The Danube River mouth is a main source for fresh water used for water supply and irrigation purposes in the arid southern part of Ukraine. In addition, the water of the mouth is used for filling the Danube Lakes. Climate change and numerous social and economic factors reduce the area and the water level of the Danube Lakes and increase their salinity. Under these circumstances the water exchange between the Danube River and the lakes is a very important process which allows maintenance of the water–salt balance of the latter. Such water exchange massively depends on the Danube River regime. The paper presents research of temporal variation of typical water discharges in the lower course of the Danube River associated with assessment of natural and anthropogenic factors affecting river runoff fluctuations. Time series of annual average water discharge at the water gauging stations of such cities as Reni and Izmail were considered uniform for the selected periods. They cover the period of conventionally natural river runoff regime (1840–1920), the period of the least altered river runoff regime (1921–1960) and the most altered river runoff regime under a heavy anthropogenic influence (1961–1989). The analysis of the impact of climate changes on the river runoff allowed introduction of a new, modern period of the Danube River hydrological regime (1990–2015). It is established that the series of maximum and minimum water discharges are characterized by the lack of their uniformity from the beginning of the period of initial influence of hydraulic engineering structures on the hydrological regime (since 1961). The analysis of the aggregate multi-year series (1840–2015) of average annual, maximum and minimum water discharges of the Danube River across its length from Reni to Izmail showed the presence of positive trends. At the same time the period of climatic changes (after 1989) is characterized by a less intensive growth of maximum water discharges. The research resulted in establishing the periodicity and synchronism of water discharge fluctuations in the lower course of the Danube River from Reni to Izmail within the territory of Ukraine. In particular, the study showed for the first time that from the 2000s onwards the redistribution of river runoff between the Danube River branches led to unsynchronization of minimum river runoff in different parts of the Danube Delta. The research shows that comprehensive analysis of the Danube River hydrological regime under modern climatic conditions is required to ensure efficient engineering regulation of the Danube Lakes filling pattern. Such regulation, in its turn, will ensure that the water supply and irrigation purposes are achieved.

Katlabukh Lake is a part of the Danube Lakes system and is one of the surface water sources for water supply, for agricultural needs and irrigation of the region. Changing the conditions of operation and regulation of the reservoir led to a decrease of water levels and an increase of salinity, which makes it impossible to use water for different management needs. Calculations of the water and salt regimes of the lake based on the solution of the equation of balance said that in the water balance of Katlabukh Lake the main volume of the revenue part for the period 1980-2018 was precipitation (36.1%) and water inflow from the Danube River (38%), and the expenditure part – evaporation together with transpiration (50.5%). Salt flow into the lake is mainly due to surface inflow (53.4%) and water of the Danube River (25.5%), and loss of irrigation (45.1%) and water discharges to the Danube River (31.9%). Simulation modeling of the water-salt regime of the Katlabukh lake under different conditions of exploitation of the reservoir showed that corrective management measures are needed to improve the qualitative indicators of the water in the reservoir. They consist in the fact that for three summer months it is necessary to carry out forced pumping of poorly mineralized water from the Danube River to compensate for evaporation from the water surface (on average in volumes of the order of 55 million m3) or to carry out fences of water from the lake for irrigation in 60 million m3. This will allow to reach the design values of water mineralization in the lake equal to 1.0-1.5 g/dm3. Thus, addressing a range of problems to conserve and restore the rational use of the natural resources of Katlabukh Lake requires effective managerial water management activities that require additional feasibility studies.