3.3.1 Defining the zone of potential hydrological change


The zone of potential hydrological change is the area within the subregion where changes in hydrology due to additional coal resource development exceed defined thresholds for groundwater and surface water hydrological response variables. The impact and risk analysis presented in Section 3.4 and Section 3.5 focuses on landscape classes and assets that intersect this zone. Any landscape class or asset wholly outside of the zone of potential hydrological change is considered very unlikely (less than 5% chance) to be impacted due to additional coal resource development.

The zone of potential hydrological change is defined as the union of the groundwater zone of potential hydrological change (Section 3.3.1.1) and the surface water zone of potential hydrological change (Section 3.3.1.2). It is presented in Section 3.3.2.

3.3.1.1 Groundwater

The groundwater zone of potential hydrological change is defined as the area around the coal resource development that has a greater than 5% chance of exceeding 0.2 m drawdown in the regional water table due to additional coal resource development. This 5% chance is determined based on the uncertainty analysis, described in Section 2.6.2.8 of companion product 2.6.2 for the Namoi subregion (Janardhanan et al., 2018). It means that, for each individual model cell within the groundwater zone, at least 5% (95th percentile) of groundwater model predictions exceeded 0.2 m of drawdown.

The deeper groundwater layers could be used as economic assets by extraction bores or ecological assets as the source water for springs. The drawdown in the confined parts of the Pilliga Sandstone does not exceed 0.2 m more than 2 km outside of the zone of potential hydrological change. There are no extraction bores or springs within this area so defining the zone based upon the drawdown at the regional watertable is appropriate.

The groundwater use and management for coal resource developments are regulated under state legislation and regulatory frameworks. The 0.2 m drawdown threshold adopted in bioregional assessments (BAs) is consistent with the most conservative minimal impact threshold in the NSW Aquifer Interference Policy (NSW Office of Water, 2012).

The 95th percentile of groundwater drawdown under the baseline and the CRDP are shown in Figure 9. The extent of this drawdown under the baseline is 407 km2 (1.1% of the assessment extent). This increases to 2590 km2 (7.3% of assessment extent) under the CRDP, which represents the combined extent of drawdown under baseline and due to additional coal resource development.

It is the area where the drawdown due to the additional coal resource development has at least a 5% chance of exceeding 0.2 m that forms the basis of the groundwater zone of potential hydrological change (Figure 10). The groundwater zone of potential hydrological change coincides approximately with a 10 km buffer around the mine footprints except in the Pilliga area. The combined effect of Narrabri mine and the Narrabri Gas Project results in an extensive area with a probability of greater than 5% of exceeding 0.2 m drawdown due to additional coal resource development. The groundwater zone of potential hydrological change covers an area of 2299 km2, or 6.1% of the assessment extent.

Figure 9

Figure 9 95th percentile of drawdown under (a) the baseline and (b) the coal resource development pathway

The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 1)

Figure 10

Figure 10 Groundwater zone of potential hydrological change

The groundwater zone of potential hydrological change is defined as a 5% chance of exceeding 0.2 m drawdown due to additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 1)

3.3.1.2 Surface water

The threshold hydrological change adopted for each hydrological response variable for defining the zone of potential hydrological change is given in Table 4 (from companion submethodology M06 for surface water modelling (Viney, 2016)). Together, these nine hydrological response variables represent potential changes across the full flow regime. The low flows are represented by the daily flow rate at the 1st percentile (P01), zero-flow days (ZFD), low-flow days (LFD), low-flow spells (LFS) and length of longest low-flow spell (LLFS), while the high flows are represented by the daily flow rate at the 99th percentile (P99) and high-flow days (FD). Two remaining hydrological response variables represent changes in flow volume (AF) and interquartile variability (IQR). A location on the river is deemed to be in the zone if the change in at least one of the nine variables exceeds the given threshold. Probability estimates are derived from the predictions of 300 model replicates, each of which uses a unique set of model parameter values. A 5% significance threshold implies that at least 15 of the 300 replicates have modelled changes that exceed the relevant change threshold. If fewer than 15 replicates have modelled changes that exceed the threshold at a particular location, then the change in that hydrological response variable at that location is considered very unlikely to impact water-dependent landscape classes and assets. Table 12 and Figure 27 in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018) identify the model nodes and links in the river modelling network where the hydrological change exceeds these thresholds.

Table 4 Surface water hydrological response variables and the thresholds used in defining the zone of potential hydrological change


Hydrological response variable

Units

Description

Threshold

AF

GL/year

Annual flow volume. The volume of water that discharges past a specific point in a stream in a year, commonly measured in GL/year. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102).

