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a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 2
Hydrogeological assessment of serial biologicalconcentration of salts to manage saline drainage
S. Khan a,b,c,*, A. Abbas c, J. Blackwell a, H.F. Gabriel a,d, A. Ahmad a
aCharles Sturt University (CSU), Wagga Wagga, NSW 2678, AustraliabUNESCO IHP-HELP, Australiac Irrigation Systems, CSIRO Land and Water, Wagga Wagga, NSW 2678, AustraliadNIT, National University of Sciences & Technology (NUST), NUST Risalpur Campus, Risalpur Cantt. 24080, Pakistan
a r t i c l e i n f o
Article history:
Accepted 14 May 2007
Published on line 28 June 2007
Keywords:
Serial biological concentration
Irrigation
Saline drainage
Regional groundwater
Groundwater model
Waterlogging
a b s t r a c t
Serial biological concentration (SBC) of salts is an innovative technology to manage salts in
agricultural drainage. This approach utilises saline drainage water as a resource to produce
marketable crops and, therefore, provides a method to manage salts in a viable manner.
However, there are associated risks of development of a groundwater mound beneath the
treatment facility and the consequent threats of groundwater contamination. The water
table in the shallow aquifers often rises to the ground surface following irrigations and
rainfall events. In the SBC system, the intensive drainage system manages these events and
enables the water table to be lowered rapidly. This paper describes the hydrogeological
assessment of an SBC system to quantify the water table mound and the effect on the local
groundwater. The deep leakage rates and lateral flows to adjoining lands are determined in
order to assess the on-site and regional impacts under typical SBC operation. Modelling
results show that the net water table rise under a 50 ha site, in the first year of the system
operation, is about 1.3 m. However, there is no further water table rise during 25 years of
simulated operation, mainly because of the high drainage efficiency of the tile drainage
operation in the SBC system. The water table under the SBC site reaches quasi equilibrium
with periodic rise and fall around the tile drain depth. The deep leakage beneath the SBC
bays is approximately 1 mm/day which is around 10% of the saturated groundwater flow
above the tile drains. Simulation scenarios of various sizes of the SBC system in its present
hydrogeological settings suggest that the lateral extent of the groundwater mound does not
extend beyond 50 m from the outer edge of its bays. In order to develop SBC systems at other
locations, a GIS-based site suitability assessment model is recommended to evaluate the
SBC effect under different soil and hydrogeological conditions.
# 2007 Elsevier B.V. All rights reserved.
avai lable at www.sc iencedi rec t .com
journal homepage: www.e lsev ier .com/ locate /agwat
1. Introduction
Contaminated surface waters have been treated using a series
of vegetated wetlands where intense biological processing
* Corresponding author at: CSIRO Land and Water, Locked Bag 588, Wfax: +61 2 6933 2647.
E-mail addresses: Shahbaz.Khan@csiro.au (S. Khan), Akhtar.Abbashgabriel@csu.edu.au (H.F. Gabriel), aahmad@csu.edu.au (A. Ahmad).
0378-3774/$ – see front matter # 2007 Elsevier B.V. All rights reservedoi:10.1016/j.agwat.2007.05.011
occurs (Kivaisi, 2001). Although effective, this approach is a
non-production system. One option for managing inevitable
drainage water from irrigation or other contaminated surface
waters is to sequentially use and re-use it to grow increasingly
agga Wagga, NSW 2678, Australia. Tel.: +61 2 6933 2927;
@csiro.au (A. Abbas), jblackwell@csu.edu.au (J. Blackwell),
d.
