This study analyzes the potential impacts of climate change on Sydney's water supply system. It uses downscaled climate projections from global climate models under three emissions scenarios to assess changes in rainfall, evaporation, and river flows. The results suggest that under a high emissions scenario by 2030, average annual rainfall may decrease and evaporation may increase, reducing water inflows and the system's yield by around 8%. By 2070, yield reductions could reach 11%. However, the downscaled projections lack persistence in drought conditions seen in historical data, representing a key limitation. Further research is needed to improve statistical downscaling methods and incorporate multiple climate models to better assess impacts on Sydney's water supply system.

1. Climate Change Impact Assessment
for Sydney’s Water Supply
S. Maheswaran (Mahes), J. Martin and G. Kibria
Sydney Catchment Authority
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2. Study background
This Study is a collaboration between
•Commonwealth Scientific and Industrial Research Organisation (CSIRO)
•Australian Government Department of Climate Change and Energy Efficiency
•Office of Environment and Heritage (OEH)
•NSW Office of Water (NOW)
•Sydney Catchment Authority (SCA)
•Sydney Water (SWC)
•University of New South Wales (UNSW)
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3. Sydney Catchment Authority
• Owns 21 storage dams and weirs
• Stores 2.6 million megalitres
• Covers 16 000 square kilometres
• Supplies bulk water for treatment and supply
to 4.5 million people (60% of NSW)
• Supplies raw water to 11 water filtration plants
• Supplied 404 000 ML in 2011
• 250 staff
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4. Greater Sydney’s Water Supply System
Main Storages
• Warragamba
• Upper Nepean Dams (Avon,
Nepean, Cataract and
Cordeaux)
• Shoalhaven System
(Wingecarribee, Fitzroy Falls
and Tallowa Dam)
• Blue Mountains dams,
• Woronora Dam
• Prospect Dam
Desalination Plant 250ML/d
Upgradable to 500ML/d
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5. Long term planning challenges
• Climate variability – Wet and dry cycles
• Population increase – Population to reach 5.7 Millions by
2036
• Water for environment –Environmental releases (80/20)
commenced for Upper Nepean dams, Woronora dam and
Tallowa dam and decisions of release regime from
Warragamba Dam to be made in 2014.
• Degree of success of water conservation, demand
management and recycling projects
• Climate Change Uncertainty
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6. Historic rainfall and inflow variability
• Water Supply system is exposed to high rainfall and inflow
variability
• Warragamba supplies almost 80% of the demand, estimated
inflows and rainfall is shown below for last 120 years
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7. GCM and Downscaling Method used
• Rainfall, Evaporation and Temperature data were downscaled to the
local/regional level using statistical techniques developed by
UNSW
• Outputs from CSIRO Mk3 GCM runs for three greenhouse gas
emission scenarios—representing low, mid and high emission futures
(B1, A1B, and A2 respectively)
To better represent the uncertainty in possible climates, 100
replicates of rainfall and evaporation estimates were produced
for the current climate and for each the future climate change
scenarios in 2030 and 2070
The climate period for 1960–2002 is used as the baseline in this
study as it is representative of the recent average climate in the
Sydney region. It is referred to as the current climate
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8. Climate Scenarios
In this analyses, three scenarios were
used - B1, A1B, A2 (Only A2 scenario
presented here)
Three time frames
were used
 Current climate
between 1960 and
2003; and
 Future climates
21-year period
centred around 2030
(2020-2040) and
2070 (2060-2080)
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9. Observed and Current Climate Rainfall Data
Downscaled and
observed annual
average rainfall
distribution is
shown in the LH
Figure.
In general the
average GCM
downscaled rainfall
and evaporation are
a reasonable
representation of
the current climate
(RH Figure)
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10. Rainfall-Runoff Model
HSPF catchment models were calibrated using traditional techniques
with an emphasis on ensuring that dry periods are accurately
represented. Calibration was undertaken to observed stream flow
estimates of periods up to 20 years and then validated with a balance of
up to 80 years.
Warragamba Annual inflows
8000
Obs
7000
HSPF
6000
5000
GL/a
4000
3000
2000
1000
0
1885 1905 1925 1945 1965 1985 2005
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11. Yield Assessment
Yield is the maximum amount of water that can be extracted on a
sustainable basis from the water supply system within the constraints of
the design criteria – reliability, robustness & security.
Design Criteria in SCA’s Operating Licence
Security: the SCA’s storages do not approach emptiness (defined
as 5% of water in the storage) more often than 0.001% of
the time.
Reliability: restrictions last no longer than 3% of the time. That is,
restrictions do not last for too long during any one drought
event.
Robustness: restrictions occur no more often than once in every
ten years. That is, restrictions are not too frequent
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12. Scenario A2 : Annual Average Rainfall
Downscaling
process was used
to generate 100
rainfall and
evaporation daily
sequences for
climate scenarios
A2 for the 2030 and
2070 assessment
periods
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13. A2 Scenario – Likely changes
•Inland regions consisting of the majority of the
Warragamba and Shoalhaven catchments tend to
become drier with less average rainfall and more
evaporation
•Coastal catchments slightly higher rainfall
•The results showed that small reductions of average
rainfall and small increases in evaporation could
result in disproportionate reductions to inflows. This
is mostly due to concurrence of rainfall reduction and
evaporation increase.
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14. Low persistence in future CC scenarios
• Persistence is the relationship between
rainfall and inflows in one year and those in
adjacent years.
•This factor leads to long periods of drought
and is a key factor in the assessment of
supply systems with many years of supply
storage capacity.
•The persistence is statistically measured
as a correlation between each year’s flow
and flows for previous years.
•Figure shows the lag-1 coefficient of the
Warragamba Current Climate and A2
2030 inflows
•Key finding
• GCM downscaled rainfall for the future
climate and resultant inflows did not show
any persistent droughts and lacked any lag-
1 correlation on annual scales.
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15. Conclusion
The yield results were published in the NSW Office of Water Web Water
for Life
Climate change impacts on water supply were assessed based on a
specified water supply system configuration that represents a range of
government policy decisions and planned infrastructure
Yield of the Greater Sydney Water Supply System
•For A2 scenario,
in 2030 the yield has been reduced down by around 8% and
in 2070, yield is reduced by around 11%.
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16. Findings
• Bias in the annual level persistence in the downscaled rainfall is
recognized as a major issue in assessing climate change impacts on
water supply yield and is a key knowledge gap.
• Further research required to investigate the issue of persistence in the
GCM simulated variables and develop a methodology to correct the
GCM variables so that rainfall obtained using these variables has the
proper variability
• SCA is partnering a study to address the shortcomings
• The new study will
 Improve statistical downscaling method
 Incorporate Multiple GCM’s and RCM’s
 Explore dynamic downscaling methods
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