Response to the Industry Panel’s request for further information, 25 August
2014, part A, question 2
On 25 August 2014 the Industry Panel requested further information from ACTEW. This return provides a
response to part A, question 2 of this information request, which was for:
A description of the methodology used by ACTEW to calculate the yield for each water source and the
current yield estimate
ACTEW response
1. Introduction
Planning for the Canberra Region’s water supply security requires extensive modelling of scenarios relating
to water demand, water supply and infrastructure availability. The results of such modelling depend on the
water security target required and the assumptions input into the model.
Six key planning variables influence the level of Canberra’s water supply security during the planning
horizon. These are:
1.
2.
3.
4.
5.
6.
Climate variability and climate change;
Bushfire impacts;
Population growth and service area;
Per capita water consumption;
Environmental flow requirements; and
Level of service requirements.
This paper discusses ACTEW’s planning approach to these six key variables and reports on ACTEW’s current
estimate of system yield.
2. Climate Variability and Climate Change
ACTEW’s approach to climate planning for water security is documented in the ACTEW Future
Climate Update1, available on our website. Future climate scenarios are fundamental to ACTEW Water’s
planning for future water security because the spatial and temporal variability of temperature, evaporation
and rainfall largely determine the level of urban water supply security.
Climate Variability
ACTEW has used stochastic data to determine water security in all studies since 2004. Stochastic data are
large amounts of data that are developed to reproduce the statistical properties of the observed data (e.g.
average rainfall, rainfall variability, number of raindays, relationship between rainfall at different sites).
Stochastic data should be able to approximately reproduce these relationships on all timescales from daily
through to multi-year. Stochastic data has a number of advantages over historical record data, including:


1
It provides better resolution around extreme events and can therefore provide a better estimation
of the probability of water restrictions.
Because there is much more data than the historical record, it contains far worse droughts than
observed and can therefore be used to evaluate the risk of running out of water altogether.
ACTEW Water, ACTEW Future Climate Update, August 2014
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
It enables probabilistic analysis by examining the outcomes that could occur in many different
weather scenarios that could occur during the planning horizon.
ACTEW has revised its stochastic data in the Climate Update. The new stochastic data have been updated
to:




Use the most recent stochastic data generation techniques.
Update the historical data to include recent observations, including the Millennium Drought.
Increase the amount of stochastic data from 10,000 years to 50,000 years (1000 equally likely
versions of the next 50 years known as replicates)
Produce estimates of flow in Murrumbidgee River catchments.
Climate Change
ACTEW has also included climate change planning in all water security studies since 2004 and recently
updated its climate change projections. Four climate change scenarios are adopted in the Climate Update
to consider the full range of possible future climate:




Dry climate change, stochastic data produced using outputs from the second driest of the 15 global
climate models included in the South-East Australian Climate Initiative (SEACI) project.
Medium climate change, stochastic data produced using outputs from the median global climate
model included in the SEACI project.
Wet climate change, stochastic data produced using outputs from the second wettest of the 15
global climate models included in the SEACI project.
Last 20 Years, stochastic data produced so that the seasonal means of rainfall in each catchment
and evaporation are adjusted to match the means observed in the past 20 years from June 1993 to
May 2013.
These four scenarios will provide different yield estimates for ACTEW’s water supply system. In this paper
the dry climate change scenario has been used to estimate system yield.
3.
Bushfire Impacts
In 2003, severe bushfires burnt most of the Cotter catchment as well as causing loss of life and widespread
property destruction in Canberra. Bushfires are likely to impact upon the amount of runoff generated by
the catchment because regrowth in the catchment uses more water than an existing forest. ACTEW
incorporates the impact of the 2003 bushfires and possible future fires in the catchment using a bushfire
yield reduction model. Bushfire impacts are not modelled in the Googong or Murrumbidgee catchments.
The model may randomly apply a future bushfire event when all of the following are met:



There has not been a severe fire in the catchment in 20 years
The month is January or February
Dryness thresholds for triggering a fire are met
A yield reduction curve is applied when fires are triggered and to include the impact of the 2003 event.
Inflows for the relevant catchment are altered by the factors shown in Figure 8, based on modelling
conducted by DHI following the 2003 event2. The maximum yield reduction applied is 16%, 10 years after
the bushfire. For the 2003 event, we are currently near the maximum reduction so this is applied to the
beginning of the flow sequences.
