David M. Hannah

Hydrological droughts in the 21st century, hotspots and uncertainties from a global multimodel ensemble experiment

Significance Increasing concentrations of greenhouse gases in the atmosphere are widely expected to influence global climate over the coming century. The impact on drought is uncertain because of the complexity of the processes but can be estimated using outputs from an ensemble of global models (hydrological and climate models). Using an ensemble of 35 simulations, we show a likely increase in the global severity of drought by the end of 21st century, with regional hotspots including South America and Central and Western Europe in which the frequency of drought increases by more than 20%. The main source of uncertainty in the results comes from the hydrological models, with climate models contributing to a substantial but smaller amount of uncertainty. Increasing concentrations of greenhouse gases in the atmosphere are expected to modify the global water cycle with significant consequences for terrestrial hydrology. We assess the impact of climate change on hydrological droughts in a multimodel experiment including seven global impact models (GIMs) driven by bias-corrected climate from five global climate models under four representative concentration pathways (RCPs). Drought severity is defined as the fraction of land under drought conditions. Results show a likely increase in the global severity of hydrological drought at the end of the 21st century, with systematically greater increases for RCPs describing stronger radiative forcings. Under RCP8.5, droughts exceeding 40% of analyzed land area are projected by nearly half of the simulations. This increase in drought severity has a strong signal-to-noise ratio at the global scale, and Southern Europe, the Middle East, the Southeast United States, Chile, and South West Australia are identified as possible hotspots for future water security issues. The uncertainty due to GIMs is greater than that from global climate models, particularly if including a GIM that accounts for the dynamic response of plants to CO2 and climate, as this model simulates little or no increase in drought frequency. Our study demonstrates that different representations of terrestrial water-cycle processes in GIMs are responsible for a much larger uncertainty in the response of hydrological drought to climate change than previously thought. When assessing the impact of climate change on hydrology, it is therefore critical to consider a diverse range of GIMs to better capture the uncertainty.

An approach to hydrograph classification

The stream hydrograph is an integration of spatial and temporal variations in water input, storage and transfer processes within a catchment. For glacier basins in particular, inferences concerning catchment-scale processes have been developed from the varying form and magnitude of the diurnal hydrograph in the proglacial river. To date, however, such classifications of proglacial diurnal hydrographs have developed in a relatively subjective manner. This paper develops an objective approach to the classification of diurnal discharge hydrograph ‘shape’ and ‘magnitude’ using a combination of principal components analysis and cluster analysis applied to proglacial discharge time-series and to diurnal bulk flow indices. The procedure is applied to discharge time-series from two different glacier basins and four separate ablation seasons representing a gradient of increasing hydrological perturbation as a result of (i) variable water inputs generated by rainstorm activity and (ii) variable location and response of hydrological stores through a systematic decrease in catchment glacierized area. The potential of the technique for application in non-glacial hydrological contexts is discussed. Copyright

Citizen science in hydrology and water resources: opportunities for knowledge generation, ecosystem service management, and sustainable development

The participation of the general public in the research design, data collection and interpretation process together with scientists is often referred to as citizen science. While citizen science itself has existed since the start of scientific practice, developments in sensing technology, data processing and visualisation, and communication of ideas and results, are creating a wide range of new opportunities for public participation in scientific research. This paper reviews the state of citizen science in a hydrological context and explores the potential of citizen science to complement more traditional ways of scientific data collection and knowledge generation for hydrological sciences and water resources management. Although hydrological data collection often involves advanced technology, the advent of robust, cheap and low-maintenance sensing equipment provides unprecedented opportunities for data collection in a citizen science context. These data have a significant potential to create new hydrological knowledge, especially in relation to the characterisation of process heterogeneity, remote regions, and human impacts on the water cycle. However, the nature and quality of data collected in citizen science experiments is potentially very different from those of traditional monitoring networks. This poses challenges in terms of their processing, interpretation, and use, especially with regard to assimilation of traditional knowledge, the quantification of uncertainties, and their role in decision support. It also requires care in designing citizen science projects such that the generated data complement optimally other available knowledge. Lastly, we reflect on the challenges and opportunities in the integration of hydrologically-oriented citizen science in water resources management, the role of scientific knowledge in the decision-making process, and the potential contestation to established community institutions posed by co-generation of new knowledge.

