Widespread dispersal of Icelandic tephra: how does the Eyjafjöll eruption of 2010 compare to past Icelandic events?

The Eyjafjöll AD 2010 eruption is an extraordinary event in that it led to widespread and unprecedented disruption to air travel over Europe – a region generally considered to be free from the hazards associated with volcanic eruptions. Following the onset of the eruption, satellite imagery demonstrated the rapid transportation of ash by westerly winds over mainland Europe, eventually expanding to large swathes of the North Atlantic Ocean and the eastern seaboard of Canada. This small‐to‐intermediate size eruption and the dispersal pattern observed are not particularly unusual for Icelandic eruptions within a longer‐term perspective. Indeed, the Eyjafjöll eruption is a relatively modest eruption in comparison to some of the 20 most voluminous eruptions that have deposited cryptotephra in sedimentary archives in mainland Europe, such as the mid Younger Dryas Vedde Ash and the mid Holocene Hekla 4 tephra. The 2010 eruption, however, highlights the critical role that weather patterns play in the distribution of a relatively small amount of ash and also highlights the spatially complex dispersal trajectories of tephra in the atmosphere. Whether or not the preservation of the Eyjafjöll 2010 tephra in European proxy archives will correspond to the extensive distributions mapped in the atmosphere remains to be seen. The Eyjafjöll 2010 event highlights our increased vulnerability to natural hazards rather than the unparalleled explosivity of the event. Copyright © 2010 John Wiley & Sons, Ltd.     

Earthshine: not just for romantics

Peter Thejll, Chris Flynn, Hans Gleisner and Andrew Mattingly explain how classic astronomical techniques, married to the latest technology, can be used to observe earthshine, and learn about Earth's climate, among many other things.     

The isotope hydrology of the Muskoka River Watershed,Ontario, Canada

Stable isotope tracers of δ¹⁸O and δ²H are increasingly being applied in the study of water cycling in regional-scale watersheds in which human activities, like river regulation, are important influences. In 2015, δ¹⁸O and δ²H were integrated into a water quality survey in the Muskoka River Watershed with the aim to provide new regional-scale characterization of isotope hydrology in the 5,100-km² watershed located on the Canadian Shield in central Ontario, Canada. The forest dominated region includes ~78,000 ha of lakes, 42 water control structures, and 11 generating stations, categorized as “run of river.” Within the watershed, stable isotope tracers have long been integrated into hydrologic process studies of both headwater catchments and lakes. Here, monthly surveys of δ¹⁸O and δ²H in river flow were conducted in the watershed between April 2015 and November 2016 (173 surface water samples from 10 river stations). Temporal patterns of stable isotopes in river water reflect seasonal influences of snowmelt and summer-time evaporative fractionation. Spatial patterns, including differences observed during extreme flood levels experienced in the spring of 2016, reflect variation in source contributions to river flow (e.g., snowmelt or groundwater versus evaporatively enriched lake storage), suggesting more local influences (e.g., glacial outwash deposits). Evidence of combined influences of source mixing and evaporative fractionation could, in future, support application of tracer-enabled hydrological modelling, estimation of mean transit times and, as such, contribute to studies of water quality and water resources in the region.     

Pressure‐driven,shoreline currents in a perennially ice‐covered,pro‐glacial lake in Antarctica,identified from a LiCl tracer injected into a pro‐glacial stream

The distribution of streamwater within ice‐covered lakes influences sub‐ice currents, biological activity and shoreline morphology. Perennially ice‐covered lakes in the McMurdo Dry Valleys, Antarctica, provide an excellent natural laboratory to study hydrologic–limnologic interactions under ice cover. For a 2 h period on 17 December 2012, we injected a lithium chloride tracer into Andersen Creek, a pro‐glacial stream flowing into Lake Hoare. Over 4 h, we collected 182 water samples from five stream sites and 15 ice boreholes. Geochemical data showed that interflow travelled West of the stream mouth along the shoreline and did not flow towards the lake interior. The chemistry of water from Andersen Creek was similar to the chemistry of water below shoreline ice. Additional evidence for Westward flow included the morphology of channels on the ice surface, the orientation of ripple marks in lake sediments at the stream mouth and equivalent temperatures between Andersen Creek and water below shoreline ice. Streamwater deflected to the right of the mouth of the stream, in the opposite direction predicted by the Coriolis force. Deflection of interflow was probably caused by the diurnal addition of glacial runoff and stream discharge to the Eastern edge of the lake, which created a strong pressure gradient sloping to the West. This flow directed stream momentum away from the lake interior, minimizing the impact of stream momentum on sub‐ice currents. It also transported dissolved nutrients and suspended sediments to the shoreline region instead of the lake interior, potentially affecting biological productivity and bedform development. Copyright © 2014 John Wiley & Sons, Ltd.     

