Model sequencing

As of mid-2015, the baseline includes one existing coal mine (Jeebropilly Mine, west of Ipswich) and the coal resource development pathway (CRDP) includes the Jeebropilly Mine and one additional coal resource development for CSG (Metgasco Limited’s West Casino Gas Project). This CSG development is located in the Richmond river basin near Casino, NSW. 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. The focus on this area is due to the presence of highly gas-saturated coal seams that have a relatively high permeability, which are located along the western side of the Casino Trough at depths as shallow as 250 m.

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 lease 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 (as listed in Table 1) for developing a coal resource development pathway (Lewis, 2014), once the CRDP is determined, it is not changed for bioregional assessment (BA) purposes, even in cases such as this where Metgasco have discontinued their operations in the Clarence-Moreton bioregion.

In order to simulate hydrological changes caused by the additional coal resource development (i.e. the West Casino Gas Project), a model sequence is needed that simulates the impacts on the regional groundwater system, the alluvial groundwater system and the stream network. The Clarence‑Moreton Bioregional Assessment adopts a model sequence that consists of a rainfall-runoff model that simulates the surface water system, and a numerical groundwater model that simulates the groundwater system. The surface water model is used to generate river stage heights, which are then used as an input to the groundwater model. The groundwater model then predicts the flux of water between the groundwater and surface water systems.

Potential groundwater impacts of CSG development in the geological Clarence‑Moreton Basin were simulated using a regional-scale numerical groundwater model (MODFLOW–NWT; Niswonger et al., 2011) (Bioregional Assessment Programme, Dataset 2), chosen because it deals well with instabilities arising from dry cells in the unconfined model layers. The groundwater model predicts the change in surface water – groundwater flux through the MODFLOW River package. This flux is taken into account in the AWRA‑L surface water model generated streamflow. The change in a number of hydrological response variables is modelled at various surface water model nodes.

The model sequence that was adopted to simulate the impacts of the CRDP is depicted in Figure 3. Figure 3 shows the relationship between the AWRA‑L surface water model and the groundwater model. The stage height time series that is derived from AWRA‑L drives the MODFLOW River package, and depending on the head differences and the streambed conductance, the river either loses water to, or gains water from, the alluvial aquifer.

The CRDP impacts on daily streamflow at each model node are estimated as the baseflow impact. The hydrological changes to baseflow are simulated using the numerical groundwater model, which is described in detail in Section of companion product 2.6.2 for the Clarence‑Moreton bioregion (Cui et al., 2016). The numerical groundwater model estimates monthly baseflow for each model node under the baseline and CRDP. The difference between CRDP and baseline simulations is taken as the monthly hydrological changes in baseflow, which is then equally partitioned to obtain the daily changes. The technical details of the model conceptualisation, parameterisation and implementation, together with the uncertainty analysis of the simulated impacts, are documented in companion product 2.6.2 for the Clarence‑Moreton bioregion (Cui et al., 2016).

Figure 3

Figure 3 Model sequence for the Clarence-Moreton bioregion

AWRA-L = Australian Water Resources Assessment landscape model; baseline = baseline coal resource development; CRDP = coal resource development pathway; CRDP = baseline + additional coal resource development; green rectangles represent models; orange rectangles are input and/or output data of models

As outlined in companion submethodology M06 (as listed in Table 1) for surface water modelling (Viney, 2016), nine hydrological response variables have been chosen to characterise the impacts of coal resource development. These variables are intended to be representative of the flow characteristics that are important for assessing impacts on economic and ecological assets. Five of the hydrological response variables characterise low streamflow, two characterise high streamflow, and two characterise long-term flow variability.

The low-streamflow hydrological response variables are:

  • P01: the daily streamflow rate at the 1st percentile (ML/day)
  • ZFD: the number of zero-flow days per year. Zero flow is identified using the minimum detectable flow. For ease of applicability, a threshold of 0.01 ML/day is set for determining the number of ZFD for all surface water nodes
  • LFD: the number of low-flow days per year. The threshold for LFD is the 10th percentile from the simulated 90-year period (2013 to 2102)
  • LFS: the number of low-flow spells per year (perennial streams only). A spell is defined as a period of contiguous days of streamflow below the 10th percentile threshold
  • LLFS: the length (days) of the longest low-flow spell each year.

The high-streamflow hydrological response variables are:

  • P99: the daily streamflow rate at the 99th percentile (ML/day)
  • FD: flood (high-flow) days, the number of days with streamflow greater than the 90th percentile from the simulated 90-year period (2013 to 2102).

In addition, two hydrological response variables that represent streamflow volume and variability are:

  • AF: the annual streamflow volume (GL/year)
  • IQR: the interquartile range in daily streamflow (ML/day). That is, the difference between the daily streamflow rate at the 75th percentile and at the 25th percentile.

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

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