Classification of river regimes: a context for hydroecology

Over the past 30 years, ecologists have demonstrated the importance of flow and temperature as primary variables in driving running water, riparian and floodplain ecosystems. As it is important to assess the size and timing of discharge variations in relation to those in temperature, a method is proposed that uses multivariate techniques to separately classify annual discharge and temperature regimes according to their shape and magnitude, and which then combines the classifications. This paper: (i) describes a generally applicable method; (ii) tests the method by applying it to riparian systems on four British rivers using a 20-year record (1977-97) of flow and air temperature; (iii) proposes a hydroecological interpretation of the classification; (iv) considers the degree to which the methodology might provide information to support the design of ecologically acceptable flow regimes. Regimes are defined for discharge and air temperature using monthly mean data. The results of applying the classification procedure to four British rivers indicates that the typical regimes for each of the four catchments are composite features produced by a small number of clearly defined annual types that reflect interannual variability in hydroclimatological conditions. Annual discharge patterns are dominated by three shape classes (accounting for 94% of the station years: class A, early (November) peak; class B, intermediate (December-January) peak; and class C, late (March) peak) and one magnitude class (70% of the station years fall into class 3, intermediate), with two subordinate magnitude classes: low-flow years (18%) and high flow years (12%). For air temperature, annual patterns are classified evenly into three shape and four magnitude classes. It is argued that this variety of flow-temperature patterns is important for sustaining ecosystem integrity and for establishing benchmark flow regimes and associated frequencies to aid river management.

Wood storage within the active zone of a large European gravel-bed river

Wood storage within the active zone of the dynamic, gravel-bed, Fiume Tagliamento, Italy, was investigated at eight sites along the rivers main stem. The quantity, nature, and mode of wood storage revealed a number of trends related to active zone morphology, cover type, and distance from the rivers source. Relatively small quantities of wood were stored on open-gravel surfaces (estimates ranged from 1 to 21 t ha−1), intermediate quantities were associated with established islands (24–186 t ha−1), and large quantities were associated with pioneer islands (293–1664 t ha−1). Thus, variations in the geomorphological style of the river, which are associated with changes in these three cover types, are reflected in variations in the amount of wood that is stored in different reaches. In addition, although wood was found in many locations within the active zone, it was preferentially stored in three specific locations: (i) bar crests (the main open-gravel location for wood accumulations and pioneer islands); (ii) the margins and (iii) surfaces of established islands. The proportion of the stored wood that was living (sprouting) increased downstream and was higher on the open gravel than in association with established islands. There was a downstream gradient in the dominant type of wood accumulation. Individual logs predominated at the most upstream site. Thereafter, on the open gravel, whole shrubs and trees dominated the more confined sites in the headwaters and middle reaches, whereas, jams were the most frequent form of accumulation in the downstream reaches. Jams were the most frequent type of accumulation associated with established islands throughout the river. In contrast to small streams, where debris dams constitute the major type of wood accumulation, complex patterns and trends of wood storage were revealed along the Tagliamento. Although further studies are needed, it is clear that erosion of woody vegetation, its subsequent transport and deposition, play a major role in structuring the geomorphological and ecological character of this relatively natural, large European river-system. Insight into the mechanisms underlying the observed spatial patterns will contribute to a better understanding of the dynamic processes involved, and is essential for more effective management of river ecosystems.

Alpine Stream Habitat Classification: An Alternative Approach Incorporating the Role of Dynamic Water Source Contributions

Abstract We review current understanding of dynamic interactions between environmental variables and stream benthic communities within glacierized alpine catchments to provide a context for the central theme of habitat complexity within alpine streams. We present a conceptual model summarizing the important links between environmental variables, from large-scale (regional/catchment) to stream-reach/patch-scale processes, to illustrate this physical habitat complexity. Existing alpine stream classification and zonation criteria are examined, and the environmental characteristics representative of the different stream classes are identified. The theme of habitat complexity is developed to assess the applicability of traditional (principally temperature-based) alpine stream classifications. These traditional classifications do not take into account spatial and temporal variations in water source contributions to alpine streams unless associated temperature changes occur. However, different hydrological stores and pathways impart other physical and chemical influences upon stream benthic communities that are overlooked by traditional classifications. We propose a new classification system to better describe spatial and temporal variability in glacial, snowmelt, and groundwater inputs to alpine streams, based upon the mix of proportions of water contributed from each of these sources. Field data collected in the French Pyrénées are used to support this new alpine stream classification, which we propose as a tool for further research in alpine river catchments.

