Generation of runoff in an alpine meadow hillslope underlain by permafrost

Permafrost plays an important role in hydrological processes of alpine regions. The frost table in the active layer on the permafrost acts as an impermeable boundary and regulates water generation from hillslopes and its routing to streams. Past studies focused on modes or critical conditions of flow generation, rather than on the capacity of the active layer on the permafrost to recharge flow. This study aimed to characterize the role of supra-permafrost groundwater in the generation of runoff on hillslopes during the active layer thawing processes. The study focused on an alpine meadow permafrost hillslope located in the northeastern Tibet Plateau during the months of July and August in both 2021 and 2022. Hydrometeorological variables, including precipitation, air temperature, soil temperature, soil moisture, thaw depths, supra-permafrost groundwater level, and runoff generation were monitored in field. Partial Least Squares Path Modeling was selected to analyze the relations between the above variables. The results showed that infiltrated rainwater tended to move into deep thawed soil, following which the frozen layer forced horizontal transport along the hillslope. This indicated that thaw depths along the soil profile regulated the dominant runoff path. The accumulated precipitation of the previous days had a significant impact on runoff generation. There was minimal lateral subsurface flow when the saturated zone was absent, whereas lateral subsurface flow increased with increasing thickness of the saturated zone. Runoff generation on the hillslope was regulated by both thaw depths and the thickness of the saturated zone along the soil profile. This study can act as a reference for runoff generation processes of permafrost hillslopes.