Transboundary water security challenges in the Upper Heilong-Amur River Basin under climate stress: Insights from a hydrological perspective

● Grid-based method with correction factor improves spatial attribution of climate and human impacts on runoff ● GHRA method corrects spatial anomalies in historical and future flow attribution. ● Future water distribution projected in China–Russia–Mongolia transboundary region. ● Climate stress intensifies water security risks in future flow attribution results. The Heilong-Amur River Basin (HARB), the largest transboundary basin in northeastern Asia, is increasingly vulnerable to water-security risks arising from climate change and human activity, yet robust quantitative spatiotemporal attribution remains challenging. In this study, we developed a grid-based hydrological response attribution (GHRA) framework that integrates the Variable Infiltration Capacity (VIC) model with a cumulative slope change approach. By coupling a distributed hydrological model with attribution analysis and incorporating a physically based correction coefficient, the framework enables spatially explicit and physically consistent streamflow attribution. The GHRA was applied to the upper HARB for historical (1992–2017) and future periods (2025–2099). Results show that climate change and human activity contributed 49.57% and 50.43% to historical streamflow changes, respectively. Climate change is projected to dominate during the future period, contributing 71.11% under SSP2-4.5 and 76.91% under SSP5-8.5. Compared with uncorrected results, the GHRA reduced the standard deviation of attribution values by 11.33% and 20.74% under SSP2-4.5 and SSP5-8.5, respectively. Regionally, climate change dominates the Kherlen and Argun Rivers, whereas human activity—particularly land-cover change—remains the primary driver in the Zeya and Shilka Rivers. Projections based on CMIP6 data indicate that Russia contributes the largest proportion of water resources (63% and 67% under SSP2-4.5 and SSP5-8.5, respectively), followed by China (26% and 21%) and Mongolia (11% and 12%). Climate-driven stress increases spring floods along the Argun, Heilong-Amur, and Zeya Rivers and intensifies summer flooding in the Shilka and Zeya Rivers, exacerbating transboundary water security challenges.