Northern basins are projected to continue warming at rates higher than the global average, with the impacts of warming compounded by concomitant deglaciation, permafrost thaw and vegetation shifts. The Mackenzie River Basin drains headwaters in the glaciated Canadian Rockies to the Arctic Ocean and is mostly underlain by permafrost. Scenarios of future change in this basin were simulated using the MESH distributed hydrological-cryospheric land surface model. MESH was forced with bias-corrected, downscaled RCM forcings and parameterized with a deep subsurface profile, organic soils, and glaciers. The model, validated against discharge, snowpack, and permafrost observations, was used to simulate 21st century hydrology and permafrost dynamics under the RCP8.5 emissions scenario, incorporating projected land cover change applied at two discrete time steps (2021 and 2065). The findings indicate a rapid acceleration of permafrost thaw. By the 2080s, most of the basin will be devoid of permafrost. By late century, river discharges shift to earlier and higher peaks in response to projected increases in precipitation, temperature and snowmelt, despite increases in evapotranspiration from longer snow-free seasons. Baseflow discharges increase in winter, due to higher precipitation and increased basin connectivity from permafrost thaw resulting in enhanced groundwater flow. Subsurface moisture storage rises slightly but the liquid water fraction increases dramatically, increasing subsurface runoff and river discharge. Canadian Rockies' deglaciation reduces summer and annual discharge in the Athabasca and Peace headwaters. Downstream and northward of the mountain headwaters the direct impacts of climate change on river discharge dominate those of changing land cover and glaciers.

The source area of the Yangtze River (SAYR), part of the Tibetan Plateau, is an ecologically fragile alpine region sensitive to climate change. Current research has predominantly examined hydrological and ecological responses as isolated systems, failing to address the coupled mechanisms through which permafrost degradation mediates water-carbon interactions. In this study, we used a fully coupled eco-hydrological model that integrates permafrost processes, along with multi-source remote sensing data, experimental monitoring, and machine learning, to quantify the water retention and carbon sequestration capacity over the past 20 years. The region was categorized into three risk zones based on changes in soil moisture, net ecosystem productivity (NEP), and dissolved organic carbon (DOC) fluxes in streams. We evaluated eight factors, including precipitation, temperature, vegetation phenology and cover, and their contributions to changes of water retention and carbon sequestration using an interpretable machine learning approach. Results show that the central and eastern regions of the study area face the highest risk of declining water retention and carbon sequestration capacity. The changes of temperatures and precipitation have led to depletion of soil water and carbon reserves. This depletion raises concerns about the potential shift from a carbon sink to a carbon source considering land-to-river carbon loss. Our study provides critical insights into the water and carbon flux dynamics and offers valuable guidance for water resource and ecological management in alpine river systems.

Study area: The Binggou and adjacent Yakou catchments in the northeastern Tibetan Plateau. Study focus: Hillslope flow paths were studied using hydrochemical data of various water types in the spring snowmelt and summer rainfall periods based on hydrochemical tracers and endmember mixing analysis. New hydrological insights for the study region: End-member mixing analysis confirmed the dominance of surface and near-surface runoff during the spring snowmelt. Specifically, the spring Binggou stream water had 61 % surface runoff, 22 % shallow groundwater, and 17 % near-surface runoff. The spring Yakou stream water had 64 % snowmelt, 25.5 % near-surface runoff, and 10.5 % riparian saturated soil water at a depth of 20 cm. The application of end-member mixing analysis failed in the summer rainfall period, and shallow subsurface flow contributed the most to the streamflow (similar to 100 %). The average acid-neutralizing capacity of the spring Yakou stream water was 611 mu eq/L, increasing to 841 mu eq/L in the summer, and for the Binggou stream water, the values were 747 mu eq/L and 1084 mu eq/L, respectively, indicating that the thawed soil layers had a significant buffering effect on stream water chemistry. This study revealed seasonal shifts in flow paths and stream sources, with a transition from surface to subsurface flow influenced by meteorological conditions and the active layer thickness. Future climate change may enhance subsurface flow recharge, leading to less diluted streamflow and stronger water-soil interactions.