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 2 65
salt-tolerant crops while concentrating the drainage to a
manageable level (Oron, 1993; Tanji and Karajeh, 1993). This
treatment system is known as ‘‘serial biological concentration
(SBC)’’. SBC systems have emerged as a viable option for using
saline waters for irrigated cropping. The system makes
productive use of waters considered unfit for general irrigation
and avoids the associated risks of saline water irrigation which
often proves non-sustainable environmentally (Jayawardane
et al., 1997; Jayawardane, 2000; Blackwell, 2000). Specifically,
the concept of SBC of salts aims to reduce drainage effluent
volumes from irrigated lands (Bethune et al., 2004). In 2003,
SBC was being applied on some 63,000 ha in the San Joaquin
Valley, California, where it is the basis of an Integrated On-
Farm Drainage Management System (Pratt Water Solutions,
2003). Blackwell (2000) and Jayawardane (2000) provided
design guidelines and the field performance of a land-based
sewage treatment system, Filtration, Land Treatment and
Effluent Reuse (FILTER) in Australia. The SBC system described
and analysed herein was based on the function and operation
of the FILTER system. FILTER is an effective treatment system
producing low nutrient drainage waters which meet Environ-
mental Protection Agency (EPA) criteria for discharge to
surface water bodies (Jayawardane, 2000). A schematic view
of the SBC system based on the FILTER approach is shown in
Fig. 1. With either FILTER or SBC there are associated risks
caused by the required high hydraulic loading which may have
a deleterious effect on the surrounding groundwater quality.
These risks could also bring about a water table rise at local
and sub-catchment level causing waterlogging and saliniza-
tion. This would lead to production losses in surrounding
irrigation lands and other potentially harmful off-site envir-
onmental impacts (Su et al., 2005).
With the developing tension between future global fresh
water needs and diminishing water resources, agricultural
water management should consider alternative sources of
water (green, grey and black) as a replacement for fresh water.
Fig. 1 – Schematic view of possible layout, flow
Technologies such as SBC extract beneficial use from water
previously considered a waste and unfit for irrigation.
Irrigation globally uses 70% of all freshwater withdrawals
and comes under heavy scrutiny in any discussion of fresh-
water governance (UNESCO, 2006). This scrutiny generally
results in fresh water leaving irrigation to satisfy increased
demands for urban, industrial and environmental water. The
effect of this redistribution will be felt severely in regions
already water stressed. About 40% of the world’s population is
experiencing water stress, and about 30 countries are suffering
from water scarcity during a large part of the year (Kivaisi,
2001). Drainage must be an integral aspect of irrigation
schemes but discharge of this drainage and its management
is one of the critical issues facing agriculture today. Such
situations are amenable to a system re-use approach, which
may include the shandying of fresh water with drainage
effluents to increase available water supply for the sustain-
ability of future agricultural systems. However, Rhoades et al.
(1999) warn against this approach because of the potential for
harmful salt build-up in the system. Drainage water should
not be returned to the land drained, rather, it should be used
sequentially on other well managed drained lands until it is
too salty for productive use, the classic SBC approach.
The high hydraulic loading of water applied in an SBC
system to achieve a 33% leaching fraction could affect the
aquifer systems. Water table monitoring and groundwater
modelling are required at the local and sub-regional level to
provide in-depth knowledge and concise understanding of the
hydrogeological behaviour. In particular, the operation of the
SBC site demands an integrated monitoring set-up to help
understand the development of the water table mound and
associated salinization hazards. Gallichard et al. (1997)
described the importance of a monitoring system for ground-
water table control, soil salinity and collector discharge. Gupta
et al. (1997) studied the spatial variability and developed
appropriate monitoring strategies for non-uniform soils.
s and concentrations of the SBC system.
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 266
Benyamini et al. (2005) installed monitoring systems perpen-
dicular to the parallel subsurface drains to study the
mechanisms of salinization and the dynamics of the water
table regime.
This paper describes the hydrogeological assessment of an
SBC trial in the Murrumbidgee Irrigation Area (MIA) in
southeast Australia. Owing to the hydrogeological system of
the MIA, the region has inherent drainage problems that have
been aggravated by intensive irrigation applications leading to
problems of rising water tables and salinization. By 2001, more
than 80% of the MIA had water tables within 2 m from the land
surface (Khan et al., 2001). The average volume of irrigation
water used per irrigation season (270 days) in the MIA is about
1000 GL with an average electrical conductivity (EC) of 0.1 dS/
m. Shallow groundwater salinity varies from less than 2 dS/m
to over 20 dS/m. In the MIA, the salt load diverted from the
Murrumbidgee River at Berembed Weir and delivered to
farmers’ paddocks each year is about 115,000 tonnes. The
estimated average (1999–2003) drainage volume of MIA is
212,340 ML per year (Pratt Water Solutions, 2003) with an
average winter salinity of 1.2 dS/m. These drainage volumes
are well below the long time average mainly because of
drought conditions. This study focuses on understanding the
groundwater dynamics of the SBC system with the shallow
aquifer and its interactions with the deeper groundwater. The
specific objectives are to determine groundwater movements
(both lateral and vertical) using a detailed monitoring network
and to evaluate regional impacts of the SBC system at different
scales using a groundwater model.
