There is 78 % permafrost and seasonal frozen soil in the Yangtze River's Source Region (SRYR), which is situated in the middle of the Qinghai-Xizang Plateau. Three distinct scenarios were developed in the Soil and Water Assessment Tool (SWAT) to model the effects of land cover change (LCC) on various water balance components. Discharge and percolation of groundwater have decreased by mid-December. This demonstrates the seasonal contributions of subsurface water, which diminish when soil freezes. During winter, when surface water inputs are low, groundwater storage becomes even more critical to ensure water supply due to this periodic trend. An impermeable layer underneath the active layer thickness decreases GWQ and PERC in LCC + permafrost scenario. The water transport and storage phase reached a critical point in August when precipitation, permafrost thawing, and snowmelt caused LATQ to surge. To prevent waterlogging and save water for dry periods, it is necessary to control this peak flow phase. Hydrological processes, permafrost dynamics, and land cover changes in the SRYR are difficult, according to the data. These interactions enhance water circulation throughout the year, recharge of groundwater supplies, surface runoff, and lateral flow. For the region's water resource management to be effective in sustaining ecohydrology, ensuring appropriate water storage, and alleviating freshwater scarcity, these dynamics must be considered.

The Tibetan Plateau (TP) is a region rich in extensive frozen ground and the source of many major Asian rivers. However, how soil freeze/thaw (F/T) dynamics influence runoff production at the catchment scale in the TP is poorly understood. This study employs a process-based permafrost hydrology model with a new soil parameterization to investigate soil F/T dynamics and their impact on runoff in a TP permafrost watershed, i.e., the source region of Yangtze River (SRYR). The revised model separates soil evaporation and plant transpiration, and accounts for the influence of soil gravel and organic carbon content, as well as variation in saturated hydraulic conductivity along the soil profile. Validation results demonstrate that the revised model accurately simulates daily soil temperature (mean RMSE of 1.3 degrees C), soil moisture (mean ubRMSE of 0.05 cm3 cm-3), and runoff discharge (NSE = 0.82). The results reveal different altitudinal patterns of warming trend between permafrost and seasonally frozen ground (SFG). Warming rates in SFG area increase monotonously with elevation, while a turning point is observed in permafrost region around 4800 m. With active layer deepening, deep-soil water content increases but primarily replenishes soil water storage rather than directly contributing to runoff recharge, while rootzone and the middle part of the active layer become drier. Soil F/T cycles in the permafrost region exert stronger influences on runoff process compared to SFG. Delayed soil thaw onset generally results in higher spring runoff coefficient, while delayed soil freeze onset is related to slower runoff recession. The freezing zero-curtain period is likely to impact the continuity of runoff recession processes by affecting the connectivity of groundwater flow channels. These findings uncover the regulatory mechanisms of soil F/T dynamics on runoff production and river discharge characteristics, providing a fundamental basis for predicting permafrost hydrology responses to future climate change in the TP.

Permafrost is strongly associated with human well-being and has become a frontier of cryospheric science. Professor Guodong Cheng is one of the most outstanding geocryologists in China. He was elected as an academician of the Chinese Academy of Sciences in 1993 and served as the president of the International Permafrost Association from 1993-1998. In the early 1980s, Professor Cheng proposed the hypothesis of the repeated-segregation mechanism for the formation of thick-layered ground ice near the permafrost table. Subsequently, in the early 2000s, he proposed the proactive roadbed cooling concept and led the successful development of a series of specific engineering measures that were fully applied in the Qinghai-Tibet Railway Project. Furthermore, he developed a conceptual model to describe the influences of changing permafrost on the groundwater system and discovered the sink-holing effect (channeling with improved hydraulic conductivity of warming permafrost). Professor Cheng has also developed theories on the three-dimensional zonation and proposed a classification system and an altitude model for high-altitude permafrost distribution. On this special occasion of Professor Cheng's 80th birthday, this paper summarizes his outstanding achievements on permafrost science, hoping the permafrost research community will carry forward the momentum to further advance permafrost science worldwide.

