The analysis of current scientific work on the use of statistical methods in hydrochemical research has shown that this approach is sufficiently substantial, both in Ukraine and abroad. The purpose of this work is to determine the main statistical parameters and to research the possibility of applying theoretical laws of distribution to the time series of water mineralization.This research presents the results of the application of standard statistical methods of hydrometeorological information processing for data on water mineralization at 28 gauges of the Dnipro basin (within Ukraine) for the period from 1990 to 2015. The dynamics of the obtained statistical parameters (long-term annual average, coefficients of variation, asymmetry and autocorrelation) within the Dnipro basin in Ukraine has been analyzed. The average annual values of mineralization vary substantially within the studied part of the Dnipro basin - in the northern part the maximum value of the annual average mineralization is 447 mg/l, as it moves to the south, the mineralization increases and in the sub-basin of the Middle Dnipro it reaches a maximum of 971 mg/l; the highest values are observed in the south (sub-basin of the Lower Dnipro), where they can reach extremely high values for particular small rivers (the Solon River - Novopavlivka village, 3356 mg / l). The long-term variability of mineralization in the rivers of the studied area is insignificant, and the autocorrelation coefficients of the mineralization series are quite high, in most cases they are significant and tend to decrease from the sub-basin of the Prypyat’ river in the north to the sub-basin of the Lower Dnipro river in the south. Within the framework of the presented research, the possibility of using theoretical distribution curves known in hydrology to describe the series of river mineralization, using the example of the Dnipro basin, has also been analyzed. Using Pearson’s fitting criterion, the Pearson type III distributions and the three-parameter distributions by S.M.Krytsky and M.F.Menkel have been verified on their correspondence with the empirical series of mineralization. As a result, it was found that in 85% of cases the Pearson type III distribution can be used, and the three-parameter by S.M.Krytsky and M.F.Menkel can be used in 60% of cases.

Introduction. This investigation aims at studying the individual components of the hydrological and hydrochemical regimes of the Danube River (within Ukraine) in connection with the widespread use of the river's water for water supply and irrigation in the southern region, as well as to ensure more effective regulation of water-salt regime of the Danube lakes using the Danube River as a main source for their water renewal. One of important aspects includes the study of the regime of the Danube River's suspended sediments brought to its delta due to their impact on the formation of the delta at the river's mouth of as well as due to the impact on siltation of deltaic and pre-delta lakes and canals connecting the lakes with the Danube River. Purpose. The purpose of the work is to study long-term and current trends related to changes in hydrological (water levels and discharges, suspended sediments runoff) and hydrochemical (mineralization) regimes of the Danube River within the Ukrainian interval from Reni to Izmail, internal annual distribution of water runoff, as well as suspended sediments runoff and mineralization during the years of varying water content.  Results. The statistical analysis of long-term series of observations over the Danube River within the interval from Reni to Izmail resulted in discovering that time trends of average, maximum and minimum water levels (1921-2015) and discharges (1840-2015) indicate the presence of a weak increase in their growth over time, with a more pronounced increase in maximum water levels or discharges. The annual distribution shows the general synchronicity of fluctuations in runoff characteristics, and the maximum monthly values of water discharge differ have more pronounced seasonal fluctuations. The study shows the presence of a pronounced trend to reduction of suspended sediments runoff of the Danube River at Reni (for the period of 1840-2015), with their most intensive decrease over the period of 1990-2015. Annual distribution of average monthly suspended sediments runoff of the Danube River for the years with typical water content (for the period of 1978-2015) showed that they have seasonal fluctuations. At the same time, there is a decrease in the suspended sediments runoff along the length of the river from Reni to Izmail. The long-term course of average annual mineralization values of the Danube River at Izmail (1981-2015) is characterized by their decrease against the background of a small increase in average annual discharges. As per the annual distribution of mineralization values associated with all water content groups there are the periods related to the phases of the river's yearly water regime during and the economic use of water. Conclusion. Thus, the increase in the long-term period of the Danube River runoff within the interval from Reni to Izmail will contribute to the development of the region's economy and water supply, irrigated farming, regulation of the Danube Lakes filling with weakly mineralized river water. At the same time, the reduction of the suspended sediments runoff will restrain the siltation of the inlet canals connecting the lakes with the Danube River, which will improve the water renewal of the lakes with the river's fresh waters.