technical bulletin # 5 groundwater trends in the victorian dryland
TRANSCRIPT
Groundwater levels change over time
as a result of a combination of the
weather (climate), land use patterns
and management activities. Regular
review and updating of trends and
projected groundwater levels is needed
to assess the threat posed by shallow
groundwater across the Mallee.
This technical bulletin summarises the
findings and methodology of the recent
‘Reforecast of Groundwater Trends in the
Mallee Region for 2030 and 2050’ project.
This project produced current (2007)
and predicted 2030 and 2050 depth to
groundwater maps.
Consulting company Sinclair Knight
Merz (SKM) was engaged by the Mallee
Catchment Management Authority (CMA)
to undertake this project with funding
provided by the Australian Government’s
National Action Plan for Salinity and Water
Quality (NAP).
The aim of this project was to use the
‘Groundwater Trend Tool’ developed in
2006 to accurately predict changes in
the groundwater levels so future risk
from changes in the watertable may be
determined. This included mapping the
current depth-to-groundwater 2007 and
the predicted depth-to-groundwater for
2030 and 2050.
Groundwater Trends in the Victorian Dryland Mallee Region
Method
This project combines a variety of new
data sets and innovative approaches to
groundwater trend and level projection to
provide the most up to date assessment
of the current watertable level and
projections of future levels.
The initial step in this assessment was to
collate and evaluate the available data for
groundwater level trends. SKM reviewed
the bores in the Mallee CMA region and
in addition bore and water level data
obtained from New South Wales, South
Australia and Victoria outside of the Mallee
CMA boundary.
At a glance
These maps will help to •
determine future risk from
changes in watertable depth and
help target onground works.
In most scenarios the watertable •
is declining as a result of the past
12 years of very dry conditions.
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Swan Hill
Robinvale
Mildura
Ouyen
Nyah
Manangatang
Woomelang
Birchip
Sea Lake
Red Cliffs
Irymple
Merbein
Underbool
Murrayville
Hopetoun
RainbowBeulah
Cullulleraine
0 20 40 6010
Kilometres
Legend
d Towns
Mallee CMA! Bore Locations
Road
Saline AreaWatertable Depth (m)
< 2
2 - 5
5 - 10
10 - 20
20 - 40
> 40
Mallee Catchment Management Authority
Reforecast of Groundwater Trendsin the Mallee Region
for 2030 and 2050
Watertable Depth 2007and Mapped Saline Areas
1:902,027Map Scale:
Figure 1. Current Depth-to-groundwater with mapped saline area.
Technical Bulletin # 5
m a l l e e c a t c h m e n t
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T e c h n i c a l B u l l e t i n
A total of 328 bores were used to map
the 2007 depth-to-groundwater. Reduced
numbers of bores were used for the
predicted maps as trend data was not
available or reliable in all bores.
Mapping the Watertable Surface
The distribution of shallow watertables
in the Mallee region is complex and
is influenced by a number of natural
features including topography, native
vegetation, and the Murray River, as well
as human processes such as land use
and irrigation.
A geostatistical method known as
Sequential Gaussian Simulation (SGS)
was used to incorporate some of these
processes into the predicted watertable
surfaces by the use of a “constraint layer”.
Figure 2 (above) provides a simplified
schematic representation of the steps
involved in modelling the watertable
surface. As shown in figure 2, the
following steps were undertaken.
1. Input of data – Bore data, constraints
layer, digital elevation model, topography.
2a. The model then randomly chose a
grid cell within the modelling domain.
It identified neighbouring bore data and
applied the correlogram to obtain an
estimate of the watertable elevation at
the grid cell.
2b. The model then randomly selected a
new grid cell to estimate, incorporating
the previous grid cells created as a data
point.
2c. The process described above was
repeated until all grid cells had been
estimated.
3. Once an estimate had been obtained
for all grid cells. The model started again;
the process was repeated 100 times and
produces 100 watertable surfaces.
4. The 100 watertable surfaces provided a
statistical distribution of potential values
for a grid cell; this allowed a range of
probability products to be generated. The
median watertable depth taken from each
grid cell was then used to develop the
depth to watertable surfaces and produce
a depth to watertable map.
