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relationship between inputs and outputs on the one hand
and landforms on the other, one would describe the
situation as equilibrium. The Upper Wharfedale land-
scape, like most others, however, has both landscape
features which appear to be adjusted to modern inputs
and outputs, and also relict features which still show past
environmental conditions. Throughout the Quaternary
period changes in climate, sea level, tectonics and human
activities affect, and have affected, the processes going on
in the system. Note that these changes operate at time
scales ranging from minutes to millennia, as was discussed
earlier.
If the environment of a landform, a soil or a vegetation
community changes dramatically through a change in
climate, tectonics or human impact, for example, it is likely
that the landform, soil or vegetation community will alter
dramatically. In systems terminology, the inputs have
changed. The time required to return to equilibrium after
a change or perturbation is the total response time . This is
the sum of two other time periods, namely the relaxation
time (the time required to reach equilibrium after a
perturbation) and the reaction time (the time between
the perturbation and the response). Modern physical
geography recognizes that many different physical systems
can coexist and interact in the real world. The systems
involve different physical materials (minerals, gases,
liquids, organic material) operating over very different
time scales and over very different spatial scales. Some
systems are dominated by negative feedback and will
return to their original state after a minor perturbation.
This is steady state equilibrium in Chorley and Kennedy's
(1971) classification as shown i n Figure 1.8a . Where part
of the landscape is undergoing a gradual and progressive
change over the medium to long term yet preserves
equilibrium in the short term, the situation is described
as dynamic equilibrium , as shown in Figure 1.8b . An
example would be a river which attempts to maintain the
relationship between channel geometry and discharge
whilst at the same time lowering its longitudinal profile.
The study of parts of the physical landscape in a
systems framework has inevitably brought forward many
examples where there appears not to be an equilibrium
situation, neither steady state nor dynamic. Renwick
(1992) classifies these situations into two types, disequilib-
rium and non-equilibrium . Disequilibrium features are
those that tend towards equilibrium but have not had
enough time to reach it. Either the perturbation has been
quite recent or the processes operate at a low intensity.
Non-equilibrium features do not appear to move
towards any equilibrium even with long periods of
stability in the environment. These features change so
rapidly and so dramatically that it is difficult to identify
an average or equilibrium condition. Non-equilibrium
features are inherently unstable. Three causes of their
instability are identified (Renwick 1992) and are illus-
trated in Figure 1.10 . First, there is the situation where a
landscape or part of it is affected by thresholds or sudden
changes in the magnitude of the rates of processes, so
that the processes change quickly over several orders of
magnitude. In other words, the threshold is a major
discontinuity caused by high-magnitude, low-frequency
events. Infrequent and atypical weather events, high-
magnitude floods, large mass movements and tectonic
events all cause a system to become unstable and to shift
across a critical threshold. The movement across the
threshold is irreversible, and negative feedback is no
longer able to restore the system to its original form. This
condition has also been called dynamic metastable
equilibrium (Chorley et al. 1984).
(a) Threshold dominated nonequilibrium
Time
(b) Positive feedback nonequilibium
Time
(c) Chaotic nonequilibrium
Time
Figure 1.10 Non-equilibrium conditions in physical geography.
Source: After Renwick (1992)
 
 
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