10 Outputs from groundwater modelling


10.1 Outputs for product 2.6.2 (groundwater numerical modelling)

Product 2.6.2 (groundwater numerical modelling) reports the potential impacts of coal resource development on water resources at the selected model nodes within the groundwater model domain. This is done by comparing model simulations that account for the coal resource development pathway (CRDP) with those that only consider the baseline.

10.1.1 Hydrological response variables

The groundwater modelling outputs hydrological response variables, the hydrological characteristics of the system or landscape class that potentially change due to coal resource development. These outputs from the groundwater modelling can be either fluxes or stores. They need to be decided before the sensitivity analysis begins and also need to be defined precisely ‒ for example, drawdown at location (x, y, z) at time t.

The primary hydrological response variables for groundwater are shown in Table 4.

Table 4 Primary hydrological response variables for groundwater


Shortened form

Description of hydrological response variable

Units

Tmax

year of maximum change

year

dmax

maximum difference in drawdown for one realisation within an ensemble of groundwater modelling runs, obtained by choosing the maximum of the time series of differences between two futures

metres

Figure 15 shows an example of output for hydrological response variables for the Clarence-Moreton bioregion (see Cui et al. (2016) for full explanation and interpretation of these results). Uncertainty analysis has been undertaken for these results as well (as per Chapter 8).

Other outputs from groundwater modelling include:

  • groundwater fluxes to or from the stream network, which are fed back to the surface water modelling (Viney, 2016) and are reported as surface water hydrological response variables in product 2.6.1 (surface water numerical modelling)
  • the volume of co-produced water and mine water make, which is reported in product 2.5 (water balance assessment)
  • interpolated surfaces of percentiles of drawdown and probability of exceeding thresholds of 0.2 and 2 m for the baseline, CRDP and additional coal resource development.

Some groundwater models will be capable of generating many gigabytes of output data from a single model run. When such models are run thousands of times, the storage space required may become infeasible and file transfers may become prohibitive or impossible. For this reason, only the model outputs that will actually be used in evaluating the potential impacts of coal resource development on assets and landscape classes will be stored.

Figure 15

Figure 15 Example of the groundwater model output time series of model nodes pdm_324 (a) and (c) and pdm_1291 (b) and (d)

Example only; do not use for analysis. This is an early draft of a figure published in Cui et al. (2016). See Cui et al. (2016) for full explanation and interpretation of these results.

Additional drawdown is the maximum difference in drawdown (dmax) between the coal resource development pathway (CRDP) and baseline, that is due to additional coal resource development

Coal resource development pathway = baseline + additional coal resource development

10.1.2 Content for product 2.6.2 (groundwater numerical modelling)

Table 5 shows the recommended content for product 2.6.2 (groundwater numerical modelling).

The outline for product 2.6.2 (groundwater numerical modelling) can be flexibly adapted where there are multiple groundwater models. There are several reasons why there could be multiple groundwater models within a subregion or bioregion including:

  • where the development occurs in two distinct geographical regions without overlap
  • a hybrid approach with models feeding in to one another, or
  • if child models are used for detail in an area of a regional model.

In the Bioregional Assessment Technical Programme only the Gloucester subregion has multiple groundwater models. Two models were built for the Galilee subregion, although only one is used directly for the Bioregional Assessment Technical Programme analysis.

Table 5 Recommended content for product 2.6.2 (groundwater numerical modelling) when there is one groundwater model


Section number

Title of section

Main content to include in section

2.6.2.1

Methods

Summary

This section identifies the models used, the interactions between the different models, the sequence in which they need to be run and for which model nodes they simulate the impact of coal resource development.

2.6.2.2

Review of existing models

Summary

This section reviews the previous groundwater models developed for coal resource development in the subregion or bioregion. Level 5 headings can cover individual projects.

2.6.2.3

Model development

Summary

This section describes how the model was developed. The following Level 5 headings are recommended but not mandatory.

2.6.2.3.1 Objectives

2.6.2.3.2 Hydrogeological conceptual model

2.6.2.3.3 Design and implementation

2.6.2.3.4 Model code and solver

2.6.2.3.5 Modelling approach

2.6.2.4

Boundary and initial conditions

Summary

This section characterises the boundary and initial conditions. The following Level 5 headings are recommended but not mandatory. 2.6.2.4.1 Lateral

2.6.2.4.2 Recharge

2.6.2.4.3 Surface water – groundwater interactions

2.6.2.5

Implementation of coal resource development pathway

Summary

This section describes how the coal resource development pathway (as specified in product 2.3 (conceptual modelling)) is implemented in the groundwater model. The following Level 5 headings are recommended but not mandatory.

