Geography Reference
In-Depth Information
vegetation. Transmission refers to the fluxes of water,
energy and matter through the catchment. These fluxes
are strongly dependent on the connectivity between the
different parts of the catchment and will significantly vary
over time in many cases, depending on the moisture state
of the system. Finally, release refers to the mechanisms by
which water, energy and matter are released from the
catchment through atmospheric, surface and subsurface
fluxes. Fluxes of water, energy and matter include evapor-
ation, transpiration, channel flow, sediment transport and
groundwater exchange.
The co-evolution of climate, vegetation, landscape and
soils, through the self-organised landscape patterns it tends
to produce, gives rise to evident fingerprints on the catch-
ment runoff responses. Since the structure of the landscape
determines the heterogeneity and organisation of pathways
that water can follow, and associated residence times, these
also govern the richness of the catchment
chooses to look at. This is because catchments exhibit
the characteristics of complex systems, so different pat-
terns emerge at different time scales. At time scales of
seconds one may recognise the effects of turbulence and
wave action in the runoff. At time scales of millennia, if
such data were available as in the case of Jefferson et al.
(
2010
), one would recognise long-term climate and land-
scape evolution trends. There may be several emergent
patterns in the time domain and they are all inter-connected
because they are all the result of the same complex system
and co-evolutionary processes.
Depending on the collective behaviour of the catchment
processes and the underlying drivers, the runoff signatures
may differ. Therefore they can be seen as windows that
enable us to look into the catchment dynamics at different
time scales. They help us to understand the system holistic-
ally. Signatures provide insights into catchment processes,
and are thus outward manifestations of the internal dynam-
ics of the catchment. The runoff signatures examined in
this topic are annual runoff, seasonal runoff, flow duration
curves, low flows, floods and runoff hydrographs (
Figure
2.4
). In a preface to a special journal issue on the down-
ward approach to hydrological prediction, Sivapalan et al.
(
2003a
) said, inter alia:
'
s hydrological
responses. This includes the emergent connectivity of path-
ways, the appearance of thresholds and tipping points, all
leading to a holistic response that is harder to prescribe
a priori, let alone predict on the basis of traditional simple
system approaches. Indeed, Knighton and Nanson (
2001
)
have documented complex patterns of event-scale runoff
variability at a range of time and space scales for the
Channel Country of Australia, including Lake Eyre, which
overlaps with the geographic region presented earlier in
Figure 2.1
. The work raises interesting questions about
how the amazingly complex spatial patterns shown in
Figure 2.1
are mirrored in the runoff variability, and
whether it can be explained hydrologically to enable pre-
dictions. Understanding these connections is particularly
important when humans increasingly become a major part
of this co-evolutionary system, with the possibility of
generation of new emergent dynamics hitherto unobserved
(Winder et al.,
2005
; Kallis,
2007
).
In this topic, following Jothityangkoon et al.(
2001
) and
Eder et al.(
2003
), the temporal patterns of the observed
runoff response of catchments, when viewed at different
time scales, are termed runoff
The Budyko curve, inter-annual and mean monthly variability of
water balance,
ow duration curves, and the spatial organisation
of these signatures
…
are the key signatures that embody the
hydrological organisation or hidden order, and a quest for
identifying them seems promising.
For example, annual runoff is a reflection of the catchment
dynamics at relatively long time scales, which is particu-
larly evident in the between-year variability of annual
runoff. Seasonal runoff reflects the within-year variability,
i.e., how the catchment organises itself at the sub-annual
time scale. The flow duration curve represents the full
spectrum of variability in terms of flow magnitudes. Low
flows focus on the low end of that spectrum, and so
provide a window into catchment dynamics when there is
little water in the system, and floods are at the opposite
end, when there is much water in the system. Hydrographs
are the complex combination of all of these signatures.
They are the most detailed signatures of how catchments
respond to water and energy inputs.
In this topic, the signatures are the starting point for
making runoff predictions in ungauged basins as they are
the manifestations of the catchment functioning at different
time scales. They are also the focal point of the predictions,
and predictions of all the signatures in ungauged basins are
reviewed in this topic in their own right. In fact, they are
fully consistent with the time scales at which runoff pre-
dictions in ungauged basins are needed from a societal
perspective, as illustrated in
Table 1.1
.
'
'
signatures
, and deemed
emergent patterns. We term them
because they
are considered as reflections of the overall functioning of
the catchments, including the co-evolutionary features of
the catchments
'
signatures
'
surface and subsurface architecture. The
spatial signatures (or fingerprints) of catchments, such as
soil catena, stream network topology and soil moisture
patterns, are all intimately related to the temporal patterns
of runoff at different time scales, and the focus here is on
advancing and exploiting our understanding of
'
their
interrelationships.
Runoff variability at any location is a temporal con-
tinuum covering a wide range of time scales, but the
characteristics one sees depend on the temporal scale one
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