The Atacama Plateau in the Central Andes (28-22 degrees S) is characterised by a dry and cold periglacial tundra due to the high altitude, low precipitation, and high evaporation. Endogenous freshwater sources - e.g.: seasonal streams and lakes, subsurface reservoirs, surface snow/ice patches - are available, though they are highly sensitive to climatic changes. The near surface hydrological network is highly modified by the distribution and seasonal evolution of perennial frozen ground, i.e. permafrost, which is also expected to change in the future. The interplay between permafrost and hydrology, especially in relation to future climate change, is poorly explored. To address this issue, we carry out long-term ground temperature measurement and modelling, snow coverage survey, tritium- and stable isotope analysis of surface waters on the Ojos del Salado Massif, which is representative of high altitude mountains on the Atacama Plateau. According to our results, a highly transient surface hydrological network - lakes, springs and streams - forms during each summer where permafrost is widespread and ground thawing (i.e. active layer) is present (similar to 4900-6500 m a.s.l.). In this system, the water is of meteoric origin and relatively young (<10 years). The development of the network is strongly influenced by the active layer, which plays a crucial role in storing, seeping, and discharging groundwater. However, future permafrost degradation is expected to reduce the seasonal presence of shallow water, and hence, modify groundwater recharge patterns.

Global warming is altering soil moisture (SM) droughts in Europe with a strong drying trend projected in the Mediterranean and wetting trends projected in Scandinavia. Central Europe, including Germany, lies in a transitional zone showing weaker and diverging change signals exposing the region to uncertainties. The recent extreme drought years in Germany, which resulted in multi-sectoral impacts accounting to combined drought and heat damages of 35 billion Euros and large scale forest losses, underline the relevance of studying future changes in SM droughts. To analyze the projected SM drought changes and associated uncertainties in Germany, we utilize a large ensemble of 57 bias-adjusted and spatially disaggregated regional climate model simulations to run the hydrologic model mHM at a high spatial resolution of approximately 1.2 km. We show that projections of future changes in soil moisture droughts over Germany depend on the emission scenario, the soil depth and the timing during the vegetation growing period. Most robust and widespread increases in soil moisture drought intensities are projected for upper soil layers in the late growing season (July-September) under the high emission scenario. There are greater uncertainties in the changes in soil moisture droughts in the early vegetation growing period (April-June). We find stronger imprints of changes in meteorological drivers controlling the spatial disparities of SM droughts than regional diversity in physio-geographic landscape properties. Our study provides nuanced insights into SM drought changes for an important climatic transition zone and is therefore relevant for regions with similar transitions.

The Qinghai-Tibetan Plateau (QTP) has undergone significant warming, wetting, and greening (WWG) over decades, alongside substantial alterations in hydrological regimes. These changes present great challenges for safeguarding water resources and ecosystems downstream. However, the lack of field observation and systematic research has obscured our understanding of how hydrological processes respond to the combined influences of climate-permafrost-vegetation. This study focuses on the source regions of the Yangtze River, one of the highest permafrost-covered basins on the QTP, and employs a process-based hydrological model to quantify the effects of WWG on hydrological processes. We show that the increasing precipitation dominates subsurface runoff changes while rising temperature primarily affects surface runoff changes by reducing the frozen duration (-52 days/century) and thickening the active layer (+2.4 cm/year). Greening vegetation primarily affects transpiration and interception evaporation. Warming, wetting, and greening will cause a transition in runoff dynamics from surface runoff dominance to subsurface runoff dominance in permafrost basins, and reduce the risk of both flooding and water shortage indicated by the decreased maximum low flow duration and maximum high flow duration of 11.0 and 5.0 days/year, respectively. Moreover, cold permafrost regions exhibit a greater propensity for generating runoff, as indicated by a higher annual increase in runoff coefficient (0.005/year) and total runoff (4.81 mm/year), compared to warm permafrost regions (with increase of 0.001/year and 1.20 mm/year, respectively). These findings enhance the understanding of hydrological changes due to WWG and provide insights for water resources management in permafrost regions under climate change.

