The results of the analysis of long-term regional trends of the intra-annual distribution of river flow in the river Ural are presented. A database on the discharge of 9 rivers has been compiled, including information on the monthly average, maximum and minimum flow rates. The duration of the data series is 45–78 years; the maximum number of observations reaches 91 years for the Ural River (Orenburg). To identify the dynamics of seasonal runoff, standard methods of statistical analysis and classical methods adopted in hydrology (supply curves, differential-integral curves, etc.) were used. The long-term dynamics of the intra-annual distribution of river runoff is considered taking into account phases of different water content. It was established that the water regime of the rivers of the studied basin is characterized by a stable tendency to reduce the proportion of spring runoff and increase the proportion of low-flow runoff, especially in winter. It was revealed that despite the presence of large artificial reservoirs on the rivers of the studied basin (including the Iriklinsk reservoir of long-term regulation), the weather-climatic factor in the formation of river flow and the transformation of intra-annual flow remains the leading one. The increase in the share of winter runoff is due to an increase in the frequency of positive winter anomalies, the values and duration of winter thaws. The results of the study confirm a significant transformation of the annual flow of the rivers of the river basin. Ural, similar for other rivers in the European territory of Russia. Despite the commonality of the revealed trends, the extremely irregular long-term and intra-annual flow dynamics remain characteristic of the rivers of the basin under consideration, a specific feature of the rivers of the Kazakhstan type of water regime. These trends are most clearly identified for rivers whose catchments are in arid conditions of runoff formation (Ilek, Or, Kumak rivers, etc.), less clearly for the main river (Urals). At the same time, under favorable conditions for runoff formation (forest and forest-steppe lowlands of the Southern Urals, which perform a barrier-cyclone function), there is an almost complete absence of intra-annual transformations for the entire period under consideration. This applies to river basins – the right tributaries of the river Ural (r. Sakmara, Big Ik, etc.). The results obtained indicate an increase in the risks of problems in the use of water resources of the steppe regions, which requires increasing the efficiency of water use in all sectors of the economy.

Sivokhip Zhanna Tarasovna, Candidate of Sciences (Geography), Institute of the Steppe UB RAS, 11, Pionerskaya st., Orenburg, 460000, Russian Federation, e-mail: sivohip@mail.ru 

Pavleichik Vladimir Mihaylovich, Candidate of Sciences (Geography), Institute of the Steppe UB RAS, 11, Pionerskaya st., Orenburg, 460000, Russian Federation, e-mail: pavleychik@rambler.ru

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