2.6.1.6.4 Defining thresholds of hydrological change


The consequences of the changes to streamflow characteristics described in this product on landscape classes and water-dependent assets are considered in companion product 3-4 for the Hunter subregion (as listed in Table 2). In order to rule out water-dependent landscape classes and assets that are unlikely to be impacted by changes in surface water hydrology, it is necessary to define the magnitude of change in hydrology below which reaches of the stream network are assumed to experience no significant hydrological change due to the additional coal resource development. A threshold has been defined conservatively for each of the nine hydrological response variables in Section 8.1.4 of Viney (2016). For:

  • the flux-based hydrological response variables AF, P99 and IQR, this is a 5% or greater chance of a 1% or greater change in the variable (i.e. if at least 5% of model replicates show a maximum difference between the CRDP and baseline of at least 1% of the baseline value)
  • the flux-based hydrological response variable P01, this is a 5% or greater chance of a 1% or greater change in the variable and the change in runoff depth is greater than 0.0002 mm. Note that the addition of a runoff depth threshold is a departure from Viney (2016) and is designed to exclude reaches where the absolute change in runoff is negligible.
  • the frequency-based metrics FD, LFD, LLFS and ZFD, this is a greater than 5% chance of there being a change in the variable of at least 3 days in any year
  • the frequency-based metric LFS, this is a greater than 5% chance of there being a change in the variable of at least two spells in any year.

If results from the surface water modelling indicate that for all nine variables at a model node there is a less than a 5% probability the hydrological changes will exceed the thresholds, then landscape classes and assets that depend on streamflow at that location are very unlikely to be impacted due to the additional coal resource development.

Streams, predicted to experience changes that exceed these thresholds, will not necessarily be adversely impacted by these changes. Rather they are retained in the group of ‘potentially impacted’ streams for which more local information and analysis are needed to assess the implications of the changes on ecological, economic and sociocultural values. Thus these thresholds form the basis for defining the zone of potential hydrological change (see companion product 3-4), within which the potential for impacts cannot be ruled out.

Table 11 summarises for each surface water modelling node in the Hunter subregion whether the hydrological change due to the additional coal resource development exceeds the threshold for each hydrological response variable. At nodes 2–5, 15, 19, 23–24, 33–34, 39, 57–63, there are no significant hydrological changes due to the additional coal resource development; at nodes 7–9, 11, 26–27, 29, 35, 52 and 55, changes in all nine hydrological response variables exceed their respective thresholds; at all other nodes, there are above-threshold changes in some hydrological response variables, but not others. The last row in Table 11 gives the number of nodes for which the hydrological response variable was modelled to exceed its specified threshold. The majority of nodes (46) experience changes in three of the low-streamflow hydrological response variables (LFD, LFS and LLFS) and in the IQR hydrological response variable (43); less than a third exceed the specified threshold for the ZFD hydrological response variable; about half (33) exceed the specified threshold for AF.

Table 11 Change in hydrological response variable (column) relative to its threshold at each model node (row) due to additional coal resource development


