# 2.5.1.1 Spatial and temporal extent of the water balances

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A water balance provides a summary of the inflows, outflows and change in storage for a defined area and period of time. A crude water balance was provided in Section 2.3.2.4 of companion product 2.3 for the Hunter subregion (Dawes et al., 2018), which included estimates of rainfall, recharge, baseflow and a residual term for the Hunter subregion, based on observation data and various approximation methods. Results from the numerical modelling reported in companion product 2.6.1 (surface water modelling; Zhang et al., 2018) and companion product 2.6.2 (groundwater modelling; Herron et al., 2018) for the Hunter subregion are reported as water balances in this product to provide accessible summaries of the effect of coal resource development on key variables of the regional water balance. The coal resource development pathway (CRDP) in the Hunter subregion comprises open-cut and underground coal mines only. Coal seam gas (CSG) development is not currently part of the CRDP for the Hunter subregion.

In the Hunter subregion, groundwater and surface water have been modelled largely independently, but come together in the river model via a 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 Hunter 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 water balance equations can be written for:

a. Groundwater as: Re = ET + ExL + ExM + Qbf + B + ΔS

where Re denotes recharge, ET is evapotranspiration, ExL and ExM are licensed and mining extractions, respectively, Qbf is surface water – groundwater flow, B is boundary flow, ΔS is change in storage; and

b. Surface water: P + M = Q + Ex + Res

where P is rainfall and M is mine water discharge, both inputs to the system, and Q is streamflow, Ex is extractions from the river and Res is a residual term, which includes evapotranspiration, leakage and change in storage. Streamflow, Q, includes the surface water – groundwater flux, Qbf, from the groundwater model.

Figure 3 Water balance terms for the Hunter subregion (a) groundwater balances and (b) surface water balances

Groundwater balances are shown in cross-section, whereas surface water balances are shown in plan view.

ET = evapotranspiration

Most of the coal resource developments in the Hunter subregion occur around the Hunter River from Muswellbrook and Singleton, south of Maitland, in the headwaters of the Goulburn River and inland and around Lake Macquarie and Tuggerah Lakes. These coal resource developments are detailed in Section 2.3.4 of Dawes et al. (2018). Five additional coal resource developments were not included in the groundwater modelling: three due to insufficient information to represent them in the model (West Muswellbrook, Wambo and Wilpinjong); and two due to the small scale of the proposed changes (Austar and Mount Arthur). Five additional coal resource developments were not included in the surface water modelling: two due to no significant change in the area of disturbance at the surface (Austar and Wambo); one due to insufficient information to represent in the model (West Muswellbrook); one due to mining under Lake Macquarie (Chain Valley); and one due to lack of surface water – groundwater flux data from the groundwater model (Mandalong). The potential hydrological changes and impacts on assets from additional coal resource developments that were not modelled using surface water and/or groundwater models are discussed in companion product 3-4 (impact and risk analysis) for the Hunter subregion (as listed in Table 2).

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), which align with modelled temperature increases of 1.0, 1.5 and 2.0 °C under a future climate projection from the Japanese Meteorological Research Institute global climate model (GCM). 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. These are not directly comparable to the hydrological response variables reported in Herron et al. (2018) and Zhang et al. (2018), which were based on the maximum difference between the CRDP and baseline time-series for each hydrological response variable over the simulation period, but are derived from the same sets of model simulations.

The groundwater modelling domain for the Hunter subregion encompasses an area greater than the subregion (Figure 4). As described in Herron et al. (2018), the groundwater model represents groundwater in the Hunter subregion as a whole-of-subregion aquifer, overlain by alluvial aquifers along the Hunter River and its tributaries and along the streams that drain the Macquarie-Tuggerah lakes basin (see companion product 1.1 for the Hunter subregion (McVicar et al., 2015)). A groundwater balance can be generated for the entire Hunter subregion, which comprises an inflow from recharge, outflows from evapotranspiration, mine and non-mine groundwater extractions, discharges to streamflow (baseflow) and boundary flows, and change in storage (Figure 3(a)). 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 34,000 km2. Groundwater balances for subdomains of the Hunter subregion are not presented because the model was not configured to do this.

The high connectivity between alluvial aquifers and streams in the Hunter 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 river model (AWRA-R), hence this groundwater term is included in the surface water balances. 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 Hunter subdomains are rainfall, river diversions and mine discharges and river outflows, and were obtained from the AWRA landscape model (AWRA-L), AWRA-R and groundwater modelling.

Surface water balance reporting points 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.

Two areas were defined in the Hunter river basin (Figure 4) – the contributing areas to:

1. node 41 (stream gauge 210006) on the Goulburn River, which is just downstream of the tributaries that drain the most eastern mine in the western group, Bylong mine. This node represents the cumulative changes of the Ulan West, Moolarben, Wilpinjong and Bylong additional coal resource developments (but excludes changes to baseflow from Wilpinjong mine, which was not represented in the groundwater modelling). The surface water contributing area for this basin is 3400 km2
2. node 6 (stream gauge 210001) on the Hunter River, just downstream of Singleton. This node represents the cumulative changes from the Goulburn River coal mines, as well as the additional coal resource developments in the Hunter River basin upstream of this point: the open-cut mines at Ashton, Bengalla, Bulga, Drayton South, Liddell, Mount Arthur, Mount Owen, Mount Pleasant and Mount Thorley-Warkworth; and underground mine at Mount Arthur (but excludes changes to baseflow from West Muswellbrook open-cut and Wambo underground; and reductions in surface runoff from West Muswellbrook mine). The surface water contributing area for this basin is 16,485 km2.

From a surface water perspective, the Macquarie-Tuggerah lakes basin is not hydrologically contiguous with the Hunter river basin, nor are the individual subcatchments hydrologically connected, although some drain to the same coastal lakes. This basin contains three additional coal resource developments: Mandalong and Chain Valley, which were not included in the surface water modelling, and did not have surface water – groundwater fluxes calculated by the groundwater model; and Wallarah 2, which had surface water changes quantified using AWRA-L and groundwater fluxes to the Wyong River generated at two model nodes. Due to the incompleteness of the numerical modelling of the Mandalong and Chain Valley additional coal resource developments, a surface water balance is not presented for this area.

Figure 4 Reporting areas for groundwater and surface water balances

baseline = baseline coal resource development, ACRD = additional coal resource development

Footprints represent the maximum impacted area at the surface and include both groundwater and surface water footprints.

Footprints of the coal resource development pathway (CRDP) equal the union of footprints for baseline and ACRD.

Data: Bioregional Assessment Programme (Dataset 1, Dataset 2, Dataset 3, Dataset 4, Dataset 5)

Last updated:
18 January 2019