2
DHI, ACT Catchments Hydrological Modelling: Cotter and Googong Catchments Model, Development and
Bushfire Effects, 2005
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10
5
3.5
% Change in Yield
0
-5
-10
Bushfire Yield Reduction Curve
-15
Current Year
-16
-20
-25
0
2
4
6
8
10
12
14
16
18
20
Years since Bushfire
Figure 1 – Projected impact of severe bushfires on Cotter catchment streamflows (DHI, 2005)
4.
Population Growth and Service Area
Population growth and per capita water demand are not key determinants of the GL/year yield of the
water supply system, which is mainly determined by infrastructure, inflow assumptions and environmental
flow requirements. However, they are key planning variables and are included for completeness and to
give a better understanding of the modelling methodology.
The ACT region is predominantly a residential and commercial region; it does not have any significant water
use intensive industries. As such, future water demand is largely driven by climate (in Canberra, water
consumption can vary from 90 ML/day during a wet winter to over 300 ML/day during a dry summer),
future population, and the level of individual water consumption (i.e. the per capita demand).
The current service area covers Canberra and Queanbeyan in NSW. ACTEW considers Canberra and
Queanbeyan as one and applies the same growth assumptions to each. Comparison of the two shows the
Queanbeyan population has been a constant 10 to 12% of Canberra's population for the last 12 years.
In the future the service area may include additional areas in NSW, such as Yass, Murrumbateman,
Palerang and/or Goulburn. This possibility has been raised in the past, particularly during the recent
drought, and is included in an ACT, NSW and Commonwealth inter-governmental Memorandum of
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Understanding (MoU) covering the supply of ACT water to NSW3. The MoU formalises the existing
arrangements to supply Queanbeyan City Council and provides a framework for supply to NSW regions
close to ACT over the next 30 years.
Based on the possible additional NSW population, ACTEW makes an allowance of 1.5% of the combined
Canberra / Queanbeyan population from 2020.
Figure 2 shows the observed and projected population growth for the ACT. The 2013 Australian Bureau of
Statistics high, medium and low growth projections4 are shown along with the ACT Government
projection5.
The ACT Government recommends ACTEW be prudent and use the ABS high population growth projections
for its water security planning6. ACTEW does this by applying the ABS projected high growth rate to the
current population; effectively updating the population projection on the basis of observed population.
That is:
2014 ACTEW & Queanbeyan population = 2013 recorded population × 2014 Series A value / 2013
Series A value (and so on for each year)
Table 1 shows the projected water supply population for the next 30 years.
ACT Population Projections: 2013 to 2062
Historical and Projected Population
1,000,000
900,000
800,000
Population
700,000
600,000
500,000
400,000
300,000
Historical
ACT Government Projection
200,000
ABS Series A
ABS Series B
100,000
ABS Series C
0
Year
Figure 2 – Observed and projected population growth of the ACT
3
Chief Minister's Department, ACT Government, ACT-NSW Cross-Border Water Resources Agreement, 2006
ABS, 3222.0 - Population Projections, Australia, 2012 (base) to 2101, November 2013
5
ACT Government, Chief Minister, Treasury and Economic Development Directorate, ACT Population
Projections: 2013 to 2062, January 2014
6
ACT Government, Think Water, Act Water, 2004
4
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Table 1 – Projected ACTEW water service area population
5.
Year
Projected ACT & Queanbeyan population
2014
434,020
2024
549,705
2034
665,874
2044
787,406
Per Capita Water Consumption
As shown in Figure 3, per capita water consumption has reduced significantly in the past 20 years following
the Millennium Drought, prolonged water restrictions, introduction of a range of demand management
measures, increases in water efficiency, changes in housing stock and increases in the real unit price of
water. The ACT Government adopted a target reduction in per capita consumption of 25% by 2023, relative
to 1990s levels7. With current per capita consumption approximately 35% below 1990s levels (after
adjusting for weather conditions) the ACT Government considers this target to have been achieved8.
In its long term planning, ACTEW currently assumes that per capita consumption will remain at current
levels. This assumption allows for the possibility that it will not be possible to achieve further reductions
beyond the significant reduction already achieved. While ACTEW does not anticipate an increase in per
capita demand, ongoing water efficiency measures may be less effective due to demand hardening and
could also be balanced by a return to higher outdoor water use following the breaking of the drought and
demographic issues such as a reduction in the number of people per household.
The influence of weather on demand is significant in Canberra. ACTEW uses the Breusch-Ward demand
model to allow for the impacts of seasonality, rainfall, evaporation and water restriction level9. For longerterm planning, the model output is multiplied by population growth relative to present population to apply
demand in the water resources model (population growth is not considered in short-term planning).
