Geoscience Reference
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of the landscape into these preconceived time-dependent
conceptual models.
Although in the late nineteenth century G. K. Gilbert
had proposed that a delicate balance exists between parts
of the landscape and present processes operating on them,
it was not until the middle of the twentieth century that
these ideas were formalized in the concept of
dynamic
equilibrium
; this states that parts of the landscape rapidly
adjust to the processes acting on them. Once equilibrium
is reached, the form or shape of the landform, vegetation
community or soil is constant as long as the basic controls
remain constant. If inputs and outputs remain constant
the system will reach a
steady state
.
Equilibrium
is often regarded as an ideal theoretical
state, rarely found in the real world, as the controlling
variables are in a constant state of flux. Climate will
fluctuate on an annual basis, as well as over several years,
and also in trends over decades or longer-term periods.
Human activity brings about land use changes, and, again,
this can be through annual changes or longer-term trends.
This dynamism does not make the equilibrium concept
wrong, but it means that equilibrium is critically depen-
dent upon the time interval being considered.
It is convenient to envisage three different types of time
interval, namely
cyclic time, graded time
and
steady time
.
Cyclic time
changes over long time periods, possibly
millions of years. Changes are progressive in the average
state of the system to give a condition of
dynamic equilib-
rium
. Changes in landforms over possibly 100 to 1,000
years occur over
graded time
. Because of negative feed-
back, the system is maintained in a constant condition. In
the short
steady-time
intervals of days and months, most
elements in the landscape are unchanging. Steady-state
equilibrium exists over steady time, whilst dynamic
equilibrium occurs over cyclic time (
Figure 1.8
).
Environmental systems are complex systems in which
the relations between variables are characterized by
multiple feedback. Some elements of the system respond
rapidly to changes in external controls, whilst other
elements will change only slowly. For example, in the
alluvial channel system of the river Wharfe the channel
shape, channel bed forms and flow velocity adjust quickly
to hydrological changes in the catchment, whilst other
characteristics of the catchment, for example valley
slopes and channel pattern, take much longer to adjust.
Hydrological changes are often the result of changing
land use in the catchment.
The value of 'time' in studying the environmental
system lies also in the causative factors which are sought
to explain the landscape. Factors which govern the
landscape have a different relative importance depending
Kenneth Hare and (d) Sir Ghillean Prance.
Photos: Rosemary Chorley, John Houghton, Hazel and Robin Hare and
Ghillean Prance
figure's subjectivity!) The contribution of many of these
scholars will become clear in the pages in this topic.
Environmental systems are constantly changing, as energy
flows through them, and material within them is in a
mobile state. The systems are continually adapting to
the changing controls of climate, hydrology, ecological
processes and geochemical reactions. Change in land-
scape systems has been studied in the past by means of
'ideal cycles' such as the
cycle of erosion
proposed by W.
M. Davis or the
succession-to-climax
vegetation model of
F. Clements. The prevailing ideas on change in the first half
of the twentieth century were that elements in the
landscape continuously and slowly evolve through a series
of stages, each with distinct characteristics. Each cycle
had a predictable end point, whether a low peneplain in
geomorphology, a climax vegetation in biogeography or
a mature soil in pedology. Much of the study of the
landscape focused on the relevant
evolutionary
model, and
effort was directed at attempting to 'pigeonhole' elements