≥5% chance of ≥1% change in AF

P99

ML/day

Daily flow rate at the 99th percentile (ML/day). This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102).

≥5% chance of ≥1% change in P99

IQR

ML/day

Interquartile range in daily flow (ML/day); that is, the difference between the daily flow rate at the 75th percentile and at the 25th percentile. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102).

≥5% chance of ≥1% change in IQR

FD

days/year

Number of high-flow days per year. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102). The threshold for high-flow days is the 90th percentile from the simulated 90-year period. In some early products, this was referred to as ‘flood days’.

≥5% chance of a change in FD ≥3 days in any year

P01

ML/day

Daily flow rate at the 1st percentile (ML/day). This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102).

≥5% chance of ≥1% change in P01 and change in runoff depth >0.0002 mm

ZFD

days/year

Number of zero-flow days per year. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102).

≥5% chance of a change in ZFD ≥3 days in any year

LFD

days/year

Number of low-flow days per year. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102). The threshold for low-flow days is the 10th percentile from the simulated 90-year period.

≥5% chance of a change in LFD ≥3 days in any year

LFS

number/year

Number of low-flow spells per year. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102). A spell is defined as a period of contiguous days of flow below the 10th percentile threshold.

≥5% chance of a change in LFS ≥2 spells in any year

LLFS

days/year

Length of the longest low-flow spell each year. This is typically reported as the maximum change due to additional coal resource development over the 90-year period (from 2013 to 2102).

≥5% chance of a change in LLFS ≥3 days in any year

The surface water zone of potential hydrological change includes reaches that make up the AWRA‑R link-node network (see Figure 5 in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018)); however, the zone also includes reaches that were not modelled, but that could potentially be impacted due to additional coal resource development. They include perennial and temporary streams within the groundwater zone of potential hydrological change. It is assumed that within the groundwater zone of potential hydrological change, streams connected to regional groundwater could potentially be affected by additional coal resource development. Streams tagged as ‘perennial’ or ‘temporary’ in the modelled flow regime spatial layer for the Namoi subregion (Bioregional Assessment Programme, Dataset 3) are assumed to be connected to groundwater because the data do not allow separation of ephemeral streams (i.e. no groundwater connection) from intermittent ones (i.e. connected to groundwater). The temporary stream reaches in the modelled flow regime that intersect the surface water maximum footprint areas for open-cut mines for additional coal resource development (Bioregional Assessment Programme, Dataset 4) are included in this zone, as disruption to surface water drainage on coal mining sites can potentially affect them.

In the Namoi subregion, except for the Namoi River, all affected streams are temporary and assumed to have a varying degree of intermittency. These streams, when they pass through the groundwater zone of potential hydrological change, were extended further downstream of the zone to where they join a reach already in the surface water zone of potential hydrological change, and added to the network of potentially impacted streams. In all, about 5521 km of streams were identified as potentially impacted. These 5521 km of potentially impacted streams were used to select the 1 km x 1 km assessment units (Bioregional Assessment Programme, Dataset 5) that intersect the stream network (Bureau of Meteorology, Dataset 3) or contain surface water – dependent ecosystems (Bioregional Assessment Programme, Dataset 6), to define the surface water zone of potential hydrological change. These ecosystems include floodplain riparian forest/wetlands or floodplain riparian forest/wetland groundwater-dependent ecosystems (GDEs) within 1 km of the stream network. This selection was inspected and adjusted using a precautionary principle to ensure that the riparian vegetation that could potentially be impacted by changes in surface water hydrology is included in the zone. The surface water zone of potential hydrological change (Figure 11) shows the mine footprints that formed part of the surface water modelling.

The surface water zone of potential hydrological change covers an area of 6430 km2 (about 22% of the assessment extent) and includes all streams intersecting the groundwater zone of potential hydrological change. It includes the whole of Namoi River downstream of the confluence with the Peel River, all of the Namoi River anabranches including Pian and Gunidgera creeks, Mooki River, the Back Creek, Merrygowen Creek, Bollol Creek, Driggle Draggle Creek and other smaller tributaries of the Namoi River. An unnamed creek that flows into Lake Goran is also part of the surface water zone of potential hydrological change.

Figure 11

Figure 11 Surface water zone of potential hydrological change

The extent of the coal resource developments in the coal resource development pathway (CRDP) is the union of the extents in the baseline and in the additional coal resource development (ACRD).

Data: Bureau of Meteorology (Dataset 3); Bioregional Assessment Programme (Dataset 5)

Last updated:
6 December 2018
Thumbnail of the Namoi subregion

Product Finalisation date

2018
PRODUCT CONTENTS

ASSESSMENT