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General circulation models (GCMs) suggest that rising concentrations of greenhouse gases may have significant consequences for the global climate. What is less clear is the extent to which local (subgrid) scale meteorological processes will be affected. So-called 'downscaling' techniques have subsequently emerged as a means of bridging the gap between what climate modellers are currently able to provide and what impact assessors require. This article reviews the present generation of downscaling tools under four main headings: regression methods; weather pattern (circulation)-based approaches; stochastic weather generators; and limited-area climate models. The penultimate section summarizes the results of an international experiment to intercompare several precipitation models used for downscaling. It shows that circulation-based downscaling methods perform well in simulating present observed and model-generated daily precipitation characteristics, but are able to capture only part of the daily precipitation variability changes associated with model-derived changes in climate. The final section examines a number of ongoing challenges to the future development of climate downscaling. I The rationale for downscalingEven if global climate models in the future are run at high resolution there will remain the need to 'downscale' the results from such models to individual sites or localities for impact studies. Downscaling methodologies are still under development and more work needs to be done in intercomparing these methodologies and quantifying the accuracy of such methods (DOE, 1996: 34).The present generation of general circulation models (GCMs) of the climate system are restricted in their usefulness for many subgrid scale applications by their coarse spatial Source: Modified after Hostetler (1994) and temporal resolution (Wigley et al., 1990;Carter et al., 1994). For example, hydrological models are frequently concerned with small, subcatchment (even hillslope) scale processes, occurring on spatial scales much smaller than those resolved in GCMs (see Figure 1). GCMs deal most proficiently with fluid dynamics at the continental scale and parameterize regional and smaller-scale processes. These scale-related sensitivities and mismatch problems are further exacerbated because they usually involve the most uncertain components of climate models, water vapour and cloud feedback effects (Rind et al., 1992). As Hostetler (1994) has observed, the greatest errors in the parameterizations of both GCMs and hydrological models occur on the scale(s) at which climate and terrestrial impact models interface. These mismatch problems, which affect both the temporal and spatial dimensions, have important implications for the credence of impact studies derived by the output of models of climate change, especially as research into potential human-induced modifications to hydrological and ecological cycles is assuming increasing significance. Downloaded from 532 change. A major focus of the BAHC (Biological Aspects of the Hydrol...

[1] A probabilistic framework is presented for combining information from an ensemble of four general circulation models (GCMs), two greenhouse gas emission scenarios, two statistical downscaling techniques, two hydrological model structures, and two sets of hydrological model parameters. GCMs were weighted according to an index of reliability for downscaled effective rainfall, a key determinant of low flows in the River Thames. Hydrological model structures were weighted by performance at reproducing annual low-flow series. Weights were also assigned to sets of water resource model (CATCHMOD) parameters using the Nash-Sutcliffe efficiency criterion. Emission scenarios and downscaling methods were unweighted. A Monte Carlo approach was then used to explore components of uncertainty affecting projections for the River Thames by the 2080s. The resulting cumulative distribution functions (CDFs) of low flows were most sensitive to uncertainty in the climate change scenarios and downscaling of different GCMs. Uncertainties due to the hydrological model parameters and emission scenario increase with time but were less important. Abrupt changes in low-flow CDFs were attributed to uncertainties in statistically downscaled summer rainfall. This was linked to different behavior of atmospheric moisture among the chosen GCMs.

It is now accepted that some human-induced climate change is unavoidable. Potential impacts on water supply have received much attention, but relatively little is known about the concomitant changes in water quality. Projected changes in air temperature and rainfall could affect river flows and, hence, the mobility and dilution of contaminants. Increased water temperatures will affect chemical reaction kinetics and, combined with deteriorations in quality, freshwater ecological status. With increased flows there will be changes in stream power and, hence, sediment loads with the potential to alter the morphology of rivers and the transfer of sediments to lakes, thereby impacting freshwater habitats in both lake and stream systems. This paper reviews such impacts through the lens of UK surface water quality. Widely accepted climate change scenarios suggest more frequent droughts in summer, as well as flash-flooding, leading to uncontrolled discharges from urban areas to receiving water courses and estuaries. Invasion by alien species is highly likely, as is migration of species within the UK adapting to changing temperatures and flow regimes. Lower flows, reduced velocities and, hence, higher water residence times in rivers and lakes will enhance the potential for toxic algal blooms and reduce dissolved oxygen levels. Upland streams could experience increased dissolved organic carbon and colour levels, requiring action at water treatment plants to prevent toxic by-products entering public water supplies. Storms that terminate drought periods will flush nutrients from urban and rural areas or generate acid pulses in acidified upland catchments. Policy responses to climate change, such as the growth of bio-fuels or emission controls, will further impact freshwater quality.Key words climate change; water quality; rivers; catchments; lakes; estuaries; ecology; hydrochemistry Une revue des impacts potentiels du changement climatique sur la qualité des eaux de surface Résumé Il est maintenant admis qu'un certain changement climatique d'origine anthropique est inévitable. Les impacts potentiels sur l'alimentation en eau ont fait l'objet de nombreuses attentions, mais peu de connaissances sont disponibles sur les changements associés en termes de qualité de l'eau. Les changements prévus en termes de température de l'air et de précipitations pourraient affecter les écoulements des rivières et par conséquent la mobilité et la dilution des substances polluantes. Une augmentation des températures de l'eau affectera la cinétique des réactions chimiques et, par combinaison avec les dégradations de la qualité, l'état écologique des hydrosystèmes. Une augmentation des écoulements aura pour conséquences des changements dans la puissance des cours d'eau et donc aussi des charges sédimentaires qui pourront altérer la morphologie des rivières et le transfert de sédiments vers les lacs, ce qui aura des impacts sur les habitats hydrobiologiques dans les systèmes lacustres et fluviatiles. Cet article fait une revue de tels imp...

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