The evaluation, attribution and ecological implications of downstream flow regime changes in a large transboundary basin in Central Asia

Study region The Ili River is a typical transboundary river between China and Kazakhstan. Study focus This study applied the Indicators of Hydrologic Alteration to investigate downstream discharge changes after Kapchagay Reservoir (KR) impoundment. Then a hybrid model was proposed to reconstruct the downstream flow under the hypothetical condition of no human activities in the KR interval catchment (HAKR), and the contribution of HAKR to change in downstream discharge was quantified. New hydrological insights for the region Significant decreases in annual maximum streamflow and summer streamflow were detected during the FS (former-Soviet) period at both stations, where substantially increased annual minimum streamflow at short- and medium- durations and winter streamflow during the PS (post-Soviet) period were observed. The hybrid model increased the Nash-Sutcliffe efficiency by 0.05–0.08 (calibration) and 0.04–0.06 (validation) over extreme gradient boosting (XGBoost). HAKR led to the homogenization of the intra-annual runoff. However, the degree of discharge homogenization induced by HAKR varied with time. Mean annual discharge reduction during the FS period was mainly driven by HAKR at both stations, whereas increased annual streamflow during the PS period was primarily attributed to other factors such as climate change and upstream inflow. HAKR dominated the decrease in summer streamflow regardless of time period or hydrological station. This study can support sustainable water management and provide insights for local ecological restoration in the Ili River.