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ABSTRACT The fraction of precipitation converted to stream discharge within a watershed, termed as runoff efficiency, may shift as climate changes. Runoff efficiency is known to be temperature‐sensitive in some watersheds, but temperature sensitivity is unquantified in many other watersheds. We identify regions where runoff efficiency is temperature‐sensitive using 942 watersheds, minimally influenced by anthropogenic activity, across the continental United States and Canada. Stepwise regression using historical discharge and climate records shows that runoff efficiency in 10 of 16 hydrologically similar hydro‐regions is sensitive to temperature, expanding the number of locations expected to experience temperature‐driven water stress, particularly in the North American continental interior. Runoff efficiency in all hydro‐regions demonstrates sensitivity to precipitation, but during wet years, runoff efficiency temporarily decreases, likely reflecting increasing groundwater storage. The temporary decrease in runoff efficiency is followed by an increase in the following year, likely due to the release of stored groundwater. This effect suggests changes in runoff efficiency help to stabilise watersheds, making it more difficult to both enter and leave drought as climate changes. The latter effect may partially explain observations of hydrologic drought persistence after meteorological drought ends. Understanding regional temperature sensitivity and the multiple‐year effect of precipitation will improve the ability to forecast runoff efficiency. 

ABSTRACT The Northeast United States exhibits significant spatial heterogeneity in flood seasonality, with spring snowmelt‐driven floods historically dominating northern areas, while other regions show more varied flood seasonality. While it is well documented that since 1996 there has been a marked increase in extreme precipitation across this region, the response of flood seasonality to these changes in extreme precipitation and the spatial distribution of these effects remain uncertain. Here we show that, historically, snowmelt‐dominated northern regions were relatively insensitive to changes in extreme precipitation. However, with climate warming, the dominance of snowmelt floods is decreasing and thus the extreme flood regimes in northern regions are increasingly susceptible to changes in extreme precipitation. While extreme precipitation increased everywhere in the Northeastern United States in 1996, it has since returned to near pre‐1996 levels in the coastal north while remaining elevated in the inland north. Thus, the inland north region has and continues to experience the greatest changes in extreme flooding seasonality, including a substantial rise in floods outside the historical spring flood season, particularly in smaller watersheds. Further analysis reveals that while early winter floods are increasingly common, the magnitude of cold season floods (Nov‐May) have remained unchanged over time. In contrast, warm season floods (June‐Oct), historically less significant, are now increasing in both frequency and magnitude in the inland north. Our results highlight that treating the entire Northeast as a uniform hydroclimatic region conceals significant regional variations in extreme discharge trends and, more generally, climate warming will likely increase the sensitivity of historically snowmelt dominated watersheds to extreme precipitation. Understanding this spatial variability in increased extreme precipitation and increased sensitivity to extreme precipitation is crucial for enhancing disaster preparedness and refining water management strategies in affected regions. 

Abstract Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments. 

Fluvial geomorphic analyses frequently require knowledge of bankfull channel geometries, which are thought to be related to characteristic stream discharges. However, relating bankfull geometry to characteristic discharge is challenged by spatially limited stream discharge measurements, which may also lack extensive temporal records. Because of these limitations, discharge is commonly assumed to scale linearly with watershed drainage area. Here we evaluate the assumption of a linear relationship between discharge and drainage area for watersheds across the United States and Canada with limited anthropogenic disturbance. Using machine-learning to objectively cluster hydrologically similar gauges, we find that discharge scales linearly with drainage area for most of North America. However, regions with low average runoff efficiency tend to have non-linear dischargescaling. In regions with non-linear discharge scaling, bankfull channel dimensions increase more rapidly with drainage area than in regions with linear discharge scaling. These results suggest that the recurrence interval of the characteristic discharge that sets channel geometry may be larger in regions where discharge scales nonlinearly with drainage area compared to those regions where linear discharge scaling applies. 

