Indicator: Water supply
What the results tell us for the ACT
The reticulated water supply system continues to meet ACT and Queanbeyan consumers' water needs from sources in the Cotter and Queanbeyan River systems. Around 99% of the combined populations of the ACT and Queanbeyan are supplied water by ActewAGL Corporation Ltd.
The dams supplying water to the ACT and Queanbeyan (see Table 1) were at more than 80% capacity for most of the reporting period, with a low of less than 70% during April–July 1998, after which 100% capacity was reinstated in August 1998, with the heaviest winter rains on record. Dam capacities are shown in Table 2.
|Cotter||4.7||Mass Concrete (gravity)||1915–51 (upgraded 1999)|
|Bendora||10.7||Double Arch Concrete||1961|
|Corin||75.5||Earth and Rock Fill||1967|
|Googong||124.5||Earth and Rock Fill||1979 1|
GL = gigalitres, PMF = probable maximum flood
1. Googong Dam wall was raised by 4.5 metres and spillway upgraded to PMF and strengthened with anchors in 1991–92
|Full volume (ML)||10 720||75 455||4 697||124 510|
|Level when full (m)||778.20||955.54||500.69||663.00|
ML = megalitres, m = metres
No water restrictions were placed on the ACT during the reporting period, nor indeed, for many years. However, as demand for water is projected to increase with population growth, smaller household units and increased housing, the water resource supply will come under increasing pressure.
The primary source for water in the ACT is the reticulated water supply system, which covers the majority of urban Canberra and Queanbeyan. Table 3 shows the number of domestic water service connections provided by ActewAGL, and the population these support.
|Population serviced||% of total population||Total domestic properties serviced||Total of non-domestic properties serviced|
|337 000||99%||132 000||6 000|
The non-reticulated sources of water in the ACT are bores, which are often used for rural and commercial applications and rainwater tanks used in rural and urban areas.
Environment ACT implemented a system of bore licensing in December 1999. Since then, 47 licences have been issued for groundwater users with one currently pending. These bore licences may represent one or multiple bores. There are, additionally, a number of unused bores for which there are no formal records, as well as a number of bores intended for use for which licence applications are expected. The number of bore licences does not yet accurately represent the overall number of bores in the ACT, as there are still many, mostly in residential areas, that remain unlicensed.
The ActewAGL/ACT Government Rainwater Rebate Scheme was set up to encourage use of rainwater tanks in conjunction with the reticulated water supply system. One hundred and five rebate applications were processed between 20 August 1997 and 1 October 2000. ActewAGL's Customer Advisory Service received 1380 enquiries in that time. Tanks between the sizes of 4.5 kilolitres and 8.9 kilolitres attract a rebate of $200 and tanks over 9 kilolitres attract a rebate of $500. There are no data on tanks smaller than 4.5 kilolitres.
As stated in the 1997 State of the Environment Report, the combined storage capacity of the existing reservoirs Queanbeyan and the ACT use (more than 215 000 megalitres) is sufficient to provide water for a population of 400 000. The basis for that assessment is the average daily water use during the early 1980s, which included a major drought. The present population supplied is about 340 000, so it would appear that the capacity would be adequate on present rates of consumption for at least the next 10 years.
There are strong inter-annual and inter-seasonal factors that affect both the filling and drawdown of Canberra's water supply dams. Recent historical record of water levels and volumes of water stored in the two independent storage systems on the Cotter and Queanbeyan Rivers are depicted in Figure 2 and Figure 3.
Water restrictions were last invoked in 1967–68 before Googong Dam was commissioned. During the severe drought of 1982–83 the Corin–Bendora system was heavily drawn down (Daniel and Flutter, 1984). Cotter River storages were close to critical drawdown: while there was ample water in Googong Dam (77% full), Bendora Dam (75%) fed by Corin Dam (38%) was within weeks of failure. Heavy rains in March 1983 and then again in April restored normal base flow runoff. Future risks of restrictions will be affected by population growth, development of significant industries and by any changes to the pattern of normal domestic, urban green-space and commercial use.
There are many other factors that constrain water use from storages. These include pumping regimes required to meet peak daily use—neither the Bendora nor the Googong mains have sufficient hydraulic capacity to supply hot dry summer water demand alone—and lack of water treatment capability for the Cotter. The Cotter Dam is maintained in an operational condition for emergency or extreme drought requirements. Below specified dam levels, water quality is very poor and there are level limits for pump-down.
The Cotter River System
There are two dam systems on the Cotter River: the Corin–Bendora system with the Bendora main supplying the bulk of Canberra's drinking water under normal operating conditions, and the backup small (emergency supply only) Cotter Dam system which, unlike the gravitated Bendora system, must be pumped.
Corin Dam (75.4 gigalitres; catchment area 196 square kilometres) and Bendora (10.7 gigalitres, catchment area 91 square kilometres) are on the upper Cotter River (see Figure 4). The majority of their storage catchment is in wilderness condition. However, grazing in this area in the earlier part of the 20th century has deeply incised filtering sphagnum swamps. Feral animals and increasing visitor pressure currently represent the main threat to the integrity of raw water quality, but normally water quality is exceptional. Bendora is an integrator dam, that is, it is filled to a fairly constant level, in accordance with demand, from Corin Dam.
