Four databases were consulted: Web of Science, Scopus, Google Scholar, and the Chinese National Knowledge Infrastructure (CNKI). The search covered the period from 1986–2025, with a particular focus on studies published after 2015 to capture recent advances in the field. Core search terms were used in various combinations, including “permafrost”, “active layer”, “hydrology”, “runoff”, “baseflow”, “thermokarst”, “surface water–groundwater interaction”, and “climate change”. Boolean operators (AND, OR) were applied to refine the search. For example, (“permafrost” OR “frozen ground”) AND (“hydrology” OR “runoff” OR “baseflow”) was used to identify studies focusing on hydrological responses. In the Chinese databases, equivalent Chinese terms were employed (e.g., “”, “”, “”, “”, “”). To minimize publication bias, no restrictions were placed on study outcomes (i.e., positive, negative, or non-significant results were all considered). In addition to database searches, the reference lists of key review articles and the included studies were manually screened to identify potentially relevant publications not captured by the electronic search (snowball sampling).

Studies were selected based on several considerations. Priority was given to peer-reviewed articles, books, and major reports (e.g., from AMAP, IPCC) that focused on hydrological processes in permafrost-affected regions (including high-latitude and high-altitude areas). The screening process was conducted in two stages. In the first stage, titles and abstracts were evaluated against the inclusion criteria, leading to the exclusion of a large number of records that were clearly off-topic (e.g., permafrost engineering studies lacking hydrological components). The full texts of the remaining records were then assessed in the second stage, during which further records were excluded, primarily due to the absence of quantitative hydrological data or only incidental relevance to permafrost conditions. Ultimately, 215 peer-reviewed studies were retained for the formal evidence synthesis, and their full list is provided in Table S1 in the Supplement. In addition, Table S2 in the Supplement lists seven other items cited in the main text – conference abstracts, methodological references, and non-permafrost comparative/background studies – which were not included among the 215 studies used for the formal evidence synthesis.

The selected studies were categorized according to spatial scale (including plot, hillslope, watershed, and regional scales), permafrost type (continuous, discontinuous, sporadic, and isolated), hydrological process focus (surface runoff, active layer processes, and permafrost thaw impacts), and methodology (field observations, numerical modeling, isotopic tracers, and geophysical techniques). This categorization scheme follows established frameworks for synthesizing permafrost hydrological research (Walvoord and Kurylyk, 2016; Tananaev et al., 2020) and ensures comparability across studies.

For each included study, key information was extracted and recorded in a standardized spreadsheet. This information included study location and climatic setting; permafrost characteristics (type, ground ice conditions, active layer thickness); observed hydrological variables and their temporal resolution; quantitative results (e.g., contribution percentages, trend magnitudes, statistical significance); methods used; and reported uncertainties and limitations.

The synthesis followed a four-stage procedure, moving from individual study coding to the development of a cross-cutting framework.

We began by coding and thematically grouping the extracted information. An initial coding scheme was developed iteratively during a first reading of 30 key papers, and was then applied to capture hydrological process type (e.g., infiltration, runoff generation, baseflow, groundwater–surface water interaction), controlling factors (permafrost type, climate, topography, vegetation), reported mechanisms, quantitative relationships, and stated uncertainties. These codes were later aggregated into higher-level themes corresponding to the three SAP domains: surface processes, active layer functions, and permafrost degradation effects. The coding scheme was refined through co-author discussion; a second researcher independently coded a random sample of 20 studies, yielding an inter-coder agreement of >85%.

After the initial coding, we synthesised the evidence within each thematic group by first summarising consensus findings and then flagging divergences. Where multiple studies reported comparable metrics (e.g., ground ice melt contribution to runoff), we compiled the values to identify ranges and patterns. These compiled results are presented in a summary table (Table 1) in the results section. Contradictory results were examined in relation to differences in permafrost type, climate, topography, or methodology, and are discussed in Sect. 5.3.3. To reduce interpretation bias, we followed a narrative synthesis approach, consistent with systematic review guidelines for environmental sciences (Haddaway and Bayliss, 2015).

In the third stage, we integrated qualitative and quantitative evidence. Where comparable metrics existed, we summarised quantitative data (percentages, trend magnitudes). Qualitative evidence – descriptions of processes and conceptual models – was used to explain the observed patterns and to define the boundary conditions under which the reported responses occur. Where quantitative evidence was sparse or inconsistent, we gave more weight to qualitative information in building the conceptual framework.

Finally, we conducted a cross-cutting synthesis that brought together findings from the three domains and examined interactions among surface, active layer, and permafrost processes. This integration underpins the SAP framework presented in Sect. 6, which links climate forcing to hydrological responses via permafrost degradation, changing hydraulic properties, enhanced subsurface connectivity, and modified runoff partitioning. We did not perform a formal meta-analysis, owing to the heterogeneity in study design, metrics, and spatial scale across the literature. Nonetheless, the structured categorisation and data extraction allowed for a consistent and transparent synthesis that supports the proposed framework.

Spring snowmelt is a primary driver of surface runoff in permafrost catchments. During the early thaw phase, the shallow thaw front and cold soil temperatures cause infiltrating meltwater to refreeze, greatly reducing infiltration capacity (Bayard et al., 2005). As a result, meltwater accumulates at the surface or within the uppermost soil layers and flows overland, generating saturation-excess runoff that dominates the early spring hydrograph (Wang et al., 2009, 2017). This mechanism is particularly pronounced in continuous permafrost regions where the frozen substrate acts as a near-impermeable barrier.