Abstract Spatial complexity impacts the resilience of river ecosystems by mediating processes that control the sources and sinks of sediment and organic material. Using four independent geochemical tracers and three morphometric indices, we show that downstream spatial gradients in stream power (Ω) predict storage of material in the channels and margins and/or floodplains. A field test in a 48 km2 watershed demonstrates that reaches with downstream decreases in Ω coincide with wider floodplains and elevated inventories of 137Cs, 210Pbex (ex—excess), and organic matter in locations of the ~3 to 20 yr floodplain. In contrast, reaches with downstream increases in Ω coincide with narrower floodplains and decreased inventories of 137Cs, 210Pbex, and organic matter. The occurrence of in-channel bedrock exposures and the activity of short-lived 7Be in within-channel sediments also correlate with downstream Ω gradients, demonstrating a link, over both short and long time scales, between withinchannel processes and floodplain-forming processes. The combined geochemical and physical characteristics demonstrate the importance of downstream gradients in sediment transport, characterized by downstream changes in stream power rather than at-a-point stream power, in determining spatial complexity in carbon and sediment storage at intermediate scales (102 to 103 m) in river systems. 

Water ice Ih exhibits brittle behavior when rapidly loaded. Under tension, it fails via crack nucleation and propagation. Compressive failure is more complicated. Under low confinement, cracks slide and interact to form a frictional (Coulombic) fault. Under high confinement, frictional sliding is suppressed and adiabatic heating through crystallographic slip leads to the formation of a plastic fault. The coefficient of static friction increases with time under load, owing to creep of asperities in contact. The coefficient of kinetic (dynamic) friction, set by the ratio of asperity shear strength to hardness, increases with velocity at lower speeds and decreases at higher speeds as contacts melt through frictional heating. Microcracks, upon reaching a critical number density (which near the ductile-to-brittle transition is nearly constant above a certain strain rate), form a pathway for percolation. Additional work is needed on the effects of porosity and crack healing. ▪ An understanding of brittle failure is essential to better predict the integrity of the Arctic and Antarctic sea ice covers and the tectonic evolution of the icy crusts of Enceladus, Europa, and other extraterrestrial satellites. ▪ Fundamental to the brittle failure of ice is the initiation and propagation of microcracks, frictional sliding across crack faces, and localization of strain through both crack interaction and adiabatic heating. 

Abstract. New systematic experiments reveal that the flexural strength of saline S2 columnar-grained ice loaded normal to the columns can be increased upon cyclic loading by about a factor of 1.5. The experiments were conducted using reversed cyclic loading over ranges of frequencies from 0.1 to 0.6 Hz and at a temperature of −10 ∘C on saline ice of two salinities: 3.0 ± 0.9 and 5.9 ± 0.6 ‰. Acoustic emission hit rate during cycling increases with an increase in stress amplitude of cycling. Flexural strength of saline ice of 3.0 ± 0.9 ‰ salinity appears to increase linearly with increasing stress amplitude, similar to the behavior of laboratory-grown freshwater ice (Murdza et al., 2020b) and to the behavior of lake ice (Murdza et al., 2021). The flexural strength of saline ice of 5.9 ± 0.6 ‰ depends on the vertical location of the sample within the thickness of an ice puck; i.e., the strength of the upper layers, which have a lower brine content, was found to be as high as 3 times that of lower layers. The fatigue life of saline ice is erratic. Cyclic strengthening is attributed to the development of an internal back stress that opposes the applied stress and possibly originates from dislocation pileups. 

Abstract. The flexural strength of ice surfaces bonded by freezing, termedfreeze bond, was studied by performing four-point bending tests of bondedfreshwater S2 columnar-grained ice samples in the laboratory. The sampleswere prepared by milling the surfaces of two ice pieces, wetting two of thesurfaces with water of varying salinity, bringing these surfaces together,and then letting them freeze under a compressive stress of about 4 kPa. Thesalinity of the water used for wetting the surfaces to generate the bondvaried from 0 to 35 ppt (parts per thousand). Freezing occurred in air under temperatures varyingfrom −25 to −3 ∘C over periods that varied from 0.5 to∼ 100 h. Results show that an increase in bond salinity ortemperature leads to a decrease in bond strength. The trend for the bondstrength as a function of salinity is similar to that presented in Timco andO'Brien (1994) for saline ice. No freezing occurs at −3 ∘C oncethe salinity of the water used to generate the bond exceeds ∼ 25 ppt. The strength of the saline ice bonds levels off (i.e., saturates)within 6–12 h of freezing; bonds formed from freshwater reach strengthsthat are comparable or higher than that of the parent material in less than0.5 h.