2.5.1.2 Water balance uncertainty

Water balances represent a long-term mean averaged over a yearly period. Changes in water components such as groundwater discharge may appear small when compared to other fluxes (e.g. groundwater recharge) but may cause significant changes in indices such as number of zero-flow days in surface water systems. Further information on specific components of water balance, such as baseflow, is outlined in Karim et al. (2018).

2.5.1.2.1 Surface water

The water balance terms summarised in this product are a different set of model outputs to the hydrological response variables generated at model nodes, which were reported in companion product 2.6.1 (Karim et al., 2018) for the Galilee subregion. To estimate the uncertainty of the water balance terms, the posterior parameter distributions for the model chain are sampled a limited number of times to calculate the water balance components. From a set of 347 model runs that met the acceptance criteria for the uncertainty analysis reported in companion product 2.6.1 (Karim et al., 2018) for the Galilee subregion, a total of 30 runs were selected to assess the uncertainty in the components of the water balance. The sample size of 30 was used because it is commonly considered as the threshold sample size separating small-sample statistics from large-sample statistics (Davis, 2002). Although the 30 samples will not provide the rigorous uncertainty analysis presented in companion product 2.6.1 (Karim et al., 2018) for the Galilee subregion, it is expected to provide a statistically significant estimate of the 10th, 50th and 90th percentiles of the various water balance components.

2.5.1.2.2 Groundwater

There are likely to be significant uncertainties associated with most components of the groundwater balance presented in Section 2.5.2.1.2 (e.g. evapotranspiration (ET), recharge, groundwater flow out of modelling area, spring discharge). These uncertainties have not been quantified as the approach taken does not have stochastic outputs. There is likely to be a greater level of confidence around components such as bore water use as an estimate can be made of water usage. However, bore water use, by volume, is one of the smaller components of the water balance.

In environmental impact statement (EIS) documentation for six of the seven mines included as additional coal resource developments it is reported that there will be a requirement for water to be sourced external to mine site areas. The external water volumes have not been incorporated in the water balance estimates presented here, as the sources and final volumes for external water were yet to be determined as of February 2016.