Published online by Cambridge University Press: 06 December 2010
INTRODUCTION
A model is an entity built to reproduce some aspect of the behaviour of a natural system. In the context of groundwater, aspects to be reproduced may include: groundwater flow (heads, water velocities, etc.); solute transport (concentrations, solute fluxes, etc.); reactive transport (concentrations of chemical species reacting among themselves and with the solid matrix, minerals dissolving or precipitating, etc.); multiphase flow (fractions of water, air, non-aqueous phase liquids, etc.); energy (soil temperature, surface radiation, etc.); and so forth.
Depending on the type of description of reality that one is seeking (qualitative or quantitative), models can be classified as conceptual or mathematical. A conceptual model is a qualitative description of ‘some aspect of the behaviour of a natural system’. This description is usually verbal, but may also be accompanied by figures and graphs. In the groundwater flow context, a conceptual model involves defining the origin of water (areas and processes of recharge) and the way it flows through and exits the aquifer. In contrast, a mathematical model is an abstract description (abstract in the sense that it is based on variables, equations and the like) of ‘some aspect of the behaviour of a natural system’. However, the motivation of mathematical models is not abstract, but to aid quantification. For example, a mathematical model of groundwater flow should yield the time evolution of heads and fluxes (water movements) at every point in the aquifer.
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