Current Depth to Watertable 2007
The depth to watertable maps for 2007
generally showed that the depth-to-
groundwater across the Mallee has
increased when compared with the maps
produced in 2004 (REM 2005).
Areas of shallow watertable were
mapped at the following locations:
The Tyrrell Basin•
The Raak Plains and Hattah Kulkyne •
National Park
The Pink Lakes•
Surrounding Ouyen•
Murray Trench, particularly Lindsay •
Island and the irrigation area between
Mildura and Colignan
Morkalla•
Lake Agnes•
Spatial layers of mapped saline areas
and salt tolerant species (Low Chenopod
Shrubland and Samphire Shrubland) were
overlaid onto the 2007 map to compare
with the mapped shallow watertable.
(Refer to Figure 2). From these maps
it can be seen that major areas of
saline discharge or salt tolerant species
correspond well with areas of mapped
shallow groundwater.
It is important to remember that a
shallow watertable is not always an
indication of salinised water discharge.
Predicted 2030 and 2050 Depth to
Watertable (dry, average and wet
climates)
The same process was used to produce
predicted depth to watertable maps for
2030 and 2050 incorporating climate
sequencing.
The predicted 2030 and 2050 depth to
watertable maps were produced using
three different climate scenarios, wet, dry
and average rainfall.
Climate Sequencing
Climate sequencing takes into account
climate variability, that is the periods
of above and below average rainfall,
however it does not take into account
climate change as a result of increased
greenhouse gas concentrations.
For the climate sequencing, continuous
monthly rainfall records from 1885
Figure 2. Modelling simplified schematic representation of the steps involved in modelling the watertable surface.
Map: SKM.
through to 2007 were obtained from the
Bureau of Meteorology for rainfall districts
76 (South Mallee) and 77 (North Mallee).
Statistical analysis of the district rainfall
data showed that:
The Mallee rainfall data contains •
significant decadal and longer-term
variability.
The rainfall in both districts shows •
a strong coherence with the Inter-
decadal Pacific Oscillation (IPO.
The IPO is a climate mode that influences
rainfall at the decadal and inter-decadal
time scales across much of Eastern
Australia, and is closely associated with
the El Nino –Southern Oscillation (ENSO)
phenomenon.
A technique known as stochastic
generation was used to model possible
future rainfall sequences that are
representative of this observed variability.
Stochastic generation refers to the
generation of data which maintains the
observed historical data and projects
forward based on these trends.
Predicted 2030 Depth to Watertable
(dry, average and wet climates)
The mapped waterlevel for 2030 ‘average
climate scenario’ shows some areas of
shallow watertable that have expanded
from the 2007 map, while others have
contracted.
The distribution of watertable depths in
the 2030 wet climate scenario prediction
is similar to that of the 2007 levels and the
2030 average and dry climate prediction,
however in comparison to the other
maps the 2030 wet prediction is actually
showing deeper watertables.
Although this may seem odd, this has
been attributed to a period of low rainfall
around 2030 in the wet sequence.
Although the wet climate scenario is
on average a wet 23 year period it has
variability within this period and around
2030 is low rainfall variability.
The majority of the Mallee CMA region
is predicted to have waterlevels in the 10
to 40 metres below groundlevel range in
the 2030 dry climate scenario prediction.
Although the spatial distribution of the
areas of shallow watertable is similar to
the 2007 watertable, the depth to water is
expected to become greater.
Predicted 2050 Depth to Water Table
(dry, average and wet climates)
Areas of concern for shallow water tables
in 2050 are still
The Tyrrell Basin•
The irrigation area between Red Cliffs •
and Colignan
Raak Plains and Hattah Kulkyne National •
Park
Morkalla•
Pink Lakes•
Lake Agnes•
Hopetoun•
The 2050 wet and dry climate scenario
maps are both similar to the average
climate scenario prediction in terms of
the area of the region that is predicted to
have watertables of less than five metres
below groundlevel.
Probability of Watertable Depth
Greater than 5m – 2007, 2030 and
2050 (Average Climate Scenarios)
The 2007, 2030 and 2050 average climate
scenarios have been post processed to
calculate the probability of the watertable
being less than five metres below the
ground level at each grid cell.
