Surface water – groundwater interactions

The MODFLOW River package is used to represent surface water – groundwater interactions in the major alluvial systems and watercourses (Figure 7). In alluvial areas outside the Condamine Alluvium, river stages are set at modelled ground level, which ‘effectively assumes that all surface watercourses act as discharge boundaries and hence cannot leak is considered to be a conservative assumption from an impact point of view. This assumption is consistent with work undertaken by Hillier (2010) which suggested that alluvial strata within the GAB typically act as a drain for the underlying sediments’ (GHD, 2012, p. 57). The MODFLOW River package reports baseflow (discharge to stream) water balances for each River cell. In this way, the OGIA model water balance can be used to make conservative estimates of changes to baseflow volumes for major alluvial systems and watercourses represented in the model.

The Condamine Alluvium is modelled using an integrated approach, where calibrated values of aquifer thickness and hydraulic parameters, and time-variant water level conditions from the detailed transient Condamine Model (KCB, 2011) are imported into the regional model (QWC, 2012). Groundwater levels calibrated to historical water level conditions in the detailed Condamine Alluvium model are used to define MODFLOW River cell elevations in the regional model. The Condamine Model is used to estimate impacts on groundwater levels in the Condamine Alluvium that result from the change in flow from the Condamine Alluvium to the Walloon Coal Measures predicted by the regional model (QWC, 2012).

Temporary watercourses that are losing or losing – disconnected with respect to the underlying aquifer are represented in the model by Drain cells that cover the rest of the active model domain. Drain cell elevations are set at modelled ground level, which assumes that all temporary watercourses represented by Drain cells act as groundwater discharge boundaries.

Last updated:
17 October 2018
Thumbnail of the Maranoa-Baloone-Condamine subregion

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