Basics of GroundWater Modelling Part 1

Groundwater modelling is a way to represent a system in another form to investigate the response of the system under certain conditions, or to predict the behaviour of the system in the future. Groundwater modelling is a powerful tool for water resources management, groundwater protection and remediation. Decision makers use models to predict the behaviour of a groundwater system prior to implementation of a project or to implement a remediation scheme. Clearly, it is a simple and cheap solution compared to project establishment in reality.

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.

Objectives of Modelling

Groundwater models are normally used to support a management decision regarding

groundwater quantity or quality. Depending on the objectives of modelling, the model extent, approach and model type may vary.

Groundwater models can be interpretive, predictive or generic. Interpretive models are used to study a certain case and to analyse groundwater flow or contaminant transport.

Predictive models are used to see the change in groundwater head or solute concentration in the future. Generic models are used to analyse different scenarios of water resource management or remediation schemes.

Objectives of groundwater modelling can be listed as:

• Prediction of groundwater flow and groundwater head temporally and spatially.

• Investigating the effect of groundwater abstraction at a well on the flow regime and

predicting the resulting drawdown.

• Investigating the effect of human activities (e.g. wastewater discharge, agricultural

activities, landfills) on groundwater quality.

• Analysis of different management scenarios on groundwater systems, quantitatively

and qualitatively.

Depending on the objectives of study and the intended outcome, selection of model

approach and data requirements can be made to suit the area of study and the objectives. For example, if the objective is a regional groundwater flow assessment, then a coarse model may satisfy this objective, but if the area of study is small then a fine-grid model with high datadensity should be used.

Conceptual Model

A conceptual model is a descriptive representation of a groundwater system that incorporates an interpretation of the geological and hydrological conditions. Information about water balance is also included in the conceptual model. It is the most important part of groundwater modelling and it is the next step in modelling after identification of objectives. Building a conceptual model requires good information on geology, hydrology, boundary conditions, and hydraulic parameters. A good conceptual model should describe reality in a simple way that satisfies modelling objectives and management requirements (Bear and Verruijt 1987). It should summarise our understanding of water flow or contaminant transport in the case of groundwater quality modelling. The key issues that the conceptual model should include are:

• Aquifer geometry and model domain

• Boundary conditions

• Aquifer parameters like hydraulic conductivity, porosity, storativity, etc

• Groundwater recharge

• Sources and sinks identification

• Water balance

Once the conceptual model is built, the mathematical model can be set-up. The

mathematical model represents the conceptual model and the assumptions made in the form of mathematical equations that can be solved either analytically or numerically.

Boundary Conditions

Identification of boundary conditions is the first step in model conceptualisation. Solving of groundwater flow equations (partial differential equations) requires identification of boundary conditions to provide a unique solution. Improper identification of boundary conditions affects the solution and may result in a completely incorrect output. Boundary conditions can be classified into three main types:

• Specified head (also called Dirichlet or type I boundary). It can be expressed in a

mathematical form as: h (x,y,z,t)=constant

• Specified flow (also called a Neumann or type II boundary). In a mathematical form

it is: Ñh (x,y,z,t)=constant

Head-dependent flow (also called a Cauchy or type III boundary). Its mathematical

form is: Ñh (x,y,z,t)+a*h=constant (where “a” is a constant).

In addition to the above-mentioned types there are other sub-types of boundaries. These will be explained later.

In groundwater flow problems, boundary conditions are not only mathematical constraint, they also represent the sources and sinks within the system (Reilly and Harbaugh 2004).

Selection of boundary conditions is critical to the development of an accurate model (Franke et. al. 1987).

It is preferable to use physical boundaries when possible (e.g., impermeable boundaries, lakes, rivers) as the model boundaries because they can be readily identified and

conceptualised. Care should be taken when identifying natural boundaries. For example groundwater divides are hydraulic boundaries and can shift position as conditions change in the field. If water table contours are used to set boundary conditions in a transient model, in general it is better to specify flux rather than head. In transient simulation, if transient effects (e.g. pumping) extend to the boundaries, a specified head acts as an infinite source of water while a specified flux limits the amount of water available. If the groundwater system is heavily stressed, boundary conditions may change over time. For this reason, boundary conditions should be continuously checked during simulation.

End of part 1