Uncertainty analysis


The uncertainty analysis is consistent with the approach described in companion submethodology M09 (as listed in Table 1) for propagating uncertainty through models (Peeters et al., 2016). The same 37 parameters investigated in the sensitivity analysis are considered in the uncertainty analysis of the groundwater model for the Namoi subregion.

Prior distributions for each parameter assume a uniform distribution, with no covariance of parameters. Groundwater level observations from a limited set of observation sites, observed streamflow data and estimates of coal resource development water makes are used to constrain the parameter space. Parameter sets for groundwater levels are considered acceptable when they result in average groundwater level predictions that are within 10 m of observed historical averages at observation sites within 30 km of the prediction site. Acceptable parameter sets for modelled surface water – groundwater fluxes are those in which the average of the simulated historical surface water – groundwater flux is less than the 20th percentile of observed streamflow. Acceptable parameter sets for the mine water makes are those in which the simulated coal resource development water makes are within an order of magnitude of estimations from the proponents’ environmental impact statement (EIS) modelling.

Predictions close to Narrabri and Gunnedah are well constrained, while the observation data are not able to greatly reduce the predictive uncertainty of drawdown due to additional coal resource development in the Pilliga area.

Drawdown due to additional coal resource development was analysed at 14,209 model nodes within the model. The predicted drawdown is less than 2 m for three-quarters of the model nodes and less than 0.2 m for two-thirds of these model nodes. Drawdown due to additional coal resource development is localised around the additional coal resource development mines. At a distance of about 10 km from the mine sites, there is only about a 5% chance of additional drawdown exceeding 0.2 m. In general, tmax occurs relatively quickly in the immediate vicinity of the mines, but progressively later with increasing distance from the mines. In most cases the drawdown is attenuated at the alluvium boundary due to the high transmissivity and so the largest magnitude drawdowns occur in the consolidated rock units rather than the alluvium.

Last updated:
6 December 2018
Thumbnail of the Namoi subregion

Product Finalisation date