3.3 Climate Change Impacts on the Water Regime
for Central Europe
3.3.1 Precipitation, Evaporation and Climatic Water Balance
In Central and Eastern Europe the mean temperature is projected to increase
between 1 and 3
C in the next decades and up to 5
C until the end of the century
(see Chap. 2). A general pattern is that the higher temperatures lead to an intensification of the water cycle (EEA 2008). Based on the Clausius-Clapeyron expression for saturation vapour pressure, the moisture holding capacity of air increases
by about 7 % per 11990). As a result, climate warming will lead to an increase of the evaporative
demand in the air, or “potential evaporation”. Generally a higher moisture potential
in the atmosphere ultimately leads amongst others to changes in rainfall patterns.
Key changes to the hydrological cycle in Central Europe associated with an
increased concentration of greenhouse gases in the atmosphere include (Goudie 2006):
Changes in the seasonal distribution and amount of precipitation. Generally,
for all scenarios, the projected annual mean precipitation increases in northern
Europe and decreases in the south of Europe. In doing so, the changes in
precipitation patterns vary from season to season and across regions in response
to changes in large-scale circulation and water vapour loading (Bates et al. 2008).
A substantial decrease is projected in summer precipitation for most parts of
Central Europe. Because precipitation comes primarily from moisture convergence, an increase in the atmospheric water holding capacity increases the
potential for intense precipitation (Trenberth et al. 2003). At the same time this
leads to a decrease in the frequency and duration of precipitation events, making
way for longer dry periods between precipitation events (IPCC 2007).
Increased evapotranspiration and a reduction in soil moisture. As a result of
higher temperature the water vapour deficit in the atmosphere increases. In areas
with sufficient (surface) water availability this leads to an increase in actual
evapotranspiration. With scarce precipitation, this enlarges the risk of drought as
surface drying and hence, a reduction on soil moisture is forced up (Bates
et al. 2008). Additionally, an increased atmospheric CO 2 -concentration directly
alters plant physiology and thus transpiration rates (especially C3 plants).
Changes in the balance between snow and rain. As temperature rises, the
likelihood of precipitation falling as rain rather than snow increases, especially
in areas where temperatures are near freezing point, and at the beginning and end
of the snow season. Yet a warmer climate leads to a shorter snow season with
more rains but reduced snow packs, earlier snowmelt and greater ablation. Such
changes are already observed in many places, especially over land in high
latitudes of the Northern Hemisphere (Bates et al. 2008). As for some areas in
Central and Eastern Europe a general increase in winter precipitation is projected,
this effect could partly compensate a reduction of the total amount of snow, even
if the percentage of precipitation falling as snow is decreasing.
34
J. Stagl et al.
for Central Europe
3.3.1 Precipitation, Evaporation and Climatic Water Balance
In Central and Eastern Europe the mean temperature is projected to increase
between 1 and 3
C in the next decades and up to 5
C until the end of the century
(see Chap. 2). A general pattern is that the higher temperatures lead to an intensification of the water cycle (EEA 2008). Based on the Clausius-Clapeyron expression for saturation vapour pressure, the moisture holding capacity of air increases
by about 7 % per 11990). As a result, climate warming will lead to an increase of the evaporative
demand in the air, or “potential evaporation”. Generally a higher moisture potential
in the atmosphere ultimately leads amongst others to changes in rainfall patterns.
Key changes to the hydrological cycle in Central Europe associated with an
increased concentration of greenhouse gases in the atmosphere include (Goudie 2006):
Changes in the seasonal distribution and amount of precipitation. Generally,
for all scenarios, the projected annual mean precipitation increases in northern
Europe and decreases in the south of Europe. In doing so, the changes in
precipitation patterns vary from season to season and across regions in response
to changes in large-scale circulation and water vapour loading (Bates et al. 2008).
A substantial decrease is projected in summer precipitation for most parts of
Central Europe. Because precipitation comes primarily from moisture convergence, an increase in the atmospheric water holding capacity increases the
potential for intense precipitation (Trenberth et al. 2003). At the same time this
leads to a decrease in the frequency and duration of precipitation events, making
way for longer dry periods between precipitation events (IPCC 2007).
Increased evapotranspiration and a reduction in soil moisture. As a result of
higher temperature the water vapour deficit in the atmosphere increases. In areas
with sufficient (surface) water availability this leads to an increase in actual
evapotranspiration. With scarce precipitation, this enlarges the risk of drought as
surface drying and hence, a reduction on soil moisture is forced up (Bates
et al. 2008). Additionally, an increased atmospheric CO 2 -concentration directly
alters plant physiology and thus transpiration rates (especially C3 plants).
Changes in the balance between snow and rain. As temperature rises, the
likelihood of precipitation falling as rain rather than snow increases, especially
in areas where temperatures are near freezing point, and at the beginning and end
of the snow season. Yet a warmer climate leads to a shorter snow season with
more rains but reduced snow packs, earlier snowmelt and greater ablation. Such
changes are already observed in many places, especially over land in high
latitudes of the Northern Hemisphere (Bates et al. 2008). As for some areas in
Central and Eastern Europe a general increase in winter precipitation is projected,
this effect could partly compensate a reduction of the total amount of snow, even
if the percentage of precipitation falling as snow is decreasing.
34
J. Stagl et al.