2. Hydrogeology of the study area
The study area is situated in the Murrumbidgee Irrigation Area
on the northern side of a fluvial plain formed by the
Murrumbidgee River. The climate of the MIA is characterised
as semi-arid with annual rainfall ranging from 256 to 609 mm
and average annual evaporation of 2100 mm. Average rainfall
gets closer to average evapotranspiration during the winter
months of June and July. The total irrigable area for the MIA is
156,605 ha and the main agricultural products are rice, grapes
and citrus. Rice is the dominant water user with more than
32,000 ha (14% of the total land use) in 2000. Rice growing
Fig. 2 – Experimental set-up to mo
started in the MIA in 1924, although rapid development of rice
areas occurred in the 1970s and 1980s. Rice is grown in bays
using flood irrigation, having the highest applied water of any
crop in the MIA of 15 ML/ha or 1500 mm. Previous studies have
shown groundwater accessions of 1.2 ML/ha under rice and
0.58 ML/ha under horticulture (Neeson et al., 1995).
The topography consists of flat plains with a mean
elevation of less than 200 m above sea level although low
hilly outcrops, extending to 240 m above sea level exist on the
north-eastern flank. The land surface has an average slope of
0.04% in an east-west direction. The geology of the MIA is
described by three major aquifer systems, i.e. Shepparton,
Calivil and Renmark Formations (Brown and Stephenson,
1991).
The Shepparton formations mainly consist of unconsoli-
dated to poorly consolidated, mottled, variegated clays, silty
clays with lenses of polymictic, coarse to fine sand and gravel,
partly modified by pedogenesis.
The Calivil formations are mainly poorly consolidated, pale
grey, poorly sorted coarse granular quartz and conglomerate
with a white kaolinitic matrix.
The Renmark formations overlay the basaltic bedrock and
are distinguished from the Calivil formations by the presence
of grey, carbonaceous sand, coal and lignite.
3. Methodologies
3.1. Experimental set-up
A detailed groundwater monitoring set-up was used to
understand the dynamics of the groundwater mound. The
experimental layout consisted of 11 experimental SBC bays
(totalling 11 ha) and 1 full sized bay of 4 ha as shown in Fig. 2.
The area of each of the 10 experimental SBC bays was 1 ha
(250 m � 40 m) and 1 (bay 11) was 0.33 ha. Use of holding ponds
was to store drainage water to adjust its salinity to required
application level. The instrument set-up for this hydrological
investigation comprised 2 replicates of 12 piezometers with a
depth of 3 m perforated at the bottom over a length of 30 cm
and 6 test wells perforated over the whole length. One set in
each replicate is located within the full-scale SBC bay and the
remaining are located between the full-scale SBC bay and the
nitor groundwater dynamics.
Fig. 3 – Vertical section of deep drainage experiment in SBC
bay 2.