Rivers originating from the Tibetan Plateau (TP) provide water to more than 1 billion people living downstream. Almost 40% of the TP is currently underlain by permafrost, which serves as both an ice reserve and a flow barrier and is expected to degrade drastically in a warming climate. The hydrological impacts of permafrost thaw across the TP, however, remain poorly understood. Here, we quantify the permafrost change on the TP over 1980-2100 and evaluate its hydrological impacts using a physically-based cryospheric-hydrological model at a high spatial resolution. Using the ensemble mean of 38 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), the near-surface permafrost area and the total ground ice storage are projected to decrease by 86.4% and 61.6% during 2020-2100 under a high-emission scenario, respectively. The lowering of the permafrost table and removal of permafrost as a flow barrier would enhance infiltration and raise subsurface storage capacity. The diminished water supply from ground ice melt and enhanced subsurface storage capacity could jointly reduce annual runoff and lead to exacerbated regional water shortage when facing future droughts. If the most severe 10-year drought in the historical period occurs again in the future, the annual river runoff will further decrease by 9.7% and 11.3% compared with the historical dry period due to vanishing cryosphere in the source area of Yellow and Yangtze River. Our findings highlight the importance to get prepared for the additional water shortage risks caused by pervasive permafrost thaw in future water resources management across the TP.

The Yangtze River Source Region (YaRSR) is located in the third polar region, the most threatened zone by global warming after the Arctic. Permafrost covers eighty percent of the total area of YaRSR, while the rest is seasonally frozen ground. Due to a significant rise in air temperature, degradation of the permafrost could occur. Permafrost coverage in a river basin greatly controls its hydrology. This study focuses on hydrological modeling in this permafrost environment using the Soil and Water Assessment Tool (SWAT). The SWAT model was calibrated (1985-2000) and validated (2001-2015) on a daily time step. The results were also compared on a monthly time scale. An impermeable layer was introduced within the SWAT model to represent the permafrost conditions. The streamflow is strongly dependent on the seasonal variation of precipitation and temperature, and the rising limb of the hydrograph shows the melting of snow, the contribution of soil water, and thawing of permafrost during the spring-summer season. The permafrost layer well restricted the deep percolation of water. During the spring season, streamflow mainly consists of surface runoff because of the frozen soils. Permafrost and frozen ground thawing lead to an increase in the contribution of groundwater flow to streamflow. Ultimately, the frozen ground depletes as the temperature gets close to the freezing point. This study also describes the SWAT model appli-cation to better analyze and understand the hydrology of the permafrost/frozen ground with limited data availability.

Permafrost thaw due to climate change is altering terrestrial hydrological processes by increasing ground hydraulic conductivity and surface and subsurface hydrologic connectivity across the pan-Arctic. Understanding how runoff responds to changes in hydrologic processes and conditions induced by permafrost thaw is critical for water resources management in high-latitude and high-altitude regions. In this study, we analyzed streamflow recession characteristics for 1964-2016 for the Tahe watershed located at the southern margin of the permafrost region in Eurasia. Results reveal a link between streamflow recession and permafrost degradation as indicated by the statistical analyses of streamflow and the modeled ground warming and active layer thickening. The recession constant and the active layer temperatures at depths of 5, 40, 100, and 200 cm simulated by the backpropagation neural network model significantly increased during the study period from 1972 to 2020 due to intensified climate warming in northeastern China. The onset of seasonal active layer thaw was advanced by 10 days, and the modeled active layer thickness increased by 54 cm in this period. The average annual streamflow recession time increased by 11.5 days (+ 53 %) from the warming period (1972-1988) to the thawing period (1989-2016), with these periods determined from breakpoint analysis. These hydrologic changes arose from increased catchment storage and were correlated to increased active layer thickness and longer seasonal thawing periods. These results highlight that permafrost degradation can significantly extend the recession flow duration in a watershed underlain by discontinuous, sporadic, and isolated permafrost, and thereby alter flooding dynamics and water resources in the southern margin of the Eurasian permafrost region.

Antarctica is highly susceptible to climate and environmental change. In particular, climate change can lead to the warming of permafrost and the development of active layers in permafrost areas, resulting in variations in hydrological characteristics. This study investigated the hydrological process associated with a stream in a snow-dominated headwater catchment on King George Island, maritime Antarctica, during austral summer using the chemical and isotopic compositions. During the cold period, as the snowmelt rate decreased, the amount of new water also decreased. Hence, the electrical conductivity (EC) increased because the contribution of supra-permafrost groundwater (old water), which occurs in the active layer, increased more during the cold period than during the warm period. Moreover, diel variations in the stable isotopic compositions (delta O-18 and delta D) of snowmelt (new water) were clearly observed in the stream water, indicating that runoff was the dominant flow path of snowmelt during the cold period. In contrast, during the warm period, the amount of snowmelt increased and the EC value decreased as a result of the dilution effect. In addition, compared with the cold period, diel variations in the isotopic compositions of the stream water were attenuated during the warm period. This attenuation effect was not due to the increased contribution of old water; instead, it was due to the contribution of new water with a low-amplitude signal in the diel variations of the isotopic compositions. Thus, the observed diel variations in the isotopic compositions of the stream water during cold and warm periods suggest that this catchment is dominated by new water. These findings are helpful for improving our understanding of climate-related changes in the hydrological pathways and water-related ecosystems of polar catchments.

