Catchments in the future North: interdisciplinary science for sustainable management in the 21st Century

Abstract

The “North” is an elusive, relative geographical concept which in global terms usually refers to an undefined area in the higher latitudes. More specific definitions can be made, such as for areas within the arctic circle, but this excludes extensive northern sub-arctic regions above the 50o line of latitude, and boreal regions in continental interiors that may extend slightly further south. The effects of recent climate warming have been particularly dramatic in many such “northern” regions. The impacts of climate change on ice sheets and ice caps of the arctic have been well-documented (e.g. Chen et al., 2006), as have the dramatic changes to landscapes where permafrost is thawing due to increased temperatures (e.g. Jorgenson et al., 2006). However, expanding the focus to the wider northern sub-arctic region with its increased heterogeneity, the effects of climatic change become more complex. Changes to climatic drivers can vary, inherent landscape sensitivity to climatic change is diverse and the effects of other anthropogenic agents of change can be confounding as human impacts tend to be greater in mid-latitudes. Thus, the overall implications for hydrology and water resources are difficult to synthesise as they are so varied. Nevertheless, a common feature of the hydrology of these high-latitude regions and their vulnerability to climate change is the influence of snow and the importance of 0 °C as a critical environmental threshold. Small temperature changes determine whether precipitation falls as rain or snow and influence accumulation of winter snow pack and rate of melt. Snow-influenced regions are characterized by a high degree of spatial variation; in more northern and montane regions, snowpack accumulation occurs throughout the winter, with a single, clear spring-melt and a short summer. At lower altitudes, coastal regions or the more southerly parts of “the North”, snow accumulation may be transient in the winter and interspersed with periods of rain, with melt events frequent, and often influenced by rain. Projected climate changes between these geographic areas, generally show warming with lower snow influence, but variable effects on precipitation totals. Regardless of the changes, there will be a cascade of impacts on stream flow regimes and the magnitude, timing and nature of snow-melt influence (e.g. Barnett et al., 2005). This, in turn, will affect biogeochemical processes and surface water quality (e.g. Haaland et al., 2010; Pourmokhtarian et al., 2012) as well as in-stream hydroecology (e.g. Parmesan and Yohe, 2003; Kruitbos et al., 2012). Consequently, an interdisciplinary framework of catchment science is needed to understand the rapid changes occurring in many Northern regions and provide a basis for sustainable management. This Special Issue of Hydrological Processes on “Catchments in the Future North” is part of the scientific activities of the North-Watch international network (http://www.abdn.ac.uk/northwatch) which focused on catchment inter-comparison studies at well-known, long-term experimental sites in the wider sub-arctic – boreal zone. The papers presented come from an associated international workshop held in May 2012 in Potsdam, Germany. The overall aim of North-Watch was to better understand the integrated consequences of climate change on the physical, chemical and biological characteristics of water resources across northern regions to facilitate inter-catchment comparisons that will synthesize a comprehensive, interdisciplinary and regional understanding of the recent effects of climatic change and provide a stronger scientific basis for predicting what further changes are likely to be (see Tetzlaff et al., 2013). The North-Watch project used a space for time substitution to hypothesise how catchments may change in response to climate forcing. The motivation behind the Potsdam Workshop and this subsequent Special Issue is to demonstrate the value of integrated catchment science gaining a broader view of comparative hydrology and integrating with biogeochemistry and freshwater ecology. Truly interdisciplinary studies are crucial to be able to assess possible implications of climate change projections and provide an evidence-base for policy development. In this preface, we introduce 11 papers spread across a number of themes to provide examples of interdisciplinary studies focused on snow-influenced regions in the wider North. The development of the volume has emphasised to us that catchment science in northern regions has dramatically increased our knowledge base over the past two to three decades. Critical to this success have been long-term catchment studies at experimental sites that have provided detailed process data that has also informed model developments. That many such study sites struggle for funding, or in some cases are threatened with closure (e.g. the Canadian Experimental Lakes Area), in the present challenging economic times is a matter for great concern, because despite significant scientific advances, it is clear that in most cases we are poorly equipped to understand exactly how catchment systems will respond across large northern regions, as climate change advances. The prospect of unpredictable non-linearities in the climate system itself and catchment responses means that our science base for policy development is weak. Ironically, this is at a time when climatic changes and technological advances are accelerating resource extraction and economic development in over increasingly large parts of North America and Eurasia. This synergy of climate change and increasing economic development has many unknowns. Science will need to contribute advice on the implications for building societal resilience to climatic change as well as offering guidance on how to avoid serious environmental degradation as development occurs. The first set of papers in this Special Issue examine fundamental processes which are critical to understanding