Central Lockyer g/w model
Background and Purpose
This numerical groundwater model was developed for the alluvial aquifer under the Central Zone (Clarendon Sub-artesian Area). This zone incorporates the areas that are benefited from surface storages. Such a numerical model can be used to simulate aquifer responses to recharge and groundwater extraction. These computer models can be then used as a tool for water resource management.
The model was produced by Department of Natural Resources & Mines, and completed in June 2000.
Main Components of Model
The model covers the Central Zone of the main Lockyer Creek alluvium, and is based on a mesh with a series of cells. The model was developed in three stages,
- calibration of aquifer parameters so the model can replicate historical water level responses.
- estimation and calibration of recharge locations along the creeks and major drainage lines (this is determined by using constant aquifer properties).
- recharge locations determined in (2) were used to calibrate recharge over a long period.
The model needs to be based on real data. The period 1987-1997 was selected as continuous data were available, and the average rainfall in the period was similar to the long term average. The model was then calibrated against these conditions.
The software used is the MODFLOW program develop by the US Geological Survey, plus additional support and testing software. MODFLOW is widely used internationally by both government and private sectors.
Requirements of the Model
Following good standard practice, a conceptual groundwater model was first developed to understand the system. Many cross sections were produced based on drillhole geological logs. Four example cross-sections (location map) give an idea of the structure of the aquifer within the alluvium.
Note in the cross-sections:
- they are all at the same scale
- a vertical exaggeration of 50 x
- the yellow-coloured zone represents the "aquifer" material (sands and gravels)
- the overlying grey-coloured zone represents the non-aquifer material (silts and clays)
It is obvious there are many variations in the shape of the aquifer material, and of the surface of the sandstone.
For the model design, a one layer model was chosen (i.e. the yellow zone) as geological logs do not indicate the common existence of two separate water-bearing layers. There are, however, known to be variations in permeability within both the aquifer layer, and the overlying (grey) zone.
Outcomes
This model enables simulations of the groundwater resource under different conditions of recharge and water loss. It therefore becomes an essential component of management for an intensively utilised groundwater system. However, to use the model effectively, water use data must be obtained and entered.
One of the major findings from this modelling is that it is not possible to assign a blanket value of extraction across the whole Central Zone. This is due to the variable conditions of the aquifer, such as depth, thickness of water-bearing gravels, and variation in permeability. When recharge occurs it is also quite variable spatially. As a consequence, different optimum extraction rates are recommended for different zones.
It is most likely that such variabilty in optimum pumping rates will be the case for the whole valley.
With such groundwater models, they continue to be improved and developed as more data and understanding of the system become available.
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Further information
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A1: Satellite image showing alluvial boundary (NR&M) [732.96 kB] |
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A2: Hydrology and water supply system (NR&M) [485.01 kB] |
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B1: Model grid and boundary conditions (NR&M) [550.47 kB] |
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B2: Observation bores (NR&M) [483.84 kB] |
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B3: Stage 1 parameter zonation (numbers are different groundwater zones) (NR&M) [544.27 kB] |
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B4: Model cells with a modelling run showing extraction from selected bores (NR&M) [553.1 kB] |
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Locations of cross-sections (NR&M) [548.63 kB] |
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Cross-section A-B [428.26 kB] |
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Cross-section C-D [432.53 kB] |
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Cross-section E-F [432.23 kB] |
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Cross-section G-H [431.56 kB] |
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