Geography Reference
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of the floods (
Figure 9.2
bottom), however, may differ from
that of the extreme rainfall. In the mountain ranges this is
due to snowprocesses. For example, even if there are October
rainfall maxima, the flood maxima may cluster around
June and July due to snowmelt. In the lowlands this is due
to the interplay of soil moisture with extreme rainfall. For
example, even if there are July rainfall maxima, the flood
maxima may occur in December and January when the soil
moisture is largest in the catchments due to low evaporation.
Not only is there often considerable intra-annual vari-
ability of rainfall and floods but also inter-annual (e.g., El
Niño and La Niña variability), and even inter-decadal
variability (e.g., Inter-decadal Pacific Oscillation (IPO),
Pacific Decadal Oscillation etc.) of precipitation, all of
which can impact the shape of the flood frequency curve.
Figure 9.3a
presents the flood frequency curves in Eastern
Australia under El Niño and La Niña conditions along with
the associated 90% confidence limits (Kiem et al.,
2003
).
Much
higher flood risk is associated with La Niña events
as opposed to El Niño. In
Figure 9.3b
the flood frequency
curves are shown for IPO negative (
<
−
0.5) against non-
negative IPO phases. It can be seen that IPO negative phases
correspond to a much increased flood risk when compared
to the non-negative phases of IPO. It is therefore clear that
monitoring of the multi-decadal IPO phase may provide
valuable insight into flood risk on multi-decadal scales,
whilst the joint occurrence of inter-annual La Niña events
within the IPO negative phase represents further elevated
flood risk in Eastern Australia. Depending on the region
there are a number of processes related to climate that
modulate the inter-annual precipitation variability (Kund-
zewicz,
2012
) including soil moisture, and snow storage
3.8
a)
3.6
3.4
La Niñas
3.2
3.0
2.8
2.6
El Niños
2.4
3.8
b)
3.6
La Niñas in
IPO < -0.5
3.4
3.2
3.0
2.8
All other
La Niñas
2.6
2.4
1
10
100
Return period (years)
Figure 9.3. Regional flood frequency curves in New South Wales,
Australia, with 90% confidence bounds (dashed). RI is, for each
year, the regional average of the flood runoff normalised by its
long-term mean. (a) Floods under El Niño and La Niña conditions;
(b) floods during negative (
Runoff generation
Rainfall and snowmelt run off the land surface or infiltrate
into the soil through a variety of mechanisms including
infiltration excess, saturation excess and subsurface storm-
flow (see
Chapters 4
and
10
). Mechanisms that involve
surface flow paths such as infiltration excess and saturation
excess produce a quick response, whereas mechanisms that
involve subsurface flow paths in general produce a rela-
tively slower response, and yet can make a significant
impact on the shape of the flood frequency curve (Samuel
and Sivapalan,
2008
). The mechanism that operates in a
specific catchment, or during a specific event depends on
the rainfall intensity and depth, soils, vegetation and top-
ography and especially on the antecedent wetness of the
catchment, which reflects a carry-over or memory of pre-
vious events. The effect of antecedent wetness on the flood
frequency curve can be significant (Wood,
1976
; Komma
et al.,
2007
). In arid regions where infiltration excess run-
off often dominates, antecedent soil moisture tends to be
76) and non-negative
Inter-decadal Pacific Oscillation phases (
1946
-
~
1924
-
43, 1979
-
97).
~
From Kiem et al.(
2003
).
mainly random. On the other hand, in many catchments
around the world that exhibit strong seasonality in climate
forcing, e.g., humid or temperate climates in Europe or
North America, and in Mediterranean catchments in south-
ern Europe, Western Australia, and western USA, antece-
dent soil moisture exhibits a strong (systematic) seasonal
component, and has been shown to have a significant
impact on the flood frequency curve (e.g., Sivapalan
et al.,
2005
). In parts of Western Australia, floods with a
return period less than 10 years are typically winter floods,
whereas floods with a return period longer than 30 years
tend to be summer floods, despite generally drier soils in
summer (Sivandran,
2002
). This arises due to different
mechanisms of rain-producing events (frontal events in
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