4 Boundary conditions

4.1 Model extent

Ideally the model domain should extend to geological boundaries so that boundary effects on model predictions can be minimised. This is especially the case in those subregions or bioregions that have an off-shore component to the geological basin, such as the Hunter subregion.

4.2 Recharge

Temporal distributions of diffuse recharge to groundwater will be obtained from the Australian Water Resources Assessment (AWRA) landscape model (AWRA-L) that is used for the surface water modelling in BA (Viney, 2016). Since AWRA-L is calibrated to streamflow observations, recharge outputs will most likely be of different magnitudes to those determined using hydraulic and hydrochemical methods (e.g. chloride mass balance, water balance, tracers, etc.). For this reason, these recharge outputs will require scaling before use. In addition, AWRA-L outputs are produced over a 0.05 degree grid at a daily time step. These will need to be aggregated temporally to match the monthly time steps used in the groundwater models. Similarly, AWRA-L outputs will need to be aggregated spatially to a single temporal sequence to be applied to all recharge grid cells in the groundwater models. This simplification to a single temporal pattern for a subregion was shown to be appropriate for Clarence-Moreton bioregion in Crosbie et al. (2015).

The landscape model will be run under historical conditions for the 30-year period from 1 January 1983 to 31 December 2012. Climate forcing data for the forward modelling will be constructed from the historical climate time series repeated three times to create a 90-year time series and modified to be consistent with a median future climate projection. Further details of the future climate time series is given in the companion submethodology M06 (as listed in Table 1) for surface water modelling (Viney, 2016).

Localised recharge due to river losses, overbank flooding and irrigation will be modelled in the AWRA river model (AWRA-R). The overbank flooding and irrigation recharge are used directly and the river losses are calculated by the groundwater model using the river stage from the AWRA-R model (see Chapter 6). These outputs from the river model will be provided as daily time series and will need to be aggregated temporally to match the monthly time steps used in the groundwater models and matched spatially from the river reach to the irrigated or flooded portion of that river reach in the groundwater model.

4.3 Extraction

Rates of groundwater extraction for stock, domestic, irrigation, industry and town water supplies will be treated as constant and equal to the rates specified in the water sharing plan (or other equivalent instrument) that was enacted for the last quarter of 2012 (unless actual metered data are available). Any future developments associated with agriculture or other industries have been excluded from the scope of the BAs and so these extractions will be consistent between the baseline and coal resource development pathway (CRDP).

Groundwater extractions associated with coal seam gas (CSG) and large coal mining development are determined based on target groundwater levels rather than extraction rates. For example, the target groundwater level for a CSG operation could be specified as the elevation located approximately 35 m above the top of the target coal seam. Similarly, the target groundwater level for large coal mining operations would be the pit floor for open-cut operations and atmospheric pressure for longwall mining operations. Using this approach, the rate of groundwater extracted is a function of hydraulic properties of the aquifers and aquitards involved (which are uncertain) and will be estimated as a probability distribution rather than as a discrete value.

4.4 Evapotranspiration

In areas featuring shallow watertables, or where shallow watertables might develop due to irrigation developments (associated with co-produced water), the parameterisation of evapotranspiration will require use of a depth-dependent boundary condition in the groundwater models in order to account for the loss of groundwater via evapotranspiration. In MODFLOW this is implemented as the EVT package (Harbaugh et al., 2000) with other model codes having something similar. In this manner, terrestrial groundwater-dependent ecosystems have been incorporated into the groundwater models.