In summer, surface runoff becomes intermittent and is largely limited to high-intensity rainfall events. Observations on the Tibetan Plateau show that summer runoff events typically last less than 24 h and are often absent when antecedent soil moisture is low (Gao et al., 2018a). During this period, the active layer has deepened, increasing infiltration capacity and reducing the proportion of rainfall that becomes surface runoff. However, when rainfall intensity exceeds the soil infiltration rate – a condition that can occur even in a thawed active layer – infiltration-excess (Hortonian) overland flow can occur locally, especially on sparsely vegetated or compacted surfaces (Zhang et al., 2017a).

The relative dominance of saturation-excess versus infiltration-excess surface runoff thus shifts seasonally: saturation-excess prevails in spring and autumn when the active layer is either partially frozen or freezing, while infiltration-excess becomes more relevant during summer storms on soils with limited infiltration capacity. This seasonality is modulated by soil properties, vegetation cover, and the depth of the thaw front.

Permafrost strongly influences how water moves across hillslopes. Because the permafrost table restricts vertical percolation, water that does not evaporate or transpire is forced to flow laterally near the surface, creating shallow subsurface flow paths that can emerge as surface runoff in downslope areas (Aas et al., 2019; Tananaev and Lotsari, 2022; Del Vecchio et al., 2024). Watersheds in permafrost regions exhibit lower drainage densities than those in non-permafrost regions, likely because frozen soil inhibits channel development. Climate warming is expected to lower the incision thresholds and promote the growth of channel networks in permafrost landscapes (Nitzbon et al., 2020; Tananaev and Lotsari, 2022; Del Vecchio et al., 2024). Thawing permafrost, melting ground ice, and changing hydrological regimes can expand channel networks and enhance hydrological connectivity (Rowland, 2023). Studies on the Niaqunguk River watershed in Nunavut, Canada have revealed that the hillslope structure and thaw processes create distinct fill-and-spill domains. Surface and subsurface domains influence sporadic hillslope-stream connections during periods of sufficient rainfall and saturation (Chiasson-Poirier et al., 2020). Observations from 11 headwater streams in Alaska have shown that hillslopes with continuous permafrost have shallower flow paths and higher evapotranspiration rates than non-permafrost hillslopes, underscoring the role of permafrost in sustaining surface soil moisture and promoting surface runoff generation (Sjöberg et al., 2021). These findings indicate that freeze–thaw processes are closely linked to hillslope hydrological connectivity and runoff patterns.

In addition, water tracks – unchannelized depressions that concentrate surface and subsurface flow down hillslopes – represent an important but understudied component of permafrost hillslope hydrology (Del Vecchio and Evans, 2025). These features dominate hillslope hydrology in some permafrost landscapes, serving as critical conduits that modulate water and sediment flux from uplands to stream channels (Tananaev et al., 2020; Del Vecchio and Evans, 2025). Water tracks occupy a flow regime transitional between purely hillslope drainage and fluvial channels, and their hydrological behavior is strongly influenced by permafrost conditions, including active layer thickness, ground ice content, and subsurface hydraulic connectivity (Sjoberg et al., 2021; Del Vecchio and Evans, 2025). Under warming conditions, permafrost degradation can enhance water track development by increasing subsurface flow and connectivity, potentially accelerating the delivery of water, sediment, and nutrients to streams (Rowland, 2023). However, the global occurrence, morphology, climate relationships, and geomorphic roles of water tracks remain understudied, representing an important avenue for future research.

Seasonal changes in channel morphology are becoming more pronounced in permafrost-controlled valleys, with important implications for hydrological connectivity, flow routing, and sediment transport. Higher low bank and reduced high bank retreat rates lead to accelerated floodplain subsidence (Tananaev and Lotsari, 2022). Permafrost degradation weakens riverbanks, increasing lateral channel mobility, although large Arctic rivers have experienced a decrease in lateral migration over the past half-century due to indirect effects such as bank shrubification and reduced overland flow (Ielpi et al., 2023). In the Low Arctic, permafrost thaw and talik formation alter streamflow regimes, with riparian shrub expansion indicating widespread change (Liljedahl et al., 2020). Aufeis (icing) formed from groundwater discharge is an important component of water storage and can influence channel processes. Although aufeis is primarily a winter phenomenon, its spring melt can contribute to channel scour, sediment transport, and modification of channel geometry (Yoshikawa et al., 2007). For example, winter aufeis volumes account for 27 %–30 % of the annual groundwater discharge in the Arctic Kuparuk River (Yoshikawa et al., 2007), representing a substantial water flux that, when released in spring, can affect channel stability and floodplain inundation. These findings underscore the stable, long-term, and continuous contribution of aufeis to surface-water recharge.

The presence or absence of permafrost significantly influences soil water storage capacity on a large scale (basin to regional scales, typically >102-104km2), determining whether water cycles are driven by hydrological or ecological processes (Ding et al., 2000; Young-Robertson et al., 2021). At smaller scales (plot to hillslope), hydrological processes are primarily controlled by local factors such as soil properties, vegetation, and microtopography. At larger watershed to regional scales, the integrative effects of permafrost distribution, climate gradients, and landscape heterogeneity become dominant factors, leading to emergent hydrological behaviors that are not simply the sum of local-scale processes (Tananaev et al., 2020; Shirley et al., 2022). In continuous permafrost regions, hydrological processes are primarily controlled by the active layer depth and soil water storage capacity (Fig. 1a). In discontinuous permafrost zones, hydrological processes exhibit a transitional shift from being dominated by active layer thickness and soil water storage capacity to increasing ecological regulation (Fig. 1b), with varying hydrological-ecological linkages contingent upon the degree of permafrost degradation. In sporadic or isolated permafrost and seasonal frost regions, ecological processes play a significant role in influencing the surface runoff capacity (Fig. 1c). In low-gradient watersheds underlain by continuous permafrost, evapotranspiration often exceeds precipitation during the summer months, leading to the drying of extensive wetlands and the fragmentation of drainage networks (Kane et al., 2003). The modelling results indicate that rapid permafrost thaw can lead to soil drying, surface warming, and reduced relative humidity in the Arctic–Subarctic regions (Kim et al., 2024). Land-cover changes resulting from permafrost degradation have a substantial impact on runoff. For example, studies in northwestern Canada have shown that land-cover change is the single most important factor contributing to increased runoff, accounting for approximately 35 % of the total increase (Connon et al., 2014). Increased rainfall accelerates permafrost thaw, with observations in Alaska indicating that each additional centimetre of rain leads to 0.7±0.1cm of additional thaw, particularly in disturbed and wetland sites (Douglas et al., 2020). Permafrost thaw can also alter river discharge, but the sign and magnitude of change are region-dependent. Arctic-wide studies have reported significant changes in water export, spring freshet, and summer stream intermittency (Feng et al., 2021).

