Environmental Engineering Reference
In-Depth Information
Figure 1.8
The alluvial channel system.
Source: After Richards (1982).
whilst others regard systems thinking as a useful framework which should not preclude
the appreciation of an historical element in the study of landscapes (e.g. Richards 1982).
Systems thinking emphasizes the spatial interactions which take place between parts of
the landscape. For example, in Upper Wharfedale processes of weathering, mass
movement and erosion on hill slopes interact with the ability of the river Wharfe and its
tributaries to move sediment down the river channel. In other words, hill-slope processes
interact with fluvial processes. However, these interactions take time before there will be
a perfect balance between the processes and the landforms. If there was a consistent
relationship between inputs and outputs on the one hand and landforms on the other, one
would describe the situation as equilibrium (see page 11). The Upper Wharfedale
landscape, 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).