The Cotter Dam (4.7 gigalitres; catchment area 192 square kilometres) on the lower Cotter River is significantly affected by forestry activity (sediment transport and increased turbidity) and to a lesser extent by recreational activity, including runoff from unmade roads. There are a number of picnic areas in the lower Cotter and one camping area at Blue Range on Condor Creek that feeds into the Cotter River. Water is available from this storage for emergency supply, for example, following severe drought or failure in the Bendora and Googong trunk mains, but there is no facility for full treatment to bring it up to the quality of water supplied from Bendora Dam or from the fully treated Googong supply. The Cotter pumping station is maintained in operational condition, but water quality is relatively poor. Recreation in any area upstream of the Cotter Dam wall is prohibited.
The Queanbeyan River System
The Googong Dam (124.5 gigalitres; catchment area 890 square kilometres on the Monaro–Tinderry–Gourock Ranges), is located 50 km south-east of Canberra (see Figure 5). Catchment landuse is a mix of nature reserve, low-intensity forestry (mainly native hardwood), rural residential, grazing (cattle and sheep) and a few small enterprises, such as a native crayfish farm, and recreation. A detailed profile of the jurisdictional and land management framework is provided by Starr (1999).
A more recent analysis of vegetation and stream condition dating from 1944 is also provided by Starr (2000) The latter report describes spatial and trend temporal conditions in the catchment. This report is being supplemented by specialised catchment attribute maps showing digital elevation, soils, geology, vegetation and land use, farm dams and contour banks and septic tanks. The report also shows other points of potential contamination associated with rural residential development and landuse. Other on-the-ground studies are being used to validate the mapping database and the information is being made freely available to Shire Councils and other parties (for example, Landcare Groups) to facilitate best management practice in the catchment.
Trends in the state of drainage networks within the Googong catchment, and the intensity of rural land use, imply that water quality will be maintained or may improve.
Many of the drainage networks have transitioned from eroding sources of sediment and efficient transport systems to depositional systems that may filter much of the water flowing through the catchment. Modifications made to upper slope drainage networks, principally by earthworks constructed under the Lake Burley Griffin Protection Scheme, may be important in protecting the quality of water emanating from rural residential areas.
Changes in tree cover have been dominated by natural regeneration, mainly of areas cleared in 1944. In all, the total area of regeneration in the catchment is three times that of the area cleared during the same period. These figures support a general analysis that grazing land use in the catchment has become less intense over the past 55 years.
These trends can be further clarified by modification of sediment budgets to reflect current trends and by development of pathogen budgets. Both forms of budget should be valuable for risk assessment. Further study of the impact of modifications to drainage networks by earthworks is warranted, as these may be a means by which water quality can be improved in particular areas.
The dynamic water yield from the Cotter River scheme is much higher than from Googong reflecting different rainfall regimes and differently-sized catchments. Googong is larger by a factor of two but rainfall is lower, and costs of pumping and treating water from this storage are about five times those from the Cotter Catchment. In essence, there is a highly varied annual rainfall pattern both temporally and spatially across both catchments, although the Brindabella Range (Cotter Catchment) tends to be much wetter than the Gourock–Tinderry Range (Great Divide) Catchment feeding the Queanbeyan River.
'Percentage fullness' does not relate well to security of supply since a few dry years is reflected in problems of balancing hydraulic demand from the two systems. In practice, both systems must be operated to meet drier-than-average peak summer demand. The impact of environmental flow releases on security of supply, and how that brings forward the requirement to construct a new dam, has been a matter of ongoing discussion. Demand management and reuse programs (mainly the former) would appear to have the greatest potential to reduce the pressure of extracting water for urban needs.
About the data
Data, including maps were provided by ActewAGL Corporation Ltd from its records.
Dam percentage fullness is monitored on the 15th day of each month, with records stored and maintained in a database.
Data on population/properties were provided by ActewAGL from the ActewAGL Annual Report 1998–99, using the ActewAGL customer database.
Data were also sourced from:
- Daniel, T M and Flutter, J July 1984, Drought and operation of Canberra's Water Supply, Department of Housing and Construction internal report, Canberra, 28 pp.
- Starr,B March 2000, Soil Conservation Priority Plan for the Australian Capital Territory, Department of Urban Services, Canberra.
- Starr, B June 1999, Googong Dam Catchment Area Management: A Review, prepared for ActewAGL Corporation, 58 pp.
- Starr, B January 2000, Googong Catchment: the state of drainage networks and trends in catchment condition since 1944, prepared for ActewAGL Corporation, 61pp. (This study is being augmented by 1944 and current maps of vegetation and stream and drainage channel condition. As with other elements of digital maps being prepared queries can be made to show change in catchment condition or to highlight critical features.)
Description: What does 'water supply' measure?
Which data are collected?
- reticulated and non-reticulated sources of water in each jurisdiction, as a proportion of urban and non-urban populations
- the combined %-fullness of water supply dams over time
- number of days with water restrictions
Why do we report this indicator?
Water supply for human and agricultural consumption can be derived from a number of sources - either reticulated to consumers by water supply authorities, or non-reticulated (i.e. abstracted by individuals). Sources of water to populations include in-stream reservoirs, direct offtakes from flowing streams, utilisation of groundwater reserves and rainfall.
When reticulated, the reliability of this supply is one indicator of the productivity of the system from an anthropogenic perspective. It also indicates the extent to which the Region's water resources are committed, and thus the urgency with which new water supply reservoirs must be constructed.
Additional information about the level of commitment of water supply, and likely quality, can be derived from a knowledge of the proportion of a population (both urban and rural) relying on non-reticulated sources of water in addition to those relying on reticulated sources, as well as the extent of that reliance.