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| Titel |
Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications |
| VerfasserIn |
E. Zehe, M. Sivapalan  |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 13, no. 7 ; Nr. 13, no. 7 (2009-07-22), S.1273-1297 |
| Datensatznummer |
250011945
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| Publikation (Nr.) |
 copernicus.org/hess-13-1273-2009.pdf |
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| Zusammenfassung |
| In this paper we review threshold behaviour in
environmental systems, which are often associated with the onset of floods,
contamination and erosion events, and other degenerative processes. Key
objectives of this review are to a) suggest indicators for detecting
threshold behavior, b) discuss their implications for predictability, c)
distinguish different forms of threshold behavior and their underlying
controls, and d) hypothesise on possible reasons for why threshold behaviour
might occur. Threshold behaviour involves a fast qualitative change of
either a single process or the response of a system. For elementary
phenomena this switch occurs when boundary conditions (e.g., energy inputs)
or system states as expressed by dimensionless quantities (e.g. the Reynolds
number) exceed threshold values. Mixing, water movement or depletion of
thermodynamic gradients becomes much more efficient as a result.
Intermittency is a very good indicator for detecting event scale threshold
behavior in hydrological systems. Predictability of intermittent
processes/system responses is inherently low for combinations of systems states and/or
boundary conditions that push the system close to a threshold. Post hoc
identification of "cause-effect relations" to explain when the system
became critical is inherently difficult because of our limited ability to
perform observations under controlled identical experimental conditions. In
this review, we distinguish three forms of threshold behavior. The first one
is threshold behavior at the process level that is controlled by the
interplay of local soil characteristics and states, vegetation and the
rainfall forcing. Overland flow formation, particle detachment and
preferential flow are examples of this. The second form of threshold
behaviour is the response of systems of intermediate complexity – e.g.,
catchment runoff response and sediment yield – governed by the
redistribution of water and sediments in space and time. These are
controlled by the topological architecture of the catchments that interacts
with system states and the boundary conditions. Crossing the response
thresholds means to establish connectedness of surface or subsurface flow
paths to the catchment outlet. Subsurface stormflow in humid areas, overland
flow and erosion in semi-arid and arid areas are examples, and explain that
crossing local process thresholds is necessary but not sufficient to trigger
a system response threshold. The third form of threshold behaviour involves
changes in the "architecture" of human geo-ecosystems, which experience
various disturbances. As a result substantial change in hydrological
functioning of a system is induced, when the disturbances exceed the
resilience of the geo-ecosystem. We present examples from savannah
ecosystems, humid agricultural systems, mining activities affecting rainfall
runoff in forested areas, badlands formation in Spain, and the restoration
of the Upper Rhine river basin as examples of this phenomenon. This
functional threshold behaviour is most difficult to predict, since it
requires extrapolations far away from our usual experience and the
accounting of bidirectional feedbacks. However, it does not require the
development of more complicated model, but on the contrary, only models with
the right level of simplification, which we illustrate with an instructive
example. Following Prigogine, who studied structure formation in open
thermodynamic systems, we hypothesise that topological structures which
control response thresholds in the landscape might be seen as dissipative
structures, and the onset of threshold processes/response as a switch to
more efficient ways of depleting strong gradients that develop in the case
of extreme boundary conditions. |
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