how climate affects the water balance and runoff yields of catchments. The fundamentals of snow hydrology are still poorly understood. In catchments where snow plays a dominant role in the water balance, the snowpack can facilitate lateral flow and redistribution of melt and rain water. Crucially, if lateral water flow occurs within the snowpack, lateral transfer may deliver the water to the stream more rapidly, altering the resulting downstream hydrograph and that water may not be available to terrestrial ecosystems which depend on soil moisture. Eiriksson et al., evaluate the hydrologic significance of lateral flow through snow in the context of runoff generation and moisture redistribution in a semi-arid snow dominated mountainous catchment in southwest Idaho, USA. They use an integrated approach of dye tracers, geophysical methods, and hydrometric measurements to show that cold, midwinter snowpacks tended to promote flow and re-distribution within the snowpack, while highly metamorphosed ripe snowpacks displayed water movement in the basal layers. Importantly, the volumes of downslope redistribution can be sufficient to impact soil moisture distributions and the sources and composition of streamflow. The size and longevity of snowpack accumulation can be compromised under a warming climate when the rain-snow threshold elevation rises. Uncertainty surrounds the relative sensitivity of catchments to climate change. At the regional scale a key control on sensitivity can be geology, and how it affects water storage, movement and release in the landscape. Safeeq et al., present an analytical framework to link the sensitivity of catchments to changing climate to underlying geology, through the concept of drainage efficiency. Using long-term observed streamflow data that incorporates both snowpack dynamics and drainage efficiency from 81 unregulated catchments distributed across a wide range of precipitation regimes (rain, snow, mixed) and geological settings (i.e. drainage efficiencies) in the western US. They define precipitation- and streamflow-based metrics, and use these for catchment classification with respect to snowpack influence and drainage efficiencies. The differences in drainage efficiency are largely due to intrinsic topographic and geologic characteristics. These are likely to be constant under the timescales of anthropogenically-forced climate change, but nonetheless exert a first-order control on the magnitude and direction of climate change impacts on streamflow. Safeeq et al. argue that understanding the full range of hydro-geological processes is essential to be able to predict streamflow response to climate change. At a more local, or sub-regional, scale more specific small-scale features of catchments can determine sensitivity to climatic variability and stream flow response. Input-output data for catchments usually combine non-stationary signals in terms of anthropogenic climate trends and quasi-stationary variability in terms of natural climate oscillations. One key limitation is the lack of long-term data to distinguish directional climatic warming trends from naturally occurring climate oscillations. In their paper, Mengistu et al., provide an analysis of catchment sensitivity to climate forcing and change in four headwater catchments in Ontario, Canada. They present an analytical framework for discriminating non-stationary and stationary signals in water yield responses in a forested landscape where climatic variability is evident but little is known about its effect on water yields. Using wavelet analyses and wavelet coherence to understand relations between hydrological time series they evaluate the predominant climate forcing. Their findings show that the investigated forested headwater catchments exhibited a general non-stationary decline in water yields caused by regional climate warming. However, individual catchments varied in their responsiveness to climate change, with catchments that have higher water loading and lower water storage capacity (in terms of lakes and wetland) being the most sensitive. Headwater catchments showed stationary cycles in water yields caused by interactive impacts of multiple global climate oscillations. Mengistu et al. propose that in landscapes that are not impacted by human activities, headwater catchments could serve as sentinels of climate change if one is able to discriminate climate trends from climate oscillations. Clearly, long-term experimental sites are the most likely to have such long-term data to facilitate such analysis. The influence of catchment sensitivity on projected stream flow responses to climatic variation and change was explored in a modelling framework by Capell et al. for three Scottish sites from the North-Watch (Northern Watershed Ecosystem Response to Climate Change: http://www.abdn.ac.uk/northwatch/) network. They used a declining wetness gradient on a west-east climatic transect in the Scottish Highlands where the three upland catchments give a regional perspective on the responses to climate change projections based on the UK Climate Projections 2009. Derived synthetic time series were used to force conceptual hydrological models specifically developed for the Scottish Highlands with parameterizations identified as behavioural using a tracer-aided approach developed previously. The patterns of change projected for the three catchments were largely consistent with other studies in UK upland catchments with increasing temperatures throughout the year, increased winter and decreased summer precipitation. However, the study highlights subtle differences of likely regional climate change and their relevance for hydrological function. The study also indicated that impacts are mediated by the form and structure of montane catchments themselves. Similar to the paper by Mengistu et al. (2013) the magnitude of different water storages and the associated partitioning of precipitation along different flow paths is suggested as a key influence on sensitivity. Given the close relationships between water flow