Alstonville plateau groundwater model

A two-layered MODFLOW model (Harbaugh et al., 2000) for the Alstonville plateau was developed by the NSW Department of Sustainable Natural Resources (Bilge, 2003). Bilge (2003) stated that the model was based on very limited data and hence was only suited for educational purposes. The model covers an area of 445 km2 that lies within the Alstonville Groundwater Management Area and only contains the basalt aquifers that were represented by a shallow unconfined upper aquifer (layer 1) and an unconfined/confined lower aquifer (layer 2). A grid spacing of 300 m was used to discretise the model, whose extents are shown in Figure 7.

A monthly stress period was implemented during the simulation period. The initial heads in the upper layer were assumed at a depth 10 m below the ground level due to a lack of historical data and a very irregular topography. The initial water level of the lower layer was interpolated using the first measured value on or after 1 July 1987. A constant-head boundary was used along the western side of the plateau. All other boundaries were assumed to be impermeable. Three rainfall zones were defined depending on rainfall patterns and magnitudes. Across all three zones, it was assumed that 8% of rainfall infiltrates into the aquifers as recharge, although a large part of it would discharge to the local surface water features.

The model was calibrated to data derived from two bores in the lower aquifer with observations from 1 July 1987 to 31 June 2001. The calibration was conducted via a trial-and-error manual approach. Parameters that were adjusted during the calibration include groundwater usage, hydraulic conductivity, specific storage for the lower aquifer and the constant heads along the western boundary. With the calibrated parameter set, the model was able to match the major trends observed in the hydrographs derived from the two calibration bores.

Based on the calibrated model, an annual mean storage loss of 459 ML/year throughout the simulation period was observed with a long-term rising trend from 1987 to 2001. The lower aquifer receives 1637 ML/year from the upper aquifer, while 306 ML/year flows back into the upper aquifer. Additionally, the modelling report suggests that data gaps need to be filled by installing new monitoring bores, running pumping tests and metering groundwater usage.

Last updated:
23 October 2018
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