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The aim of the study is to substantiate the probabilistic-forecasting method for forecasting the low flow discharge and its implementation for the Pivdennyi Buh, Black Sea area and Lower Dnieper rivers, taking into account climatic patterns of precipitation distribution and establishing probabilistic characteristics of low flow discharge in multi-year period. The study area is in a zone of significant risk due to the shortage of water resources, the formation of extremely low runoff in the dry flow period, which requires its definition and forecasting. The methodological basis of forecasts is to solve the equation in determining the components of the low flow of rivers by constructing regional dependences for forecasting the average decade summer-autumn low flow from previous water discharge (in runoff modules), establishing their probabilities water discharge for a number of intakes. Results. The paper substantiates the probabilistic-forecasting method for forecasting low flow discharge (in summer, autumn and winter periods) based on the construction of regional dependences of average decadal runoff modules on previous water discharge for groups of basins of studied rivers taking into account climatic dependences of precipitation distribution in the territory and the establishment of probabilistic characteristics of the low flow water discharge in a multi-year period. The methodology of territorial short-term forecasts of average decade water discharge of low flow of summer, autumn and winter river runoff is assessed as satisfactory with a margin of error of 70 % to 97 %, with a number of members of more than 500 points. To determine the cumulative probability of the forecast values of the average decade water discharge of the summer, autumn and winter dry weather flow, the empirical distribution of the average monthly water discharge in the limited seasons, which are generalized in the basins of the studied rivers of Ukraine, is established. Scientific novelty. For the first time for the zone of insufficient natural water content of rivers the method of territorial forecasts of low flow discharge, determination of their probability of occurrence in a multi-year period and cartographic representation of prognostic values have been developed and practically implemented. The practical importance is the use of forecast modules maps of low flow for spatial monitoring and assessment of low water levels on rivers in the whole region, including ungauged rivers, and their probabilistic characteristics – to assess the possible occurrence of low runoff, even when it reaches values close to the environmental runoff, which are critical for the functioning of the river ecosystem.

The Southern Buh river basin is located in three natural zones of Ukraine and is heterogeneous in terms of physical and geographical conditions of river runoff formation. Purpose. Carry out hydrological zoning of the Southern Buh river basin with the allocation of areas with the same conditions for the spring floods formation by a set of morphometric characteristics of rivers and their basins and hydrometeorological and agrometeorological factors. Methods. Using a statistical model of factor analysis (R-modification) the most significant factors from the totality of all features were identified. There are two factors – the first describes 38% of the total variance of all factors (latitude of catchment centers, average height of catchments, wetlands, water reserves in the snow cover, precipitation of snowmelt and maximum depth of soil freezing), and the second – 21% (river length, catchment area and the amount of precipitation that fell after snowmelt). Factor loads were used for further grouping of basins using the method of cluster analysis. Results. As a result of territory zoning, two hydrological districts with sub-districts (district I and district II with sub-districts IIa, IIb, IIc) were identified. Area I covers the area from the source to hydrological post on the Southern Buh river basin – Trostyanchik village. Downstream and to the river mouth, the territory covers area II, which is divided into three sub-areas (sub-area IIa, IIb, IIc). Data from small rivers between the Dniester and the Southern Buh are involved in the hydrographical network. The boundaries of hydrological districts in the Southern Buh river basin were drawn along the watersheds of rivers, taking into account the physical and geographical zoning of the territory and involving in the analysis of maps of soil cover and vegetation in the basin. The hydrological zoning of the Southern Buh river basin under the conditions of spring flood formation is in good agreement with the zoning of the plain territory of Ukraine, which was performed by different authors over time. Conclusions. The use of statistical methods in the work allowed to clarify the boundaries of existing districts and identify new sub-districts in the Southern Buh river basin. The analyzed natural conditions have shown that within the limits of hydrological zoning they have certain features of spring floods formation. Such features will be used in the substantiation of the regional method of long-term forecasting of the characteristics of spring flood in the Southern Buh river basin.

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