In the spatial domain we are to make similar decisions about representation of space in the model. Is there much spatial heterogeneity in the system to justify a spatially explicit description? Or can the system be considered spatially homogeneous? Or are there large segments in which the system parts are uniform and the whole system can be described as a collection of these spatially homogeneous compartments? How big are these compartments? Can we describe the system spatially with a map of a 1:1 000 000 scale or should it be 1:100?
In all these decisions diagrams can be helpful to understand the system and to communicate our understanding to others. Figure 2 describes how a lake ecosystem is to be modeled in space. It shows that there are large parts of the system that may be considered homogeneous and presented by some average values. The geometry of the lake is described; the central deeper part is considered as a separate compartment and is subdivided in depth. The shallow littoral segments are assumed to be entirely mixed to the bottom and are therefore described by one layer.
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