2.6.2.4.3 Discharge

Topographic control of groundwater levels is established using the MODFLOW Drain package (Figure 7). Drain cells are assigned at modelled ground level in the uppermost active layer in each model cell to remove recharge in excess of the modelled flux to the deeper confined system from the model. Drain cells are assigned a conductance value of 2500 m²/day, which is a relatively high value. ‘Given that all of the modelled drain and river cells have been parameterised as discharge boundaries and hence cannot leak then modelled impact predictions are considered unlikely to be sensitive to the conductance value assigned’ (GHD, 2012, p. 58).

Evapotranspiration is not modelled explicitly in the OGIA model. Instead, Drain cells remove excess recharge that is not transmitted to the deeper model layers in order to estimate net recharge during the steady-state model calibration. A consequence of this modelling approach is that hydrological response variables related to terrestrial groundwater-dependent ecosystems, cannot be predicted by the OGIA model as discussed in Section 2.6.2.1.

Groundwater extractions

The MODFLOW Wells package is used to represent all groundwater extractions, except CSG depressurisation and mine dewatering. ‘Estimated extraction rates in 1995 from approximately 19,000 abstraction bores have been included in the model via the MODFLOW well package’ (GHD, 2012, p. 59). These include:

‘Total water extraction is about 215,000 ML/year, of which about 85,000 ML/year is from the GAB formation and 130,000 ML/year is from other aquifers’ (QWC, 2012, p. 42). Companion product 1.5 for the Maranoa-Balonne-Condamine subregion (Cassel et al., 2015) details the number of bores and estimated non-petroleum and gas groundwater extractions represented in the OGIA model. The OGIA model includes water extraction from ‘154 conventional oil and gas wells extracting water from GAB formations and 83 extracting water from older Permian and Devonian formations underlying the Bandanna Formation’ (QWC, 2012, p. 42).

Coal seam gas production

The MODFLOW EVT package is used to represent groundwater extraction associated with coal seam gas (CSG) production in the OGIA model. The EVT package allows extraction rates to gradually reduce as the head falls toward a user-defined extinction depth. This is useful for predictive simulations since the actual rate of extraction is not known in advance. The timing and volume of water extraction is based on annually updated actual extraction data for the historical period and estimated future production rates provided to OGIA by the tenure holders (OGIA, 2014). ‘It is estimated that over the life of the industry, water extraction will average about 95,000 ML/year’ (QWC, 2012, p. 59).

Water management for coal resource development in the Maranoa-Balonne-Condamine subregion is described in greater detail in companion product 2.3 (conceptual modelling) (Holland et al., 2016).

The MODFLOW Drain package is used to represent water extraction for open-cut coal mines, which is described in Section 2.6.2.5.