Climate change and water in the UK - past changes and future prospects

Climate change is expected to modify rainfall, temperature and catchment hydrological responses across the world, and adapting to these water-related changes is a pressing challenge. This paper reviews the impact of anthropogenic climate change on water in the UK and looks at projections of future change. The natural variability of the UK climate makes change hard to detect; only historical increases in air temperature can be attributed to anthropogenic climate forcing, but over the last 50 years more winter rainfall has been falling in intense events. Future changes in rainfall and evapotranspiration could lead to changed flow regimes and impacts on water quality, aquatic ecosystems and water availability. Summer flows may decrease on average, but floods may become larger and more frequent. River and lake water quality may decline as a result of higher water temperatures, lower river flows and increased algal blooms in summer, and because of higher flows in the winter. In communicating this important work, researchers should pay particular attention to explaining confidence and uncertainty clearly. Much of the relevant research is either global or highly localized: decision-makers would benefit from more studies that address water and climate change at a spatial and temporal scale appropriate for the decisions they make.

Longitudinal variations in exposed riverine sediments: a context for the ecology of the Fiume Tagliamento, Italy

1. A key component of physical habitat along braided river systems is the exposed riverine sediment within the active zone. The relatively unmanaged, gravel-bed Fiume Tagliamento, Italy, provides the focus for exploring two ecologically important properties of exposed riverine sediments: their within-patch and between-patch variability in calibre. 2. To characterize between-patch variation in exposed riverine sediments, replicate (within-patch) samples were obtained from three geomorphologically distinct locations along 130 km of the river: bar heads along the margin of the low-flow channel, the heads of major bars across the exposed surface of the active zone, and floodplain surfaces. A photographic technique enabled rapid and consistent field sampling of the coarse sediments at bar heads along the low-flow channel margin and on major bars across the dry bed. 3. A downstream decrease in particle size and an increase in within-patch heterogeneity in sediment size were observed within bar head sediments along the margin of the low-flow channel. Comparisons between major bar and low-flow channel samples revealed greatest within-patch variability in individual sediment size indices (D50, A- and B-axes of the larger particles) at headwater sites, greatest between-patch variability in the three measured indices in the central reaches, and lowest between-patch variability at downstream sites. However, there was a distinct increase in the overall heterogeneity in particle size, which was sustained across all patches, in a downstream direction. 4. There was a clear downstream decrease in the size of floodplain sediments in the headwaters, but thereafter there was no distinct downstream trend in any of the calculated particle size indices. 5. The geomorphological controls on the observed patterns and the potential ecological significance of the patterns, particularly for plant establishment, are discussed in relation to the relative relief of the active zone, and the highly variable hydrological and climatic regime along the river.

Groundwater–surface water interactions in upland Scottish rivers: hydrological, hydrochemical and ecological implications

Synopsis Contrary to previous hydrogeological assumptions, we now know that drift deposits and fracture systems in crystalline rocks can constitute important aquifers in the Scottish Highlands and other montane environments. Groundwater from these aquifers usually has an important influence on the hydrology, hydrochemistry and ecology of upland river systems. Tracer-based research in the Girnock burn catchment in the Cairngorms revealed that groundwater comprises at least 30% of annual runoff. Groundwater often enters stream channels via drift deposits in valley bottom areas, which appear to be fed from recharge areas on the catchment interfluves. A range of groundwater sources exist in the catchment reflecting the complex solid and drift geology. These account for spatial differences in stream hydrochemistry and the spatial delineation of groundwater discharges to rivers and riparian zones. Areas where groundwaters enter the stream channel directly can have profound ecological implications. Most obvious are low rates of salmonid egg survival where chemically reduced groundwater discharges through the hyporheic zone. However, it is argued that only further research will reveal the full significance of groundwater–surface water interactions to the ecological status of Scottish rivers.

Stability and persistence of alpine stream macroinvertebrate communities and the role of physicochemical habitat variables

Macroinvertebrate communities in alpine streams have rarely been examined over more than two consecutive years or at sub-monthly temporal resolution during the summer melt season, in relation to a range of stream physicochemical habitat measurements. This paper addresses these research gaps by investigating the inter- (late melt season, 1996–2003) and intra-annual (bi-weekly; June–September, 2002–2003) community compositional stability and persistence of three alpine streams fed from different water sources (snow, glaciers and groundwater) in the Taillon–Gabiétous catchment, French Pyrénées. Inter-annual community stability and persistence decreased from 1996 to 2003; however, groundwater stream communities changed less than those in the main glacial stream. Intra-annual community stability varied spatially and temporally, particularly in relation to water quality variables (water temperature and suspended sediment concentration); water quantity (stream discharge) was less important perhaps due to taxa possessing adaptations to flow variability. The 15 most abundant taxa were consistently more stable and persistent than the entire stream community suggesting a common pool of taxa in these streams. Overall, the results support the view that streams originating from different alpine water sources are characterised by distinct benthic macroinvertebrate assemblages, and demonstrate the value of sampling at nested temporal scales (inter-annual to bi-weekly) for understanding how these stream ecosystems function.