| 16 Dec 2025
Integrating surface-active layer-permafrost hydrological processes: A systematic review and research framework
Abstract. 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 the current knowledge through an extensive literature review and systematic analysis, establishing a foundational framework for permafrost hydrology. The framework integrated three critical dimensions: surface hydrological processes, hydrological functions of the active layer, and the hydrological effects of permafrost changes. Subsequently, the current state, trends, and challenges in permafrost hydrology research were summarised, and a holistic overview was provided. 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 active layer thawing, slope hydrological processes, river channel dynamics, large-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. Finally, this study outlined future research directions, emphasising three key areas: the application of novel observational methods, integrated surface-subsurface investigations, and the ecological and environmental impacts of permafrost hydrological changes.
General comments
Thanks for the opportunity to review this manuscript. The authors aim to establish an integrated framework for permafrost hydrology, structured around (1) surface hydrological processes, (2) hydrological functions of the active layer, and (3) hydrological effects of permafrost change. The manuscript covers a wide range of topics, including freeze thaw cycle, thermokarst, surface–groundwater interactions, and baseflow changes. With the increasing attention to cold-region hydrology, this work could become a useful compilation for readers interested in hydrological processes in permafrost regions
While a framework for permafrost hydrology is introduced, the organization of the manuscript does not always clearly reflect this framework. In the manuscript, the section titles do not fully match the content. Similar processes are discussed more than once, but without clear distinction. In addition, many results on baseflow and runoff changes are listed, but they are not clearly summarized.
Major comments
The manuscript describes surface runoff, baseflow, ground ice, the active layer, and thermokarst mainly through text. A conceptual diagram or flowchart would be helpful to clearly illustrate the proposed framework, highlight key processes and important fluxes, and show how different runoff components change under permafrost degradation.
In Section 2.1 Impact of freeze–thaw cycles on surface runoff, the manuscript mainly discusses processes such as infiltration, active-layer storage, and subsurface runoff. Much of this discussion focuses on hydrological processes within the active layer, rather than surface hydrological processes. In addition, this section partly overlaps with Section 3. In Section 4.3 Impact of climate change on hydrology in permafrost regions: Increasing discharge and recharge to surface runoff, the section still focuses mainly on changes in baseflow and ground ice, while surface runoff is not analyzed in sufficient detail. As a result, the content does not fully correspond to the section title.
In the Introduction, lines 49–64, the authors emphasize the impacts of permafrost degradation on ecosystems and hydrology, while the topics of this manuscript surface runoff processes and the role of the active layer in runoff generation are briefly addressed. In addition, the Introduction first discusses permafrost degradation and then introduces surface runoff and active layer, which does not fully match the structure of the manuscript.
The manuscript discusses baseflow changes in different regions under permafrost degradation. However, the discussion mainly focuses on describing findings from the literature, while the mechanisms controlling baseflow change are not sufficiently analyzed. For example, Line 152 suggests that permafrost degradation leads to increased runoff in the Arctic, whereas Line 514 reports a decreasing runoff trend in the Yarlung Zangbo River region. The manuscript does not sufficiently discuss the conditions under which different runoff trends occur.
Sections 4.2 and 4.3 partly overlap, as both address mechanisms of baseflow increase associated with permafrost degradation. Although Section 4.2 focuses on temporal trends and Section 4.3 emphasizes spatial variability related to permafrost coverage, the distinction between these perspectives is not always clear. Further clarification or synthesis of regional baseflow responses would improve the presentation.
Although many studies are cited in Sections 2.1 and 4.2 to support the effects of permafrost degradation on surface runoff and baseflow, it would be helpful to include figures of runoff changes from typical catchments or analyses based on observation data. This would strengthen the conclusions and improve reader understanding.
In Section 5.1, the authors provide a future direction on the development of many observation approaches. However, the manuscript does not clearly identify which key hydrological fluxes remain poorly observed within the proposed framework, what the main limitations of current observational methods are, or how future techniques are expected to address these core challenges.
Detailed comments:
In Lines 204–206, the authors refer to data from 2011–2012, but it is not clear what these data represent. The author should clarify these data and include an appropriate figure to help readers better understand their significance.
Line 110: The statement that “Increased spring-summer runoff is primarily driven by snowmelt and active layer thaw, whereas permafrost limits deep infiltration, leading to an increased groundwater contribution during thaw” is confusing. The manuscript suggests that limited infiltration under permafrost conditions leads to increased groundwater contribution. However, it is unclear whether “groundwater” here refers to shallow suprapermafrost water or deeper groundwater systems. The authors should clarify the hydrological compartment and explain the physical mechanism.
Line 134: The introduction of aufeis (icing) in this paragraph appears abrupt. While aufeis is an important indicator of winter groundwater discharge and water storage, there is not clearly direct relevance to the channel morphology and lateral migration. The authors should clarify the connection.
Line 281: The manuscript states that permafrost type, underlying surface characteristics, topography, and soil composition are key variables controlling hydrological process. However, the subsequent discussion focuses mainly on soil and vegetation. The roles of permafrost type, underlying surface characteristics, and topography are not sufficiently analyzed. The authors should expand the other factors.
Lines 240~254, the authors describe runoff generation during freeze thaw cycles. However, this discussion is not supported by appropriate references. The relevant citations should be added in this section. Section 5.2 also lacks the related references.
Some technical terms are used inconsistently throughout the manuscript. For example, Line 24, it is unclear whether active layer thawing refers to the same process as the freeze–thaw cycle discussed later. Similarly, in Line 27, the relationship between permafrost thawing and permafrost degradation is not clearly defined. The authors are encouraged to review the manuscript and standardize the terminology where appropriate to avoid confusion.
The heading structure is inconsistent across the manuscript. Subheadings in Section 3 line 241~254 lack numbering, whereas subheadings in other sections are numbered. A consistent heading numbering scheme is recommended.