Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7
Water resources (continued)
Surface water resources (continued)
River systems with high water extractions for human uses were identified in the previous section. Although water extraction from a river system may be low in comparison to other river systems, it may be affecting the 'health' of the aquatic ecosystems because of their unique ecological and environmental characteristics. In other river systems, aquatic ecosystems may be more resilient to higher volumes of water extraction. The sustainable yield is an estimate of the volume of water that can be extracted from a river system while maintaining an acceptable level of aquatic ecosystem health. It is important to recognise that 'an acceptable level' of aquatic ecosystem health will vary between rivers. For example, a higher level of ecosystem health would be expected in a pristine river in a national park compared with a highly modified and polluted river in a city.
A good indicator to assess the availability and condition of surface water resources is comparing the volume of water that has been allocated for human use (or developed yield) with the sustainable yield of a river system. If the volume of water allocated exceeds the sustainable yield, it is likely that the health of the aquatic ecosystems will be affected.
Other terms that are important in assessing water availability and condition are (NLWRA 2001):
- mean annual run-off - the average yearly volume of water in rivers and streams due to direct rainfall on their catchment
- developed yield - average annual volume of surface water that can be diverted for human use with existing infrastructure (e.g. dams and pumps)
- divertible yield - average annual volume of surface water that can be feasibly diverted for human use with both existing infrastructure and new infrastructure that has not been built. The divertible yield does not include environmental water requirements.
- sustainable yield - volume of surface water that can be diverted for human use after taking account of environmental values and making provision for environmental water needs. The sustainable yield is generally lower than the divertible yield. The sustainable yield should also consider the timing of extraction to ensure environmental flows meet the needs of aquatic ecosystems.
The mean annual run-off from Australian catchments is 391 664 GL/year (Table 5) and is highest in the tropical northern areas of Australia (drainage divisions 1, 8 and 9) and the mountainous regions along the south-east coast and Tasmania (drainage divisions 2, 3 and 4). The mean annual run-off of water regions with the highest water use is presented in Table 6.
|Drainage division||Mean annual run-off (GL/yr)||Developed yield(GL/yr)||Divertible yield
|1 North-east Coast||73 411||3 540A||22 900A||NAB|
|2 South-east Coast||42 390||4 280A||15 100A||NA|
|3 Tasmania||45 582||3 543||10 900A||NA|
|4 Murray-Darling||23 734||10 000A||12 400A||NA|
|5 South Australian Gulf||952||149||237||164|
|6 South-west Coast||6 785||507||2 935||1 608|
|7 Indian Ocean||4 609||27||739||440|
|8 Timor Sea||83 320||1 980A||43 638||9 983|
|9 Gulf of Carpentaria||95 615||78A||13 200A||NA|
|10 Lake Eyre||11 866||26A||204A||NA|
|11 Bulloo-Bancannia||1 914||0A||41A||NA|
|12 Western Plateau||1 486||1||1 461||76|
|13 Island Territories||NA||NA||NA||NA|
|TOTALC||391 664||24 131||123 755||NA|
A NLWRA (2001a) data incomplete and was replaced with AWRC (1987) data.
B NA data incomplete and no alternative values available.
C Total of drainage divisions for which data were available.
Source: NLWRA 2001a.
