2.5.2.2 Water balance based on groundwater model


The water balance is reported for the entire groundwater model domain and is based on the modelling results from the top 30 best-fit models (Bioregional Assessment Programme, Dataset 4). The water balance is summarised in Table 9 for the three reporting periods of 2013 to 2042, 2043 to 2072 and 2073 to 2102, respectively. For each time period the 10th, 50th (median) and 90th percentiles of mean annual values of different water balance components are reported for both the baseline and CRDP. The net impact of the West Casino Gas Project is the difference between the results for the CRDP model and the baseline, and is also reported in the tables. Downwards, upwards and up-down arrows are used to indicate inflow, outflow and inflow/outflow water balance components, respectively.

Based on the current groundwater model conceptualisation, groundwater storage exhibits a future decline even without the impacts of the West Casino Gas Project, although the median annual reduction is well below 0.5% of the corresponding median annual recharge for all the three reporting periods. This can be partly attributed to the climate change signal used in the model; it can also be a result of extraction for irrigation and other usage. Overall, diffuse recharge from precipitation and discharge to surface water systems are the two dominant water balance components for both the baseline and CRDP. Among the 30 sets of modelling results, only the net balance of the General-Head Boundary (GHB) (a package used to simulate head-dependent flux boundaries in MODFLOW) (Harbaugh, 2005) varies from discharge to recharge. This implies that the adopted observations do not constrain the GHB related parameters. Water balance components associated with the GHB boundary are less reliable than other components. Additional observations are required to improve the estimate of flux exchange between the model domain and its surrounding aquifers.

Groundwater extraction due to the West Casino Gas Project mainly occurs between 2013 and 2042 with a median mean annual pumping of 328 ML/year. The median additional mean annual reduction in storage due to the West Casino Gas Project from 2013 to 2042 is 327 ML/year (6.62% of the median mean annual storage change for the baseline). A very minor change is also seen in the discharge component to surface water (Table 9) to compensate for CSG water production. No other changes can be identified for the other water balance components. The water production by the West Casino Gas Project reduces to 58 ML/year (median mean annual extraction) for the second reporting period (Table 9). However, the median mean annual discharge difference grows from 3 to 24 ML/year due to the time lag. The storage change difference is mitigated to some extent through the reduction of discharge component to surface water from 2043 to 2072. The impact on the discharge to the surface water continues until the end of the simulation (Table 9).

The overall change in groundwater balance due to the West Casino Gas Project over the entire model domain is very minor based on the modelling in companion product 2.6.2 for the Clarence‑Moreton bioregion (Cui et al., 2016) given the current model conceptualisation. For example, for the first reporting period (2013 to 2042), the median mean annual extraction due to the West Casino Gas Project only corresponds to 0.02% of the median mean annual recharge of the groundwater model domain (Table 9). There, however, can possibly be a more significant change in groundwater water balance components on a local scale close to the CSG development, especially during a dry period when more groundwater is required for other usage such as irrigation. Given the current regional model, a refined model could be developed in order to evaluate the potential impact on a local scale.

Table 9 Mean annual groundwater balance for the entire groundwater model domain for the three reporting periods in the Clarence‑Moreton bioregion (ML/year)

aThe net impact of the additional coal resource development (ACRD) is defined as the difference between results for the coal resource development pathway (CRDP) and the baseline. Note that the difference is calculated individually for each of the 30 model runs because the median under baseline may not correspond to the same realisation with the median under CRDP. The difference column is the summary of individual differences (i.e. median of the differences, not difference of the medians).


Period

Water balance term

Under baseline

Under CRDP

Differencea

2013 to 2042

↓Recharge (diffuse)

1,600,025 (859,946; 2,621,302)

1,600,025 (859,946; 2,621,302)

0 (0; 0)

↓Recharge (streams)

11,628 (4,043; 55,712)

11,628 (4,043; 55,712)

0 (0; 1)

↕GHB

8,592 (–47,758; 117,411)

8,592 (–47,758; 117,411)

0 (0; 0)

Discharge (surface water)

1,607,220 (920,573; 2,643,897)

1,607,215 (920,359; 2,643,851)

–3 (–38; 0)

↑Non-CSG bores

11,059 (10,678; 11,187)

11,059 (10,677; 11,187)

0 (0; 0)

↑CSG wells

NA

328 (88; 651)

NA

Storage change

–5,161 (–9,271; –2,968)

–5,482 (–9,671; –3,130)

–327 (–613; –82)

2043 to 2072

↓Recharge (diffuse)

1,590,460 (854,806; 2,605,634)

1,590,460 (854,806; 2,605,634)

0 (0; 0)

↓Recharge (streams)

11,659 (4,065; 55,745)

11,660 (4,065; 55,747)

0 (0; 3)

↕GHB

9,053 (–46,935; 118,597)

9,053 (–46,935; 118,597)

0 (0; 0)

Discharge (surface water)

1,597,290 (915,104; 2,628,197)

1,597,240 (914,749; 2,628,098)

–24 (–123; –1)

↑Non-CSG bores

11,058 (10,671; 11,186)

11,058 (10,670; 11,186)

0 (0; 0)

↑CSG wells

NA

58 (16; 121)

NA

Storage change

–4,216 (–9,073; –2,437)

–4,272 (–9,111; –2,425)

–31 (–80; 86)

2073 to 2102

↓Recharge (diffuse)

1,579,535 (848,934; 2,587,734)

1,579,535 (848,934; 2,587,734)

0 (0; 0)

↓Recharge (streams)

11,681 (4,083; 55,766)

11,682 (4,083; 55,769)

0 (0; 2)

↕GHB

9,462 (–46,130; 119,759)

9,462 (–46,130; 119,759)

0 (0; 0)

Discharge (surface water)

1,587,365 (909,639; 2,612,398)

1,587,300 (909,561; 2,612,361)

–25 (–83; –2)

↑Non-CSG bores

11,055 (10,664; 11,183)

11,055 (10,663; 11,183)

0 (0; 0)

↑CSG wells

NA

0 (0; 0)

NA

Storage change

–4,723 (–10,163; –2,478)

–4,721 (–10,158; –2,440)

25 (2; 87)

The first number is the median, and the 10th and 90th percentile numbers follow in brackets. Water balance terms impacted by the West Casino Gas Project are shown in bold.

GHB = general-head boundary

CSG = coal seam gas

NA = data not available

↓: inflow; ↑: outflow; ↕: inflow and outflow

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4)

Last updated:
11 July 2017