Surface runoff generation in permafrost regions is controlled by freeze–thaw cycles through their seasonal modulation of infiltration capacity. Saturation-excess overland flow dominates in spring and autumn, when the active layer is partially frozen, whereas infiltration-excess overland flow occurs during summer storms on soils with limited infiltration capacity. The presence of a frozen layer near the surface restricts infiltration and promotes overland flow; as the active layer thaws, infiltration capacity increases, surface runoff diminishes, and subsurface flow is enhanced. The relative dominance of these runoff mechanisms depends on permafrost type (continuous versus discontinuous), soil properties, vegetation cover, and precipitation intensity. The timing and magnitude of surface runoff responses to permafrost degradation remain poorly constrained, particularly in mountainous regions where observational data are sparse.

Studies have demonstrated that the annual processes within the active layer exert significant hydrological impacts (Cooper et al., 2011; Lacelle and Vasil'chuk, 2013; Wang et al., 2017; Gao et al., 2018b; Ding et al., 2020b; Cao et al., 2021a; Qin et al., 2024). Water within the active layer comprises surface snowmelt, rainfall infiltration, soil water, and suprapermafrost water (Chang et al., 2015; Yang et al., 2017; Cao et al., 2018; Xiao et al., 2025b; Grewal et al., 2025). Rainfall infiltration serves as the primary driver of soil hydrological processes, with the freeze–thaw front acting as the main limiting factor (Xu et al., 2024). Figure 2 shows the hydrological dynamics within the active layer in different freezing-thawing periods. During spring, as the surface temperature rises above zero, the active layer begins to thaw, which coincides with snowmelt. In the initial thaw phase, the shallow thaw front and low soil temperatures caused infiltrating snowmelt to refreeze, significantly reducing infiltration rates. During this stage, the still-frozen portion of the active layer functioned as an impermeable barrier, leading to high surface runoff coefficients and substantial snowmelt runoff. In the mid-thaw phase, as soil temperatures increase and the thaw front deepens, water from surface snowmelt and soil thaw accumulates above the thaw front, forming shallow groundwater within the active layer. As the active layer deepens, the soil infiltration capacity increases, allowing snowmelt or rainfall to rapidly infiltrate. This process elevates soil moisture levels and triggers a rapid increase in shallow groundwater levels. Groundwater storage and flow capacity generally increase with active layer depth, although the relationship may be nonlinear and is modulated by factors such as soil texture, organic matter content, and ground ice distribution (Wang et al., 2017; Sjoberg et al., 2021). Consequently, subsurface runoff gradually increases, and surface runoff coefficients decrease as the active layer deepens. In the late thaw phase, as the active layer continues to deepen, the soil pore space expands, and precipitation infiltration becomes the primary source of groundwater recharge, replacing meltwater. This shift has led to the formation of precipitation-dominated subsurface runoff. During this stage, groundwater levels gradually decrease, and the upper soil layers begin to dry. By September, the active layer is fully thawed, and suprapermafrost water accumulates above the permafrost table. The height of the suprapermafrost water table is strongly influenced by the amount of precipitation recharge during the earlier stages. From the mid-thaw to the completely thawed stage, the available storage capacity within the suprapermafrost zone increases with active layer deepening. Where recharge does not keep pace with enhanced lateral drainage, a portion of the stored water is progressively released as baseflow, leading to a deepening of the water table (i.e., an increase in water table depth below the ground surface) over the thaw season. Yet, the water-table response to active-layer deepening is not universal and may be offset by local meltwater supply.

The impact of the active layer on surface runoff is most pronounced during the spring thaw and autumn freeze periods (Qin et al., 2024). During the spring thaw, surface snowmelt or precipitation often generates runoff through saturation-excess overland flow, which dominates surface runoff in the early thaw phase. In the mid-thaw phase, as the soil infiltration capacity increases, the surface runoff transitioned from saturation-excess to infiltration-excess overland flow. During the late and fully thawed phases, soil drying allows rapid precipitation infiltration, making surface runoff intermittent and limited to heavy rainfall events. In autumn, as the surface begins to freeze, soil infiltration decreases, and surface runoff reverts to saturation-excess overland flow (Wang et al., 2017; Zhang et al., 2017a; Gao et al., 2022). Driven by the temperature gradient, unfrozen water in the active layer migrates from bottom to top during the freezing stage. These dynamics demonstrate that runoff generation in permafrost regions involves multiple mechanisms controlled by temperature, and exhibits distinct seasonal variations. Observations and simulations of small watersheds on the Tibetan Plateau have revealed distinct seasonal hydrological dynamics (Cao et al., 2021b). During the spring flood period, the soil temperature and moisture at depths above 65 cm play a dominant role. In contrast, during the summer dry period, the soil moisture above 40 cm is negatively correlated with runoff, whereas deep soil temperature and moisture are the primary controlling factors. Air temperature and precipitation are the main drivers of hydrological processes during the summer flood period (Li et al., 2009; Chang et al., 2024; Xu et al., 2024). During the spring thaw, saturation-excess overland flow accounts for 66 %–85 % of the total spring runoff, and interflow within the active layer contributes 14 %–34 % (Wang et al., 2017). In the autumn runoff recession period, suprapermafrost groundwater runoff decreases rapidly as the active layer freezes, leading to an increase in the proportion of subsurface to total runoff and higher surface runoff coefficients (Bai et al., 2012; Wang et al., 2017). In the northern wetlands of Canada, the storage capacity in continuous permafrost regions increases during summer as evaporation reduces the peat moisture content (Roulet and Woo, 1986). The subsurface flow remains minimal owing to low hydraulic gradients and conductivity (Quinton et al., 2009). Consequently, when the water table lies within the peat layer, the contribution of wetlands to streamflow is negligible. However, during intense storms, when the water table rises above the ground surface, streamflow increases rapidly owing to the surface flow across the wetlands (Carpino et al., 2025; Lhosmot et al., 2025).

