The relevance of the work is related to the study of water and salt regimes of the Danube Katlabukh Lake, which is one of the sources of water supply in the southern region of Ukraine. In this case, the hydrological regime of the lake is poorly understood, and some components of the water and salt balance do not have systematic observations, which requires the development of methods for their assessment and determination. In recent decades, water exchange in the lake, as an artificially regulated reservoir, has deteriorated, leading to a critical decrease in water levels and an increase in the mineralization of water in it. This has led to restrictions on the use of lake water for water management and drinking water supply. Therefore, an important task is the restoration and rational use of the natural resources of Katlabukh Lake, improving its hydrological and hydrochemical regimes in the interaction of natural and anthropogenic factors and in the conditions of present and future climate fluctuations, developing recommendations for improving the conditions of the reservoir's functioning.
Studies of the natural conditions of the Danube region of Ukraine, in particular, the Katlabukh basin, revealed that the formation of increased mineralization of the lake and, in particular, of the rivers that feed it (up to 6.0-7.5 g/dm3) is due to the high mineralization of groundwater (from 0.3 to 8.4 g/dm3), by the distribution of chloride-sulphate salt soils and salt marshes.
Climatically, the study area is characterized by a temperate continental climate with insufficient humidity. The annual rainfall for the Bolgrad and Izmail WGSs are 504 mm and 469 mm (respectively), and their greater mass (from 64-66% of the annual sum) falls in the warm period (IV-X). The average annual air temperatures at these stations are 10.8°C and 11.0°C and are higher by 0.5-0.4°C than the standard climate standard.
Analysis of the data on observations of air temperature and precipitation at the meteorological stations Bolgrad and Izmail as a whole confirms the available data on the current warming of the climate - the increase in annual air temperatures over the period 1961-2018 averages 1.5-1.7°C, synchronization is observed annual rainfall and temporal trends are insignificant. Regarding the evaporation values from the water surface (WGS Bolgrad), which are an expense component of water balances of reservoirs, with the average annual (for the period 1960-2018) the evaporation value for the year 800 mm, they tend to increase - for the period 1960 -2018. Evaporation from the water surface increased by 108 mm.
The research of the water regime of Katlabukh Lake, its rivers and the Danube have shown that they have long-term trends and seasonal variations, and for Katlabukh Lake they have a regulatory influence on hydraulic structures. Against the backdrop of not significantly decreasing water levels in the lake (1980-2018), in recent years (since 2012), there have been long-term seasonal reductions, with minimal levels sometimes below the dead volume (LDV = 0,7mBS). In the hydrologic regime of the rivers Veliki Katlabukh, Yenika and Tashbunar, which flow into Katlabukh lake, spring waterfalls are released, and after its passage the rivers often dry up. The development of anthropogenic use of rivers, as well as the current climatic conditions have led to a decrease in the water content of the rivers and a deterioration of water quality in them.
The paper investigates long-term and current trends in changes in the hydrological regime of the Danube River (on the Reni-Izmail area) as the main source of freshwater supply to the lake. The performed research has shown that positive trends in river water content (in the period 1840-2015), in particular maximum runoff (in the period 1921-2015), will contribute to the effective regulation of the filling regime of the Katlabukh lake.
In the studies of long-term series of average annual costs of suspended sediment it was found that there is a pronounced trend (in the period 1840-2015) to a decrease in the outflow of suspended sediment, especially in the present period of water of the Danube (1990-2015). This has the positive effect of reducing the siltation of the artificial channels that connect the lake to the Danube River and, if artificially cleared, may improve the water exchange of the lake with the freshwater of the Danube River.
The results of the water balance calculations showed that the major part of the revenue part for 1980-2018 was atmospheric precipitation (35.9%) and water inflow from the Danube River (41.2%), the volume of river inflow equal to 9.0%, the volumes of lateral inflow, ground and drainage waters are 2.2%, 4.9%, 6.5% respectively. The major part of the expenditures is evaporation together with transpiration (52.1%), water intakes for irrigation, water discharges to the Danube River and filtration are 21.2%, 20% and 5.9% in accordance.