This paper discusses the potential response of fluvial processes and landforms to the projected permafrost degradation and related hydrological change. Fluvial system structure is presented in the first of the paper along with permafrost controls over its functioning, which vary across fluvial system compartments. The distinction is drawn between primarily fluvial landforms that are expected to adjust to future hydrology with less permafrost constraints, and primarily cryogenic landforms evolving in line with permafrost disturbances. The influence of permafrost on fluvial action varies across compartments: on hillslopes, permafrost mostly controls the occurrence of surface runoff, in river valleys and channels, sediment erodibility, while thermal interaction is essential for growing thermo-erosional gullies. Observed and projected changes in permafrost and hydrology are outlined, and their relevance for cryo-fluvial evolution of fluvial systems is reviewed. Based on these projections, future changes in fluvial action in each compartment are discussed. On hillslopes, where permafrost exerts important controls on hillslope hydrology, fluvial activity of overland flow is expected to decrease following the active layer deepening and decreased overland flow duration. In erosional networks, controlled by thermal interaction between runoff and permafrost terrain, higher water temperature is expected to increase the occurrence and rates of thermo-erosional gully development. In river valleys and channels, where permafrost controls the erodibility of bed and bank material, the expected fluvial feedbacks vary across scales and stream orders, and include changes in seasonality of channel deformations, increased retreat rates in lower river banks and decreased, in higher banks, along with floodplain subsidence, and minor potential for complete destabilization of existing channel patterns. Future collateral effects of fluvial change include alterations of terrestrial biogeochemical cycles and societal impact that must be accounted for in climate change adaptation and mitigation strategies.

Arctic slope hydrology studies suggest that water follows preferential subsurface flow paths known as water tracks. While subsurface flow is usually expected to transport only dissolved solids, periglacial studies have indicated some evidence of lessivage associated with flow through sorted patterned ground. We investigated the transport of dissolved and suspended sediments in water tracks on a polar desert slope, and linked this transport to slope and flow path geomorphology. Solute transfer was dominated by carbonate weathering products, and concentrations of other ions increased disproportionately when the active layer thawed. Suspended sediment transport occurred in water tracks, but fluxes were supply-limited, indicating competent subsurface mechanical erosion. Solute mass fluxes were 5-10 times greater than sediment fluxes. In this dry landscape dominated by snowmelt, surface seepage leads to sediment deposition, while subsurface flow promotes lessivage. A conceptual model of nivation slopes is presented, taking into consideration the influence of flow path morphology and adaptation of the hydrological system to localized water sources from wind-drifted snowbanks. Climate-driven permafrost degradation and the increased frequency of rainfall events may result in new sediment sources and changes in flow pathways, modifying the physico-chemical properties and ecology of downstream receiving waters.

Permafrost hydrology is an emerging discipline, attracting increasing attention as the Arctic region is undergoing rapid change. However, the research domain of this discipline had never been explicitly formulated. Both 'permafrost' and 'hydrology' yield differing meanings across languages and scientific domains; hence, 'permafrost hydrology' serves as an example of cognitive linguistic relativity. From this point of view, the English and Russian usages of this term are explained. The differing views of permafrost as either an ecosystem class or a geographical region, and hydrology as a discipline concerned with either landscapes or generic water bodies, maintain a language-specific touch of the research in this field. Responding to a current lack of a unified approach, we propose a universal process-based definition of permafrost hydrology, based on a specific process assemblage, specific to permafrost regions and including: (1) Unconfined groundwater surface dynamics related to the active layer development; (2) water migration in the soil matrix, driven by phase transitions in the freezing active layer; and (3) transient water storage in both surface and subsurface compartments, redistributing runoff on various time scales. This definition fills the gap in existing scientific vocabulary. Other definitions from the field are revisited and discussed. The future of permafrost hydrology research is discussed, where the most important results would emerge at the interface between permafrost hydrology, periglacial geomorphology, and geocryology.