Integrating surface-active layer-permafrost hydrological processes: A systematic review and research framework

Climate warming has accelerated permafrost degradation, leading to significant challenges in water circulation and transformation. Since the early 21st century, especially during the last ten years, permafrost hydrology has garnered substantial attention, yielding a wealth of research outcomes. However, a comprehensive and systematic understanding of the permafrost hydrology remains limited. This study synthesised current knowledge through a comprehensive systematic review and established a Surface-Active layer-Permafrost (SAP) framework that integrated three critical domains: surface hydrological processes, hydrological functions of the active layer, and the hydrological effects of permafrost changes. The SAP framework reveals that permafrost hydrological responses are governed by a causal chain linking climate warming to permafrost degradation, active layer thickening, altered hydraulic properties, enhanced subsurface connectivity, and can ultimately increase baseflow and modified runoff partitioning in many regions. Regarding surface hydrological processes in permafrost regions, this study examined the impacts of freeze–thaw cycles on surface runoff from multiple perspectives, including the influence of slope hydrological processes, river channel dynamics, basin- to regional-scale permafrost hydrology, and the hydrological consequences of thermokarst formation. For hydrological processes within the active layer, the study identified the hydrological role of the active layer, key factors influencing its hydrological behaviour, and the interactions between suprapermafrost water and soil water. Concerning the hydrological impacts of permafrost thaw, this study investigated the transformation dynamics between surface and groundwater in permafrost regions by analysing the effects of climate change through increased baseflow, groundwater recharge, and subsurface runoff. The current state, trends, and challenges in permafrost hydrology research were summarised, and a holistic overview was provided. Finally, future research directions were outlined, emphasising three key areas: the application of novel observational methods, integrated surface-subsurface investigations, and the ecological and environmental impacts of permafrost hydrological changes.