Europe has a very diverse hydrological background, reflecting its varied climate and topography. In the south there is very significant variation in flow through the year, with long, dry summers. To the west there is less extreme variation, and in catchments underlain by absorbent aquifers flows remain reasonably constant through the year. In the north and east, much precipitation falls as snow, so much flow occurs during the spring snowmelt period. Major rivers such as the Rhine, Rhone, Po, and Danube distribute water from the "water tower" of the Alps. Superimposed on this varied hydrological base is a wide variety of water uses, pressures, and management approaches. A succession of floods and droughts has illustrated Europe's vulnerability to hydrological extremes. There are many other water-related pressures on Europe's environment, however (see the Dobris Assessment-Stanners and Bourdeau, 1995), such as increasing demand for water, particularly in the south, and subsequent increases in abstractions. River ecosystems and wetlands are increasingly at risk. The quality of Europe's rivers, lakes, and groundwater is being threatened by the discharge of sewage and industrial waste (often a legacy from past industrial development) and by excessive application of pesticides and fertilizers. At the same time, the institutional aspects of water resources management are changing. There is a shift in many countries from "supply-side" solutions to a "demand-side" approach, aimed at reducing demand for water or exposure to risk. Water managers in many European countries have adopted (at least in principle) a sustainable approach, and forthcoming EU directives are likely to encourage this further. Environmental demands are being taken increasingly seriously across most of Europe. Climate change adds another set of potential pressures on European water resources and their management.
Climate and land-use change influence the structure of the water balance (Ryszkowski and Kedziora, 1987; Ryszkowski et al., 1990; Olejnik and Kedziora, 1991). Calculations at the European scale (Arnell, 1999) indicate that under most climate change scenarios, northern Europe would see an increase in annual average streamflow, but southern Europe would experience a reduction in streamflow. In much of mid-latitude Europe, annual runoff would decrease or increase by about 10% by the 2050s, but the change resulting from climate may be smaller than "natural" multidecadal variability in runoff. To the south and north, it may be substantially larger (Hulme et al., 1999). Table 13-1 lists catchment-scale studies into potential hydrological changes in Europe that have been conducted since the IPCC's Second Assessment Report (SAR).
|Table 13-1: Catchment-scale studies into effects of climate change on runoff regimes in Europe (since SAR).|
|Albania||Bruci and Bicaj (1998)|
|Belgium||Gellens and Roulin (1998), Gellens et al. (1998)|
|Czech Republic||Dvorak et al. (1997), Hladny et al. (1997), Buchtele et al. (1998)|
|Danube||Starosolszky and Gauzer (1998)|
|Denmark||See Nordic region|
|Estonia||Bálint and Butina (1997), Jaagus (1998), Jarvet (1998), Roosaare (1998)|
|Finland||Vehviläinen and Huttunen (1997); see Nordic region|
|France||Mandelkern et al. (1998)|
|Germany||Daamen et al. (1998)|
|Greece||Panagoulia and Dimou (1996)|
|Hungary||Mika et al. (1997)|
|Latvia||Butina et al. (1998), Jansons and Butina (1998)|
|Nordic region||Sælthun et al. (1998)|
|Norway||See Nordic region|
|Poland||Kaczmarek et al. (1997)|
|Rhine basin||Grabs (1997)|
|Romania||Stanescu et al. (1998)|
|Slovakia||Pekárová (1996), Szolgay (1997), Hlavcová and Cunderlík (1998), Petrovic (1998), Hlavcová et al. (1999)|
|Spain||Avila et al. (1996)|
|Sweden||Xu (1998); see Nordic region|
|Switzerland||Seidel et al. (1998)|
|United Kingdom||Arnell (1996), Arnell and Reynard (1996), Reynard et al. (1998), Roberts (1998)|
The consequences of climate change for the variation of flow through the year vary across Europe. In Mediterranean regions, climate change is likely to exaggerate considerably the range in flows between winter and summer. In maritime western Europe, the range also is likely to increase, but to a lesser extent. In more continental and upland areas, where snowfall makes up a large proportion of winter precipitation, a rise in temperature would mean that more precipitation falls as rain and therefore that winter runoff increases and spring snowmelt decreases. The timing of streamflow therefore alters significantly. Further east and at higher altitudes, most of the precipitation continues to fall as snow, so the distribution of flow through the year is altered little (Arnell, 1999). The substantial projected change in flow regime resulting from reduction in snowfall and snowmelt across large parts of CEE has been noted widely (e.g., Hladny et al., 1996; Kasparék, 1998; Hlavcova and Cunderlik, 1998; Starosolszky and Gauzer, 1998).
Low-flow frequency generally will increase across most of Europe (Arnell, 1999), although in some areas where the minimum occurs during winter the absolute magnitude of low flows may increase because winter runoff increases: The season of lowest flow shifts toward summer. An implication of simulated changes in streamflow is that riverine flood risk generally would increase across much of Europe and that in some areas, the time of greatest risk would move from spring to winter. Effects on groundwater recharge (a major resource for many Europeans) are less clear because the general increase in winter rainfall may be offset by a reduction in the recharge season. Studies in the UK (Arnell and Reynard, 1999) and Estonia (Jarvet, 1998) indicate that groundwater recharge could be increased by climate change.
Implications of climate change for river water quality have been less well studied, but an increase in water temperature and widespread reductions in flow during summer are likely to lead to deterioration in many determinants of water quality (particularly dissolved oxygen concentrations). Jansons and Butina (1998) estimate nitrate and phosphate loads in a catchment in Latvia from streamflow; they use the relationships to infer an increase in nitrate and phosphate loads in winter (when flow increased) and a decrease in spring. Their model, however, did not account for possible temperature-related effects on nitrate and phosphate concentrations. Higher water temperatures are likely to increase the risk of blue-green algal blooms in rivers and lakes (e.g., Zalewski and Wagner, 1995).
The impacts of climate change on European water resources and their management depend not only on the change in hydrology but also (perhaps more particularly) on characteristics of the water management system. In general terms, the more stressed the system is under current conditions, the more sensitive it will be to climate change. Table 13-2 summarizes the key potential impacts on European water resources. These impacts should be considered against the background of other pressures and drivers on European water. Most of the potential impacts are self-explanatory.
Changes in the water resource base affect many sectors within Europe, including agriculture, industry, transport, power generation, the built environment, and ecosystems. Similarly, changes in many of these sectors will affect hydrology and the resource base. Changes in agricultural practices resulting from climate change, for example, will affect volumes and, more likely, quality of streamflow.
In many European countries, industrial water use has declined as a result of legislation, environmental protection, and economic change. The warmer climate may lead to significant increase in water demands in southern Europe-leading to possible overexploitation of groundwater resources, decrease of baseflow, and environmental degradation. Climate change is but one of the pressures facing European water managers. They will be played out against varying socioeconomic backgrounds, political demands for environmental improvements, and new trends in integrated water management. More detail on this and the foregoing conclusions appear in the water chapter of the European ACACIA report (Arnell, 2000).
Other reports in this collection