The recession of a sandy bluff was investigated in a controlled laboratory wave flume, replicating the complex interactions between hydrodynamic forcing, sediment transport processes, and bluff slope stability. A comprehensive monitoring approach measured water levels, pore water pressures, moisture content, and detailed bathymetric-topographic data, providing a thorough understanding of the governing mechanisms and their interrelationships within the beach-bluff system. Bluff recession occurred through notch formation at the bluff toe, followed by a series of minor and major episodic bluff failures. Pore-water pressure variations within the bluff were closely linked to morphological changes on the beach and the bluff's instability. The final beach profile exhibited distinct characteristics: near the shoreline, it was steeper than the equilibrium beach profile due to the sediment supplied by bluff recession. Cross-spectral analysis between water level fluctuations and pore water pressure signals revealed a strong coupling between incident wave energy and pore water pressure responses within the beach-bluff system. The rapid rise in saturation, along with the formation and expansion of the notch, contributed to bluff instability and episodic failure events.

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 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.

The relationship between groundwater and discharge in Arctic and sub-Arctic regions is strongly controlled by permafrost. Previous work has shown that catchments with thawing frozen soils due to the warming climate are expected to show changes in their storage-discharge relationship. In this study, we use a mechanistic modelling approach to demonstrate that a thawing catchment underlain with continuous permafrost undergoes a dramatic change in storage-discharge relationship. We demonstrate that the effect of permafrost thaw, conceptualized as a reduction of an impermeable layer in the subsurface, will likely only be clearly observable as a change in slope of the recession curve of catchments with hillslope gradients>5 %. For flat catchments (<1% hillslope gradient), we find no relation with permafrost extent and change in recession curve slope will likely be dominated by changes in active layer parameters, such as in shallow surface permeability (Hydraulic Conductivity, above permafrost) and shallow surface and subsurface water retention (Specific Yield of Groundwater & Specific Yield of Surface). For mildly sloped Arctic catchments (5 % hillslope gradient), change in recession curve slope is controlled both by changes in permafrost extent and the subsurface flow length and subsurface hydraulic properties for the shallow flow, with minimal impact from overland flow properties and changes in meteorological factors. For moderately sloped Arctic catchments (10 % hillslope gradient), change in recession curve slope change is dominated by changes in permafrost extent, and secondly by changes in the subsurface flow length and sub-surface hydraulic properties, with no impact from meteorological factors or changes in overland flow properties. Several of the parameters found to be driving shifts in recession curve slopes in our modeling, such as changes in active layer thickness and the formation of taliks, are more likely than others to evolve with the ongoing Arctic climate change in hillslopes, helping us understand what drives the real-world increases in non-linearity of storage-discharge relationships.

Due to polar amplification of climate change, high latitudes are warming up twice as fast as the rest of the world. This warming leads to permafrost thawing, which increases the thickness of the overlying active layer and modifies the subsurface hydrologic regime of the draining watershed, therefore affecting baseflow to surface water and modifying recession characteristics. The active layer thickening and the subsurface flow modification are assumed to be linearly correlated. The objective of this study is to test this assumption by quantifying the correlation between the temporal evolution of hydrologic parameters (recession slope and initial recession outflow) and 11 controlling factors (all linked to surface, subsurface and climatic conditions) for 336 Arctic catchments from 1970 to 2000. Contrary to previous studies, we demonstrate a clear decrease in recession slope and initial recession outflow over 1970-2000 for a majority of catchments at any significance level. We explain this result by identifying high topography and low permafrost extent as controlling factors that complexify the relationship between trends in recession parameters and active layer thickness evolution. The study goes further by identifying the mechanisms behind the complexification of the relationship: permafrost-extent loss, hydrologic-connectivity increase, flow-path-diversity increase, contributing drainage area multiplication. The novel aspect of the study lay behind the large number of studied catchments and the large range of controlling factors tested.