The HYDMEAS database (NSW Office of Water, Figure 20, Table 11) were used for evaluating the objective function that was used for constraining the model predictions as described in Section 220.127.116.11.1.2. Agricultural extractions are represented in the model from the year 1983 onwards at a constant rate, therefore the model simulations do not accurately capture the expansion of groundwater from the 1970s onwards and hence the water levels are not comparable to the observations during the first ten years of simulation. Considering this, the observation data for the period prior to 1993 were discarded. As the agricultural extraction is implemented in the model at a constant rate throughout the year, the seasonal dynamics of extraction and therefore groundwater levels are not captured, and so only those monitoring with enough records to form a representative annual average water level were used. It is noteworthy that this assumption does not grossly affect the primary objective of the model development (i.e. simulation of that occurs after 2012).) contains the observations of the regional monitoring network. Groundwater level measurements from 170 monitoring sites across the (see Figure 17 in Section 2.1.3 in companion product 2.1-2.2 for the Namoi subregion ( )) were selected as having at least one reliable spatial attribute (location or elevation). Of these, 36 monitoring sites did not have reliable observations and were removed from the observation used to constrain the model parameters. From the remaining monitoring sites, those sites which have at least four records in any year between 1993 and 2012 were identified; this left 134 monitoring sites for use in constraining the model. The groundwater level data from these 134 sites (
Table 11 Summary of groundwater level observations
Number of bores
Average number of observations per bore
Minimum groundwater level
Maximum groundwater level
m AHD = metres Australian Height Datum
Local monitoring information, including monitoring data from mining companies, is not included in the analysis to avoid biasing the objective function used for constraining the model with measurements predominantly affected by local hydrogeological conditions that are not captured in the regional model. Mining companies install monitoringin the vicinity of the mine sites to capture information about local hydrogeological conditions, features or local lenses of more or less permeable strata. Within a site-scale groundwater model, these local features and stresses can be represented with sufficient spatial detail for the groundwater level observations to be used to infer local parameter values or constrain local predictions. In a regional-scale model, the representation of local features and stresses cannot be done at a resolution sufficient to match the information from local-scale observations and the regional-scale parameters will compensate for the missing spatial detail (see companion submethodology M07 for groundwater modelling ( )). As shown by and , this can lead to a bias in the inferred parameter values, and in turn to biased predictions. Local information can be used in a regional , if the tolerance of model-to-measurement misfit is increased to account for the missing local detail. This means in the vicinity of mines where, due to the historical pumping rates, hydraulic gradients are expected to be high, large discrepancies between modelled and observed groundwater levels should be expected and tolerated. Effectively, this reduces the information content of the local observations. Establishing an appropriate weighting or tolerance for local observations is site-specific and subjective. In order to limit the propensity of biasing regional parameter values through the incorporation of local-scale observations and the inherently subjective weighting of these observations, local-scale information from mine groundwater-monitoring networks is not used.
Data: Bioregional Assessment Programme ()
Section 18.104.22.168.1 provides the details of how these groundwater level observations are integrated into the objective function to constrain the groundwater model.
Themodel was not designed to simulate total streamflow in the river system within its model domain, but rather to generate estimates of the change in – groundwater flux due to coal resource development at points along a prescribed stream network. Constraining these fluxes requires regional estimates of the surface water – groundwater flux, which, as discussed in Section 2.1.5 in companion product 2.1-2.2 for the Namoi subregion ( ), are not easy to determine.
As an alternative, mean daily streamflow at 31 gauges (Table 12) based on long-term observed streamflow hydrographs (NSW Office of Water, ) has been used as a proxy to constrain estimates of surface water – groundwater flux by the groundwater model. Parameter combinations that result in long-term historical surface water – groundwater fluxes in excess of the mean daily surface water flow were deemed unacceptable. This constraint is considered to be conservative because in the perennial stream reaches in the the majority of the flow is sourced from upstream outside of the groundwater model domain and so would be expected to have a low percentage of flow sourced from groundwater within the model domain. The majority of the headwater streams in the Namoi subregion are ephemeral and so this criterion will accept any stream modelled as being losing in the groundwater model.
Section 22.214.171.124.1 provides more detail on how these streamflow observations are integrated in the objective function to constrain the groundwater model.
Table 12 Mean daily streamflow at 31 gauges used to contrain the groundwater model
These gauges are also displayed in Figure 11 of companion product 2.6.1 for the Namoi subregion ()
Data: NSW Office of Water ()
126.96.36.199.1.3 Water production for coal resource developments
Another type of observation to constrain the 188.8.131.52.1.model is the expected annual total water production during coal seam gas (CSG) production from the Narrabri Gas Project and other coal mining projects as reported in companion product 2.1-2.2 for the Namoi subregion ( ). In the groundwater model, CSG production and water from the mines are implemented using the Drain package of MODFLOW-USG. The volume of water that needs to be extracted to achieve the specified at the cells with a drain boundary will therefore depend on the hydraulic parameters of the model. While it is by no means a goal of the (BA) modelling to reproduce these values exactly, the simulated water extraction rates across multiple model runs should be within a range that is consistent with these estimates, especially since these are based on detailed local information and pilot production testing. The usage of the data in objective functions is further described in Section
Table 13 shows the water production rates, estimated by the various proponents, associated with the planned coal mining or coal seam gas . These estimates integrate local geological information as well as operational detail. The implementation of the through drain boundary conditions ensures that water production rates are simulated by the groundwater model as well. The simulated water production rates are compared with the proponents’ estimates to verify that the simulated stress imposed on the is consistent with the planned stress.
Table 13 Summary coal resource water production rates
Product Finalisation date
- 184.108.40.206 Methods
- 220.127.116.11 Review of existing models
- 18.104.22.168 Model development
- 22.214.171.124 Boundary and initial conditions
- 126.96.36.199 Implementation of the coal resource development pathway
- 188.8.131.52 Parameterisation
- 184.108.40.206 Observations and predictions
- 220.127.116.11 Uncertainty analysis
- 18.104.22.168 Limitations
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product