Geography Reference
In-Depth Information
D
400
N
O
S
300
A
J
200
J
M
A
100
M
F
J
0
J
FM
A
M
J
J
A
S
O
N
D
1
2
5
10
20
50
100
D
N
100
O
S
80
A
J
60
J
M
40
A
M
20
F
J
0
J
FM
A
M
J
J
A
S
O
N
D
1
2
5
10
20
50
100
Return period (yrs)
Figure 9.4. Seasonality of monthly runoff (shaded areas), precipitation (dashed lines) and flood frequency (points). (Top) Gilgel Abbay River,
Blue Nile near Merawi, Ethiopia (1664 km²); (bottom) Thompson River at Davis City, Iowa, USA (1816 km²).
winter, thunderstorms and tropical cyclones in summer),
and their interaction with different flooding processes
dominating in different times of the year.
Figure 9.4
shows the net effect of soil moisture on the timing of
the flood frequency curve. In the Gilgel Abbay River, a
tributary of the Blue Nile in Ethiopia, rainfall maxima
occur in July. Runoff lags a little behind rainfall due to
storage effects and the largest floods occur in August.
For the Thompson River in Iowa, USA, rainfall maxima
occur between June and August and monthly runoff
peaks occur between March and June, i.e., earlier
because of the higher soil moisture in spring due to
snowmelt. This interplay of rainfall and soil moisture
produces a pattern of floods mostly in May, but the
largest floods may occur later in the season, e.g., Sep-
tember. This is because antecedent soil moisture will be
less important if the rainfall depths are very large. Rain-
fall characteristics become increasingly important as the
magnitude of an event increases, and the largest rain-
storms occur in September.
The precipitation intensity and depth have a very strong
effect on what mechanisms operate during a specific event.
Low or moderate intensity rainfalls with a longer duration
generally tend to produce subsurface stormflow and satur-
ation excess overland flow, while high intensity events
tend to favour infiltration excess runoff. Two or more
mechanisms may operate in different parts of the same
catchment during the same event. Alternatively, different
mechanisms may dominate during different events. With
increasing intensity or event depth that goes beyond a
threshold, there could be a switch from saturation excess
to infiltration excess runoff, or from subsurface stormflow
to saturation (storage) excess, and both of these may be
reflected in a sudden increase in runoff in the flood fre-
quency curve (Sivapalan et al.,
1990
; Samuel and Sivapa-
lan,
2008
; Gioia et al.,
2008
). Historically, such steep
increases have often been treated as outliers, but an inter-
pretation in terms of the flood generation processes may be
more insightful. This is illustrated in
Figure 9.5
for an
Alpine catchment in Austria. The figure shows different
areas contributing to fast surface runoff for events of
different magnitudes. During smaller events (events 1 and
2) only very few areas contribute to direct surface runoff,
such as sealed areas and rocks. As the event magnitude
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