Ongoing and amplified climate change in the Arctic is leading to glacier retreat and to the exposure of an ever-larger portion of non-glaciated permafrost-dominated landscapes. Warming will also cause more precipitation to fall as rain, further enhancing the thaw of previously frozen ground. Yet, the impact of those perturbations on the geochemistry of Arctic rivers remains a subject of debate. Here, we determined the geochemical composition of waters from various contrasting non-glacial permafrost catchments and investigated their impact on a glacially dominated river, the Zackenberg River (Northeast Greenland), during late summer (August 2019). We also studied the effect of rainfall on the geochemistry of the Zackenberg River, its non-glacial tributaries, and a nearby independent non-glacial headwater stream Gr ae nse. We analyzed water properties, quantified and characterized dissolved organic matter (DOM) using absorbance and fluorescence spectroscopy and radiocarbon isotopes, and set this alongside analyses of the major cations (Ca, Mg, Na, and K), dissolved silicon (Si), and germanium/silicon ratios (Ge/Si). The glacier-fed Zackenberg River contained low concentrations of major cations, dissolved Si and dissolved organic carbon (DOC), and a Ge/Si ratio typical of bulk rock. Glacial DOM was enriched in protein-like fluorescent DOM and displayed relatively depleted radiocarbon values (i.e., old DOM). Non-glacial streams (i.e., tributaries and Gr ae nse) had higher concentrations of major cations and DOC and DOM enriched in aromatic compounds. They showed a wide range of values for radiocarbon, Si and Ge/Si ratios associated with variable contributions of surface runoff relative to deep active layer leaching. Before the rain event, Zackenberg tributaries did not contribute notably to the solute export of the Zackenberg River, and supra-permafrost ground waters governed the supply of solutes in Zackenberg tributaries and Gr ae nse stream. After the rain event, surface runoff modified the composition of Gr ae nse stream, and non-glacial tributaries strongly increased their contribution to the Zackenberg River solute export. Our results show that summer rainfall events provide an additional source of DOM and Si-rich waters from permafrost-underlain catchments to the discharge of glacially dominated rivers. This suggests that the magnitude and composition of solute exports from Arctic rivers are modulated by permafrost thaw and summer rain events. This event-driven solute supply will likely impact the carbon cycle in rivers, estuaries, and oceans and should be included into future predictions of carbon balance in these vulnerable Arctic systems.

Climate change has resulted in significant changes to subsurface hydrological processes in permafrost regions. Lateral subsurface flow (LSF) represents the dominant flow path in hillslope runoff generation. However, the contributions of runoff components to LSF, such as precipitation, soil water, and ground ice, remain unclear. This study aimed to characterize LSF generation processes in an alpine permafrost hillslope of Northeastern Tibetan Plateau, using stable isotopes and total dissolved solids (TDS) as tracers. Samples of precipitation and soil water [including mobile soil water and supra-permafrost groundwater (SPG)], LSF, and ground ice samples were collected from different thaw depths of the active layer in 2021. The results showed that LSF came directly from SPG in the active layer. Two-source partitioning using delta H-2 or TDS suggested that the dominant source of LSF gradually shifted from ground ice during the initial thaw period to precipitation with increasing thaw depths. The contributions of ground ice to LSF were 70 % and 30 % at thaw depths of 0-30 cm and >30 cm, respectively. The results of three-source partitioning indicated ground ice, precipitation, and SPG to be the dominant sources of LSF at thaw depths of 0-30 cm, 30-150 cm, and >150 cm, respectively. SPG largely regulates hillslope hydrologic processes at thaw depths >= 250 cm. Therefore, with continuing climate warming, SPG will play an increasing role in hydrological processes of alpine meadow permafrost hillslopes.

Climate-change driven degradation of permafrost and changes in precipitation have resulted in significant changes to hydrological processes in permafrost areas. Previous studies on hillslope-stream connectivity and associated runoff-recharge to rivers have mainly focused on the threshold conditions and processes. In contrast, there has been limited study on the capacity of the permafrost active layer to recharge rivers and the relationships between river channel precipitation and river runoff, needed to predict flood events. This study aimed to characterize river runoff generation processes in the Yakou Catchment, northeastern Tibetan Plateau. Continuous monitoring of meteorological variables (precipitation and air temperature) and hillslope hydrological elements (thaw depths, supra-permafrost groundwater, and the thickness of the saturated zone) was conducted between June-August 2021-2022. The results showed using the thickness of the saturated zone (TSZ) to determine wet and dry conditions yielded significantly higher low flow (average of 0.153 m3 s- 1) and lower low flow (average of 0.049 m3 s- 1) with average TSZ depths of 0.40 m and 0.12 m under wet and dry conditions, respectively. However, no significant difference was noted in quick flow. Precipitation during typical rainfall events deter-mined the generation of quick flow, with low flow constituting the main component of river runoff. The application of a partial least squares path model showed that TSZ on the permafrost determined the generation of river low flow which mainly originated from hillslope lateral subsurface flow. Conversely, river channel precipitation determined the generation of quick flow, which can contribute up to 80 % of the peak runoff during extreme rainfall events. Specifically, this study enhances the understanding of the connectivity between hill -slopes and rivers and the storage-discharge relationship in permafrost catchments. This study provides a new theoretical reference for simulations of hydrological processes in the permafrost region.