Seasonal Redistribution of Precipitation and Streamflow and Their Relationships in the Upper Heihe River Basin, China

Climate change is altering hydrological processes in alpine regions, and changes in streamflow seasonality and its relationship with precipitation inputs have important implications for regional water resource management. Based on monthly streamflow records from three hydrological stations (Zhamashike, Qilian, and Yingluoxia) in the upper Heihe River Basin during 1970–2025, together with corresponding precipitation data, this study investigated long-term variations in seasonal precipitation and streamflow distributions and their relationships using seasonal contribution analysis, Mann–Kendall tests, Sen’s slope estimation, and regression analysis. The results showed that: (1) precipitation remained strongly concentrated in summer, with a broadly stable seasonal pattern despite a significant increase in the spring precipitation fraction (0.946 percentage points per decade); (2) streamflow exhibited significant seasonal redistribution, with decreases in the summer streamflow fractions at Qilian and Yingluoxia and an increase in the spring fraction at Yingluoxia; (3) precipitation–streamflow relationships varied among seasons, with stronger contemporaneous relationships generally occurring in summer and weaker relationships in winter; and (4) the observed differences between precipitation and streamflow seasonality suggest that precipitation timing alone cannot fully explain seasonal runoff variability. Catchment storage and delayed water-release processes may contribute to these differences, although their individual contributions were not quantitatively assessed. These findings highlight the importance of considering seasonal runoff redistribution in water-resource assessments of alpine river basins.