Observations and predictions


Historical groundwater level and streamflow data are used to constrain groundwater model predictions. Through a rigorous quality control process, 64 of 583 groundwater monitoring sites are selected to constrain the numerical modelling. In the absence of reliable, regional estimates of surface water – groundwater flux, observed streamflow data are used to impose upper and lower bounds on the modelled surface water – groundwater flux.

At each model node in the model domain, the model simulates the time series of groundwater level for the baseline and coal resource development pathway (CRDP). One baseline mine and five additional coal resource developments were not modelled, so predictions do not represent changes from the full CRDP. The maximum difference in drawdown between the modelled CRDP and baseline, due to additional coal resource development (dmax), and the year of maximum change (tmax), are calculated as the difference between the two time series. At points along the prescribed stream network, the model also generates baseline and CRDP time series of the surface water – groundwater flux. These outputs become change in baseflow inputs to the river model and are encapsulated in the streamflow hydrological response variables reported in companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018).

Predictions of the change in the surface water – groundwater flux arising from coal resource development indicate that increases in baseflow are possible. These are caused by hydraulic enhancement in the goaf above longwall mines, immediately following their collapse. A simple two-dimensional model illustrates how drawdown and hydraulic enhancement interact to produce a time series of baseflow increases and decreases. Local watertable increases have been reported in the literature, but there is no published account of baseflow increases.

A subset of ten groundwater model parameters is allowed to vary stochastically to form the basis for the sensitivity and uncertainty analysis. The groundwater levels and surface water – groundwater flux are most sensitive to depth of the river channel (d_riv), which determines the drainage level, the hydraulic properties (Kh, K_lambda, KvKh) and the recharge multiplier (RCH). The surface water – groundwater flux is also sensitive to riverbed conductance (C_riv).

The main prediction, dmax, the drawdown due to additional coal resource development, is sensitive to Kh as well, but is also sensitive to the baseline porosity parameter (ne) and the parameters that define the impact due to the additional coal resource development, such as Q_mine (the mine water extraction rate) and K_ramp (the magnitude of hydraulic enhancement). As the groundwater level and streamflow observations are not sensitive to these parameters, these parameters will not be constrained greatly in the uncertainty analysis.

Last updated:
18 January 2019
Thumbnail of the Hunter subregion

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