The active layer acts as a dynamic hydrological regulator. Its seasonal thaw progression controls the partitioning between surface and subsurface flow, with threshold behavior related to ground ice content. The active layer is the critical interface that mediates water exchange between the surface and the permafrost.

The primary factors influencing the hydrological processes in the active layer include permafrost type, underlying surface characteristics, topography, and soil composition. Understanding the hydrological processes in permafrost regions requires an integrated analysis of both water and heat balance relationships (Wang et al., 2017; Zhang et al., 2017b; Gao et al., 2021, 2022). Permafrost type exerts a first-order control. In continuous permafrost, the active layer is typically shallow and the permafrost table acts as a near-impermeable boundary, promoting lateral subsurface flow (Yoshikawa et al., 2006). In discontinuous and sporadic permafrost, the permafrost table is deeper and discontinuous, allowing more vertical connectivity and deeper groundwater circulation (Evans et al., 2015; Song et al., 2022). Topography modifies these effects: slope angle and aspect influence solar radiation, thaw depth, and drainage pathways. Hillslope position affects water accumulation and flow path development, with convergent areas often exhibiting deeper active layers and enhanced subsurface flow (Sjöberg et al., 2021; Chiasson-Poirier et al., 2020). Underlying surface characteristics – including geology, organic layer thickness, and land cover – determine surface infiltration capacity and heat transfer. For example, peat plateaus with thick organic layers insulate permafrost, while exposed mineral soils promote deeper thaw (Quinton and Baltzer, 2013). Soil composition, particularly texture and organic matter content, directly influences hydraulic conductivity and water-holding capacity. Soil physical properties play critical roles in regulating soil water and heat dynamics in these regions (Zhang et al., 2017b; Yang et al., 2019a).

Observations indicate that as vegetation cover decreases, soil moisture decreases at depths of 0.2–0.6 m in the active layer but increases at depths of 0.6–0.8 m. In seasonally frozen ground, soil moisture increases with depth throughout the profile (0.2–1.0 m). Thus, changes in vegetation cover significantly alter the soil water and heat processes, although the effects differ between permafrost and seasonal frost (Hu et al., 2009). Observations and simulations have demonstrated that the initial, stable, and cumulative soil infiltration rates are strongly influenced by the underlying surface types. The highest initial and stable infiltration rates were observed in the alpine desert steppes, whereas the lowest rates were observed in the alpine meadows. Within the same underlying surface, the infiltration parameters increase with vegetation cover, whereas soil moisture and infiltration rates exhibit opposite trends (Wang et al., 2014; Hu et al., 2018). Soil organic matter plays a dual role by providing both insulating and water-retaining effects. Its content in the active layer not only mitigates deepening, but also significantly influences water movement (Roulet and Woo, 1986; Karra et al., 2014; Hu et al., 2015; Kalyuzhnyi and Lavrov, 2017; Tananaev et al., 2022). Simulations of soil moisture in permafrost regions revealed that despite increases in net atmospheric surface water flux (P-ET), most models predict long-term drying of surface soils (0–20 cm) owing to active layer deepening or permafrost thaw (Liu et al., 2020). This suggests that future increases in precipitation may not fully offset the effects of active-layer deepening on surface soil moisture reduction (Andresen et al., 2020). In summary, water movement within the active layer is a complex process shaped by the interactions between water and heat dynamics, active-layer thickness, permafrost type, soil composition, and vegetation cover.