|Drainage division||Water region||Mean annual run-off(GL/yr)||Divertible yield
|Developed yield(GL/yr)||Sustainable yield(GL/yr)|
|1 North-east Coast||1 Princess Charlotte Bay||13 897||4 335A||11A||NAB|
|2 Barron||24 432||6 457A||612A||NA|
|3 Burdekin||8 982||5 490A||1 210A||NA|
|4 Whitsunday||4 032||653A||154A||NA|
|5 Shoalwater Bay||3 841||502A||10A||NA|
|6 Fitzroy (Qld)||5 380||2 570A||74||NA|
|7 Curtis||1 502||512A||44A||NA|
|8 Burnett||3 611||797A||373A||NA|
|9 Mary||4 663||650A||136A||NA|
|10 Brisbane||1 839||563A||555A||NA|
|11 Gold Coast||1 233||395A||109A||NA|
|2 South-east Coast||1 Coffs Harbour||17 646||5 470A||87A||NA|
|2 Hunter||3 603||1 115A||417A||NA|
|3 Sydney||4 133||1 020A||581A||NA|
|4 Snowy-Shoalhaven||8 000||3 545A||1 460A||NA|
|5 Gippsland||4 248||2 090||737||859|
|6 Melbourne||1 826||653||558||633|
|7 Otway||1 213||371||118||321|
|8 Hamilton||1 341||106||84||106|
|9 Millicent Coast||380||85||0||85|
|4 Murray-Darling||1 Upper Murray||6 538||4 050A||873||NA|
|2 Goulburn-Loddon||4 412||2 838||2 205||2 205|
|4 Mid-Murray||0C||2 103A||3 553||NA|
|5 Murrumbidgee||4 184||2 505A||2 140A||NA|
|6 Lachlan||1 054||680A||570A||NA|
|7 Menindee Lakes||106||409A||354A||NA|
|8 Border Rivers||1 172||342A||150A||NA|
|9 Namoi-Gwydir||1 626||777A||627A||NA|
|10 Macquarie||1 656||713A||417A||NA|
|11 Condamine||1 500||286A||172A||NA|
|13 Lower Murray||132||735A||602||NA|
|6 South-west||1 Esperance||136||4||0||4|
|3 Warren-Blackwood||2 496||1 388||60||544|
|4 Busselton-Harvey||1 299||684||191||492|
|5 Perth-Mandurah||1 580||427||243||384|
A NLWRA (2000a) data incomplete and was replaced with AWRC (1987).
B NA not available.
C These water regions have a negligible catchment area but harvest water from upstream water regions.
Source: NLWRA 2000a.
The ability to store water in tropical areas is limited as most run-off occurs after very large rainfall events that require uneconomically large reservoirs to capture the water. The divertible yield in the Gulf of Carpentaria drainage division, for instance, is only 14% of the mean annual run-off.
The figures in Table 5 do not consider the quality of water. For instance, the sustainable yield of the South-west Coast drainage division is 1608 GL/yr, although only 1315 GL/yr of this is low salinity or fresh (WRC 2000).
The run-off and yield estimates for the Murray-Darling drainage division (Table 5) include transfers of water from the Snowy River (South-east Coast drainage division) via the Snowy Mountains Hydroelectric Scheme. Following the Snowy Water Inquiry, the Victorian and New South Wales state governments have committed to returning a proportion of the transferred water to the Snowy River (see http://www.snowywaterinquiry.org.au). The initial target is to increase the flow in the Snowy River at Jindabyne from 1% of natural flows to 28% of natural flows, which is the minimum environmental flow requirement. An initial target of 21% of natural flows is to be achieved within 10 years. The run-off and yield estimates for the Murray-Darling and South-east Coast drainage divisions will change accordingly over the next decade.
The run-off volumes in Table 5 are based on historical climatic conditions. Run-off volumes may change in the future due to changes in climate. Already the impacts of climate change have been experienced in south-west Western Australia. Long-term rainfall data indicate that the region has experienced a prolonged period of below-average rainfall, including reductions in the number of days of rainfall each year and a reduction in the amount of rainfall occurring during higher rainfall events. Since 1975 annual inflows to Perth's major reservoirs have been generally below the long-term average inflow (NLWRA 2001a). The cause for this reduction in rainfall is not known, but is being investigated. This shift in catchment yield raises important questions for long-term ecosystem and water management.
The enhanced greenhouse effect could have a significant impact on average rainfall across Australia (see the Atmosphere Theme Report). The worst case prediction is a 20% decrease in average annual rainfall in central and northern Australia and a 10% decrease in other areas by 2070. The most optimistic prediction is no change in average annual rainfall in central and northern Australia and an increase of 10-20% in other areas by 2070. It appears that there will be some reduction in the central and northern regions average rainfall and this is likely to reduce the already low catchment yields. Without more accurate rainfall predictions, it is difficult to estimate the impact of the enhanced greenhouse effect on rainfall and water resources in the eastern states. Additional research and data are required before water resource managers can begin to plan for the impacts of the enhanced greenhouse effect.