Table 1 – Hydraulic parameters for calibration of regionalscale model
Layers Horizontalconductivity
(m/day)
Drainableporosity (%)
Layerthickness (m)
Min Max Min Max Min Max
Layer 1 0.0086 0.89 5.9 9.7 12 12
Layer 2 0.00055 0.99 6.1 10.2 5 9
Layer 3 0.12 11.87 na na 0.25 52
Layer 4 7.24 37.44 na na 0.25 100
Table 2 – Hydraulic parameters for calibration of localscale model
Parameters Initialvalues
Optimizedvalues
Horizontal conductivity (m/day) 0.001 0.004
Vertical conductivity (m/day) 0.001 0.004
Deep leakage (m/day) �0.001 �0.0002
Drainable porosity (%) 6 6
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 2 67
bay 9 of the experimental bays (Fig. 2) The piezometers and
test wells were fitted with data loggers to continuously
monitor the groundwater height at each location. This
piezometric data was used to develop, calibrate and validate
a MODFLOW model to quantify the local impacts of the full-
scale bay. Another array of 21 piezometers and test wells was
installed in the experimental SBC bay 2 (Fig. 3). These
piezometers and test wells were used to determine the
internal drainage characteristics of the bays and the lateral
and vertical leakage from them. All bays were artificially
drained using perforated polyethylene drainpipe. The drain
depth was 1.0–1.2 m below ground surface and drain spacing
was 5 m. Each bay was periodically flood irrigated and drained
through the tile drainage system, on a 14-day cycle.
3.2. Groundwater model
3.2.1. Quantification of local effectsA local scale 2-D saturatedgroundwater model toexplore lateral
and vertical components of groundwater flow was developed
using MODFLOW (MacDonald and Harbaugh, 1988) under a
Processing MODFLOW for Windows (PMWIN) environment.
PMWIN is a simulation system for modelling groundwater flow
and transport processes and is the mostpowerful graphical user
interface available for MODFLOW today (Chiang and Kinzel-
bach, 1998). The model domain consists of a 30 m deep and
54.5 m long strip and is represented by a 20 intervals mesh
(varying from 0.5 to 8 m size) in thex-direction and a 13 intervals
mesh (varying from 0.5 to 5 m size) in the z-direction. This mesh
spacing ensures that the ratio of dimensions of two consecutive
mesh intervals is less than or equal to 2 to avoid any errors in
Taylor’s approximation for spatial finite difference approxima-
tion. The top model boundary is a transient water table
boundary which means groundwater pressure is zero and
water table height equals elevation (h = z). The bottom model
boundary is a leakage boundary where the rate of leakage is
specified by sinks at the centre of each cell. The left and right
hand side boundary conditions are no flow boundary (dh/dx = 0)
and a time dependent head boundary, respectively. The head
variations are specified using data from the deep piezometers.
3.2.2. Quantification of regional effectsTo determine the regional impacts of the SBC system, a
regional groundwater model was developed using spatial
zooming techniques (Szekely, 1998; Leake and Claar, 1999)
from an existing larger scale groundwater model of the MIA
region (Khan et al., 2002). The dimensions of the resulting
regional model of the SBC system are 8.25 km � 9 km. The
modelled area was represented by 8448 cells (88 cells in x and
96 cells in y-direction) with dimensions of 93.75 m � 93.75 m.
In the vertical direction, the model comprised four geologic
layers: Upper Shepparton Formation, Lower Shepparton
Formation, Calivil Formation and Renmark Group Formation.
The soil properties of each of the layers vary considerably as
shown in Table 1. The vertical thickness of the porous material
beneath the model domain varied between 17 and 166 m
depending upon the depth to bedrock. The parameters used in
the regional model calibration are given in Table 1.
3.3. Model calibration and validation
3.3.1. Deep leakage and vertical flows at local scaleThe model was calibrated by varying the initial values of
model parameters (Table 2) within reasonable limits to
minimise the difference between the observed and calculated
hydrographs. The calibration was done for the data set from
20/12/1999 to 17/01/2000.
The initial piezometric levels in the model domain were
computed by fitting a second degree polynomial to the water
table levels. The first derivative of the polynomial equation
approximated to the lateral gradient. During the calibration
process discrepancy was noted in the observed and computed
hydrographs using the estimates of the initial hydraulic
properties (Table 2). However, matching was significantly
improved using the calibrated hydraulic properties and
leakage rates (Table 2). Fig. 4 shows a good match between
computed and observed hydrographs for the calibrated model
parameters. Computed and observed hydrographs at the other
piezometric locations were in similar agreement.
The sensitivity of model outputs to the parameters is
determined by using the concept of root mean square error
Fig. 4 – Calibrated computed and observed Pz-8
hydrographs.
