Numerical groundwater flow models have become an essential tool in the solution of\ud hydrogeological problems. Their importance arises because they are the only real means by\ud which testing of hypotheses can be conducted. It is common to produce what is referred to as\ud a conceptual model, a description of the processes believed to be operating in the groundwater\ud system. However, to assess the validity of the concepts it is essential to produce results from\ud a more physically based model which can be compared with field information. If there is a\ud high degree of similarity between physical model and reality, then the conceptual model can\ud be treated with some confidence. Numerical models provide information based on the\ud physics of the supposed processes and are therefore the best approximation to a physical\ud model.\ud Fundamental difficulties arise in constructing a numerical model which is going to be able to\ud be close to the real aquifer. Amongst them are recognising the true mechanisms and finding\ud an acceptable numerical implementation. The ability to look at behaviour on different scales\ud is also important. While a relatively coarse approximation, based on a grid spacing of\ud hundreds of metres, may be acceptable for regional flow through a large expanse of relatively\ud homogeneous aquifer, this will not be adequate for small-scale local behaviour. Examples\ud such as the response of an abstraction well or the representation of a small stream will not be\ud dealt with accurately on a coarse mesh.\ud The process of model development and construction is iterative. Components of the\ud conceptual model are frequently found to be inadequate and require changes. The inability to\ud reproduce features from the real aquifer brings these inadequacies to light. Change and the\ud introduction of new processes are fundamental to the development process.\ud ZOOMQ3D is a numerical model which advances the art of model development on two\ud vitally important fronts. It incorporates a mesh refinement procedure which aids the solution\ud of problems related to scale. This is the first of its contributions. The second is that it uses\ud object-oriented techniques as the basis for the program. Whilst this is well-established in the\ud development of general commercial software, it represents a novel approach to groundwater\ud model structure. It is of considerable value in maintaining the code but it is in changing\ud model behaviour that it holds most promise for modellers. Further, the direct correspondence\ud between computer-based objects and real-world features makes the link between numerical\ud and conceptual models very easy to see, even to those with no programming expertise
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