Groundwater modelling
A. Basic Factors
There are a number of ways that groundwater levels and flow can be represented by diagrams, as well as calculated mathematically. However, although models are used to simulate flow, they need to be based on real measurements made in water bores, plus the elevation of the bores must be known.
Important in all these methods is to determine the direction of the groundwater flow and its gradient; gradient is the change in water level over distance. This is also called hydraulic head.
Obviously, the more bores used, the greater amount of data, and the better the calculated results.
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B. Conceptual Hydrogeological Model
To develop the numerical model a knowledge of the framework must first be established, and a conceptual model constructed. This is really a 3-D picture of what is there and how it operates.
The conceptual model incorporates geological data (drillhole logs), geophysics (e.g. resistivity surveys), water geochemistry (major ions, mixing, water types, processes), and various hydraulic measurements (water levels over time, pump tests, rainfall data).
The conceptual model is then used to develop the layers and parameters of the computer flow model.
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C. Groundwater Flow Nets
These are a plot of (a) contour lines groundwater levels (which show gradient), and (b) groundwater flow lines (which are at 90o to the contour lines). Flow nets can be as a horizontal map, or can be a vertical cross-section, such as through an earth dam, or under a concrete weir.
Flow nets can be simple plots, or developed with computer programs. Using programs, groundwater bores can also be incorporated, and extraction from the bore simulated; this will produced a change in the pattern of the flow net.
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D. Groundwater Flow Models
There are many types of these now, but the basic models use some form of grid network which contains a number of cells. For each cell aquifer values (e.g. K, hydraulic conductivity) are added, plus flow parameters (such as hydraulic gradient) in three directions x,y,z. Other estimated or calculated values about aquifer properties are also needed, such as transmissivity and storativity.
Widely used numerical flow models used are based on the MODFLOW program developed by the US Geological Survey. This is the model system used in the central Lockyer declared zone. We will be developing a valley-wide model over the next three years.
The MODFLOW groundwater model developed by NR&M for the central zone (Durick and Bleakley, 2000) is a very good example of a practical model developed in a hydrologically complex area of intensive groundwater use.
Shown here are some of the components developed in the groundwater model of the Tenthill-Ma Ma plain by Andrew Wilson in his MAppSc (see below D6-D9). This will also be incorporated in the valley-wide model.
These models, however, must be based on real data, the more the better. The aim of the model to simulate as close as possible a real system, and to establish how it operates in space and time. Models therefore take some time to calibrate and verify, to confirm that they are realistic and represent the system. See below D1-D5.
To develop a good groundwater flow model requires two fundamentals,
- a good conceptual hydrogeological model, and
- good data.
A useful warning about these types of models is "garbage in = garbage out".
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E. Groundwater and Aquifer Visualisation
A further step in modelling is to use maps and other diagramatic data to visual the groundwater aquifers. Such visualisation can use a 3-dimensional approach and incorporate features such a aquifer boundaries, drillholes, water levels, and salinity. In addition, animation can be utilised to demonstrate temporal changes, such as water levels for different years.
An approach using visualisation of Lockyer groundwater data has begun. This will be in conjunction with numerical modelling, and we see this as a effective means of displaying hydrological processes to water users and the wider community.
Some preliminary results of visualisation follow:
Figure 1. Topographic map using visualisation of elevation with 20 m contours, (a) wireframe of decimated triangulation, (b)surface using shading for different elevations. This shows the whole Lockyer Valley looking southward. Toowoomba is the flat area on the right.
Figure 2. Groundwater levels for a block of the alluvial aquifer on the floodplain of Tenthill and Ma Ma Creeks. The years shown are December of, 1994, 1996, 1999 and 2001. These are from an animation of this area for the period 1960 to 2001.
Figure 3. Groundwater bore logs. These are preliminary data from drillers logs recorded for alluvial groundwater bores along Lockyer Creek. Different colours show different geological materials.
Figure 4. Geology. Geology map "spread over" 3-D topography, (a) surface strata, (b) coloured surface strata, (c) coloured image to contour map, (d) bore logs inserted into a 40% transparency image.
[Groundwater data are from NR&M database; digital geology map is from GSQ-NR&M]
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Further information
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