Table 4 – Model sensitivity to deep leakage rates
Deep leakage (mm/day) ERMS (m) ESRMS (%)
�1 0.168 4.010
�0.4 0.0585 1.400
�0.2 0.0496 1.188
�0.09 0.0671 1.606
Fig. 5 – Validated computed and observed Pz-6
hydrographs.
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 268
(ERMS) and scale root mean square error (ESRMS) by the
following equations:
ERMS ¼1n
Xn
i¼1
½Wiðhi � HiÞ�2( )1=2
(1)
ESRMS ¼100ERMSPni�1ðhi � HiÞ
(2)
where n is the number of observations, Wi the weighting
factor, hi the computed head, Hi the observed head,Pn
i¼1ðhi �HiÞ is the accumulative difference between observed and
computed heads.
The ERMS and ESRMS obtained for different estimates of
vertical leakage and hydraulic conductivity parameters are
given in Tables 3 and 4. The model sensitivity is tested for four
scenarios of both vertical conductivity and deep leakage rates.
Table 3 shows that the model is not very sensitive to the
vertical hydraulic conductivity estimates in the range of 1 and
3 mm/day keeping both horizontal conductivity and deep
leakage constant for all cases: Kx = 4 mm/day and
q = �0.2 mm/day, respectively. The sensitivity to deep leakage
is tested at Kx = Kz = 4 mm/day for all cases. The guidelines on
groundwater modelling by the Murray Darling Basin Commis-
sion (Middlemis, 2001) show that the model calibration is good
if ESRMS is less than 5%. In the present study, assuming
isotropic soil hydraulic properties (Kx = Kz), the model results
show minimum ESRMS value (ESRMS < 1.2) for a vertical leakage
rate of 0.2 mm/day. The initial hydraulic heads derived from
the second order regression of the water table level on the 8th
of September 1999 were used to validate the model results.
Computed hydrographs for piezometers Pz-6 and Pz-8 (Fig. 2)
for the validation period are in close agreement with the
observed hydrographs (Figs. 5 and 6). To determine the leakage
Table 3 – Model sensitivity to vertical conductivity
Vertical conductivity (mm/day) ERMS (m) ESRMS (%)
3.0 0.0465 1.112
2.5 0.0456 1.090
2.0 0.0451 1.079
1.5 0.0454 1.087
1.0 0.0472 1.130
rate beneath the SBC system, two independent methods were
used:
(1) c
Fig
hyd
omputation of the leakage rate from the volumes of water
contained between the successive water table profiles after
the water table falls below the tile drains, and
(2) c
omputation of the time dependent vertical gradient andleakage rate using Darcy’s flow formula.
In the first method, the drainage rate (Q) from the
underlying tile drains is computed using the concept of
declining of water table (Dd) over time (Dt) by the following
equation:
Q ¼ DdPd
Dt(3)
where Q is the drainage rate per unit area based on water table
measurements (m/day), Dd the change in water table depth
(m), Pd the drainable porosity (%), and Dt is the time interval
(day).
. 6 – Validated computed and observed Pz-8
rographs.
Fig. 7 – Calibration results of the regional model for a given
piezometer.
Fig. 8 – Model simulations showing leakage flow vectors.
Fig. 9 – Fluctuations of water table between the plugged
and unplugged tile drains.
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 2 69
3.3.2. Deep leakage and vertical flows at regional scaleIn the vertical direction, the model comprises four layers
varying considerably in thickness. Initial pressures of the
groundwater in the different layers were based on the
piezometer levels of September 1995. Model parameters used
in calibration are given in Table 1.
Monthly stress periods with six time steps were used to
represent the time domain. Water balance components such
as fallow evaporation (ET—terminology used by MODFLOW
package), irrigation recharge and rainfall recharge based on
monthly rainfall data were added to the model. The ET was
used to simulate direct loss from the water table. The
maximum ET from the bare soil surface occurs when the
water table is less than 0.7 m below the soil surface and ET is
assumed to decrease linearly with the decline in water table
depth until reaching the limiting depth of 1.5 m, where ET = 0.
Regional groundwater inflow and outflow from the model
domain is represented by head boundaries. The model was
calibrated from observed data for piezometers in the model
domain from 1995 to 1999 (a stress period of 0–1440 days). The
data for this analysis was piezometric levels before and after
the winter rains. Fig. 7 shows that computed and observed
piezometric levels compare well.