Water use strategies of two co‐occurring tree species in a semi‐arid karst environment

The flow of precipitation from the surface through to groundwater in karst systems is a complex process involving storage in the unsaturated zone and diffuse and preferential recharge pathways. The processes associated with this behaviour are not well understood, despite the prevalence of karst aquifers being used as freshwater supplies. As a result, uncertainty regarding the ecohydrological processes in this geological setting remains large. In response to the need to better understand the impact of woody vegetation on groundwater recharge, annual evapotranspiration (ET) rates and tree water sources were measured for two years above a shallow, fresh karst aquifer. Water use strategies of the co‐occurring Eucalyptus diversifolia subsp. diversifolia Bonpl. and Allocasuarina verticillata (Lam.) L. Johnson were investigated using a monthly water balance approach, in conjunction with measurement of the stable isotopes of water, leaf water potentials and soil matric potentials. The results suggest that it is unlikely groundwater resources are required to sustain tree transpiration, despite its shallow proximity to the soil surface, and that similarities exist between ET losses and the estimated long‐term average rainfall for this area. Irrespective of stand and morphological differences, E. diversifolia and A. verticillata ET rates showed remarkable convergence, demonstrating the ability of these co‐occurring species to maximise their use of the available precipitation, which avoids the requirement to differentiate between these species when estimating ET at a landscape scale. We conclude that the water holding capacity of porous geological substrates, such as those associated with karst systems, will play an important role in equilibrating annual rainfall variability and should be considered when assessing ecohydrological links associated with karst systems. Copyright © 2013 John Wiley & Sons, Ltd.     

Inter‐comparison of hydro‐climatic regimes across northern catchments: synchronicity,resistance and resilience

The higher mid‐latitudes of the Northern Hemisphere are particularly sensitive to climate change as small differences in temperature determine frozen ground status, precipitation phase, and the magnitude and timing of snow accumulation and melt. An international inter‐catchment comparison program, North‐Watch, seeks to improve our understanding of the sensitivity of northern catchments to climate change by examining their hydrological and biogeochemical responses. The catchments are located in Sweden (Krycklan), Scotland (Mharcaidh, Girnock and Strontian), the United States (Sleepers River, Hubbard Brook and HJ Andrews) and Canada (Catamaran, Dorset and Wolf Creek). This briefing presents the initial stage of the North‐Watch program, which focuses on how these catchments collect, store and release water and identify ‘types’ of hydro‐climatic catchment response. At most sites, a 10‐year data of daily precipitation, discharge and temperature were compiled and evaporation and storage were calculated. Inter‐annual and seasonal patterns of hydrological processes were assessed via normalized fluxes and standard flow metrics. At the annual‐scale, relations between temperature, precipitation and discharge were compared, highlighting the role of seasonality, wetness and snow/frozen ground. The seasonal pattern and synchronicity of fluxes at the monthly scale provided insight into system memory and the role of storage. We identified types of catchments that rapidly translate precipitation into runoff and others that more readily store water for delayed release. Synchronicity and variance of rainfall–runoff patterns were characterized by the coefficient of variation (cv) of monthly fluxes and correlation coefficients. Principal component analysis (PCA) revealed clustering among like catchments in terms of functioning, largely controlled by two components that (i) reflect temperature and precipitation gradients and the correlation of monthly precipitation and discharge and (ii) the seasonality of precipitation and storage. By advancing the ecological concepts of resistance and resilience for catchment functioning, results provided a conceptual framework for understanding susceptibility to hydrological change across northern catchments. Copyright © 2010 John Wiley & Sons, Ltd.     

The relative role of soil type and tree cover on water storage and transmission in northern headwater catchments

Soil water storage and stable isotopes dynamics were investigated in dominant soil–vegetation assemblages of a wet northern headwater catchment (3.2 km²) with limited seasonality in precipitation. We determined the relative influence of soil and vegetation cover on storage and transmission processes. Forested and non‐forested sites were compared, on poorly drained histosols in riparian zones and freely draining podzols on steeper hillslopes. Results showed that soil properties exert a much stronger influence than vegetation on water storage dynamics and fluxes, both at the plot and catchment scale. This is mainly linked to the overall energy‐limited climate, restricting evaporation, in conjunction with high soil water storage capacities. Threshold behaviour in runoff responses at the catchment scale was associated with differences in soil water storage and transmission dynamics of different hydropedological units. Linear input–output relationships occurred when runoff was generated predominantly from the permanently wet riparian histosols, which show only small dynamic storage changes. In contrast, nonlinear runoff generation was related to transient periods of high soil wetness on the hillslopes. During drier conditions, more marked differences in soil water dynamics related to vegetation properties emerged, in terms of evaporation and impacts on temporarily increasing dynamic storage potential. Overall, our results suggest that soil type and their influence on runoff generation are dominant over vegetation effects in wet, northern headwater catchments with low seasonality in precipitation. Potential increase of subsurface storage by tree cover (e.g. for flood management) will therefore be spatially distributed throughout the landscape and limited to rare and extreme dry conditions. Copyright © 2014 John Wiley & Sons, Ltd.