Water balance

A water balance provides a succinct summary of the key stores and flows in a hydrologic domain for a specified period. At a minimum, a water balance will include rainfall, evapotranspiration and streamflow, and some quantification or assumptions made about storage. More detailed water balances will include estimates of catchment runoff, diffuse recharge and groundwater discharges, including the baseflow contribution to streamflow. Activities or events that lead to changes in inputs to a system, changes in storage capacity and/or changes in the pathways through the system will lead to changes in the water balance terms. Thus the impacts of coal resource development on regional hydrology can be reported in terms of changes to the regional water balance. This section provides some estimates of mean annual rainfall, recharge, streamflow and baseflow for the Hunter subregion, based on published information. Evapotranspiration is viewed as the value required to close the water balance, assuming that change in storage is zero. This is done as in many cases due to the size of this term, errors in estimating it may be larger than the small residual terms, such as recharge, that are more important to estimate accurately.

The average annual rainfall gradient across the subregion is from 640 mm at Ulan in the western Goulburn river basin, to 1100 mm at Newcastle on the coast and 1300 mm at Gosford in the Macquarie-Tuggerah lakes basin (see Figure 24 in companion product 1.1 for the Hunter subregion (McVicar et al., 2015, p. 44)). The 1982 to 2012 annual average rainfall over the Hunter subregion was 793 mm.

Groundwater recharge is commonly estimated at 2% of rainfall or less but with higher values in areas of higher regolith permeability (Mackie Environmental Research, 2006). Assuming 2% applies uniformly across the subregion, an estimate of average annual recharge of 15.9 mm is obtained from an average annual rainfall of 793 mm. The subregion-wide estimate from the spatially-variable recharge surface generated for the BA for the Hunter subregion (see Section 2.1.3 of companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018b)), which reflects the spatial variation in rainfall and regolith permeability, is 9 mm.

The average annual streamflow recorded in the Hunter River at Greta (station 210064) between 1968 and 2015 was 709.5 GL/year, which yields a depth equivalent flow of 41.0 mm across 17,320 km2 contributing area (DPI, 2015).

In a steady-state situation, assuming all groundwater recharge is discharged to the stream network, this equates to a maximum baseflow contribution from groundwater of between 22% and 39% of streamflow, using the foregoing estimates of subregion-wide average annual recharge. The gauging station at Greta is used for this estimate because it is the most downstream gauge on the Hunter River that is not tidally influenced, and hence the most downstream point represented in the river model. Other estimates of baseflow using a digital filter approach ranged from 40% to 66% for Hunter subregion rivers (see companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018b)).

A crude water balance is provided for the Hunter subregion (upstream of Greta) in Table 3. Companion product 2.5 for the Hunter subregion (Herron et al., 2018a) provides a number of water balances from the surface water and groundwater modelling of the coal resource development futures modelled in the BA for the Hunter subregion.

Table 3 Estimates of the Hunter subregion water balance

Water balance term





Percentage of rainfall

















(22%–66% of streamflow)





Residual is calculated as rainfall minus streamflow. Recharge is assumed to discharge to stream as baseflow and baseflow is a component of streamflow. Conversion from mm/year to GL/year is based on contributing area of 17,320 km2.

Last updated:
18 January 2019