Hillslope and river runoff generation in permafrost areas: a case study in the Qilian Mountains

Permafrost is widely distributed across the Tibetan Plateau and plays a significant role in regional hydrological processes. This study focuses on a permafrost catchment located at Dadongshushan Yakou, Qilian County, Qinghai Province, China. Water samples, including rainfall, soil water (encompassing mobile soil water, bulk soil water, and supra-permafrost groundwater), ground ice, and runoff from hillslope plot, were collected during various thawing stages from May to September in 2021 and 2022. Stable isotope and hydrochemical analyses were utilized to trace the sources of hillslope runoff. Additionally, a standard runoff plot on a typical hillslope and a gauging weir at the catchment outlet were established to monitor hillslope and river runoff from July to August in both 2021 and 2022. Analysis of the data yielded several key findings: (1) Ground ice meltwater and rainfall were identified as the primary sources of hillslope runoff in spring (approximately 70%) and summer (60-80%), respectively. (2) The thickness of the saturated layer emerged as the pivotal factor influencing lateral subsurface flow on permafrost hillslopes during the summer months. (3) Slow runoff in forms of subsurface flow and quick runoff originating from river channel rainfall accounted for more than 90% and less than 10% of the total river runoff during the summer period, respectively. These results indicate that the combined effects of rainfall and groundwater thawing contributed to the formation of a super-permafrost saturated soil layer, which subsequently initiates a chain process where the saturated soil layer interacts with rainfall to supply streamflow in the river channel. This study enhances our understanding of the hydrological significance of the active layer thawing process and supra-permafrost groundwater, providing a theoretical foundation for the future development of hydrological models in permafrost regions.