Two assumptions or model choices that have a large potential impact on model predictions: ‘Surface water – groundwater interactions’ and ‘Mine pit dewatering represented by MODFLOW drain cells’ are identified in the qualitative uncertainty analysis. The representation of hydrological changes in surficial aquifers that affect surface water – groundwater interactions and groundwater-dependent ecosystems are influenced by model assumptions and the availability of water quality and quantity data. Assumptions and model choices related to ‘Mine pit dewatering represented by MODFLOW drain cells’ can also have a large potential impact on model predictions. Data and resources that limit improved model predictions are related to the use of regional-scale hydrostratigraphic interpretation and pit geometry, which are appropriate for the 1.5 km × 1.5 km grid cells used in the regional model, but mean that model predictions are not comparable to those made using local-scale groundwater models, such as those used during the environmental impact assessments.
‘Model horizontal and vertical discretisation’, ‘CSG activities simulated by MODFLOW evapotranspiration cells’ and ‘Quantitative uncertainty analysis using 200 calibration constrained parameter sets’ have a medium potential impact on model predictions. The revised OGIA model developed for the Surat Underground Water Impact Report (UWIR) and released for public comment in early 2016 has addressed several of the assumptions and model choices identified in the qualitative uncertainty analysis, including:
- reinterpretation of geological logs for over 7700 water bores, including surficial aquifers
- interpretation of geologic formation contacts from geophysical logs for over 4800 petroleum and gas wells and water bores
- estimated displacements associated with 17 major fault systems
- initial parameter distributions derived from lithological data and hydraulic parameter estimates
- revised estimates of natural groundwater recharge
- hydrostratigraphy represented by 32 model layers, including 3 to 6 layers to represent target coal seams
- MODFLOW USG model code using dual-porosity functionality in target coal seams to approximate water and gas flow in and near CSG wells, where CSG extraction is represented using gradually descending MODFLOW Drain cells
- model calibration using water levels from 12,000 bores (OGIA 2012 model used 1500 bores).
The OGIA 2016 model has addressed many of the model data and resource availability and technical issues, which has reduced model prediction uncertainty of areas impacted by CSG development. However, the patterns of long-term drawdown impacts are broadly consistent between the OGIA 2012 model used for BA and the revised OGIA 2016 model, lending confidence to the BA model predictions and indicating that these improvements to the regional model have a moderate effect on model predictions. Changes to the representation of hydrological changes in surficial aquifers that affect surface water – groundwater interactions and groundwater-dependent ecosystems provide the greatest opportunities to reduce predictive uncertainty in the regional model.
Product Finalisation date
- 188.8.131.52 Methods
- 184.108.40.206 Review of existing models
- 220.127.116.11 Model development
- 18.104.22.168 Boundary and initial conditions
- 22.214.171.124 Implementation of coal resource development pathway
- 126.96.36.199 Parameterisation
- 188.8.131.52 Observations and predictions
- 184.108.40.206 Uncertainty analysis
- 220.127.116.11 Limitations
- Contributors to the Technical Programme
- About this technical product