With these general points in mind, we can now consider one of the more significant recent developments in physical geography, namely the widespread adoption of models in which the real world is viewed as a vast system or set of interlocking systems. A system can be defined as a set of objects or attributes (that is, characteristics of an object, such as size or shape) linked in some relationship. We have already stressed that the natural environment appears to operate as an entity, in which each component has connections with all the other components. It is impossible to build a model, which takes in the whole world or even a substantial part of it, so we identify various environmental sub-systems within which the connections are fairly strong. Thus weather systems, drainage systems, ecosystems and many others can be described. In analyzing these, the systems approach focuses attention on the whole system and the interrelationships within it, rather than on the individual parts.
Examples of systems familiar to us in everyday life include transport networks, the electricity grid system, or the domestic hot-water system of a house. These systems can be modelled symbolically by means of flow diagrams, consisting of the objects in the system conventionally represented by symbols, usually box-shaped, and the mass or energy flows in the system represented by lines.
Systems are normally regarded as being of two types: closed, I which no energy or matter crosses the external boundaries of systems and open, in which external factors can affect the variables within the systems. Apart from the universe, no natural system is truly closed unless we artificially make it so for the purposes of study. However, the degree of 'openness' of systems varies considerably. The earth and its atmosphere represent a partially open system, exchanging energy with outer space, but to all intent closed to material exchange. Other open systems exchange both mass and energy. For instance, a drainage basin receives energy and mass from precipitation, sunlight and the elevation of the land. These inputs pass through the system, doing work on the way to emerge as outputs of heat, water and sediment in the sea and atmosphere. A drainage system is typical of many other open systems in physical geography, which are called process-response systems, because the flow of mass or energy (the process) causes changes of responses in form (shape or arrangement) in the system. However, where plants or animals are involved, as in a forest or a pond it is called an ecosystem.
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