Transformation of the Hydrological Regime of the Shu–Talas River Basin Under the Influence of Climatic and Anthropogenic Factors

Under current climate change and intensifying anthropogenic pressure, assessing the factors determining the transformation of river hydrological regimes has become particularly relevant. This study aims to identify the role of climate change and economic activities in forming the modern hydrological regime of rivers in the Shu–Talas basin. Data from 14 gauging stations located in Kazakhstan and the Kyrgyz Republic were used for the analysis. Streamflow series reconstruction was performed using hydrological analogy methods and hydrological modeling based on the HBV-light model. Changes in air temperature, precipitation, and river discharge were evaluated using the Mann–Kendall test, Sen’s slope, and Pettitt’s test. A steady increase in air temperature was identified, which was most pronounced in winter, with no statistically significant spatially coherent changes in precipitation. Shifts in atmospheric circulation were revealed, manifested by an intensification of zonal (westerly) and a weakening of easterly circulation types. A comparison of the 1961–1990 and 1991–2020 reference periods for gauging stations with complete daily records showed relatively small changes in mean discharge (from −0.68 to +1.78%) but a more pronounced seasonal redistribution of the streamflow. Winter discharge increased by 4.02–7.11%, whereas autumn discharge decreased by 10.98–23.09%. Analysis of the normalized flow duration curves confirmed that the changes were most pronounced in the range of high and intermediate daily flows. It shows that climate change significantly affects the timing and conditions of runoff formation, whereas the nature of anthropogenic transformation varies across different river systems in the basin. The Shu River and the Asy River basin are characterized by changes in the seasonal runoff structure due to reservoir operation, whereas for the Talas River, the main consequence of economic activities is a reduction in runoff volume resulting from intensive water abstraction for irrigation. The obtained results can be used to develop strategies for adapting the water management sector to climate change and to ensure sustainable water resources management in the basin.