Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

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Husam Baalousha
scheme. Clearly, it is a simple and cheap solution compared to project establishment in
reality.
By definition, models simplify reality, and are, therefore, imperfect. The famous
statistician George Box insisted, “ all models are wrong, but some are useful” (Box and
Draper 1987). Applicability of any model and its usage depends on the objectives of that
model. Though they are imperfect, models are very useful in hydrogeology. It is a challenge
to the modeller to represent the real word problem in a simplified form without compromising
the accuracy or making invalid assumptions. Modellers try to get the best representation of
reality by collecting as much data as possible and feeding the models with new data.
Groundwater models can be classified into three categories: physical, analogue or
mathematical. Solution of mathematical models can be either analytical or numerical.
Analytical methods do not require much data, but their application is limited to simple
problems. Numerical solutions can handle more complicated problems than analytical
solutions. With the rapid development of computer processors and increasing speed,
numerical modelling has become more effective and easy to use.
The most commonly used numerical modelling approaches are the “finite difference”
method and the “finite element” method. Each method has its advantages and limitations.
Depending on the problem of concern and the objectives of modelling, the appropriate
modelling approach can be selected. Finite difference method can produce different results to
finite element method if the problem of concern is complicated. Modelling approach is not
the only factor that affects the model’s results. Other factors like boundary conditions, initial
condition, time and space discritisation, and quality of data influence the results.
This chapter outlines the stepwise methodology of groundwater modelling, differences
between modelling approaches and difficulties accompany groundwater modelling. Common
mistakes in groundwater modelling are also discussed.
2.0. Modelling Approach
Groundwater Models can be simple, like one-dimensional analytical solutions or spreadsheet
models (Olsthoorn, 1985), or very sophisticated three-dimensional models. It is always
recommended to start with a simple model, as long as the model concept satisfies modelling
objectives, and then the model complexity can be increased (Hill 2006). Regardless of the
complexity of the model being used, the model development is the same.
The stepwise methodology of groundwater modelling is shown in Figure 1. The first step
in modelling is identification of model objectives. Data collection and processing is a key
issue in the modelling process. The most essential and fundamental step in modelling,
however, is model conceptualization. Calibration, verification and sensitivity analysis can be
conducted after model completion and the first run. The following sections explain in detail
each step in groundwater modelling.