Geoscience Reference
In-Depth Information
Figure 1.2
A schematic outline of the steps in the modelling process.
how the catchment responds to rainfall under different conditions or, rather,
your
perceptions of that
response. A perceptual model is necessarily personal. It will depend on the training that a hydrologist has
had, the topics and articles he or she has read, the data sets that he or she has analysed and, particularly,
the field sites and environments of which he or she has had experience. Thus, it is to be expected that
one hydrologist's perceptual model will differ from that of another (for a typical personal example, see
Section 1.4).
An appreciation of the perceptual model for a particular catchment is important. It must be remembered
that all the mathematical descriptions used for making predictions will inevitably be simplifications of
the perceptual model, in some cases gross simplifications, but perhaps still sufficient to provide adequate
predictions. The perceptual model is not constrained by mathematical theory. It exists primarily in the
head of each hydrologist and need not even be written down. We can
perceive
complexities of the flow
processes in a purely qualititative way (see, for example, the experiments of Flury
et al.
(1994) and
Figure 1.1) that may be very difficult indeed to describe in the language of mathematics. However a
mathematical description is, traditionally, the first stage in the formulation of a model that will make
quantitative
predictions.
This mathematical description, we call here the
conceptual model
of the process or processes being
considered. At this point, the hypotheses and assumptions being made to simplify the description of
the processes need to be made explicit. For example, many models have been based on a description of
flow through the soil using Darcy's law, which states that flow is proportional to a gradient of hydraulic
potential (see Box 5.1). Measurements show that gradients of hydraulic potential in structured soils can
vary significantly over small distances so that if Darcy's law is applied at the scale of a soil profile or
