2.6.2.6.2 Summary of parameters in the groundwater model

Eighty-one parameters were used in the groundwater model. After the stress test described in Section 2.6.2.7.3.1 some of the parameters were identified to have an insignificant influence on the model predictions. These parameters were then either fixed or tied to the independent parameters leaving a total of 37 parameters that varied in the sensitivity and uncertainty analyses. These parameters can be broadly grouped by model function into parameters relating to:

land-surface fluxes: three fixed parameters for defining evapotranspiration processes (see Section 2.6.2.4.2); one parameter each for the diffuse, irrigation and flood recharge used as multipliers vary the recharge input to explore uncertainty in these components of recharge
general-head boundary behaviour: two fixed parameters to explore variability in the head and conductance of all lateral boundaries
surface water – groundwater fluxes: five parameters that define the boundary conditions for the movement of water between groundwater and the river. River-stage height varies with riverbed depth. Two parameters that limit the minimum and maximum hydraulic conductivity of the riverbed and two parameters that define the slope of the riverbed in any model cell are used to compute the conductance of the riverbed in all river cells (see Section 2.6.2.4.3)
hydraulic properties: parameters to define horizontal and vertical hydraulic conductivities and storage that varies with depth for each model layer (Section 2.6.2.6.1)
hydraulic enhancement: two parameters to characterise the magnitude of and depth over which hydraulic conductivity changes occur due to longwall mining (see Section 2.6.2.5.3)
drains: three parameters control the conductance of the open-cut mines, longwall mines and coal seam gas wells.

Table 10 summarises the groundwater model parameters, including the minimum and maximum values of the range 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 storage values explored in the sensitivity analysis and its comparison with measured data is shown in Figure 19 (which is Figure 21 in companion product 2.1-2.2 for the Namoi subregion (Aryal 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 19, which motivates the uncertainty bounds in Table 10.

Conductivity enhancement above and below mines is discussed in Section 2.6.2.5.3, and the wide range of variation (four 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).

Table 10 Groundwater model parameters: minimum and maximum values used in the uncertainty analysis