The interaction between suprapermafrost water and soil water is a critical component of hydrological processes in permafrost regions. Hydrological dynamics within the active layer are not only driven by freeze–thaw phase changes but are also closely linked to unconfined groundwater dynamics. During freeze–thaw cycles, surface and subsurface runoff undergo redistribution and transformation, resulting in frequent short-term fluctuations in surface and subsurface storage (Tananaev et al., 2020; Cao et al., 2021a, b). Simulations in the source region of the Lancang River demonstrated that cold-season warming increases liquid soil moisture in seasonally frozen ground, whereas warm-season warming primarily drives active layer thickening, increasing the liquid water content in permafrost. Cold-season warming has a greater impact because of its significant role in accelerating permafrost degradation (Xu and Gao, 2022). For liquid soil moisture, increases during cold months are attributed to rising soil temperatures and enhanced soil ice melting, whereas changes during warm months result from competition between positive precipitation and negative soil temperature effects (Cuo et al., 2015). These findings highlight the interdependence of suprapermafrost water and soil water-heat dynamics (Cao et al., 2018; Du et al., 2019; Yi et al., 2021). The interflow within the active layer migrates downward and accumulates above the permafrost table. This accumulated suprapermafrost water not only generates subsurface runoff through gravitational drainage but also interacts with permafrost through water exchange. Infiltration of groundwater from the surface carries heat, leading to the partial thawing of permafrost, whereas a portion of the groundwater freezes and is incorporated into permafrost (Wang et al., 2017, 2018d; Yang et al., 2017; Gao et al., 2018a; Zhao et al., 2019; Halla et al., 2021; Ma et al., 2021b; Hilbich et al., 2022). Under long-term stable conditions, the amounts of thawed permafrost and frozen groundwater remain balanced. Studies on the Tibetan Plateau have revealed that active-layer water accounts for 59 %–87 % of the ground ice near the permafrost table, whereas permafrost water contributes 13 %–41 %, which indicates that the active layer plays a significant role in ground ice formation, with alpine meadows contributing less (59 %–69 %) than alpine steppes (70 %–87 %) (Wang et al., 2018c). Simulations of daily runoff in the Yenisei River, the largest Arctic river in central Siberia, demonstrated that subsurface runoff within the active layer significantly contributes to river recharge (Fabre et al., 2017), underscoring the critical role of subsurface runoff in sustaining river flow. Permafrost hydrology is significantly influenced by suprapermafrost water, as a portion of ground ice is melted and integrated into the active layer hydrological processes. The seasonal dynamics and movement of suprapermafrost water are primarily regulated by soil temperature, whereas its phase changes depend on deep soil moisture and the sources of suprapermafrost water recharge. The relationship between active layer soil temperature and suprapermafrost water table dynamics can be effectively characterised by the Boltzmann function (Chang et al., 2015).

The active layer functions as a dynamic regulator of hydrological partitioning, controlling the division between surface and subsurface flow through seasonal freeze–thaw dynamics. Its influence is mediated by permafrost type, soil composition, vegetation cover, and topography. Thickening of the active layer increases storage capacity and hydraulic conductivity, promoting subsurface flow and baseflow generation, while thermal-hydrological coupling at the active layer–permafrost interface governs water exchange. The regulatory strength of the active layer is most pronounced in continuous permafrost and varies with soil texture and ground ice content. Quantitative relationships between active layer properties and hydrological responses remain poorly constrained, particularly the thresholds at which changes in the active layer trigger significant shifts in runoff generation.

In permafrost regions, the exchange between surface water and groundwater differs fundamentally from that in temperate zones because permafrost acts as a low-permeability barrier that limits vertical hydraulic connectivity. Rather than being a continuous process, this exchange is strongly compartmentalised in both space and time. During the thaw season, surface water can interact with suprapermafrost groundwater – the shallow aquifer above the permafrost table – whereas exchange with subpermafrost groundwater beneath the permafrost is largely prevented, except where taliks (local unfrozen zones) provide vertical pathways. This vertical separation defines the hydrologic behaviour of permafrost landscapes, and understanding how it may change under climate warming has become a priority research topic.

The active layer is widely recognised as the main domain for surface water–groundwater interaction. Following spring thaw and summer rainfall, infiltrating water recharges the suprapermafrost aquifer, which is later released as baseflow during dry periods and through the winter months (Chang et al., 2015; Wang et al., 2017; Qin et al., 2024). This seasonal cycle – from surface water to suprapermafrost storage and back to streamflow – operates on an annual timescale, with residence times in the suprapermafrost aquifer generally ranging from years to decades (Walvoord et al., 2012; Wang et al., 2023). The efficiency of this exchange depends mainly on active-layer depth, soil hydraulic properties, and the timing of freeze–thaw events. Warmer and longer thaw seasons tend to favour infiltration and increase suprapermafrost water storage, thereby enhancing the surface-water-to-groundwater component in summer and the groundwater-to-stream discharge component in winter (Chen et al., 2019; Ding et al., 2020a). This seasonal exchange pattern has been documented across a range of permafrost environments.

In contrast, the transformation between surface water and subpermafrost groundwater is much more restricted and occurs predominantly through taliks. These unfrozen zones, which commonly develop beneath lakes, river channels, and disturbed surfaces, create local pathways through the otherwise impermeable permafrost layer. Geophysical and isotopic studies indicate that open taliks can convey subpermafrost water upwards into surface water bodies, and conversely, allow surface water to infiltrate into deeper aquifers (Tananaev et al., 2022; Walvoord et al., 2012). However, this deeper exchange operates on much longer timescales – typically on the order of 103-104 years for subpermafrost flow systems (Walvoord et al., 2012) – and thus represents a slow but persistent component of the water cycle that is often neglected in routine hydrological assessments. In thermokarst areas, springs fed by ground-ice meltwater offer direct evidence of a transformation pathway from permafrost ice to surface water, linking ground ice, groundwater, and surface flow (Hiyama et al., 2021b; Gagarin et al., 2022). The presence and continuity of taliks therefore control whether surface water and subpermafrost groundwater are hydraulically connected, and the ongoing expansion of taliks under a warming climate is likely to enhance this deeper exchange.

Climate warming and associated permafrost thaw are progressively modifying these exchange relationships. Thickening of the active layer increases the storage capacity of the suprapermafrost aquifer and extends the seasonal window for water exchange, which in turn raises baseflow and winter runoff (Walvoord and Striegl, 2007; Walvoord et al., 2012; Wang et al., 2019; Song et al., 2022). Concurrently, talik formation and expansion gradually breach the hydraulic separation between suprapermafrost and subpermafrost aquifers, allowing previously isolated deep groundwater to contribute to surface flow (Kurylyk and Walvoord, 2021). This shift from a compartmentalised to a more integrated hydrological system is among the most significant consequences of permafrost degradation for water routing. In the Yukon River Basin, for example, long-term records show a steady increase in the groundwater fraction of streamflow, consistent with growing connectivity to deeper aquifers (Walvoord and Striegl, 2007). Similarly, in the source region of the Yellow River, permafrost degradation has been associated with a falling suprapermafrost water table and wetland shrinkage, suggesting that changes in surface-groundwater partitioning are affecting both water availability and ecosystem function (Jin et al., 2022b).