2.6.2.5.1 Open-cut mines

2.6.2.5.2 Underground mines

2.6.2.5.3 Coal seam gas wells

2.6.2.6

Parameterisation

Summary

Table 6 (in this submethodology) provides an exemplar table for listing parameters in this section.

2.6.2.7

Observations and predictions

Summary

This section provides the results, namely predictions of the hydrological response variables and the sensitivity of the results to the parameters used. The following Level 5 headings are recommended but not mandatory.

2.6.2.5.1 Predictions

2.6.2.4.2 Sensitivity analysis

2.6.2.8

Uncertainty analysis

Summary

Both qualitative and quantitative uncertainty is presented.

2.6.2.6.1 Qualitative uncertainty analysis

The qualitative uncertainty analysis lists the main model assumptions and choices and discusses their potential effect on the predictions. Table 7 (in this submethodology) provides an exemplar table.

2.6.2.6.2 Quantitative uncertainty analysis

For the quantitative uncertainty analysis, prior distributions, including covariance, are specified for all parameters from expert elicitation; constraining these prior distributions with the maximum coal seam gas (CSG) and coal mine water production rate results as well as head and flux observations in posterior probability distributions for dmax and tmax. The potential effect on the predictions are discussed along with a comparison to previous model results. Figure 16 and Figure 17 (in this submethodology) provide exemplar figures.

2.6.2.9

Limitations and conclusions

Summary

This section describes the use for which the groundwater model was developed, and limitations on its application to other uses.

Table 6 Example table to include in Section 2.6.2.6: parameters of the Avon and Karuah models for the Gloucester subregion

Example only; do not use for analysis


Parameter name

Value

Description

Unit

Minimum

Maximum

Kha

1.0

Saturated hydraulic conductivity of top alluvial layer

m/d

0.1

10.0

Khw

0.003

Saturated hydraulic conductivity of lower weathered layer

m/d

0.01

0.0001

Sy

0.15

Specific yield of the top alluvial layer

na

0.25

0.05

Dc

100.0

Hydraulic conductance of lower boundary of drain bed

m2/d

10.0

1000.0

Rmult

1.0

Multiplier for monthly recharge

na

0.1

2.0

dh

2.0

Depth to water in the lower weathered layer

m

0.0

5.0

The ‘value’ column lists the initial parameter value simulation, while the ‘minimum’ and ‘maximum’ columns show the range sampled for the design of experiment. The last two lines list non-variable parameters used in the simulations.

na = not applicable

See Peeters et al. (2016) for full explanation and interpretation of these results.

Table 7 Example table to include in Section 2.6.2.8: qualitative uncertainty analysis as used for the Gloucester subregion

Example only; do not use for analysis.


Number

Assumption / model choice

Data

Resources

Technical

Effect on predictions

1

Hybrid analytic element – MODFLOW model methodology

high

medium

high

low

2

Principle of superposition

medium

low

low

low

3

Horizontally spatially uniform hydraulic properties

high

medium

medium

low

4

Hydraulic properties vary with depth, not with stratigraphy

high

low

low

medium

5

Stochastic representation of coal seams and faults

high

low

low

low

6

Random location of CSG wells and assigning pumping interval to random coal seams

high

low

low

low

7

CSG wells as constant head wells

high

medium

high

medium

8

Open-cut mines as prescribed pumping rate

high

low

low

high

9

Specification of prior distributions

high

medium

low

low

10

River network implemented as drainage boundary

medium

low

low

low

11

Constrain model with flux estimates rather than head observations

high

low

low

low

12

Simulation period from 2012 to 2102

low

high

medium

low

CSG = coal seam gas

See Peeters et al. (2016) for full explanation and interpretation of these results.

Figure 16

Figure 16 Example figure to include in Section 2.6.2.8 Uncertainty analysis: histograms of prior and posterior distributions of the regional analytic element model for the Markov chain Monte Carlo analysis for the Gloucester subregion

Example only; do not use for analysis.

The extent of the x-axis in each plot corresponds to the range of parameters sampled during the design of experiment. Refer to Table 3 in Section 2.6.2.3.4 for definitions of terms.

See Peeters et al. (2016) for full explanation and interpretation of these results.

Figure 17

Figure 17 Example figure to include in Section 2.6.2.8 Uncertainty analysis: covariance of the posterior parameter distributions for the regional analytic element groundwater model for the Gloucester subregion

Example only; do not use for analysis.

