Geology Reference
In-Depth Information
evapotranspiration), and decreasing in the
Mediterranean coastline and in the western sub-
tropical regions of each continent. However, the
IPCC report and other global studies on climate
change impacts (e.g. Wetherald & Manabe, 2002;
Nohara
et al
., 2006) do not separate surface runoff
from total runoff; changes to surface runoff have
consequences for soil erosion processes at the
field and hillslope scales, including gully erosion,
while changes to total runoff can impact upon
channel erosion and deposition processes and
watershed sediment yield.
One of the expected impacts of an increase in
rainfall intensity is greater surface runoff genera-
tion through infiltration-excess processes, espe-
cially when coupled with soil surface crusting
(Bronstert
et al
., 2002). However, soil moisture
rates are also an important factor for runoff gen-
eration in humid and semi-arid catchments, par-
ticularly for low- and medium-intensity storms
(e.g. Cammeraat, 2002; Boix-Fayos
et al
., 2006).
The IPCC report (Meehl
et al
., 2007) points to a
global decrease in soil moisture; this is more
marked in regions where rainfall decreases, but
also predicted to occur in high latitudes despite
an increase in rainfall due to the earlier start of
snowmelt. Global modelling results obtained by
Wetherald and Manabe (2002) and Manabe
et al
.
(2004) point to a high seasonal variability of soil
moisture changes, which are expected to occur
mostly in the spring-to-autumn period. For exam-
ple, soil moisture in the Mediterranean is not
expected to decrease significantly in the winter
despite a large annual decrease, while in the mid
latitudes, soil moisture is expected to increase in
winter and decrease in summer leading to small
changes in annual averages.
These changes are likely to have an impact on
surface runoff generation and therefore on soil
erosion, especially during low- to medium-
intensity storms; in general terms, surface run-
off generation can be expected to follow overall
runoff trends. However, changes to runoff gen-
eration processes are expected to have a high
degree of spatial variability, with the spatial dis-
tribution of soil hydrological properties playing
a significant role (e.g. Bronstert
et al
., 2002; van
den Hurk
et al
., 2005; Nunes
et al
., 2008).
Another important difference could be a shift in
the most important runoff generation processes
for regions where climatic aridity surpasses
desertification thresholds (Cammeraat, 2002);
this issue is discussed further below. In sum-
mary, the processes linking rainfall, soil mois-
ture and surface runoff generation are non-linear
and often particular to a small catchment or
region, and therefore it is difficult to generalize
impacts at the continental or global scale
(Kleinen & Petschel-Held, 2007). It should be
noted that most soil erosion modelling studies
presented in this chapter also focus on surface
runoff generation.
Furthermore, Kundzewicz
et al
. (2007)
pointed to a greater irregularity of streamflow
throughout the globe, both in seasonal and daily
terms, coupled with an increase in flash flood
frequency, especially in mid to high latitudes.
Higher flow seasonality is expected due to
changes in evapotranspiration and seasonal soil
moisture patterns, coupled with shifts in rain-
fall seasonality and, in the mid to high latitudes,
by a shorter snow accumulation season and ear-
lier onset of snowmelt (Meehl
et al
., 2007).
These changes, compounded with the increases
in rainfall intensity described earlier, can com-
bine to increase significantly the probability of
occurrence of large floods (Kundzewicz
et al
.,
2007); in a global study, Kleinen and Petschel-
Held (2007) found that up to 20% of the world
population could be affected by a significant
increase in the occurrence of large-scale inunda-
tions of flood plains. Meehl
et al
. (2007) also
pointed out the impact of an increase in snow-
fall extremes on the occurrence of large spring
floods. In short, climate change could lead to an
increase of high peak flow events in many river
basins, with potential impacts on channel ero-
sion processes.
15.2.3 Vegetation cover
Climate change is also expected to have complex
impacts on both natural and agricultural vegeta-
tion, affecting the protection given by canopy
