Climate Change and the European Water Dimension - Agri ...

have recently described a method that can be used to ‘hindcast’ the seasonal
variation in the residence time of a lake. Using this system, some lakes are classified
as ‘one season’ lakes which respond very quickly to changes in the rainfall whilst
others are ‘multi-season’ lakes that are influenced by the changes recorded over
months or even years. In UK the most pronounced changes will thus be recorded in
the ‘wet’ west during the winter and the dry ‘east’ during the summer.
Figure IV.B.3 shows the effect that historical variations in the rainfall have had on the
winter residence time of Esthwaite Water, a lake with an average annual residence
time of 92 days. The results show that the winter residence time of this lake has
decreased progressively over the past 40 years with the rate of decline averaging 0.5
days a year. Lakes with long residence times are less sensitive to the effects of
changes in the rainfall since the effects of heavy winter rains may then counter the
impact of summer droughts. An increase in the water residence time in a warmer
climate due to higher evaporation and decreased stream outflow will also increase
the retention of chemical constituents, and intensify the nutrient cycle.
To summarize, in deep perialpine lakes (north and south of the Alps) climate
changes are having a double and contrasting impact on the renewal times of lake
waters. In summer, an increase in the thickness of the mixo-limnion accelerates the
renewal time, as a greater volume of water is involved. In winter, the need for more
mixing energy to trigger a complete vertical mixing, or a deeper mixing, reduces the
volume of the water mass involved in the renewal.
Water level
Because of the intimate hydrological link between river discharge and lake level,
changes in run-off will ultimately influence lake level characteristics. Increasing
trends in heavy rainfalls (Osborne and Hulme, 2002; Matulla et al., 2004) will affect
lake levels and increase the flood risk.
Climate change has introduced another level of uncertainty to the management of
most dams. The safety of large dams is affected by changes in the magnitude or
frequency of extreme precipitation events. One of the first studied in this area
concluded that the discharge of the 50-year flood on the River Severn in the United
Kingdom might increase by approximately 20% by 2050 (Tedd, 2000). There is
concern whether existing spillways of large dams can accommodate such floods in
the future.
In the flat landscapes in Estonia, N-W Russia, South Finland and North Germany
where the outflow from lakes is often limited by the small slope of rivers, changes in
precipitation result in large seasonal and interannual fluctuations of the water level.
Changes in water levels, particularly in shallow lakes, may change the mean water
column irradiance (Behrendt and Stellmacher, 1987) and shift areas of sediment
erosion, transportation, and accumulation (Bengtsson et al. 1990). The large surface
area to mean depth ratio makes the shallow lakes, especially large ones, highly
susceptible to climatic influences (Magnusson et al. 1990). The responses of phyto-
and zooplankton communities to altered environmental conditions may be complex
(Weyhenmeyer et al. 1999; Nõges and Nõges, 1999; Irigoien et al. 2000; Gerten and
Adrian 2000; Straile 2000; Straile and Adrian 2000). Generally, shorter residence
time and higher water levels in lakes will contribute to the improvement of all
common water quality parameters in these temperate lakes.
Lake levels in semi-arid and arid areas reflect a sensitive balance between water
inflow and evaporation. Increasing temperature and prolonged droughts decrease the
water level in these lakes and shift them from a permanent status to a semi-
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