The map shows that the distribution
of areas at risk of shallow watertables
remains the same as 2007. Note that
these are based on an average climate
scenario and a greater than 50 percent
probability of a grid cell mapping the water
table as less than five metres below the
groundlevel.
Based on these maps the analysis
indicates that there is a >50% probability
that 3.4%, 3.5% and 3.3% of the Mallee
CMA region has/will have a watertable
depth of less than five metres in 2007,
2030 and 2050 respectively.
Limitations of the Mapping
This mapping has been undertaken at a
regional scale, and the intent of the map
production is that they would be viewed
for the whole of the Mallee CMA region.
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Manangatang
Woomelang
Birchip
Sea Lake
Red Cliffs
Irymple
Merbein
Underbool
Murrayville
Hopetoun
RainbowBeulah
Cullulleraine
Swan Hill
Robinvale
Mildura
Ouyen
0 20 40 6010
Kilometres
Legend
d Towns
Mallee CMA! Bore Locations
Road
WaterbodiesWatertable Depth (m)
< 2
2 - 5
5 - 10
10 - 20
20 - 40
> 40
Mallee Catchment Management Authority
Reforecast of Groundwater Trendsin the Mallee Region
for 2030 and 2050
Watertable Depth 2030(average climate scenario)
1:902,027Map Scale:
Figure 3. Depth to Watertable 2030 Average.
Although the girds have been produced
at 100 metres by 100 metres spacing,
the interpolation of those grid cells may
be from groundwater bores at tens of
kilometres away from that location. The
grid data should therefore be used on
a regional scale only, and should not be
used for the determination of waterlevels
at a property scale.
This project considers the regional
watertable and not perched groundwater
systems that may result in saline land.
Uses
This report can be used by regional
agencies, such as the Mallee CMA, to
help in the design of the future research
and extension projects in the dryland
Mallee region of Victoria. The information
will greatly enhance the regions capacity
to understand current and future risk
associated with saline groundwater in the
region, which will lead future investment
and aid in the refinement of target areas
for on-ground works
Acknowledgement
Mallee CMA would like to thank all
regional organisations and personnel who
supported this project through the supply
of datasets and expert advice.
The information for this bulletin has been
taken from the “SKM 2008 Reforecast
groundwater trends in the Mallee Region
for 2030 and 2050” report. A copy of
this report can be downloaded from the
Mallee CMA website
www.malleecma.vic.gov.au, and
references listed in this bulletin can be
found within the report.
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Manangatang
Woomelang
Birchip
Sea Lake
Red Cliffs
Irymple
Merbein
Underbool
Murrayville
Hopetoun
RainbowBeulah
Cullulleraine
Swan Hill
Robinvale
Mildura
Ouyen
0 20 40 6010
Kilometres
Legend
d Towns
Mallee CMA! Bore Locations
Road
WaterbodiesWatertable Depth (m)
< 2
2 - 5
5 - 10
10 - 20
20 - 40
> 40
Mallee Catchment Management Authority
Reforecast of Groundwater Trendsin the Mallee Region
for 2030 and 2050
Watertable Depth 2050(average climate scenario)
1:902,027Map Scale:
Figure 4. Depth to Water Table 2050 Average.
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Robinvale
Mildura
Ouyen
Nyah
Manangatang
Woomelang
Birchip
Sea Lake
Red Cliffs
Irymple
Merbein
Underbool
Murrayville
Hopetoun
RainbowBeulah
Cullulleraine
0 20 40 6010
Kilometres
Legend
d Towns
Mallee CMA! Bore Locations
Road
Waterbodies
Mallee Catchment Management Authority
Reforecast of Groundwater Trendsin the Mallee Region
for 2030 and 2050
Watertable Depth Less Than 5m(>50% probability, average climate scenario)
1:902,027Map Scale:
Year 2050
Year 2030
Year 2007
< 50% (all years)
Figure 5. Greater than 50% probability of watertable less than 5m below groundlevel.
Project Partners
Published May 2009
This publication may be of assistance to you but the Mallee Catchment Management Authority refers readers to our Terms and Conditions, available from our website.
Printed with vegetable-based inks on recycled Australian paper, made from pre- and post-consumer waste.