Regional differences in these exchange processes are pronounced. In continuous permafrost, the permafrost table forms a near-impermeable boundary, and exchange is essentially restricted to the active layer. In discontinuous and sporadic permafrost zones, where permafrost coverage is patchy, the exchange pathways are more heterogeneous: some locations exhibit strong vertical connectivity through taliks, while others remain hydraulically isolated. This spatial diversity complicates regional assessments, but it also implies that the hydrological response to warming will be uneven. In lowland Arctic settings, permafrost degradation generally enhances groundwater discharge and raises baseflow (Evans et al., 2020; Koch et al., 2024). In mountainous terrain, however, the response is less clear because steep slopes, coarse deposits, and rock glaciers introduce additional storage and release mechanisms that are still poorly understood (Arenson et al., 2022). In the Andes, recent geophysical surveys have revealed that ice-rich permafrost is more extensive than previously thought, and its thaw may contribute substantially to streamflow in ways that have not yet been quantified (Hilbich et al., 2022; Halla et al., 2021). On the Tibetan Plateau, the spatial pattern of permafrost thaw correlates with variations in water-yield capacity and drainage connectivity, indicating that the conversion of ground ice to runoff is controlled locally by permafrost type and topography (Wan et al., 2019; Niu et al., 2023).

To summarise, existing studies have identified a coherent set of mechanisms governing surface water–groundwater exchange in permafrost regions: seasonal active-layer dynamics, talik-mediated vertical connectivity, and gradual ground-ice melt. Exchange pathways are vertically separated by the permafrost table, and the degree of cross-talik connection depends on the distribution and continuity of unfrozen zones. Climate warming is progressively enhancing hydraulic connectivity between surface water and deeper groundwater systems, with clear implications for baseflow, winter runoff, and water chemistry. Nevertheless, major uncertainties remain, especially in mountainous areas, where the rates and pathways of ground-ice melt contribution to streamflow are not well constrained. Future work should focus on quantifying exchange fluxes across a range of permafrost types and landscape units, and on developing predictive approaches that can handle the nonlinear, threshold-driven responses likely to accompany continued warming.

With ongoing climate warming, thermokarst processes in permafrost regions have intensified and are primarily characterised by changes in thermokarst lakes and ponds. In the Arctic, the northward expansion of beavers into tundra regions has resulted in the formation of numerous beaver ponds that exhibit hydrological and ecological effects similar to those of thermokarst lakes and ponds (Tape et al., 2022). Current research indicates that changes in thermokarst lakes are highly heterogeneous, with certain regions experiencing an increase in their number and extent (Farquharson et al., 2019; Zhou et al., 2024a), whereas others have shown a decline (Liu et al., 2023). On the Tibetan Plateau, thermokarst lakes are predominantly developed in warm permafrost regions (Liu et al., 2023; Zhou et al., 2024a), whereas thermokarst features are extensively distributed in the extremely cold permafrost areas of the High Arctic (Farquharson et al., 2019; Jorgenson et al., 2022). Despite these regional variations, the hydrological impacts of thermokarsts are profound. Thermokarst processes modify surface water dynamics, enhance water exchange between the surface and the atmosphere, influence soil water infiltration, and facilitate the transformation between surface water and groundwater (Fig. 3).

Significant changes have been observed in the ratio of maximum to minimum monthly runoff (Qmax/Qmin). Early studies investigating the relationship between permafrost degradation and runoff changes suggest that the intensity of permafrost thaw alone cannot fully explain the observed increases in runoff across large Eurasian rivers. Instead, increased precipitation has been identified as a plausible explanation (McClelland et al., 2004). Further research has confirmed that precipitation increases are the primary driver of runoff changes, whereas permafrost thaw depth primarily affects runoff coefficients and recession processes by altering the soil water storage capacity (Yamazaki et al., 2006; Wang and Zhang, 2016). Using the maximum-to-minimum monthly runoff ratio (Qmax/Qmin), subsequent studies have analysed changes in Siberian rivers and found a negative correlation between the mean annual temperature and this ratio, but a positive correlation with the recession coefficient. These findings indicate that changes in the active layer and permafrost conditions have already begun to influence the regional hydrological processes (Ye et al., 2009, 2011; Li et al., 2010). Analysis of annual hydrological processes in basins with varying permafrost coverage revealed that permafrost significantly affected intra-annual runoff distribution. Specifically, higher permafrost coverage resulted in steeper runoff hydrographs. This highlights the critical role of permafrost coverage in understanding hydrological impacts (Ye et al., 2012).

Field observations serve as the cornerstone of permafrost hydrological research. However, there is a notable lack of comprehensive, interdisciplinary observational systems that integrate permafrost, hydrology, hydrogeology, and ecology. The importance of field observations is self-evident because reliable results for mechanistic understanding, process analysis, model simulation, statistical analysis, remote sensing data interpretation, and isotopic methods ultimately depend on robust observational data. Although global permafrost monitoring, and ecological and hydrological observation networks exist, high-altitude regions present significant observational challenges. This has resulted in sparse or non-existent coverage in these critical areas. Furthermore, existing observation sites are often department-specific or focus on isolated elements and lack the integrated, interdisciplinary systems necessary to address complex scientific questions.

