2.6.1 Surface water numerical modelling for the Clarence-Moreton bioregion

Executive summary

​Rainforest waterfall in Border Ranges National Park, NSW, 2008 Credit: Liese Coulter, CSIRO

Coal and coal seam gas (CSG) development can potentially affect water-dependent assets (either negatively or positively) through direct impacts on surface water hydrology. This product presents the modelling of surface water hydrology within the Clarence-Moreton bioregion.

First, the methods are summarised and existing models are reviewed, followed by details regarding the development of the model. The product concludes with predictions of the hydrological characteristics of the system that may change due to coal resource development (referred to as hydrological response variables) also taking into account uncertainty.

Results are reported for two potential futures considered in the Clarence-Moreton Bioregional Assessment (BA):

  • baseline coal resource development (baseline): a future that includes all coal mines and CSG fields that are commercially producing as of December 2012
  • coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.

The difference in results between CRDP and baseline is the change that is primarily reported in a BA. This change is due to the additional coal resource development – all coal mines and CSG fields, including expansions of baseline operations, that are expected to begin commercial production after December 2012.

The Clarence-Moreton bioregion baseline includes one existing coal mine, the Jeebropilly Coal Mine in the Bremer river basin. An additional coal resource development is the Metgasco West Casino CSG project near Casino, NSW, in the Richmond river basin. As the baseline coal mine is far from the additional coal resource development, and there is no hydraulic connectivity between the Richmond and Bremer river basins, the conceptual hydrogeological model focuses on the geological, hydrogeological and hydrological characteristics of the Richmond river basin.

A recent decision by Metgasco (16 December 2015) to sell back their petroleum exploration licences (PELs) to the NSW Government, as well as withdraw their petroleum production license application (PPLA), effectively means that future development of any CSG resources in the Clarence-Moreton bioregion is highly uncertain. However, as per companion submethodology M04 for developing a CRDP, once the CRDP is determined, it is not changed for BA purposes, even in cases such as this where Metgasco have discontinued their operations in the Clarence-Moreton bioregion.

Surface water modelling in the Clarence-Moreton bioregion follows the approach outlined in companion submethodology M06 for surface water modelling. No river modelling has been carried out because the effects of regulation are small. There is an existing river system model of the Richmond River that uses Department of Primary Industries’ (DPI Water’s) Integrated Quantity and Quality Model (IQQM). Alternatively, the integrated modelling environment software known as Source IMS could be used to develop a Richmond River model. Neither IQQM nor Source IMS will be used in BAs. The Richmond river basin has low levels of stream regulation, so the routing parameters in IQQM are not needed for impact predictions. Instead, predicted streamflow is obtained by accumulating output from the Australian Water Resources Assessment landscape model (AWRA-L). AWRA-L has an accessible code, is relatively easy to set up and calibrate, and there is ready access to local expertise. AWRA-L performed well at estimating streamflow in the Richmond river basin and surrounding area.

The conceptual model for the Clarence-Moreton bioregion in product 2.3 (Conceptual modelling for the Clarence-Moreton bioregion) indicates, based on current information, no new coal mines are expected in the foreseeable future and CSG development is restricted to the Richmond river basin of north-eastern NSW. The surface water modelling domain comprises parts of the Richmond river basin and includes 16 model nodes, which are located where daily streamflow predictions are reported as output. The model simulation period is from 2013 to 2102. Seasonal climate scaling factors are used that result in a reduction in mean annual precipitation of 1.8% per degree of global warming for the Clarence-Moreton bioregion.

The AWRA-L model was regionally calibrated at nine unregulated streamflow gauging stations using two calibration schemes: one biased towards high streamflow and another towards low streamflow. Two parameter sets obtained from the two model calibrations were used as starting points to generate 10,000 parameter sets that can be used for the uncertainty analysis. It is noted that when the regional model is calibrated against observations from the nine streamflow gauging stations it does not generate a uniform model performance. While in general, model calibration results performed well across both the high- and low-streamflow calibrations, they both perform poorly in some areas.

Quantitative and qualitative uncertainty analyses were undertaken for surface water modelling in the Clarence-Moreton bioregion to provide a systematic overview of the model assumptions, their justifications and the effect on predictions. In the uncertainty analysis the optimised parameters are used to inform the prior parameter distributions.

The quantitative uncertainty analysis highlights the importance of constraining parameters with observations of the same type as the prediction, and it is clear that the hydrological response variables are sensitive to different parameters. For the high-flow metrics, the most important parameters are those controlling the quick-flow and interflow components of the hydrograph. The low-flow hydrological response variables are most responsive to the variable that controls the slow-flow component of the simulated hydrograph.

The qualitative uncertainty analysis provides a summary of the major assumptions and model choices underpinning the Richmond river basin surface water model.

The change in surface water hydrology predicted due to the additional coal resource development in absolute terms is predicted to have a median decrease of less than 0.01 GL/day, which corresponds to a change of about 0.01%. These changes are several orders of magnitude smaller than the observed mean streamflow. Their effect on mean and high-flow hydrological response variables will therefore be minimal. Even the effect on low-flow hydrological response variables will be very small, especially in the perennial streams.

In addition to this, such low changes in flow are extremely hard to observe as the largest uncertainties in the rating curves used to transfer measured stage heights to flows are associated with low-flow measurements.

The modelled impacts indicate that the number of zero-flow days (ZFD) across the region will not increase, with the exception of two nodes (CLM_007 and CLM_006). CLM_006 is at the downstream end of Shannon Brook where the median change in the number of ZFD is 3 days. The 95th percentile of change in zero-flow days is 120 days. As noted earlier, small changes in simulated flow can result in large changes in the number of zero-flow days, as zero-flow days are defined as days with streamflow less than 0.01 ML/day. The modelling of measurement of such low flows are problematic and uncertainty in these predicted impacts is high.

Accurately measuring and simulating low-flow conditions is very challenging and requires further efforts. The surface water numerical modelling described in this product provides input into product 2.6.2 (groundwater numerical modelling) for the Clarence-Moreton bioregion. The impact and risk analysis (product 3-4) will not be conducted in the Clarence-Moreton bioregion due to very small hydrological changes predicted at or near the surface due to the additional coal resource development. Outcome synthesis (product 5) is the final technical product being developed for the Clarence-Moreton bioregion.

Last updated:
18 October 2018
Thumbnail images of the Clarence-Moreton bioregion