Global warming is progressing more rapidly in the Arctic compared with other regions of the world, and the increasing temperature has caused gradual permafrost thaw, resulting in significant changes in hydrological processes. However, the quantitative contributions of different water sources to Arctic watersheds under ongoing climate change remain poorly understood. Therefore, this study aims to address this gap by applying a water isotope-based mixing model to better quantify the sources and pathways of water flow in permafrost-affected catchments. In this study, isotopic and chemical data were used to determine the water sources and flowpaths of the Sagavanirktok (Sag) River on the North Slope in Alaska (USA) in the summer of 2022. Results obtained using a Bayesian mixing model indicate that meltwater from permafrost ice wedges contributes 17.7% upstream and 22.2% downstream to the Sag River. At the upstream with a frozen active layer or transient layer (including seasonal intrasediment ice), lower active layer water (mineral-rich) and upper active layer water (organic-rich) accounted for 31.5% and 18.1%, respectively. By contrast, at the downstream, the contribution of active layer water was 26.9%, which was similar to that of the other sources. The sources and flowpaths of Arctic freshwater affect changes in the geochemical characteristics of the freshwater, which is channeled to the Arctic Ocean through major Arctic rivers. This study quantitatively assesses permafrost ice wedge melt in an Arctic basin and provides insights to facilitate investigations of hydrological processes and geochemical changes associated with climate change in Arctic water systems.

This study was designed to assess the potential impact of microplastic (MP) pollution on soil hydrology, specifically in retaining and releasing moisture. Herein, High-Density Polyethylene (HDPE) MP of different sizes (i.e., 0.5-1, 1-3, and 3-5 mm) and shapes (i.e., fiber, film, and fragment) were evaluated for their effects on water retention curve (WRC) of sandy loam soil, chosen for its agricultural relevance and widespread environmental presence of HDPE. Nine contamination scenarios were simulated with a low MP pollution rate, 0.01% w/w. Van Genuchten models were used to assess plant available water (PAW), wilting point (WP), and water holding capacity (WHC). Results showed that studied MP could significantly affect WRC and PAW mainly by changing WHC rather than WP and that this effect varied with MP shape and size. According to the results, fragment MP had the greatest impact on soil WHC by increasing 36.3%, followed by fibers and films by 19.8% and 15.7%. MP particles significantly increased WHC, while WP remained relatively unchanged. An observed trend indicated that the impact on WHC increased with the size of the MP particles. These findings emphasize the need to manage soil MP pollution to protect plant growth, agriculture, and water dynamics.

In the mountainous headwaters of the Colorado River episodic dust deposition from adjacent arid and disturbed landscapes darkens snow and accelerates snowmelt, impacting basin hydrology. Patterns and impacts across the heterogenous landscape cannot be inferred from current in situ observations. To fill this gap daily remotely sensed retrievals of radiative forcing and contribution to melt were analyzed over the MODIS period of record (2001-2023) to quantify spatiotemporal impacts of snow darkening. Each season radiative forcing magnitudes were lowest in early spring and intensified as snowmelt progressed, with interannual variability in timing and magnitude of peak impact. Over the full record, radiative forcing was elevated in the first decade relative to the last decade. Snowmelt was accelerated in all years and impacts were most intense in the central to southern headwaters. The spatiotemporal patterns motivate further study to understand controls on variability and related perturbations to snow water resources.

Climate change drives disturbance in hydrology and geomorphology in terrestrial polar landscapes underlain by permafrost, yet measurements of, and theories to understand, these changes are limited. Water flowing from permafrost hillslopes to channels is often modulated by water tracks, zones of enhanced soil moisture in unchannelized depressions that concentrate water flow downslope. Water tracks, which dominate hillslope hydrology in some permafrost landscapes, lack a consistent definition and identification method, and their global occurrence, morphology, climate relationships, and geomorphic roles remain understudied despite their role in the permafrost carbon cycle. Combining a literature review with a synthesis of prior work, we identify uniting and distinguishing characteristics between water tracks from disparate polar sites with a toolkit for future field and remotely sensed identification of water tracks. We place previous studies within a quantitative framework of top-down climate and bottom-up geology controls on track morphology and hydrogeomorphic function. We find the term water track is applied to a broad category of concentrated suprapermafrost flowpaths exhibiting varying morphology, degrees of self-organization, hydraulic characteristics, subsurface composition, vegetation, relationships to thaw tables, and stream order/hillslope position. We propose that the widespread occurrence of water tracks on both poles across varying geologic, ecologic, and climatic factors implies that water tracks are in dynamic equilibrium with the permafrost environment but that they may experience change as the climate continues to warm. Current knowledge gaps include these features' trajectories in the face of ongoing climate change and their role as an analog landform for an active Martian hydrosphere.