Barnett, B., Townley, L.R., Post, V., Evans, R.E., Hunt, R.J., Peeters, L., Richardson, S., Weatherill, D., Werner, A.D., Knapton, A., and Boronkay, A. (2012), Australian Groundwater Modelling Guidelines, Waterlines Report Series No.82, National Water Commission, Canberra, June, 191 pp.

The objective of the Australian groundwater modelling guidelines is to promote a consistent and sound approach to the development of groundwater flow and solute transport models in Australia. It builds on existing guidelines (Murray–Darling Basin Commission 2001) that have been adopted throughout Australia in recent years. While it is acknowledged that the term groundwater modelling refers to a variety of methods, the guidelines focus on computer-based numerical simulation models. The guidelines should be seen as a point of reference and not as a rigid standard. They seek to provide direction on the scope and approaches common to modelling projects. The continual evolution of modelling techniques through adaptation and innovation is not only acknowledged, but encouraged. It is recognised there are other approaches to modelling not covered in these guidelines and that such approaches may well be appropriate and justified in certain circumstances.

The guidelines promote an approach to model development that is underpinned by a progression through a series of interdependent stages with frequent feedback loops to earlier stages. Figure ES-1 illustrates the process.

In the planning stage the modellers and key stakeholders should agree on various aspects of the model and the process leading to its development. The process should document the agreed modelling objectives and the model‘s intended use in contributing to or providing certain outcomes required by the larger project. The model confidence-level classification should be addressed at this stage. The classification is a benchmark that illustrates the level of confidence in the model predictions and generally reflects the level of data available to support model development, the calibration process and the manner in which the predictions are formulated.

Conceptualisation involves identifying and describing the processes that control or influence the movement and storage of groundwater and solutes in the hydrogeological system. The conceptualisation should consider the physical processes and resulting heads and flows of groundwater. In this regard it provides information on how the project is expected to impact on the groundwater and the surface water bodies that depend on groundwater. The conceptual model must explain (qualitatively and quantitatively) all observed groundwater behaviour in the region. The guidelines encourage regular reassessment of the conceptual model at all stages of the project, with refinements made as other stages of the process suggest that these may be appropriate or necessary. In many cases the conceptual model may not be unique (i.e. different conceptual models can explain all observations) and it is encouraged to propose and maintain alternative conceptualisations for as long as possible through the modelling project. In some cases this may lead to the development and use of alternative numerical models.

The design and construction stage involves a series of decisions on how to best implement the conceptualisation in a mathematical and numerical modelling environment. The decisions required at this stage include selection of a numerical method and modelling software, selection of an appropriate model dimension, definition of a model domain and the spatial and temporal discretisations to be used in the model. The guidelines encourage modellers to take a pragmatic approach to these issues and to explore simple modelling options where these may be appropriate. For example, they encourage the consideration of two-dimensional (2D) rather than 3D models and consideration of steady state rather than transient models where these simpler approaches may be adequate to address the modelling objectives.

Model calibration involves an iterative process to estimate parameters describing hydrogeological properties and boundary conditions so that the model‘s results closely match historical observations. The guidelines encourage the use of as many different datasets as possible for calibration. Calibration can be achieved through a manual trial-and-error process or through an automated parameter-fitting procedure. The challenge is to find parameter values that allow a model to fit historical measurements, while preparing a model for use in predictions. A balance is needed between simplicity and complexity.

Predictive scenarios are designed to answer the questions posed in the modelling objectives. They are run with various levels of applied stresses that represent anticipated changes from the implementation of the project. The guidelines provide advice on how the climatic, pumping and drainage stresses might be implemented in the predictive scenarios. The guidelines encourage the acknowledgement of uncertainty and suggest methods to formulate predictions in which uncertainties are minimised.

Because models simplify reality, their outputs are uncertain. Model outputs presented to decision-makers should include estimates of the goodness or uncertainty of the results. Linear methods for calculating uncertainty are less computationally intensive than non-linear methods. For many decisions, linear methods are sufficient to convey expectations of uncertainty. Presentation of uncertainty results, regardless of the methods used, should include a visual depiction that the model prediction is more than a single result or set of results, and a presentation of uncertainty that most directly addresses the decision of interest.

Model reporting encompasses documentation and communication of different stages of the model through a written technical document. The report should describe the model, all data collected and information created through the modelling process. The report should be accompanied by an archive of all the model files and all supporting data so the results presented in the report can, if necessary, be reproduced and the model used in future studies.

The guidelines suggest that the model review process should be undertaken in a staged approach, with separate reviews taking place after each reporting milestone (i.e. after conceptualisation and design, after calibration and sensitivity, and at completion). Three levels of review are suggested: a model appraisal by a non-technical audience to evaluate model results; a peer review by experienced hydrogeologists and modellers for an in-depth review of the model and results; and a post-audit, a critical re-examination of the model when new data is available or the model objectives change. Examples of review checklists are provided for model appraisal and model review.

The guidelines include a detailed description of solute transport modelling where the solute of interest is non-reactive, and for problems relating only to groundwater flow and storage. These investigations involve additional difficulties and complexities and require special considerations. The guidelines promote a staged approach to model development with a step-wise increase of model complexity. They recommend the use of approximate calculations, analytical models and particle-tracking estimates before the development of a comprehensive numerical solute transport model.

Modelling of surface water–groundwater interaction requires knowledge of groundwater modelling, and an understanding of the exchange processes that occur between surface water and groundwater. These interactions can sometimes be adequately represented using boundary conditions in a groundwater-flow model while in others it is necessary to link or couple surface hydrological models with groundwater models, so that exchange of water and solutes can be computed between both models. In these type of mathematical representations, issues of scale, spatial and temporal discretisations, and head and flow variability are very important. The lag between groundwater abstraction and impacts on river baseflow can be tens of years, while event-based variations in surface water flows are of the order of minutes to weeks in duration.

A PDF version of the Guidelines is available online.