics significantly expand knowledge about the variability, magnitudes and timing of ice phenomena on water bodies. Such knowledge is important for the design and operation of hydraulic structures. The main objective of this research is the determine the probabilistic characteristics of the appearance dates of the main phases of the ice regime, as well as the duration of ice phenomena and continuous freeze-up on the rivers of the Prypiat basin. The investigation was carried out for the following indicators of the ice regime: appearance date of ice, date of freeze-up, break-up date (i.e., melt onset), date of ice disappearance, duration of continuous freeze-up, overall duration of ice. In the Prypiat River basin the data from 29 water gauges from the observations beginning to 2020 inclusive are using for research. The methodological approaches that developed by B.M. Ginzburg in 1969 were used to determine the probabilistic characteristics of the river ice regimes of the Pripyat basin. Thus, the analytical probability distribution of the appearance dates of the main phases of the ice regime were determined according to the Pearson curve of type III, and as its parameters were used the multi-annual mean—(D), the standard deviation (σD), and the asymmetry coefficient (Cs). The standard deviation was used instead of the variation coefficient, since the variation coefficient changes depending on the initial date when numerically expressing the multi-annual mean date of the ice indicator. When calculating the probabilistic distribution of the appearance dates of the main phases of the river ice regimes, their statistical series were arranged from early to late dates, i.e., low probability values were assigned to early dates, and large probability values were assigned to later dates. The calculated appearance date of ice indicator of certain probability was determined by the formula DP% =—(D) – σD F (P%, Cs), where F – the Foster number. The probabilistic characteristics of the duration of continuous freeze-up and overall duration of ice were also determined according to the Pearson curve of type III, but according to traditional parameters: the multi-annual mean, coefficients of variation and asymmetry. The empirical probability distribution for both the appearance dates of the main phases of the ice regime and duration of continuous freeze-up and overall duration of ice were determined by the Weibull formula. The parameters of the analytical curves by the method of moments are determined. The analytical curves of the dates of appearance dates of ice and freeze-up on the rivers of the Prypiat basin have a negative asymmetry, i.e., there is an asymmetry of the series relatively to the mean dates aside the later dates. At the same time, the analytical curves of the dates of break-up and of ice disappearance have the opposite trend, i.e., there is an asymmetry of the series aside the early dates. Most series of the duration of continuous freeze-up and overall duration of ice are also asymmetric aside decreasing their values. The asymmetry coefficients are little values. The values of the standard deviation of the appearance dates of main phases of the ice regime vary from ±14.4 to ±27.1 days.