4. Results and discussion
4.1. Local impacts
Fig. 8 shows the flow vectors in the model domain at the end of
the calibration period. At the edge of the re-use bay the flow
vectors are close to vertical and trending to be more inclined to
Table 5 – Rate of water table decline when above tile drain lev
Date Time (h) Area of the unsaturatedwater table declines
09 February 07:00 2.493
10 February 07:00 3.506
04 March 16:00 2.969
05 March 16:00 3.927
18 March 01:00 1.744
19 March 01:00 2.682
the horizontal axis away from the edge. This indicates that
vertical leakage occurs beneath the bay and lateral flow occurs
away from the edge of the bay. The equipotential lines in the
figure show pressure heads which show a decreasing pattern
both vertically and horizontally. Using the groundwater levels
in the model domain after 30 days of simulation, the
differential of the second-degree polynomial at x = 54 m
(distance from the bay) gives a hydraulic gradient of 0.14.
Therefore, the lateral flow is the gradient multiplied by the
hydraulic conductivity which is approximately equivalent to
0.6 mm/day for K = 4 mm/day. The model sensitivity in Table 4
reveals that the vertical leakage is 0.2 mm/day for minimum
values of both ERMS and ESRMS. This situation illustrates that
the lateral flow is three times greater than the vertical flow
from the full-scale bay.
Fig. 9 shows the time variant response in 6 test wells
(measuring shallow water tables) in bay 2 of the 11 experi-
mental SBC bays. Test wells located close to both plugged
el
zone as(m2)
Average depth towater table (m)
Drainage rate(mm/day)
0.499
0.701 12.2
0.594
0.785 11.5
0.349
0.535 11.3
Table 6 – Rate of water table decline when below tile drain level
Date Time (h) Area of the unsaturated zoneas water table declines (m2)
Average depth towater table (m)
Drainage rate(mm/day)
14 February 17:00 5.013 1.003
15 February 17:00 5.070 1.014 0.7
16 February 17:00 5.225 1.045 1.9
17 February 17:00 5.363 1.073 1.7
18 February 17:00 5.448 1.089 1.0
19 February 17:00 5.484 1.097 0.4
20 February 17:00 5.499 1.099 0.2
21 February 17:00 5.561 1.112 0.7
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 270
(every other drain was plugged to reduce the drainage rate)
and unplugged drains (at 0.5 and 5.0 m from the unplugged tile
drain) show a rapid rise and decline in the water table level
following an irrigation event. Test wells located away from the
tile drains show a steady rate of decline following the
irrigation events. There are three irrigation events shown in
Fig. 9 coinciding with water table peaks closer to the plugged
drain. It is concluded that the plugged drain and the
surrounding packing material provide temporary storage
and preferential flow and therefore act as sink lowering the
water table above it.
The average change in water table level was used to
determine the rate of drainage when the water table was
above or below the tile drain. Using Eq. (3), the average rate of
drainage (Q) was 11.67 mm/day when the water table was
above the tile drains and 0.94 mm/day when it was below the
tile drains as shown in Tables 5 and 6. This indicates that
average drainage rate to the tile drainage system was
10.73 mm/day (92% of the total flux) and 8% of water was
Fig. 10 – Net rise of pressure levels in Upper Shepparton aquifer s
1000 ha and (d) 2000 ha sites.
lost to the aquifer through the deep leakage beyond the tile
drains.
