Under conditions of increasing climatic uncertainty, this scientific study analyzes multi-year trends of seasonal distribution of river flow in the Zhaiyk–Caspian water basin. In specific natural conditions of the territory under consideration, under scarce water resources availability in the vast territory, and exceptional variability of river flow in time, the problem of water supply is especially important. The paper considers multi-year dynamics of runoff-forming factors and flow parameters taking into account phases of different water availability. River flow estimation methods include analyses of average monthly maximum and minimum values of river flow. The assessment of homogeneity of river flow series was carried out on the basis of genetic analysis and statistical analysis, application of Mann–Kendall test, Student's t test, and Fisher criterion. The identified results indicate significant transformations in the river flow in the basin: the share of spring flow has decreased, while the share of low-water flow, especially in winter, has increased. These changes are associated with higher winter temperatures and an increase in the frequency of winter thaws. In the River Elek Basin, the share of spring flow decreased to 40% of the annual value, and in the River Or Basin, to 15%.

In connection with the catastrophic floods in the spring of 2024 in the Western region of Kazakhstan, the goal was to conduct a critical analysis of the long-term dynamics of maximum water flow rates according to hydrological posts located along the main channel of the Zhaiyk River, for a retrospective assessment of the probabilistic recurrence of such values in 2024. The article discusses the main hydrological characteristics of floods on the Zhaiyk River for a long-term period up to 2021. Hydrological calculations, statistical analysis of long-term flow dynamics were carried out, and catastrophic years with high waters were identified in accordance with regulatory documents. The characteristic water levels in the years with correspondingly critical water consumption, as well as the area of flooding of territories, were studied, the time of the flood wave between hydrological posts, including the territory of Russia, was determined. In the future, it is planned to analyze the current situation for 2024, at the end of the flood and flood process in Western Kazakhstan.

In the context of intensifying climate change, it is particularly important to assess the transformation of spring floods as a key phase of the hydrological regime of rivers. This study provides a comprehensive analysis of the characteristics of maximum runoff in the Zhaiyk–Caspian basin for the modern period and projected changes for 2030, 2040, and 2050 based on CMIP6 climate scenarios (SSP3-7.0 and SSP5-8.5). Analysis of observations at 34 hydrological stations showed a reduction in spring runoff by up to 35%, a decrease in the duration of high water and a reduction in maximum water discharge on some rivers by up to 45%. It has been established that those rising temperatures, more frequent thaws, and reduced autumn moisture lead to lower maximum water discharge and a redistribution of the seasonal flow regime. Scenario projections revealed significant spatial heterogeneity: some rivers are expected to experience an increase in maximum discharge of up to 72%, while others will see a steady decline in maximum discharge of up to 35%. The results obtained indicate the need to transition to an adaptive water management system focused on the regional characteristics of river basins and the sensitivity of small- and medium-sized watercourses to climate change.

Studies were conducted aimed at the trends in the flow of the main rivers of the Zhaiyk-Caspian water basin. The average annual dynamics of river flow for a long period, their 5-year moving averages, joint tables of air temperature and precipitation values with river flow, as well as long-term dynamics of characteristic (highest, winter and lowest in open channel conditions) water flows were considered. According to the results of the research, two periods were distinguished: from 1940 to 1973 (conditionally natural) and from 1974 to 2020. (modern). In both periods, there is a tendency to reduce runoff against the background of an increase in air temperature, precipitation trends are multidirectional. Since the 1970s, there has been a decrease in the amplitude of fluctuations in annual runoff, since the 2000s there has been a decrease in both the amplitude of fluctuations and the values of maximum water flow. In the values of the lowest summer-autumn and winter water flow, on the contrary, there is a tendency to increase, which began in the late 1980s – early 1990s. For small rivers and temporary watercourses, the summer-autumn and winter periods are periods of maximum runoff depletion, during which drying and freezing phenomena (zero runoff) are characteristic.

This study examines the change in the timing of the appearance of ice phenomena in the basin of the Zhaiyk river associated with anthropogenic activity and climate change. To assess the long-term ice regime (dynamics of ice phenomena) and meteorological parameters, two periods were identified: before 1973 and 1974-2019 due to intense climate warming after the 1970s. A comparative analysis of the ice phenomena of the Zhaiyk river basin with meteorological indicators (air temperature) showed that the trend of precipitation tends to decrease slightly and there is a pronounced tendency to increase air temperature, in connection with this, the duration and date of the appearance of ice formations decreased for a later time. Estimates of changes in the considered characteristics of the ice regime of the Zhaiyk River basin for the period 1974-2019 are given. compared with the period of the conditionally stationary climatic situation (from the beginning of observations to 1973) using statistical methods.