In the field of geography, the systems theory propounded by Richard Chorley and Barbara Kennedy exemplifies a distinctive approach. They view geographical systems as open systems which exchange matter and energy with their surroundings (Chorley & Kennedy, 1971). Systems, as they define them, are structured sets of objects and/or attributes, made of components that exhibit discernable relationships, operating together as a complex whole according to some observed pattern.
Energy throughput in systems tends to produce and maintain discernable patterns of organisation, characterized by hierarchical (local-global, part-whole etc) differentiation. Open systems tend to adjust themselves to matter and energy flow by modifying the interrelationships between different elements of this system, so that input and output flows balance each other out, resulting in a steady state for the system. This kind of adjustment is called self-regulation, and Chorely and Kennedy’s project can be understood in part as the study of self-regulating geographical systems.
While they enumerate a larger class of systems, Chorely and Kennedy assert that physical geography is concerned primarily with four kinds of systems. These four systems exhibit distinct but interlocking properties, and form a progression from lower to higher levels of integration and sophistication. The systems are listed below, in the progressive order Chorley and Kennedy define for them.
A - Morphological systems: The network of structural relationships or cross-correlations between the constituent parts of systems.
P - Cascading systems: The path followed by throughputs of energy or mass.
I - Process-Response systems: The linkage of at least one morphological and one cascading system, so that the morphological form of a structure is related to the process that is energized by the cascade.
E - Control systems: These are process-response systems in which the key components are controlled by some intelligence – often a social body, where geographical units are concerned. Control causes the system to operate in some manner determined by the intelligence.
These four systems are described in more detail below.
A - Morphological Systems
These are the geographical formations we most often see. They are formations defined by physical properties, integrated at some point in time to produce a structure. The strength and direction of the connections between different parts of the system are commonly revealed by correlation analysis. Chorely and Kennedy give the example of a “beach system”, defined by such parameters as beach slope, mean grain size, range of grain sizes and beach firmness. The relationships between these parameters can be expressed by a web of correlations. The operational efficiency of the overall system depends on the degree to which the dynamic properties of these parameters are related. An analogy can be drawn between morphological systems and mechanical structures, where an externally applied stress is released by a strain, which involves a readjustment of variables to produce a new equilibrium.
P - Cascading Systems
These can be thought of as a series subsystems daisy-chained together. Like a number of forest pools linked by a long, tumbling stream, the subsystems are often characterized by thresholds, and are dynamically linked by a cascade of mass or energy, such that the mass or energy output from one subsystem becomes the input for the adjacent subsystem. The canonical structure of a cascade subsystem includes input, which is acted upon by a decision regulator such that some matter/energy may be diverted into a store and the remainder throughput to become subsystem output, serving as input into the adjacent subsystems.
I - Process-Response Systems
Morphological and cascading systems interact with each other in an integrated fashion. That is because it is morphological states (or correlates of those states) which act as the storages and regulators embedded in the cascading systems. The morphology of the forest pool determines its capacity as a storage element, and its threshold is a decision regulator. It is thus possible to view process-response systems as chains of intersecting cascading and morphological components which mutually adjust themselves to changing input-output relationships. The time required for this adjustment to a new equilibrium (i.e. the relaxation time) depends upon things like the amount and direction of change in the energy cascade and the number of links between the morphological variables.
E - Control Systems
There are key points in process-response systems, where cascading elements are close to thresholds and the morphological states that set those thresholds are underdetermined by links, leaving them some degrees of freedom. These key variables or valves (commonly involving the decision regulators) are strategic points where control systems can conveniently intervene to produce operational changes in the distribution of matter and energy within cascading systems, and consequently change the equilibrium relationships involving the morphological variables linked with them in the process-response systems. A coordinated decision-making system, such as a social agency, interacting with a physical process-response system produces a geographical control system. These systems involve flows of information and control signals as well as the basic geomorphological cascades. The decision-making layer of a control system must run a model of the controlled system to predict the outcome of potential interventions. The model will typically represent fewer linkages than the modelled system actually has. (Rosen, 1985)
More about Chorely and Kennedy's model can be found in a paper I wrote about entropy, uploaded to scribd.com, in an effort to disabuse myself of the typical misunderstandings and distortions that cloud laypersons' use of the entropy concept.