Europes ecological backbone.pdf

The water towers of Europe
Box 6.1 The hydrology of four major European mountain regions
The Alps
The Alps are located in an area of extremely high humidity owing to their close proximity to the
northern and western Atlantic Ocean, to the Mediterranean sea to the south, and due to the influence of
predominantly westerly winds. Their hydrological importance is also due to the considerable amounts of
meltwater from snow and ice originating from them during the summer months (Viviroli and Weingartner,
2004). Almost two-thirds of the Central European perennial surface ice cover is located in the Alps, with the
Aletsch Glacier being the largest valley glacier (Box 6.2). Many large and well-known European lakes are
located in the Alps including Lake Constance, Lac Leman (Lake Geneva) and Lago Maggiore.
Most of Europe's major rivers have their headwaters in the Alps and their discharge is transported via
river systems to lower-lying areas. Hence, the water system of the Alps is very important not only for the
countries of this mountain range but also for large parts of Europe (EEA, 2009a). The four main rivers
draining the Alps (Rhine, Rhône, Po and upper Danube) contribute a remarkably high amount of water
(Table 6.1), supplying up to 2–6 times more water than might be expected on the basis of catchment
size alone (Viviroli and Weingartner, 2004). The importance of the Alps in relation to water resources is
primarily based on enhanced precipitation as rainfall generally increases with altitude. A large proportion
of the precipitation falls as snow at higher altitudes, and may form glaciers, which are key features of
the hydrology of the Alps. Lower temperatures, shorter growth seasons and more shallow soils at higher
elevations also result in lower evapo-transpiration rates, causing a positive water balance in the mountains.
The Alpine rivers vary significantly in annual mean discharge per area, partly due to the positions of the
monitoring stations, but mostly because of climatic conditions and water usage (EEA, 2009a). In the
future, the combined effects of droughts and increased water consumption in the Alps could cause water
supply problems throughout Europe. Future climate change is projected to lead to a shift from summer
precipitation to winter precipitation and — together with an earlier and reduced snow melt due to lower
storage of winter precipitation as snow, as well as less glacial melt water — will lead to an essential
decrease in summer run-off all over the Alps (EEA, 2009a).
Pyrenees
The Pyrenees are the water towers for southwest France and northern Spain, particularly the basins of
the Ebro and Garonne. The western and central part of the range receives a much greater amount of
precipitation than the eastern part, due to moisture-laden air coming from the Bay of Biscay. The region
is typically divided into three climatic zones: the Atlantic (or Western); the Central; and the Eastern
Pyrenees. Precipitation falls predominantly during winter in areas adjacent to the Atlantic, and during
spring and autumn in the Mediterranean regions, with extensive and thick snow cover from December
to April in areas over 1 500 m above mean sea level, with a longer duration of snow cover at higher
altitudes and in shaded areas (García-Ruiz et al., 1986; López-Moreno and Nogués-Bravo, 2005). Snow
melt is vital for the ecological and socio-economic well-being of the region and is a major contributor to
the amount of runoff and its seasonal distribution, playing a leading role within Pyrenean river basin water
management in the semi-arid and highly populated Ebro valley (López-Moreno and García-Ruiz 2004;
López and Justribó, 2010). The Ebro River receives 50–60 % of its discharge from the Pyrenees, although
only 30 % of its catchment is in the mountains (López and Justribó, 2010). There are currently 41 glaciers
in the Pyrenees, all centrally‐located within one 100 km stretch of the range and covering a total area of
approximately 8.1 km 2 (Serrat and Ventura, 1993). These glaciers are small: the largest, Glaciar de Aneto,
is only 1.32 km², while half are 0.1 km² or less in area. All glaciated peaks are higher than 3 000 m — but
not all peaks that reach this height have glaciers — and, unlike the glaciers in the Alps, they do not descend
far down into the valleys (Serrat and Ventura, 1993). The melting of glaciers in the Pyrenees is much more
advanced than in the Alps (Box 6.2). In contrast to the Alps, there are no very large lakes in the region.
However, there are numerous smaller lakes, such as those in the Aigüestortes in the Alta Ribagorza region.
Scandinavian mountains
The distance from the top of the Scandes range to the ocean is greatest on the Swedish side, where a
dozen roughly parallel drainages run from the mountains into the Gulf of Bothnia. Most of the rivers at the
northern end of the range are above the Arctic Circle, while those at the southern end flow into the ocean
at about 60 °N. The region does not have large topographic relief, but the rivers have a number of steep
rapids interspersed with lower-gradient segments. Mean annual precipitation is 500–1 000 mm, much of
which falls as snow. The timing and level of runoff is variable and dependent on river location: the northern
rivers have low winter flows with rapid snowmelt and intense flooding during spring to early summer; rivers
draining into the central eastern coastal area have less intense spring floods; while rivers in the far south
Europe's ecological backbone: recognising the true value of our mountains
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