Number	Package	Name	Description	Note	Unit	Minimum	Maximum
1	RCH	Scale_r_fl	scaler on flood recharge	log	-	0.01	0.1
2	RCH	Scale_r_ir	scaler on irrigation recharge	log	-	0.01	1
3	RCH	Scale_r_dr	scaler on diffuse recharge	linear	-	0.5	2
4	RIV	k_min	minimum conductivity of river bed	fixed	m/d	1	1
5	RIV	k_max	maximum conductivity of river bed	fixed	m/d	3	3
6	RIV	m_min	minimum depth of river bed	fixed	m	0.5	0.5
7	RIV	m_max	maximum depth of river bed	fixed	m	5	5
8	RIV	dh	depth of river incision below topography	linear	m	2	15
9	GHB	Scale_ghb_h	scaler on general head bounday head	fixed	-	1	1
10	GHB	Scale_ghb_cond	scaler on general head boundary conductance	fixed	-	1	1
11	EVT	sc_PET	scaler on PET	fixed	-	1	1
12	EVT	minrd	minimum rooting depth	fixed	m	1	1
13	EVT	rd_vh	fraction of vegetation height added to minimum rooting depth	fixed	-	0.2	0.2
14	LPF	al1_kh	alluvium layer 1 horizontal conductivity	log	m/d	0.1	3
15	LPF	al1_ka	alluvium layer 1 horizontal conductivity decay constant	fixed	/m	0	0
16	LPF	al1_we	alluvium weathered zone parameter	fixed	-	0	0
17	LPF	al1_kv	alluvium layer 1 vertical conductivity scaler	linear	-	0.5	1.5
18	LPF	al1_S0	alluvium layer 1 Storage	log	-	0.0001	0.01
19	LPF	al1_Sa	alluvium layer 1 storage decay	fixed	/m	0	0
20	LPF	al1_SY	alluvium specific yield	log	-	0.05	0.3
21	LPF	al2_kh	alluvium layer 2 horizontal conductivity	log	m/d	1	10
22	LPF	al2_ka	alluvium 2 horizontal conductivity decay constant	fixed	/m	0	0
23	LPF	al2_we	alluvium 2 weathered zone paramter	fixed	-	0	0
24	LPF	al2_kv	alluvium layer 2 vertical conductivity	log	-	0.001	0.1
25	LPF	al2_S0	alluvium layer 2 storage	log	-	0.0001	0.01
26	LPF	al2_Sa	alluvium 2 storage decay constant	fixed	/m	0	0
27	LPF	al2_SY	alluvium layer 2 specific yield	log	-	0.05	0.3
28	LPF	IB1_k0	interburden 1 horizontal conductivity at surface	log	m/d	0.0001	0.01
29	LPF	IB1_ka	interburden 1 horizontal conductivity decay constant	linear	/m	0.003	0.01
30	LPF	IB1_we	interburden 1 weathered zone	linear	-	1	3
31	LPF	IB1_kv	interburden 1 ratio of kv to kh	log	-	0.001	0.1
32	LPF	IB1_S0	interburden 1 storage at surface	log	-	0.000001	0.0001
33	LPF	IB1_sa	interburden 1 storage decay constant	linear	/m	0	0.03
34	LPF	IB1_SY	interburden 1 specific yield	log	-	0.0005	0.05
35	LPF	pil_k0	Pilliga horizontal conductivity at surface	log	m/d	0.001	1
36	LPF	pil_ka	Pilliga horizontal conductivity decay constant	linear	/m	0.003	0.01
37	LPF	pil_we	Pilliga 1 weathered zone	linear	-	1	3
38	LPF	pil_kv	Pilliga ratio of kv to kh	log	-	0.001	0.1
39	LPF	pil_S0	Pilliga storage at surface	log	-	0.000001	0.0001
40	LPF	pil_sa	Pilliga storage decay constant	fixed	/m	0	0
41	LPF	pil_SY	Pilliga specific yield	log	-	0.001	0.1
42	LPF	IB2_k0	interburden 2 horizontal conductivity at surface	tied IB1_k0	m/d	0.0001	0.01
43	LPF	IB2_ka	interburden 2 horizontal conductivity decay constant	tied IB1_ka	/m	0.003	0.01
44	LPF	IB2_we	interburden 2 weathered zone parameter	tied IB1_we	-	1	3
45	LPF	IB2_kv	interburden 2 ratio of kv to kh	tied IB1_kv	-	0.001	0.1
46	LPF	IB2_S0	interburden 2 storage at surface	tied IB1_S0	-	0.000001	0.0001
47	LPF	IB2_sa	interburden 2 storage decay constant	tied IB1_sa	/m	0	0.03
48	LPF	IB2_SY	interburden 2 specific yield	tied IB1_SY	-	0.0005	0.05
49	LPF	hos_k0	Hoskissons horizontal conductivity at surface	log	m/d	0.001	0.1
50	LPF	hos_ka	Hoskissons horizontal conductivity decay constant	linear	/m	0.003	0.01
51	LPF	hos_we	Hoskissons weathered zone parameter	linear	-	1	3
52	LPF	hos_kv	Hoskissons ratio of kv to kh	log	-	0.01	1
53	LPF	hos_S0	Hoskissons storage at surface	log	-	0.000001	0.0001
54	LPF	hos_sa	Hoskissons storage decay constant	linear	/m	0	0.03
55	LPF	hos_SY	Hoskissons specific yield	log	-	0.001	0.1
56	LPF	IB3_k0	interburden 3 horizontal conductivity at surface	tied IB1_k0	m/d	0.0001	0.01
57	LPF	IB3_ka	interburden 3 horizontal conductivity decay constant	tied IB1_ka	/m	0.003	0.01
58	LPF	IB3_we	interburden 3 weathered zone parameter	tied IB1_we	-	1	3
59	LPF	IB3_kv	interburden 3 ratio of kv to kh	tied IB1_kv	-	0.001	0.1
60	LPF	IB3_S0	interburden 3 storage at surface	tied IB1_S0	-	0.000001	0.0001
61	LPF	IB3_sa	interburden 3 storage decay constant	tied IB1_sa	/m	0	0.03
62	LPF	IB3_SY	interburden 3 specific yield	tied IB1_SY	-	0.0005	0.05
63	LPF	mau_k0	Maules ck horizontal conductivity at surface	tied hos_k0	m/d	0.001	0.1
64	LPF	mau_ka	Maules ck horizontal conductivity decay constant	tied hos_ka	/m	0.003	0.01
65	LPF	mau_we	Maules creek weathered zone parameter	tied hos_we	-	1	3
66	LPF	mau_kv	Maules ck ratio of kv to kh	tied hos_kv	-	0.01	1
67	LPF	mau_S0	Maules ck storage at surface	tied hos_S0	-	0.000001	0.0001
68	LPF	mau_sa	Maules ck storage decay constant	tied hos_sa	/m	0	0.03
69	LPF	mau_SY	Maules creek specific yield	tied hos_SY	-	0.001	0.1
70	LPF	base_k0	basement horizontal conductivity at surface	tied IB1_k0	m/d	0.0001	0.01
71	LPF	base_ka	basement horizontal conductivity decay constant	tied IB1_ka	/m	0.003	0.01
72	LPF	base_we	basement weathered zone parameter	tied IB1_we	-	1	3
73	LPF	base_kv	basement ratio of kv to kh	tied IB1_kv	-	0.001	0.1
74	LPF	base_S0	basement storage at surface	tied IB1_S0	-	0.000001	0.0001
75	LPF	base_sa	basement storage decay constant	tied IB1_sa	/m	0	0.03
76	LPF	base_SY	basement specific yield	tied IB1_SY	-	0.0005	0.05
77	TVM	K_ramp_up	ramp function maximum impacted distance up	linear	m	100	500
78	TVM	max_dk_up	ramp function maximum change in k in up direction (orders of magnitude)	linear	-	0	4
79	DRN	Cond_mine_OC	scaler for open cut drain conductance	log	-	6,000	100,000
80	DRN	Cond_mine_LW	scaler for longwall mine conductance	log	-	2,000	25,000
81	DRN	Cond_CSG	scaler for CSG drain conductance	log	-	1,000	15,000

Package refers to the MODFLOW package that the parameter belongs to, these are: recharge (RCH), river (RIV), general-head boundary (GHB), evapotranspiration (EVT), layer properties flow (LPF), time varying materials (TVM) and drain (DRN). Note refers to the treatment of the parameter, linear is uniformly distributed between minimum and maximum, log is log transformed and then fitted uniformly between minimum and maximum, fixed means the parameter does not vary between model runs and tied means that the parameter is made equal to another parameter in the list.

PET = potential evapotranspiration; ET = evapotranspiration; GHB = general-head boundary; SW = surface water; GW = groundwater; CSG = coal seam gas

Last updated:

6 December 2018

2.6.2.6.2 Summary of parameters in the groundwater model

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