Extreme Ranges of Groundwater Level
Vince Bidwell
Lincoln Ventures Ltd
2008 NZHS & MSNZ Joint Annual Conference
Examples of groundwater level range
- observations (ECan database)
0
L36/0092 Range = 24.8 metres (1952-90)
-10
L35/0163 Range = 38.1 metres (1952-90)
Depth below ground (m)
-20
-30
-40
-50
-60
-70
-80
-90
52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04 06 08
Examples of groundwater level range - locations
L35/0163
L36/0092
What causes natural groundwater level variation?
• Primarily caused by time variation of recharge from soil-water
drainage (land surface recharge)
• Fluctuations in river recharge are damped to a relatively steady level
within a few kilometres of the river
• Big rivers are from West Coast climate
• Recharge through the land surface (Canterbury Plains climate)
causes most of the natural variation in groundwater level
Smoothed series of monthly land surface recharge
(data from D. Scott, ECan)
60
0
L36/0092
-10
L35/0163
LSR EWMA T = 20 months
50
40
-30
-40
30
-50
20
-60
-70
10
-80
-90
0
60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 04 06 08
LSR (mm/mth)
Depth below ground (m)
-20
Layout of a 1D analytical groundwater model
(Eigenmodel)
No-flow
L35/0163
16 km
L36/0092
24 km
Discharge
Groundwater model parameters
For calibration:
• Steady recharge contribution (from rivers) R
• Bulk value of aquifer storativity S
• Bulk value of aquifer transmissivity T
Preset values
• Length of aquifer (landscape drainage scale) L = 40 km
• Location of well L36/0092 from upstream boundary x = 16 km
• Vadose zone transit time Tv = 5 mths
Calibration and prediction results
100
Observed
95
Calibration
Prediction
Piezometric level (m amsl)
90
85
80
75
70
65
60
55
50
70
72
74
76
78
80
82
84
86
88
90
92
94
96
98
00
02
04
06
08
Calibrated parameter values
• Steady recharge, averages over length of the aquifer:
R = 936 mm/y (distributed over length of aquifer)
or
R = 660 mm/y (at upstream boundary)
• Aquifer storativity:
S = 0.022
• Aquifer transmissivity:
T = 30,000 m2/d
Limits on range of groundwater level
• At this site, ground level is at 119 m (amsl)
• Piezometric effect of river recharge is 58 m (amsl)
• Groundwater would have reached ground level if:
Transmissivity T decreased by 25%
Aquifer length L increased by 15%
(Storativity S has much lower sensitivity)
• Resulting range about 35 m, compared with observed 25 m
Discussion about geomorphology
• Groundwater “sapping” is an erosive factor in the formation of
landscapes
• Occurrence of groundwater at the land surface (for erosion) is
determined by a combination of:
climatically-driven recharge
aquifer properties T, S
landscape drainage scale L
• Landscape drainage scale L varies with sea level changes and
development of new drainage patterns
The role of climate in range of groundwater levels
• Geomorphological processes limit the possible extreme maximum
and minimum groundwater levels
• Recharge from soil-water drainage is the dominant cause of natural
variability in groundwater level
• The actual range of groundwater levels depends on the variability of
the hydrological climate that generates soil-water drainage
• Does Central Canterbury have a particularly variable hydrological
climate?
Comparison of climatic variation: specific yield
EWMA (1972-2006 ave.) T = 36 mths
160
Wairoa River flow EWMA (ave) 31 mths
140
Motueka River flow EWMA (ave.) 34 mths
Specific yield (mm/mth)
Canterbury Plains LSR EWMA (ave.) T = 36 mths
120
100
80
60
40
20
0
1972
1976
1980
1984
1988
1992
1996
2000
2004
2008
Comparison of climatic variation: percent of mean
EWMA (1972-2006 ave.) T = 36 mths
200
Wairoa River flow EWMA (ave) T = 31 mths
Percentage of 1972 - 2006 mean
180
Motueka River flow EWMA (ave.) T = 34 mths
160
Canterbury Plains LSR EWMA (ave.) T = 36 mths
140
120
100
80
60
40
20
0
1972
1976
1980
1984
1988
1992
1996
2000
2004
2008
Conclusions
• Groundwater is an agent in the formation of landscapes
• Extremes of groundwater level depend on the interaction between
landscape and aquifer properties
• Observed range of groundwater levels, within these extremes,
depends on the variability of the hydrological climate
• The hydrological climate of Central Canterbury is more variable in
comparison with other more humid climates in NZ
• The particular combination of climate, landscape, and aquifer
properties in Central Canterbury causes unusually large variations in
groundwater levels