The colour scale is proportional to the density of points. Refer to Table 5 in Section 2.6.2.6 of Peeters et al. (2016) for definition of terms. See Peeters et al. (2016) for full explanation and interpretation of these results.

10.2 Outputs for product 2.5 (water balance assessment)

Product 2.5 (water balance assessment) presents a quantitative water balance for the subregion. The groundwater components of this water balance are typically derived from the outputs of the groundwater modelling. Other approaches for determining groundwater balance components may be required (e.g. SKM, 2006) if the groundwater modelling undertaken for a subregion does not provide the necessary information for reporting in the water balance. Table 9 shows the recommended content for product 2.5 (water balance assessment).

The water balance will represent a defined control volume. The nature of this control volume may vary between subregions or bioregions. However, it is likely to involve a subarea of the surface water model domain. It may represent a hydrologically intact catchment area (or areas) draining to a particular point (or points) in the river network, or it may exclude external tributary inflows. Since there will be a groundwater component to the water balance, the extent of the control volume may be constrained by the spatial extent of the groundwater model. In other words, it is likely that the control volume will be a subarea of the intersection between the spatial domains of the surface and groundwater models. In product 2.5 a map will be provided that shows the location of the control volumes used for the water balance.

The following groundwater components will be reported in the water balance:

  • recharge
  • evapotranspiration
  • baseflow (discharge to stream)
  • upward flow from deeper groundwater
  • change in storage.

An exemplar for a water balance table is shown in Table 8 (see Herron et al. (2016) for full explanation and interpretation of these results).

Table 8 Example water balance table: mean annual groundwater balance for the alluvial groundwater model extent in the Avon River for 2013 to 2042 in the Gloucester subregion (ML/year)

Example only; do not use for analysis.


Water balance term

Under the baseline

Under the coal resource development pathway

Difference

Groundwater

Recharge

6893 (6067; 8191)

6893 (6067; 8191)

0

Evapotranspiration

289 (46; 866)

285 (46; 808)

–4

Baseflow (discharge to stream)

6929 (6441; 7353)

6848 (5659; 7296)

–81

Upward flow from deeper groundwater

392 (–44; 533)

340 (–368; 512)

–52

Change in storage

–11 (–180; 3)

–5 (–138; 101)

6

The first number is the median, and the 10th and 90th percentile numbers follow in brackets.

See Herron et al. (2016) for full explanation and interpretation of these results.

Table 9 Recommended content for product 2.5 (water balance assessment)


Section number

Title of section

Main content to include in section

2.5.1

Methods

2.5.1.1

Spatial and temporal extent of the water balances

Temporal resolution: The water balance is reported over three 30-year periods, namely 2013 to 2042, 2043 to 2072 and 2073 to 2102, which align with the three global warming scenarios of 1.0, 1.5 and 2.0 °C.

Spatial resolution: This will vary by subregion, but a general principle is to report the water balance over the minimum possible area which incorporates all hydrologically connected cumulative impacts. Thus more than one might be required per subregion or bioregion.

2.5.2

Water balances

Suggestions for level 4 headings are either: inflows, consumptive use and discharge, or a subheading for each water management unit.

2.5.2.1

Reporting unit #1

Number of tables: Three tables will be needed for each spatial reporting unit – one for each of the three time slices. Each will contain results under the baseline, under the coal resource development pathway (CRDP), and the difference.

Uncertainty: Within each table, for some outputs, three numbers will be required representing the median, 10th and 90th percentiles from the uncertainty analysis. For some outputs (e.g. rainfall) this will not be required.

Table 1 Water balance in [insert reporting unit name] for 2013 to 2042

Table 2 Water balance in [insert reporting unit name] for 2043 to 2072

Table 3 Water balance in [insert reporting unit name] for 2073 to 2102

2.5.2.2

Reporting unit #2

Number of tables: Three tables will be needed for each spatial reporting unit – one for each of the three time slices. Each will contain results under the baseline, under the CRDP, and the difference.

Uncertainty: Within each table, for some outputs, three numbers will be required representing the median, 10th and 90th percentiles from the uncertainty analysis. For some outputs (e.g. rainfall) this will not be required.

Table 1 Water balance in [insert reporting unit name] for 2013 to 2042

Table 2 Water balance in [insert reporting unit name] for 2043 to 2072

Table 3 Water balance in [insert reporting unit name] for 2073 to 2102

2.5.2.3

Gaps

Last updated:
8 June 2018