- Bioregional Assessment Program
- Namoi subregion
- 2.5 Water balance assessment for the Namoi subregion
- 2.5.1 Methods
- 220.127.116.11 Spatial and temporal extent of the water balances
A water balance provides a summary of the inflows, outflows and change in storage for a defined area and period of time. Results from the numerical modelling reported in companion product 2.6.1 (Aryal et al., 2018) and companion product 2.6.2 (Janardhanan et al., 2018) for the Namoi subregion are reported as water balances in this product to provide summaries of the effect of coal resource development on key variables of the regional water balance. The CRDP in the Namoi subregion is comprised of open-cut and underground coal mines as well as a coal seam gas (CSG) development.
The water balances are reported for the two potential futures considered in bioregional assessments (BAs):
- baseline coal resource development (baseline): a future that includes all coal mines and CSG fields that are commercially producing as of December 2012
- 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 additional coal resource development – all coal mines and CSG fields, including expansions of baseline operations, that are expected to begin commercial production after December 2012. For the Namoi subregion the coal resource developments modelled under the baseline and under the CRDP are listed in Table 3.
Table 3 Summary of developments modelled under the baseline and under the coal resource development pathway (CRDP)
aWerris Creek Mine is modelled in the surface water modelling but is commentary only in the groundwater modelling.
bGunnedah Precinct and Vickery South Coal Project are in the CRDP but are commentary only.
CSG = coal seam gas
In the Namoi subregion, groundwater and surface water have been modelled largely independently, but come together in the river model via a change in surface water – groundwater flux along a river network common to both models. Given this, separate groundwater and surface water balance domains are defined for the Namoi subregion, with some overlap in the surface water balances. Figure 3 summarises the water balance terms reported for (a) groundwater balances and (b) surface water balances. The surface water balance (Figure 3b) shows that evapotranspiration, leakage and change in storage are reported as a single residual term in the surface water balances. This is because the aim of the water balance report is to compare changes in water balance in three periods between baseline and CRDP. The change in overall water balance showing the rainfall, runoff, diversion and residuals is of interest rather than individual components like evaporation and leakage.
Figure 3 Water balance terms for the Namoi subregion (a) groundwater balances and (b) surface water balances
ET = evapotranspiration, Note: The groundwater balances figure is in cross-section view and the surface water figure is in plan view.
Water balance terms have been extracted from the various models for three 30-year periods (2013 to 2042, 2043 to 2072 and 2073 to 2102). These align with modelled temperature increases of 1.0, 1.5 and 2.0 °C under a future climate projection from the Institute of Atmospheric Physics (IAP) global climate model (GCM) which was the median of 15 models (more details are provided in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018)). These three time periods were generated from the 30-year historical sequence from 1983 to 2012 by modifying the historical sequence to reflect a warming trend. Thus the variability in the historical sequence is preserved, but the effect of droughts and floods does not confound the comparison between time periods. The water balance terms reported here represent the annual means for each 30-year period.
The groundwater modelling domain for the Namoi subregion encompasses an area greater than the subregion (Figure 4). As described in Janardhanan et al. (2018), the groundwater model represents groundwater in the Namoi subregion comprising the Gunnedah and Surat geological basins along with the alluvium along the major watercourses. A groundwater balance can be generated for the entire Namoi model domain, which comprises an inflow from recharge, outflows from evapotranspiration, mine and non-mine groundwater extractions, exchanges with the surface water system, boundary flows, and change in storage (Figure 3a). Boundary flows occur at the edges of the model domain and reflect inflows to or outflows from the subregion. The reported volumes are for an area of 59,000 km2. Groundwater balances for subdomains of the Namoi subregion are not presented because the model was not configured to do this.
The high connectivity between alluvial aquifers and streams in the Namoi river basin means they are managed conjunctively. It is here that exchanges between groundwater and surface water predominantly occur. The groundwater model provides the change in surface water – groundwater fluxes to the Australian Water Resources Assessment (AWRA) river model (AWRA-R), hence this groundwater term is included in the surface water modelling and reported in the groundwater balance. No other groundwater fluxes (e.g. from seeps or springs) are represented in the surface water balances as they are not modelled.
Surface water balances can be reported for subdomains of a subregion because, if surface water – groundwater fluxes are assumed to be generated within the same contributing area as the surface flows, surface water catchments can be treated as relatively closed basins (with respect to inflows) with clearly defined outflow points. Surface water balance terms for the Namoi subdomains are rainfall, river diversions and river outflows, and were obtained from the AWRA landscape model (AWRA-L), AWRA-R and groundwater modelling.
Surface water balance model nodes were selected to quantify the cumulative hydrological changes due to coal resource development over the minimum possible area that they are all hydrologically connected and for which model outputs were generated. Thus these reporting areas summarise the ‘maximum’ impact on streamflow from the main groupings of hydrologically connected mines, rather than the maximum impact around individual mines.
One area was defined in the Namoi river basin (Figure 4) – the contributing areas to model node 13 (stream gauge 419039) on the Namoi River, just downstream of the junction of the Namoi River and Bohena Creek. This model node represents the cumulative changes from all coal mines in the Namoi subregion, as well as the additional coal resource development in the Narrabri Gas Project upstream of this point. The surface water contributing area for this basin is 26,534 km2. This area includes 47% of the Namoi subregion and 63% of the Namoi river basin.
Figure 4 Reporting areas for groundwater balance and model nodes for surface water balances
AWRA-R = Australian Water Resources Assessment river model; ACRD = additional coal resource development
Data: Bioregional Assessment Programme (Dataset 1, Dataset 2, Dataset 3); Bureau of Meteorology (Dataset 4)
The water balance terms summarised here are a different set of model outputs to the hydrological response variables generated at model nodes, which are reported in companion product 2.6.1 (Aryal et al., 2018) and companion product 2.6.2 (Janardhanan et al., 2018) for the Namoi subregion. The range, as represented by the 10th and 90th percentile values for each groundwater and surface water balance term, summarises the results of selected groundwater and surface water model runs. This subset of model simulations was chosen through the uncertainty analysis described in Section 18.104.22.168 of companion product 2.6.2 (Janardhanan et al., 2018) and Section 22.214.171.124 of companion product 2.6.1 (Aryal et al., 2018). The chosen groundwater simulations are those in which the surface water – groundwater flux and the water extracted for coal resource development objective functions are met.
The range, as represented by the 10th and 90th percentile values for each surface water balance term, reflects the top 10% of the original 3000 model simulations, evaluated against annual streamflow observations at Namoi subregion gauges. The gauges chosen for evaluating model performance had at least 25 years of daily data and included unregulated and regulated parts of the river system.
Product Finalisation date
- 2.5.1 Methods
- 2.5.2 The water balances
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product