Characterizing changes in snowmelt runoff timing and their driving factors in the Upper Indus Basin

Climate warming is fundamentally restructuring the hydrological regimes of high-altitude cryospheric basins, presenting critical challenges for downstream water security. This study investigates the spatiotemporal shifts in snowmelt runoff timing across the Upper Indus Basin (UIB) and its six major sub-basins from 1971 to 2017. By integrating multiple indicators including Center Time (CT), Spring Pulse Onset (SPO), and Monthly Fractional Streamflow (MFS), we quantified the runoff response to multifaceted climatic forcing. Results indicate that while annual runoff shows an insignificant upward trend of 6.4 mm/10a, winter and spring discharge significantly increased, suggesting a systemic depletion of seasonal snowpack storage. Both CT and SPO analyses demonstrate a robust advancement of snowmelt timing across the UIB with rates of –1.1 and –2.2 days/10a respectively. Spatial heterogeneity in these shifts is driven by distinct cryospheric-climatic interactions: in the glaciated Karakoram regions and snow-dominant sub-basins such as Gilgit and Astore, runoff timing is primarily modulated by thermal forcing; conversely, the Kharmong basin, characterized by lower snowpack coverage, exhibits a more immediate runoff response to precipitation variations. These hydrological shifts characterize the increasing mismatch between meltwater supply and downstream irrigation demands during critical crop growing seasons. These findings provide a scientific basis for understanding the sensitivity of alpine hydrological processes to climate change.