Isotopic methods and geophysical exploration techniques have become indispensable tools for analysing the relationship between permafrost and hydrology. However, improving their reliability and reducing uncertainty remain key challenges for future research. Isotopic and geophysical methods play pivotal roles in quantifying ground-ice volume, estimating permafrost thaw-water recharge, tracing water sources, and identifying runoff components. Furthermore, the results derived from these methods often lack direct observational validation, raising concerns regarding their reliability and uncertainty. To address these limitations, multimethod approaches, such as combining isotopic analysis with modelling, can provide mutual validation and enhance the robustness of the findings. Additionally, as permafrost degradation increases groundwater recharge into rivers, the biogeochemical composition of river water undergoes significant changes. Leveraging biogeochemical indicators to identify the connections between permafrost thaw and changes in river runoff may offer a promising avenue for future research. For example, studies have demonstrated that dissolved organic matter (DOM) concentrations are highest in stream-draining watersheds with extensive underlying permafrost, whereas nitrate concentrations are highest in stream-draining catchments with less permafrost (Jones and Harms, 2014). This suggests that the DOM concentration could serve as a potential indicator of permafrost-related hydrological impacts, providing a novel pathway for future investigations.

Modelling has become a widely used tool for studying the hydrological impacts of permafrost change. However, significant challenges remain in parameter acquisition, process understanding, surface-groundwater coupling, and the validation of the results. Various models incorporating permafrost thermal-hydrological processes have emerged as effective tools for permafrost hydrology research and are now widely applied, representing the mainstream approach. Complexity in coupled heat-water equations, parametrization uncertainties, and data deficits for model validation are the three key challenges in permafrost change simulation. Current models exhibit high sensitivity to the parametrization of unfrozen water content (wu); however, observational data for wu remain scarce and challenging to upscale (Sun et al., 2025). For instance, in the Mackenzie Basin model, discrepancies in wu schemes induce ±25% deviations in simulated summer peak flows (Elshamy et al., 2025). Abrupt processes such as thermokarst development are largely absent in existing models, yet satellite observations indicate they contribute to over 40 % of land subsidence in the Yangtze River headwaters. While active-layer thickening significantly impacts baseflow by altering groundwater pathways, resolving these processes necessitates coupled surface-subsurface hydrological models – currently constrained by computational demands. Theoretically, hydrological changes and permafrost thawing under the influence of climate change are causally linked. However, because of the limited understanding of hydrogeological processes in the permafrost and active layer, scarcity of historic observational data, and necessary simplifications in model structures and processes for cryosphere hydrogeological modelling, process-based simulations often face substantial limitations (Kurylyk and Walvoord, 2021; Abdelhamed et al., 2022). Furthermore, the heterogeneity of permafrost and variability in thawing patterns and rates make the relationship between different permafrost thawing modes and hydrological changes highly context-dependent (Walvoord and Kurylyk, 2016). Recent advancements in permafrost hydrogeological modelling have led to the development of several mainstream groundwater models, such as SUTRA, FEFLOW, and HYDRUS, which offer advanced capabilities for simulating processes in aquifers experiencing dynamic freeze–thaw cycles (Kurylyk and Walvoord, 2021).

Recent methodological innovations are fundamentally reshaping observational and predictive capabilities in permafrost hydrology. Machine learning techniques now enable high-resolution parameterization of previously intractable processes, particularly through deep learning architectures applied to remote sensing data (Chen et al., 2024). Zwieback et al. (2025) proposed an InSAR-based approach for estimating ground ice near the permafrost table in sparsely vegetated terrain underlain by continuous permafrost. Concurrently, microfluidic experimentation has emerged as a transformative laboratory paradigm, with etched silicon chips replicating cryogenic pore networks at micrometer scales. The microcosms directly visualize ice-lens growth dynamics during freeze–thaw cycles. When coupled with X-ray microtomography, this approach has revealed hysteresis in hydraulic conductivity-ice saturation relationships that violate classical frozen soil models, explaining field observations of delayed runoff response after thaw events (Mirzaei et al., 2025).

Despite these advances, key hydrological fluxes remain poorly observed within the SAP framework. These include: (1) subsurface flow in deep active layers and taliks, where direct measurement is limited by logistical constraints; (2) ground ice melt rates, which are difficult to quantify at the watershed scale; (3) winter groundwater discharge, especially under ice cover; and (4) vertical water fluxes between suprapermafrost and subpermafrost aquifers. Current observational methods are limited by sparse spatial coverage, high cost, and inability to capture transient processes. Emerging techniques – such as distributed temperature sensing (DTS) for continuous monitoring of subsurface flow, advanced geophysics (ERT/IP) for 3-D mapping of ice content and water pathways, isotope-enabled models for source partitioning, and remote sensing (InSAR, UAV) for monitoring thaw subsidence and lake dynamics – hold promise for addressing these gaps (Ding et al., 2017; Wan et al., 2019; Tang, 2025), but require coordinated deployment and integration.

Research on permafrost hydrology has primarily focused on either the hydrological effects of surface or active layer freeze–thaw processes or the contribution of permafrost degradation to runoff. However, there is a notable lack of integrated research examining the impact of permafrost change on hydrological processes from a comprehensive SAP perspective (Fig. 4). Research on the relationship between permafrost changes and hydrological processes spans a wide range of topics, including surface freeze–thaw cycles, active layer dynamics, and the impacts of permafrost degradation on subsurface and surface runoff. Over the past decade, the widespread application of isotopic and geophysical methods, coupled with advancements in modelling and remote sensing, has significantly advanced the field of permafrost hydrology (Grewal et al., 2025). Nevertheless, most studies have focused on isolated aspects of the surface, active layer, or permafrost hydrology rather than adopting a holistic approach (Walvoord and Kurylyk, 2016; Bring et al., 2016; Tananaev et al., 2020; Kurylyk and Walvoord, 2021; Qin et al., 2024; Xiao et al., 2025b).

