2.6.1 Surface water numerical modelling for the Namoi subregion

Executive summary

Gulligal Lagoon, which is located about halfway between Gunnedah and Boggabri on the western side of the Namoi River, NSW, 2005 Credit: Courtesy of Neal Foster

Coal resource development can potentially affect water-dependent assets through impacts on surface water hydrology. This product presents the surface water modelling approach, assumptions that underpin the models and modelled hydrological changes in response to likely coal resource development in the Namoi subregion after December 2012.

To quantify impacts of coal resource development in the Namoi subregion, two potential futures are considered in a bioregional assessment (BA):

  • baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas (CSG) fields that are in commercial production as of December 2012
  • coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.

The difference in results between CRDP and baseline is the change that is primarily reported in a BA. This change is due to the additional coal resource development – all coal mines and CSG fields in the Namoi subregion, including expansions of baseline operations, that are expected to begin commercial production after December 2012.

Baseline coal mines and additional coal resource development are defined in Section 2.3.4 of companion product 2.3 (conceptual modelling) for the Namoi subregion. The baseline includes six coal mines: five open-cut coal mines – Boggabri, Rocglen, Sunnyside, Tarrawonga and Werris Creek; and one underground longwall mine – Narrabri North. The ten additional coal resource developments include nine coal mines and one CSG development: Boggabri Coal Expansion Project, Caroona Coal Project, Gunnedah Precinct, Maules Creek Mine, Narrabri South, Tarrawonga Coal Expansion Project, Vickery Coal Project, Vickery South Coal Project, Watermark Coal Project and Narrabri Gas Project. Due to insufficient information regarding the location and depth of mining, two mines, Vickery South Coal Project (open-cut coal mine) and the Gunnedah Precinct (open-cut and underground), are not being modelled. Analysis of the potential impacts and risks of these two developments will be restricted to commentary in companion product 3-4 (impact and risk analysis).

Modelling of the Namoi subregion follows the companion submethodology M06 for surface water modelling. The key features of surface water modelling are as follows:

  • The modelling domain includes all of the Namoi river basin including the Peel River.
  • The modelling includes rainfall-runoff modelling using the Australian Water Resources Assessment (AWRA) landscape model (AWRA-L) and river modelling using the AWRA river model (AWRA-R).
  • Streamflow inputs are obtained from the output of the AWRA-L model for input into the AWRA-R model.
  • Changes in baseflow due to the modelled coal resource developments from the Namoi subregion groundwater model as outlined in companion product 2.6.2 (groundwater numerical modelling) are also incorporated into the AWRA-R model along the river network.
  • The mine footprint data are obtained from mine layout maps in environmental impact statements for respective mines in order to calculate effects of mine development on runoff.
  • Daily streamflow predictions are produced at 54 model nodes for the simulation period from 2013 to 2102 using future climate input data.

Evaluation of the effects of the model assumptions on predictions shows that most assumptions are unlikely to have a significant effect on predictions. However, predictions are sensitive to the implementation of the CRDP – particularly in catchments where the mine footprint is a large fraction of the total catchment area. Predictions may also be affected by the criteria for choosing the most appropriate parameter combinations and representation of river regulation in the river model.

AWRA-L and AWRA-R are both regionally calibrated using two calibration schemes: one biased towards high streamflow, and another biased towards low streamflow. There are variations in the models’ ability to predict certain hydrological response variables such as low-flow days, zero-flow days and low-flow spells. The AWRA-R model captures relevant aspects of river management (including water resources assessment and allocations, dam storage volumes and dam releases). Recharge and other output from AWRA-R are used in the groundwater model and changes in the baseflow predicted by the groundwater model are then used to rerun AWRA-R.

The prediction results show that additional coal resource development in the Namoi subregion can cause significant changes across the flow regime. This is particularly evident for the hydrological response variables that characterise high-streamflow conditions at model nodes where the footprint forms a large proportion of the catchment.

In general, the hydrological changes attributable to the additional coal resource development are greater in the small tributaries of the Namoi River than in the model nodes along the river itself. The biggest hydrological changes (flow reductions of up to 23%) occur in the Namoi River tributaries Merrygowen Creek (downstream of the Boggabri and Tarrawonga mines) and Back Creek (downstream of the Maules Creek Mine), and in the Mooki River tributary Watermark Creek (which is affected by the Watermark Coal Project). There is no direct effect of the coal seam gas developments on surface water as the amount of water the Narrabri Gas Project proposed to release to the surface water courses is minimal at the regional scale.

The percentage changes due to additional coal resource development in the low-streamflow hydrological response variables are greater than those in the high-streamflow hydrological response variables. This especially applies to the streams with near-zero or very small baseline low-flow (e.g. for Back, Merrygowen and Bollol creeks). This is because changes to low-streamflow characteristics are caused by a combination of the instantaneous impact of interception from the additional mine footprints and the cumulative impact on baseflow over time caused by groundwater drawdown, while the changes to high-streamflow characteristics are dominated by direct interception of runoff. It is expected that there is considerable uncertainty in predicting baseflow changes due to the scale of groundwater modelling, lack of finer scale processes in riparian zones and difficulty in modelling low streamflows. As a result, the uncertainty in changes in the predicted low-streamflow hydrological response variables is also greater than that in the high-streamflow hydrological response variables.

Overall, large potential hydrological changes due to coal resource development are in catchments that are close to the developments themselves and this effect propagates to downstream locations, where the relative impact (a % of streamflow) becomes smaller because of dilution from downstream tributaries. These modelling results are used to define a zone of potential hydrological change for surface water in the Namoi subregion. The zone is used to ‘rule out’ potential impacts on landscape classes and water-dependent assets within the Namoi assessment extent. This zone includes locations where the projected change in at least one of the nine hydrological response variables has a 5% chance or greater of exceeding a specified threshold. The resulting zone of potential hydrological change for surface water includes those parts of the Namoi River and its tributaries downstream of, and including, the Mooki River in the east. Baradine Creek and the uppermost nodes of Bohena Creek and the Mooki River are outside the zone of potential hydrological change.

The surface water numerical modelling described in this product should be considered in conjunction with the groundwater numerical modelling (product 2.6.2). Together they provide key inputs to receptor impact modelling (product 2.7) and underpin the analysis of impacts on, and risks to, landscape classes and assets in product 3-4 (impact and risk analysis).

Last updated:
6 December 2018
Thumbnail of the Namoi subregion