The enhanced greenhouse effect is predicted to increase average temperatures across Australia by 0.6 to 3.4C by 2070. An increase in temperature would cause a proportional increase in evaporation rates from open waters, resulting in a reduction in the volume of water available. No estimates of the volume of water that would be lost to increased evaporation are available. Rises in average temperature could also affect aquatic flora and fauna that have adapted to historical temperature ranges (see the Biodiversity Theme Report).
In many river systems, baseflow consists primarily of groundwater inflow. Consequently, over-development of groundwater resources may result in a significant decrease in baseflows in river and stream systems. Some river systems in Murray-Darling drainage division are thought to be experiencing decreasing baseflows due to groundwater over-development. A reduction in baseflows would require an adjustment to the Murray-Darling Basin Cap (MDBMC 2000). The links between groundwater use and baseflows in rivers are currently being investigated by the Murray-Darling Basin Commission.
A good indicator of environmental stress on aquatic ecosystems from water extraction is the comparison of the volume allocated for extraction to the sustainable yield of a river system. This also highlights river systems where water use can be increased in a sustainable manner. Sustainable yields have not yet been determined for river systems in Queensland and some river systems in New South Wales and Tasmania, but are currently being developed as part of water allocation processes (see Responses - management of surface water resources). Where sustainable yields have not yet been determined, an estimate of the development status of river systems have been provided by state and territory water resource agencies. The NLWRA is the first occasion where sustainable yields have been reported nationally and represents an important step towards the adequate accounting of water resources. The methods of calculating sustainable yields were not consistent across the various states and territories and the NLWRA identified the need to agree and adopt Australia-wide comparable definitions and methods for determining sustainable flow regimes for surface waters (NLWRA 2001a). The categorisation of Australia's surface water resources into five groups is illustrated in Figure 3. It should be noted that environmental water allocations must also consider the timing of the delivery of flows to ensure that they achieve the maximum benefit to aquatic ecosystems (i.e. mimicking the 'natural' flow regime or using flows to flush or dilute saline waters or algal blooms).
Twenty-six per cent of Australia's river systems are either close to or are overused (NLWRA 2001a). Sixty-nine per cent of the total volume of water extracted from surface waters is from these stressed river systems. Most of the river systems in the Murray-Darling drainage division are over developed; however, future increases in water use are constrained at 1993/94 levels by cap in most states. Some smaller river systems in the North-east, South-east, Indian Ocean and South-west drainage divisions are highly developed. Only 3% of the sustainable yield of the Timor Sea drainage division has currently been developed and increases in water use and development are likely in this drainage division in the future.
In many highly developed river systems, the consideration of environmental water requirements has largely been retrospective because the allocation of water occurred well before the awareness of the environmental effects of water extraction. In undeveloped river systems, there is an opportunity to develop environmental water provisions before water extraction affects the health of aquatic ecosystems. Pre-emptive allocations of water for the environment have been undertaken in areas of low water use in Western Australia. Environmental and human water allocations are assessed through a formal environmental impact assessment process.
- The average annual flow or yield of some river systems may decrease due to climate change and this is already being experienced in south-west Australia. The impacts of the enhanced greenhouse on water resources are impossible to predict with current data. Reductions in river baseflows due to the over-development of linked groundwater resources may result in a decrease in catchment yields and may require current water allocations to be reviewed and reduced.
- Water allocations are close to or are exceeding the sustainable yield of most river systems in the Murray-Darling Basin and in river systems providing drinking water to major cities. The 'health' of aquatic ecosystems in these over-allocated (or developed) river systems is likely to be affected.
- The northern drainage regions, such as the Timor Sea, are likely to be the focus of new irrigation development as water use in these regions is currently low compared with the volume of water available.