Asymmetric impacts of climate warming and vegetation restoration on streamflow in the Ji-shaped bend of the Yellow River: Evidence for threshold effects

Study region Six representative basins within the Ji-shaped bend of the Yellow River. Study focus This study aims to investigate the nonstationary responses of streamflow to climate warming and vegetation restoration. A Comprehensive Ecological Index ( CEI ) was developed, and a time-varying moments model was improved to better characterize periodic variations in distribution parameters. By integrating enhanced nonstationary statistical modeling with explainable machine learning, we systematically evaluated streamflow variation characteristics and quantified the marginal contributions and threshold effects of key driving factors. New hydrological insights for the region (1) Streamflow variations exhibit pronounced spatial heterogeneity: annual streamflow in the drier northern basins showed a significant declining trend (up to 6.82 mm/10 years), whereas in the wetter southern basins, variations were dominated by periodic patterns. (2) The marginal contribution of precipitation to annual streamflow variation was 31.6–61.0%, showing a positive effect above its long-term mean. Temperature contributed 18.5–39.4%, with low temperature promoting streamflow and high temperature suppressing it in drier basins, whereas the opposite pattern was observed in wetter basins. The CEI accounted for 7.9–38.4% of the variation. (3) In the drier northern basins, the effect of vegetation restoration on streamflow shifted from promotion to suppression when CEI exceeded a threshold of approximately 0.25. These asymmetric impacts and threshold behaviors provide important insights for water resource management and ecological restoration in semi-arid regions.