Geography Reference
In-Depth Information
Figure 2.3. Simple and complex
system representation of the time
scales of floods and their process
controls. Interactions of the
processes at different time scales
have been gleaned from comparative
hydrology. From Gaál et al.(
2012
).
T
T
T
T
T
T
T
T
T
while soil depth and permeability affect flow paths and
therefore the flood response at the event scale. Even at the
landscape evolution time scale there are further interactions.
Gaál et al.(
2012
) illustrated how the comparison of catch-
ments of contrasting characteristics can help to recognise
the combined effect and interplay of flood processes on the
landscape. They showed, for example, one catchment whose
form has adapted to the flashiness of floods by producing
efficient drainage networks, which in turn enhance the flashi-
ness of the flood response. In other catchments, tortuous
drainage networks have evolved, which in turn retard the
flood response and impede the evolution of an efficient
drainage network.
Complex systems are notoriously difficult to understand,
and exhibit inherent limits to their predictability (Blöschl and
Zehe,
2005
;Kumar,
2011
). The complex interactions and
fe
edbacks of the various component processes occurring
within a catchment make it difficult to connect cause and
effect in a straightforward manner, thus presenting a signifi-
cant challenge to predictions in ungauged basins. On the
other hand, an important feature of complex systems, as
outlined above, is their tendency to generate emergent
patterns. Depending on the scale at which one looks, the
patterns the system produces may be different. If one zooms
in, one set of patterns emerges. If one zooms out, a new set
of patterns emerges. Looking at emergent patterns, one
cannot easily find causal connections between the patterns
at different scales. In the catena example above, it is not
trivial to explain how the interactions of local-scale processes
led to catena patterns at the hillslope scale and further orga-
nised patterns around the river network at the catchment
scale. The evolution of these patterns is the result of the
interaction of several component processes at a range of
space and time scales, producing patterns at many space
scales (
Figure 2.2
). Yet the fact that catchments as complex
systems create interesting spatial and temporal patterns offers
opportunities that can be exploited to advance predictions.
2.1.2 Signatures: a manifestation of co-evolution
Spatial patterns such as those presented in
Figures 2.1
and
2.2
are readily observable, and they contribute to observed
temporal patterns of hydrological response produced
by catchments. Most importantly, the observed runoff
response of a catchment constitutes an interesting, complex
temporal pattern of water fluxes, which are the result of the
collective behaviour of a great number of components of
the catchment,
including the effects of
the landscape
patterns.
When looking at the catchment behaviour in an aggre-
gate way, one can identify typical holistic characteristics of
the catchment
response, something termed
'
catchment
functioning
by Black (
1997
), by analogy with a similar
term used in ecology (Jax,
2005
). The collective or holistic
response of the catchment resulting from the component
processes can be expressed in terms of holistic behaviour
such as partitioning, transmission, storage and release of
water, energy and matter (Black,
1997
; McDonnell et al.,
2007
; Wagener et al.,
2007
). Partitioning refers to the
separation of water, energy and matter into different path-
ways at or near the land surface through processes includ-
ing interception, infiltration and surface runoff. Storage
refers to actions of the catchment to retain water, energy
and matter in different parts of the catchment and over very
different time scales. Storage can include snow and ice,
interception, soil moisture, aquifers, water bodies and also
'
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