7
ACT Government, Think Water, Act Water, 2004
ACT Government, ACT Water Strategy 2014–44: Striking the Balance, August 2014
9
ACTEW, ACTEW Main Submission to the Independent Competition and Regulatory Commission, pp. 75-80,
July 2012
8
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Canberra & Queanbeyan Water Consumption (kL/capita/year)
300
250
200
150
100
50
0
Annual Consumption
1990s Consumption
25% Reduction
Figure 3 – Reduction in Canberra and Queanbeyan Per Capita Water Demand
6.
Environmental Flow Requirements
The availability of water for urban supply is limited by ACT Government aquatic environment protection
policies. ACTEW’s Licence to Take Water (issued under the Water Resource Act 2007) includes provisions to
ensure environmental flows are protected as a first priority. Environmental flows are the flows of water in
the rivers necessary to maintain healthy aquatic ecosystems, and are defined in Environment ACT’s 2006
Environmental Flow Guidelines10. The Guidelines set out the volumes and timings of environmental flows
designed to mimic the natural flow patterns of rivers, and minimise the downstream impact of dams, pump
abstractions, other diversions, or additions to natural flows.
Draft 2011 Environmental Flow Guidelines11 were released for public comment in June 2011. They are a
revision of the 2006 Environmental Flow Guidelines, updated using scientific knowledge gained during the
past five years. Submissions closed in August 2011, but the revision remains draft at this time. The
proposed revision presents no significant change from the 2006 Guidelines in relation to ACTEW’s water
supply catchments.
7.
Level of Service Requirements
Comparing the outputs of ACTEW’s modelling – i.e. projections of future time in restrictions and net
economic benefit – to a set of water security planning objectives determines whether existing or planned
water supply systems provide an acceptable supply reliability or economic return for the community. A
10
11
ACT Government, 2006 Environmental Flow Guidelines, 2006
ACT Government, Draft 2011 Environmental Flow Guidelines, 2011
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supply system that fails to meet the objectives needs measures to better balance water consumption with
water supply, either through source augmentation, demand reduction measures or both.
Water security planning objectives are generally determined by government policies. To enable the
government to gauge ACTEW’s Future Water Major Project proposals, the ACT Government drew on the
frequency of restrictions objectives “being used elsewhere in Australia” and has based the criteria for the
ACT “around one year in 20 in temporary water restrictions”12. ACTEW has interpreted this to mean that
the average probability of water restrictions in any year should not be greater than 5%. This probability is
calculated by summing the number of months in water restrictions in the model run and dividing this by
the number of replicates (currently 1000) and the number of months in the year (12). This performance
measure is used to calculate the system yield presented below.
8.
Current Yield Estimate
The assumptions discussed above have been used to project the probability of water restrictions and
volumes of water supplied over the next 50 years. ACTEW’s REALM water resources model has been used
to make this projection by running the 1000 stochastic data replicates and averaging these results.
35%
140
30%
120
25%
100
20%
80
15%
60
10%
40
5%
20
0%
Water Consumption (GL/year)
Probability of Any Stage of Water Restrictions
Figure 4 shows the projected increase in the probability of water restrictions and average water
consumption. The level of service is exceeded in 2038, which corresponds to a serviced population of
approximately 714,000 and an average annual supply rate of 89.9 GL/year.
0
2013
2016
2019
2022
2025
2028
2031
Probability of Water Restrictions
2034
2037
2040
Consumption
2043
2046
2049
2052
2055
2058
Level of Service Reached
Figure 4 – Projected Probability of Water Restrictions and Annual Average Water Consumption
It is not possible to express this system yield in a way that provides a meaningful indication of yield from
the individual sources. However, the average amount of water supplied from each source at the point
where the level of service is exceeded is shown below in Table 2. These average volumes are not indicative
of how much water can be supplied from each individual source or how much water is supplied during a
drought.
12
Corbell, S., Debates, Weekly Hansard, Legislative Assembly for the ACT, March 2009
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Table 2 – Statistics when Level of Service is Exceeded
Probability of Water Restrictions
5%
Year
2038
Serviced Population
713547
Consumption (ML/year)
89915
Bendora (ML/year)
40631
Googong (ML/year)
29449
Lower Cotter (ML/year)
18371
Murrumbidgee to Stromlo (ML/year)
481
Stromlo to Googong (ML/year)
1135
Tantangara to Googong (ML/year)
14
13
Tantangara to Stromlo (ML/year)
Murrumbidgee to Googong (ML/year)
13
2599
9062
14
1595
Includes Tantangara to Stromlo
Includes Tantangara to Googong
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