AF

P99

IQR

FD

P01

LFD

LFS

LLFS

ZFD

1

ET

ET

ET

ET

ET

ET

ET

ET

2

3

4

5

6

ET

ET

ET

ET

ET

ET

ET

ET

7

ET

ET

ET

ET

ET

ET

ET

ET

ET

8

ET

ET

ET

ET

ET

ET

ET

ET

ET

9

ET

ET

ET

ET

ET

ET

ET

ET

ET

10

ET

ET

ET

ET

ET

ET

ET

11

ET

ET

ET

ET

ET

ET

ET

ET

ET

12

ET

ET

ET

ET

ET

ET

ET

ET

13

ET

ET

ET

ET

ET

14

ET

ET

ET

ET

ET

ET

ET

ET

15

16

ET

ET

ET

ET

ET

ET

ET

ET

17

ET

ET

ET

ET

ET

ET

ET

ET

18

ET

ET

ET

ET

19

20

ET

ET

ET

ET

ET

ET

ET

21

ET

ET

ET

ET

ET

ET

ET

22

ET

ET

ET

ET

ET

ET

ET

ET

23

24

25

ET

ET

ET

ET

­–

ET

ET

ET

26

ET

ET

ET

ET

ET

ET

ET

ET

ET

27

ET

ET

ET

ET

ET

ET

ET

ET

ET

28

ET

ET

ET

ET

ET

ET

ET

29

ET

ET

ET

ET

ET

ET

ET

ET

ET

30

ET

ET

ET

ET

ET

ET

ET

ET

31

ET

ET

ET

ET

ET

ET

ET

32

ET

ET

ET

ET

ET

ET

33

34

35

ET

ET

ET

ET

ET

ET

ET

ET

ET

36

ET

ET

ET

ET

ET

ET

37

ET

38

ET

ET

ET

ET

ET

ET

ET

ET

39

40

ET

ET

ET

ET

ET

ET

ET

41

ET

ET

ET

ET

ET

ET

ET

ET

42

ET

ET

ET

ET

ET

ET

ET

ET

43

ET

ET

ET

ET

ET

ET

ET

ET

44

ET

ET

ET

ET

ET

ET

ET

ET

45

ET

ET

ET

ET

ET

ET

ET

ET

46

ET

ET

ET

ET

ET

ET

ET

ET

47

ET

ET

ET

ET

ET

ET

ET

ET

48

ET

ET

ET

49

ET

ET

ET

ET

ET

ET

50

ET

ET

ET

ET

ET

51

ET

ET

ET

ET

ET

ET

ET

ET

52

ET

ET

ET

ET

ET

ET

ET

ET

ET

53

ET

ET

ET

ET

ET

ET

54

ET

ET

ET

ET

ET

55

ET

ET

ET

ET

ET

ET

ET

ET

ET

56

ET

ET

ET

ET

ET

57

58

59

60

61

62

63

64

ET

ET

ET

ET

ET

ET

ET

ET

65

ET

ET

ET

ET

ET

ET

ET

ET

Count

33

36

43

37

35

46

46

46

21

ET = exceeds threshold; – indicates not significant (see Viney (2016) and start of this section for definitions)

In Section 2.1.4.2.5 of companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018a), the node to link mapping for the modelled Hunter River and Wyong River networks is defined. This mapping informs the extrapolation of results from model nodes to some length of reach upstream and downstream of the node, as appropriate to do so. The information in Table 11 and the node-link mapping in Section 2.1.4 of Herron et al. (2018a) have been used to identify the reaches of the Hunter blue line river network (Dataset 3) that have modelled hydrological changes from additional coal resource development. Figure 28 shows reaches predicted to experience a change in at least one hydrological response variable above its specified threshold due to additional coal resource development.

For some reaches (e.g. node 18 to node 19; node 55 to node 59), the change from an above threshold hydrological change to a non-significant hydrological change occurs somewhere between the two nodes. These reaches are shown as dashed pink lines and other information is needed to determine where to delineate the point of change. Similarly, upstream of the pink headwater model nodes in Figure 28 (i.e. those showing with a change exceeding a specified hydrological threshold), there will be some length of stream that is also potentially affected by the additional coal resource development. To define the zone of potential hydrological change for the impact and risk analysis – that is, the area outside of which it is very unlikely that landscape classes and assets will be impacted – we need to determine the upstream extents of the stream network likely to experience a hydrological change exceeding at least one specified threshold. This final step is reported in companion product 3-4 (impact and risk analysis) for the Hunter subregion (as listed in Table 2), where drawdown results from the groundwater modelling and mine footprint data are used to identify stream reaches that are not explicit in the surface water model node-link network and where hydrological changes from additional coal resource development could impact water-dependent landscapes and assets.

What these potential changes in hydrology from additional coal resource development might mean for Hunter subregion landscape classes and assets are covered in companion products 2.7 (receptor impact modelling) and 3-4 (impact and risk analysis) for the Hunter subregion (as listed in Table 2).

Figure 28

Figure 28 Model nodes and links with changes in at least one hydrological response variable due to additional coal resource development that exceed specified thresholds

ACRD = additional coal resource development; AWRA-R = Australian Water Resources Assessment river model

Data: Bioregional Assessment Programme (Dataset 2); Bureau of Meteorology (Dataset 3)

Last updated:
15 June 2018
Thumbnail of the Hunter subregion

Product Finalisation date

2018

ASSESSMENT