It is observed that the vertical gradient between the 4 and
7 m deep piezometers at 2.5 m distance from the tile drain
varies with time where as the vertical gradient just below the
tile drain remains unaffected. This is due to the two
dimensional nature of flow at 2.5 m distance from the tile
drain (Khan and Rushton, 1996a,b,c). The average vertical
gradient just below the tile drain is computed as 0.307 and it
can be used to determine the deep leakage rate to the regional
aquifers. Using the calibrated hydraulic conductivity of 4 mm/
day and the vertical hydraulic gradient of 0.307, the vertical
leakage according to Darcy’s flow formula is 1.2 mm/day. The
difference of the deep leakage below the tile drains by two
different methods of calculation (Eq. (3) and Darcy’s Law) is
22% (the vertical leakage rate of 0.94 mm/day was obtained
from the average rate of drainage in the preceding section). It is
concluded, therefore, that the deep leakage below the tile
drains is approximately 1 mm/day.
ystem after 25 years of operation of (a) 100 ha, (b) 500 ha, (c)
a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 2 ( 2 0 0 7 ) 6 4 – 7 2 71
4.2. Regional impacts
The following six scenarios were run to evaluate groundwater
impact for the following sizes of SBC operations:
� N
o SBC operation;� 5
0 ha site;� 1
00 ha site;� 5
00 ha site;� 1
000 ha site;� 2
000 ha site.For each of the proposed scenarios the model was run for 25
years. The difference in hydraulic pressures in layer-1 (Upper
Shepparton Formation) at the end of the simulation period
(25th year) was deduced from the base scenario (no SBC
operation). This allowed calculating the rise of piezometric
levels due to each of the operational scenarios.
The modelling results show that the overall water table rise
under the SBC site is around 1.3 m for the 50 ha site. This water
table rise occurs in the first year but it does not change during
the 25 years of operation, because of induced heavy drainage
from the tile drains laid at 1.2 m depth. The lateral spread of
the groundwater mound did not extend beyond 50 m from the
outer edge of the bays.
Fig. 10 shows the net water table change of the 100, 500, 1000
and 2000 ha sizes of operation of the SBC. Similar to the 50 ha
site, the overall depth to the water table under the SBC bays
remains at the depth of the tile drains which causes a rise of 1.0–
2.5 m (depending on initial depth to the water table) under the
site and to a distance of 50 m from the site. The water table
under SBC site reaches quasi equilibrium at the depth of the
drain with periodic rise and fall around the tile drain depth. The
increasing size of SBC (for example 2000 ha) shows slightly
increased interactions with surrounding areas, this causes
wider spreading of net water table change effects exacerbated
by spatially variable hydraulic conductivity and zones of
groundwater pumping influence from a nearby abstraction
bore. The long-term 25 year modelled operation of the SBC site
shows the tiledrains maintaining the water table atdrain depth.
5. Conclusions
The detailed monitoring of groundwater dynamics and the
modelling of the SBC operations demonstrate that the vertical
and lateral leakage at the edge of the bays is around 0.2 and
0.6 mm/day, respectively. The deep leakage beneath the SBC
bays is approximately 1 mm/day. The shallow initial water
table and the very low hydraulic conductivity keep the lateral
extension of the water table mound within 50 m. During the
irrigation/ponding operations 8% of the saturated ground-
water flow above the tile drains contributes to deep leakage.
The simulations show that the water table reaches a quasi
equilibrium at the depth of the drains with periodic rise and
fall in tune with irrigation application and drainage operation
whenever an SBC system with intensive tile drainage is set up
on heavy clay soils with low saturated hydraulic conductivity.
Modelling scenarios of various sizes of SBC sites suggest
that the overall water table levels remain at the depth of the
tile drains which brings about a groundwater mound of 1–
2.5 m on-site (depending on the initial water table depth) and
to a distance of 50 m laterally over time. The local and regional
impacts of SBC systems in lighter soils and prior stream areas
are likely to be quite different than those presented in this
study. In order to develop SBC operations at other locations, a
GIS-based site suitability assessment model is recommended
to assess the SBC effect under different soil and hydrogeolo-
gical conditions.
Acknowledgements
The authors wish to acknowledge the funding support of the
National Heritage Trust and The Cooperative Research Centre
for Sustainable Rice Production (CRC-Rice), which made this
work possible. Technical support provided by the Australia’s
Commonwealth Scientific and Industrial Research Organisa-
tion (CSIRO) scientists L. Short, L. Best, J. Townsend, N.
Jayawardane, T. Biswas and J. Foley is greatly appreciated.
Scientific comments from the anonymous reviewers and Joint
Editor-in-Chief Willy Dierickx have been very useful in
improving the quality of this paper.
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