In surface hydrology, the annual freeze–thaw cycle of the active layer significantly influences snowmelt and rainfall runoff processes, with the most pronounced effects occurring during the spring thaw and autumn freeze periods (Mohammed et al., 2021; Chang et al., 2024). However, the full coupling relationship between active-layer thawing and surface hydrological processes remains poorly understood. On a multi-year timescale, as permafrost thaws and the active layer deepens, previously stable surface hydrological processes may undergo substantial or even abrupt changes (Luo et al., 2025; Spence et al., 2025). Whether a threshold active-layer depth exists to trigger such changes remains unknown. In terms of active-layer hydrological processes, the formation of subsurface runoff and its contribution to river recharge are poorly understood. It is important to distinguish between active layer hydrological processes and suprapermafrost hydrological processes. Active layer hydrology encompasses infiltration, soil moisture redistribution, and interflow within the seasonally thawed zone. The suprapermafrost aquifer, perched immediately above the permafrost table, receives recharge from the active layer and provides the primary baseflow contribution to streams in winter. Recent studies have begun to quantify this linkage: observations from permafrost catchments on the Qinghai-Xizang Plateau indicate that suprapermafrost water can contribute 57 % to 66 % of channel runoff variations, highlighting the dominant role of active-layer hydrology in streamflow generation (Qin et al., 2024). However, the specific pathways through which subsurface flow forms and recharges rivers remain largely unresolved. Similarly, the transformation between liquid and solid water in the suprapermafrost layer remains a critical knowledge gap. Regarding the hydrological impacts of permafrost degradation, the contribution of permafrost thaw to subsurface runoff and river recharge has attracted increasing attention (Xu et al., 2024). However, research in this area is still in its early stages, with significant uncertainties in both the hydrological processes and quantitative assessments (Ahmed et al., 2024; Zhou et al., 2024b).

Although research on permafrost hydrology has expanded significantly, integrated studies from a SAP system perspective under changing environmental conditions remain limited. In permafrost regions, surface water, suprapermafrost groundwater, and water released from permafrost degradation are interconnected and mutually influential (Grewal et al., 2025). Understanding these dynamic hydrological processes requires a comprehensive approach based on water and heat balance theory (Shojae Ghias et al., 2017; Fortier et al., 2023). This approach should examine the impacts of freeze–thaw cycles on surface runoff, active layer changes on subsurface runoff, and permafrost degradation on river recharge (Ahmed et al., 2024; Grewal et al., 2025). The active layer serves as a critical link in the SAP hydrological chain. Future research should investigate these changes and their hydrological effects to determine the mechanisms and interactions within the SAP system. Such efforts can provide a more holistic understanding of permafrost hydrology and its response to environmental change.

Overall, the hydrological impacts of permafrost changes can be organized within the coupled S–A–P domains: (1) surface hydrological processes; (2) active-layer hydrological functions, including suprapermafrost groundwater processes; and (3) permafrost-related subsurface changes and their hydrological effects, with thermal–hydrological interactions at the active layer–permafrost interface as a key mechanism.. Temporally, these impacts encompass intra-annual hydrological processes driven by freeze–thaw cycles and multi-year hydrological changes resulting from permafrost degradation and active layer thickening (Fig. 4). Whether examined spatially or temporally, changes in the surface and subsurface hydrological processes induced by permafrost dynamics form an integrated hydrological system. Therefore, it is essential to study permafrost hydrology from a systemic perspective by recognising the interconnected and interdependent nature of these processes.

Permafrost degradation triggers profound and far-reaching hydrological changes, with significant ecological and environmental implications. One of the most critical projected consequences of permafrost thaw is the transition of the Arctic terrestrial freshwater system from surface-water-dominated to groundwater-dominated (Frey and McClelland, 2009). On the Tibetan Plateau, permafrost degradation can disrupt the hydrological-ecological balance between suprapermafrost groundwater and alpine vegetation. This raises the question of the maximum suprapermafrost groundwater table depth necessary to maintain ecological security in permafrost regions or, specifically, the minimum groundwater depth required to sustain alpine grassland vegetation (Jin et al., 2022b). Accompanying this transition, mineral-rich groundwater may become a significant contributor to the streamflow, complementing the currently dominant contribution of mineral-poor surface waters (Frey and McClelland, 2009). These shifts highlight the need for future research focusing on the ecological and environmental effects of permafrost hydrological changes, particularly in the context of climate warming and its cascading impacts on freshwater systems and ecosystems.

Permafrost hydrology is intricately linked to the carbon and nitrogen cycles, vegetation dynamics, aquatic ecosystems, and chemical transport (Ma et al., 2019; Varner et al., 2022). Changes in permafrost hydrology not only alter water transformation and cycling but also release previously “frozen” nutrients and chemicals, thereby influencing the carbon and nitrogen cycles, terrestrial and aquatic ecosystems, river sediment dynamics, and chemical processes. These changes are primarily driven by shifts in soil moisture distribution, suprapermafrost groundwater recharge, subsurface runoff, and groundwater levels, which subsequently affect surface landscapes, vegetation, carbon and nitrogen cycles, and the transport of chemicals and nutrients (Frey and McClelland, 2009; O'Donnell et al., 2012, 2019, 2024; Drake et al., 2018; Gandois et al., 2019; Liljedahl et al., 2020; Hirst et al., 2022; Mann et al., 2022; Skovsholt et al., 2020; Rowland, 2023). Consequently, the ecological and environmental effects driven by changes in the permafrost hydrology represent a critical area for future research with far-reaching implications. These effects are closely tied to water, ecological, climate, and environmental security, making them a key focus that requires significant attention.