Climate Change and Switzerland 2050 - OcCC - SCNAT

22 Climate Change and Switzerland 2050 | Background
the runoff follows the changes in precipitation.
However, only a small part of the rainfall runs
off directly; the larger part is stored, for instance
in snow cover and glaciers, in the ground, in the
groundwater, and in natural and artificial lakes.
In the short term, runoff is influenced by the
release of water from the stores.
Different runoff types, showing different annual
runoff patterns can be distinguished by the degree
of glaciation and snow cover. A selection of runoff
types is depicted in figure 11. Watercourses that
are mainly fed by melting glaciers and snow (glacial
type) show larger runoff variations. The mean
monthly runoff can easily vary between winter
and summer by a factor of 30. Watercourses that
are mainly dependent on rainfall (elevated pluvial
type) show the least runoff variations.
The following changes can be expected by 2050
on the basis of the climate scenario presented
here:
• Less precipitation falls as snow at lower and
medium elevations due to warming. The snow
line, which separates snow-covered areas from
lower lying areas, rises by about 360 m in the
case of medium warming (see section 3).
• The frequency and intensity of flooding will
increase in winter in small and medium
catchment areas of the midlands. This is due
to the increased prevalence of rain instead of
snow at lower and medium elevations, and
the rise in heavy precipitation (see section 2).
•
In the case of medium warming, glacier area
in the Alps will decrease by about three quar-
ters by 2050 (see section 3, fig. 9). This simple
estimate is consistent with earlier studies 15 ,
which showed a rise in the equilibrium line
of glaciers by 400 m in the case of a warming
of 2.7 °C in summer.
• Evaporation will generally increase as a result
of warming. Due to soil dehydration, evaporation
may be locally and temporally restricted
and therefore reduced.
• As a result of the decrease in precipitation volume
and the increase in evaporation, annual
runoff will decrease, particularly in the south,
but also in the north. This will happen in spite
of the temporary contribution of meltwater
from the retreating glaciers. In summer, soil
moisture may be reduced for extensive periods
of time (particularly in late summer and
autumn in the south, but also in the north).
In small and medium watercourses of the midlands
and the south of Ticino, dry spells will
occur more frequently. In addition, dry spells
may occur more often in late summer in areas
where glaciers have disappeared.
• Groundwater replenishment will decrease in
summer and autumn in all non-glaciated areas.
• As a consequence of the changed accumulation
and degradation of the snow cover, the
rise of the snow line and the retreat of the
glaciers, the runoff regimes (fig. 11) at a particular
altitude will shift downwards by about
one regime level.