2.6.2.6.3 Summary of parameters in the groundwater model


There are 22 parameters in the groundwater model. They can be broadly grouped by model function into parameters relating to:

  • Land-surface fluxes: two fixed parameters for defining evapotranspiration processes (see Section 2.6.2.4.2); there is also a recharge multiplier used in the uncertainty analysis to vary the recharge input.
  • General-head boundary behaviour: one fixed parameter that is the conductance of all lateral boundaries (except the boundary to the Werrie Basin) and the ocean floor.
  • Surface water – groundwater fluxes: four parameters that define the boundary conditions for the movement of water from groundwater to the river. River stage height varies with riverbed depth. The two leakage limiter parameters are fixed in the model (see Section 2.6.2.4.3).
  • Hydraulic properties: nine parameters to define porosities and vertical and horizontal hydraulic conductivities with depth for the interburden (lithologies 1 to 6) and the alluvium (lithology 7) (Section 2.6.2.6.1).
  • Unsaturated flow: two fixed parameters in the van Genuchten unsaturated flow equation (Section 2.6.2.6.2).
  • Hydraulic enhancement: four parameters to characterise the magnitude of and depth over which hydraulic conductivity changes occur due to longwall mining (see Section 2.6.2.6.3).

Table 7 summarises the groundwater model parameters, including the reference values, ranges over which parameters are varied in the uncertainty analysis (see Section 2.6.2.8) and salient points. As identified above, a number of these parameters are dealt with in other sections of this product.

The range of conductivity and porosity values explored in the uncertainty analysis and its comparison with measured data is shown in Figure 26 and Figure 31 of companion product 2.1-2.2 for the Hunter subregion (Herron et al., 2018). As mentioned above, an upscaling analysis may be performed to yield a probability distribution for hydraulic conductivity, and the result of such an analysis is shown in Figure 21, which motivates the uncertainty bounds in Table 7. Figure 21 shows the probability distribution for the measured data in the depth interval 0 to 100 m. In this interval, conductivity measurements vary between 10–7 m/day and 30 m/day. The data have been binned into nine bins: the first lies between 10–7 m/day and 10–6 m/day, the second between 10–6 m/day and 10–5 m/day, and so on up to between 101 m/day and 102 m/day. Figure 21 shows that the data are roughly uniformly distributed into these bins, with slightly more likelihood of measurements occurring in the central bins than in the outer bins. Figure 21 also contains a probability distribution for the upscaled conductivity that is derived from the measured data, and its comparison with the uncertainty bounds for a 50 m depth from Table 7. Upscaling is discussed further in Renard and de Marsily (1997).

Conductivity enhancement above and below mines is discussed in Section 2.6.2.5.3, and the wide range of variation (5 orders of magnitude, and heights ranging between 100 m and 500 m above longwall workings) reflects the wide variation that may be experienced in different mining scenarios (Adhikary and Wilkins, 2012; Guo et al., 2014).

Figure 21

Figure 21 Scaled probability distributions for measured conductivity data in the depth interval 0 to 100 m, and the results of upscaling those data


Table 7 Groundwater model parameters: their reference values and the minimum and maximum values used in the uncertainty analysis


Process

Parameter

Units

Reference Value

Min

Max

Notes

Land-surface fluxes

ET extinction depth (d)

m

d = V/4

na

na

Fixed parameter.

Depth below surface at which ET is assumed to cease. Calculated as a function of vegetation height, V. Throughout the Hunter subregion this varies between 0 m and 10 m.

Watertable depth threshold for PET

m

–2

na

na

Fixed parameter.

Watertable depth above which ET is approximated by PET.

Recharge multiplier

na

1

0.5

1.5

Rainfall recharge to the groundwater system is multiplied by this quantity.

Outer boundary

General head conductance

ML/y/m3

10–5

na

na

General-head conditions are applied at the lateral boundaries (excepting the boundary to the Werrie Basin) and the ocean floor.

SW-GW fluxes

Riverbed conductance (C)

ML/m/y

320

32

3200

The riverbed conductance for points representing a 1 km section of river.

Riverbed depth

m

5

0

10

This parameter may also be viewed as shifting the stage height of the rivers.

River stage height (h0)

m

3

na

na

Defaults to 3 m, but varies with riverbed depth

Leakage limiter (T)

m

P: –1

E: 0

na

na

Fixed parameters.

P = perennial: leakage does not increase when groundwater head is <1 m below river stage height

E = ephemeral; T = 0 means no flow from river.

Hydraulic properties

Reference porosity – interburden (Φ1–6)

m3/m3

0.1

0.03

0.3

After multiplying by the decay parameter, porosity is constrained to always be greater than 0.0001 to ensure good convergence of the numerical model.

Reference porosity – alluvium(Φ7)

m3/m3

0.2

0.06

0.6

After multiplying by the decay parameter, porosity is constrained to always be greater than 0.0001 to ensure good convergence of the numerical model.

Decay parameter for porosity (ap)

na

0.01

0.005

0.015

An exponential decay function is used to vary porosity with depth.

Horizontal conductivity – interburden (Kh1-6)

m/day

0.5

0.05

5

After multiplying by the decay parameter and applying mining-induced changes, an upper bound of 100 m/day and a lower bound of 10–6 m/day is placed on all conductivities.

Horizontal conductivity – alluvium (Kh7)

m/day

1.0

0.1

10

Vertical conductivity – interburden (Kv1–6)

m/day

0.05

0.005

0.5

Vertical conductivity – alluvium (Kv7)

m/day

1.0

0.1

10

Decay parameter for Kh and Kv (ah and av)

na

0.025

0.01

0.04

An exponential decay function is used to vary hydraulic conductivity with depth.

Kv/Kh

na

0.1

0.01

1

Ratio of vertical to horizontal conductivity

Unsaturated flow

Capillary suction index

na

0.4

na

na

Fixed parameter.

van Genuchten capillary suction index parameter for rocks and soil

Inverse head

/m

0.1

na

na

Fixed parameter.

van Genuchten inverse-head parameter

Hydraulic enhancement

Hydraulic conductivity multiplier above seam (M)

na

LW: 9

BP: 2

1.8

0.4

9

2

Order of magnitude increase in hydraulic conductivity.

Bord-and-pillar (BP) mining has a lesser impact than longwall (LW) mining. Hydraulic enhancement occurs below, but not above open-cut (OC) mines.

Hydraulic conductivity multiplier below seam (m)

na

LW: 7

BP: 1

OC: 8

1.4

0.2

1.6

7

1

8

Order of magnitude increase in hydraulic conductivity.

Maximum height above and below the worked seam that hydraulic conductivity changes occur.

Bord-and-pillar (BP) mining has a lesser impact than longwall (LW) mining. Hydraulic enhancement occurs below, but not above open-cut (OC) mines.

Height of enhancement above worked seam (Z)

m

LW: 500

BP: 100

100

20

500

100

Depth of enhancement below worked seam (z)

m

LW: –250

BP: –50

OC: –90

–50

–10

–18

–250

–50

–90

Maximum depth below the worked seam that hydraulic conductivity changes occur.

BP = bord-and-pillar; E = ephemeral; ET = evapotranspiration; GW = groundwater; PET = potential evapotranspiration; P = perennial; LW = longwall; na = not applicable; OC = open-cut; SW = surface water

Last updated:
19 June 2018
Thumbnail of the Hunter subregion

Product Finalisation date

2018
PRODUCT CONTENTS

ASSESSMENT