paths and solute transport, as well as the importance of water availability and ambient temperatures to biogeochemical processes, the hydrological consequences of climate change will also have profound implications for stream water quality. Organic soils over extensive parts of the North represent large reservoirs of carbon that have both global and local influence. Globally, a major uncertainty in climate change predictions is the way in which increased mineralisation of this soil carbon in a warming climate will increase atmospheric greenhouse gas concentrations and further impact the climate system. Locally, such organic soils result in significant inputs of carbon into streams, particularly as a dissolved organic carbon, with implications for in-stream ecology and the potability of surface waters. Laudon et al. analyse time series of stream water and DOC fluxes along a large-scale climatic gradient using North-Watch sites. The objective of their paper was to evaluate how the seasonality and synchroneity (in terms of the coupling/decoupling of fluxes of water and DOC at different times of the year) of discharge and DOC may be affected by the contrasts and changes in winter conditions. The individual sites showed weak correlations with winter temperature, however, the combined normalized values indicated a decrease in seasonality and an increase in synchroneity during spring periods associated with warmer preceding winters. These findings suggest that while the inter-annual variability in climate provides information on individual catchment response to short-term fluctuations, the consideration of a cross-regional climate gradient offers a possible trajectory of how individual systems may respond to more lasting and large scale changes in the future. It is, however, also important to recognise that catchments are not being affected by climate change alone. Often these act synergistically with other agents of environmental change that reflect the legacy of other anthropogenic disturbances. For example, many northern areas in Europe and North America experienced high levels of acid deposition mainly resulting from S oxides produced by fossil fuel burning. The resulting acidification of surface waters in numerous sensitive areas, with concomitant ecological degradation to aquatic communities resulted in environmental regulation to control S emissions. However, the reversibility of soil and surface water acidification is complex and recovery of aquatic communities has been confounded by other environmental changes. For example, Mitchell et al. examine changes in sulphur budgets under conditions of decreasing sulphur emissions in conjunction with climate change predictions in different forested catchments in the Adirondack Mountains in the northeastern United States. They showed an increasing importance of climate controls on sulphur budgets for surface waters, which have been shown to be highly sensitive to acidification from atmospheric inputs. Catchment wetness was shown to be important in affecting sulphur budget discrepancies by increasing sulphur losses. With projected increases in precipitation and further decreases in atmospheric emissions, one would expect future annual S losses from catchments to become more strongly driven by inter-annual climate variations. Changing hydroclimatic regimes can also interact with natural variation in geochemistry to produced unanticipated water quality impacts. The study by Crouch et al. provides an example about the vulnerability of alpine and sub-alpine regions such as the Rocky Mountains to climate change. They investigated the increase in acid rock drainage in regions rich in sulfidic minerals, most notably pyrite. In their study, changing summer temperature and hydrology were found to be the drivers of increasing metals concentrations in the Upper Snake River catchment resulting from a combination of internal catchment processes. Warmer temperatures and earlier snowmelt result in drier soils and lower groundwater table in late summer causing greater exposure of pyrite to O2. It is likely that similar climate-driven changes in water quality occur in other unmonitored headwater catchments in the Rockies with acid rock drainage experiencing accelerating chemical weathering. The fact that many such areas have an industrial legacy from historic metaliferrous mines may make this a far worse and more widespread problem than is currently realised. The hydrological and biogeochemical changes that result from a warming climate in the global north will in themselves affect the composition of aquatic communities. Projected increases in air and water temperatures will further change the habitat template for most levels in freshwater ecosystems. Long-term data for stream ecological communities is extremely scarce and this is a major area of uncertainty in terms of predicting how biological resources will change and affect ecosystem services that society is dependent upon. Fundamental process-based studies, can however provide insights into how ecosystem structure and function may be affected. Friberg et al. examined how changing temperatures link with ecological systems and assessed macroinvertebrate community composition in relation to a range of environmental variables across a number of sites along a latitudinal gradient from Denmark (55oN) in the South to Greenland (69oN) in the North. They also investigated the relationship between leaf litter break-down - as a proxy of ecosystem functioning - and temperature. Their study indicates that temperature has a major influence on macroinvertebrate community composition across a latitudinal gradient and is a strong driver of ecosystem functioning. A rise in temperature is likely to be instrumental in shaping communities, however, it remains a difficult task to predict how adaptive the majority of species will be. What is predictable, however, is that metabolic rates will increase and the whole ecosystem functioning is likely to change. They therefore argue for an urgent need to explore these general patterns in mid- and high latitude, snow-influenced catchments to increase the ability to predict future changes. Sensitivity of snow-influenced catchments to change is of great concern for the freshwater life stages of salmonids which are often important in northern streams. Research has shown that the life cycle of salmon is often adapted to the seasonality of stream hydrology. In addition, these cold-water species are also ectotherms which rely on stream temperature to regulate their body temperature and have relatively narrow ranges of optimal temperatures. They are particularly vulnerable to lethal and sub-lethal effects of high temperatures. In their paper, Cunjak et al. investigate the inter-relationship of Atlantic salmon (Salmo salar) and hydrologic factors inherent to the Miramichi basin in New Brunswick, Canada and the boreal-temperate Canadian climate. Specifically, their study investigates how natural hydrologic variability in different seasons can influence the population dynamics of wild salmon. This is crucial to be able to predict changes in habitat suitability and implications for salmon populations in relation to climate change. Their findings suggest evolutionarily adaptive traits in both that spawning adults ascend small streams in response to high autumn flows and a density-dependent relationship of salmon eggs and parr fish and winter streamflows. They also emphasise that implications of climate change are highly uncertain but are likely to have significant effects on fish community dynamics. For example, rare but extreme flow events might increase in the frequency in the future as predicted under various climate-warming scenarios which are likely to have markedly negative impacts due to their destructive consequences of mid-winter mechanical ice break-up on the survival of juvenile salmon. Furthermore, predicted summer low flows directly increase the probability of high temperature stress events with major implications for growth and survival for such cold water fish. For the west coast rivers of North America, Pacific salmon species are similarly important ecological and economic resources. The consequences of warming in this general region is likely to be a greater proportion of precipitation falling as rain in the winter, which will reduce snowpack sizes, bring forward the timing of snowmelt peaks in the year and reduce flow during late spring/early summer base flow periods. These climate-related shifts in hydrology are of critical concern in the mountain basins of the northern Rocky Mountains because these rivers receive most of their runoff from snowmelt. Goode et al. examine what such changes in the flow regime mean for the geomorphic processes that shape and modulate in-channel habitat in the Middle Fork of the Salmon River, a tributary of the Columbia River. They examine channel morphological change in general and streambed scour in particular and project the consequences for incubating salmonids. They predict salmonid habitat at landscape scales using a series of nested physical models. This allows them to examine potential responses to climate-driven changes in flow and channel morphology applying top-down models. This approach can be generally applied elsewhere with four basic components: a Digital Elevation model, a daily discharge output from a hydrologic model driven by the output of downscaled climate models, empirical relationships for channel morphology characteristics, and a biological filter based on physical parameters to predict the stream segments in the basin that are suitable for spawning. In the Middle Fork Salmon, the relative risk of scour change is greatest for smaller bodied fall spawning species. However, the frequency of scouring events is relatively low indicating that individual year classes may be impacted, but extirpation of entire populations is not expected. The Commentary by Tetzlaff et al. presents a brief synthesis of the cascade of implications of possible changes in the annual water balance and seasonal streamflow distribution that can be anticipated for water quality and in-stream ecology in northern regions. A holistic hydrologic understanding that is more closely integrated with other sciences such as biogeochemistry and freshwater ecology but also social sciences is a prerequisite for more comprehensive predictions of the future (Soulsby et al., 2008). The importance of long-term study sites such as those used in the North-Watch project cannot be emphasised enough at the present time when economic constraints in many countries are resulting in reduced financial support, or even cessation of monitoring, at sites that are usually funded by public agencies. A key uncertainty is the degree to which vegetation changes – either through climatically-driven change or management decisions can ameliorate the consequences of climate change in terms of runoff response, water quality and in-stream ecology. Research to elucidate this important issue is critical if society is to develop integrated land and water policies that can build resilience to the impacts of climate change and protect ecosystems services. The societal consequences of a warming north are substantial; it is hoped that the present volume contributes to the scientific evidence base that will be needed to meet the challenges ahead. We thank the Leverhulme Trust for funding the North-Watch project (http://www.abdn.ac.uk/northwatch/) (F/00 152/AG). We also would like to thank the whole “North-Watch” team: Jim Buttle, Hjalmar Laudon, Jeff McDonnell, Kevin McGuire, Jamie Shanley, Jan Seibert for all their input during the North-watch project and particularly during this final workshop in Potsdam. We also are very grateful to all the invited workshop participants for a truly exciting and inspiring workshop, for great talks and the input to all the discussions: Ann-Kristin Bergstroem, John Buffington, Rene Capell, Irena Creed, Rick Cunjak, Richard Essery, Jim Freer, Nikolai Friberg, Gordon Grant, Jim Kirchner, Diane Mcknight, Jim McNamara, Myron Mitchell, Allan Rodhe, Christina Tague, Klement Tockner.