4 Inland water | 2 Current state and trends of the land environment | 2.1 Water flows and levels
State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.
Streamflow regime—the pattern of water flow through our rivers—is a major determinant of the environmental condition of inland waters. The regime can vary in frequency, duration and magnitude, and according to season. Natural flows are mainly altered by water resource development, such as the building of dams and weirs, diversion or extraction of in-stream flows, the alteration of flows on floodplains by levees and other structures, and the abstraction of groundwater.
Large variations in streamflow are characteristic of the Australian environment, but the past decade has seen much of the continent move from a period of water deficit to water surplus, in some cases over a short period of time. For example, in May 2010, Australia’s 231 large water storages were at 51% of capacity (total capacity is 78 449 gigalitres). One year later, they were at 73.7% of capacity and held the largest volume of water in Australia’s history: 57 817 gigalitres (Table 4.3).
|The effect of drought-breaking rains in the Murray–Darling and the South-east Coast is clear in the changes of storage.|
|Drainage division||Volume stored (GL) (% of capacity)|
|May 2010||May 2011|
|Gulf of Carpentaria||95.6 (96%)||97.6 (98%)|
|Murray–Darling||7 141 (28%)||20 265 (80%)|
|North-east Coast||7 766 (91%)||8 267 (97%)|
|South Australian Gulf||117 (60%)||135 (69%)|
|South-east Coast||3 628 (34%)||5 412 (51%)|
|South-west Coast||368 (38%)||197 (21%)|
|Tasmania||11 867 (54%)||12 852 (58%)|
|Timor Sea||9 086 (85%)||10 731 (100%)|
|GL = gigalitre; na = not available—there are no major dams to report on for the division
Source: Australian Bureau of Meteorology12
Australian Water Resources 2005 included a ‘snapshot’ of river and wetland health based on previous, relatively recent regional assessments. Key findings included confirmation that, in general, increasing diversions and extractions correlate with declining river health. Hydrological change (i.e. change related to water quality, movement and distribution) could be assessed in only 25% of the river length due to a lack of data; however, in about 20% of the regulated river length that was assessed, the flow regimes were largely unmodified from an ecological perspective.
Most rivers across northern Australia (Gulf of Carpentaria, Timor Sea and North-western Plateau divisions, and the northern third of the North-east Coast division) have unimpeded flows. The greatest diversion of water occurs in the Lower Ord system around Kununurra, to provide water for irrigation and hydro-electric power. The level of water diverted for irrigation is low, but hydro-electric power demand can require release of water from Lake Argyle of more than half the lake’s inflows. Less than 3% of the 200 000 gigalitres of mean annual streamflow discharging between Broome and Cairns is diverted for use.13 A similar low percentage of streamflow is used for irrigation in Tasmania, although a high percentage is used for hydro-electric power.
Historically, water resource development in the Murray–Darling division has caused major changes in the flooding regimes that support floodplain wetland systems in the Murray–Darling Basin. These environments are nationally and internationally important. Integrating the flow impacts through the connected rivers of the Basin shows that total flow at the Murray mouth has been reduced by 61%. The river now ceases to flow through the mouth 40% of the time; this figure would be 1% in the absence of water resource development.5 More than half the reaches of the Murray–Darling division assessed in 2001 and 2009 had modified hydrology, with the greatest changes found immediately downstream of dams and in lowland reaches used for irrigation supply.6,14
In the South-west Coast division, about 20% of surface water resources are allocated for use. About 44% of annual groundwater recharge into supply aquifers is licensed for use, representing 74% of all water supplies to the south-west.3 There are concerns that complex interactions between climate (e.g. rainfall and temperature) and vegetation are magnifying the declines that are being seen in flows.15
On average, only 3% of the surface water resources of the Tasmania division (not including the undeveloped west-coast region and releases for hydro-electricity generation) are extracted for use; the main rivers are perennial and flow more than 95% of the time. About 3% of annual groundwater recharge is extracted (38 gigalitres per year).16 Some exceptionally dry conditions were experienced in 2003–08. This led to increased demands on surface waters, and significant declines in river flows and natural inflows to water storages. Water scarcity was an increasing problem in this period, despite Tasmania’s comparatively abundant water resources. A number of streamflow sites recorded their lowest flows since records started. There is evidence that groundwater use was unsustainable in some aquifers in Tasmania due to the increasing demands on water resources, constraints on surface water availability, reduced recharge due to drought conditions, and changes in land cover.17
The Australian Government commissioned the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to provide robust estimates of current and future water yield in several regions of Australia, under internationally agreed scenarios of climate change from the Intergovernmental Panel on Climate Change. These assessments provide the science to underpin sustainable planning and management of water resources.
The first project was in the Murray–Darling Basin. In March 2008, the Council of Australian Governments agreed to extend the assessment to three other areas: northern Australia, Tasmania and south-west Western Australia. These assessments were delivered in 2009.
With these assessments and those planned for other parts of the continent,a Australia will have a comprehensive scientific assessment of water yields in most of its major water systems, providing a consistent analytical framework for national water policy decisions.
Stream hydrology was rated as poor to moderate across much of the southern section of the South-east Coast division, reflecting modified flow regimes. Groundwater management units generally had stable water levels—previous declines were arrested in four areas and continued in seven areas through the millennium drought.18 The hydrological state of coastal rivers in the northern half of the South-east Coast division was generally good.19
Water use is generally stable, where licensed, across the South Australian Gulf division. Use of groundwater in the northern Adelaide Plains and parts of the south-east is considered to be above sustainable limits, and use of surface water in the Mount Lofty Ranges is above sustainable limits in some areas.20
The degree of hydrological modification of surface water and groundwater systems varies greatly across the North-east Coast division. Groundwater levels in the northern and central parts of the division are generally stable; in the southern end, levels have been declining, mainly due to the millennium drought,21 which ended in 2010. The degree of modification of the hydrological flow regime has a similar north-south gradient.
A basin-wide coordinated approach to bore rehabilitation was proposed as part of the Great Artesian Basin Strategic Management Plan in 2000. This resulted in the development of the Great Artesian Basin Sustainability Initiative (GABSI). Under GABSI, the state and Australian governments and landholders agreed to work cooperatively over a 15-year period, and to invest significant public and private funds to repair uncontrolled artesian bores in the Great Artesian Basin and replace open bore drains with piped water reticulation systems. Pressure is recovering in artesian groundwater systems in many areas across the Great Artesian Basin. Around half of artesian bores monitored showed an increase in pressure;21 341 bores in the Queensland portion of the Great Artesian Basin showed increases of pressure of up to 8 metres, and 31 bores have increased pressures of more than 8 metres.22 These results directly reflect programs to cap free-flowing bores.
|Component: drainage division||Summary||Assessment grade||Confidence in grade||Confidence in trend|
|Very poor||Poor||Good||Very good|
|Gulf of Carpentaria||Very low levels of development, leaving flow regimes largely intact; information quite sparse|
|Indian Ocean||Low levels of development, with mostly local self-supply abstraction or mine de-watering (extraction of water from underground mines); limited and sparse data|
|Lake Eyre||Little monitoring data available, but low levels of development of surface and superficial aquifer water resources; natural flow regimes largely intact; progress on recovering pressures in the underlying Great Artesian Basin|
|Murray–Darling||High level of development of both groundwater and surface water systems, apart from Paroo and Warrego rivers; drought-breaking floods and recovery of entitlements for the environment are positive|
|North-east Coast||Low level of development in north; higher levels of development in central and south-east, with associated changes in flow regimes|
|North-western Plateau||Very low level of development, with mostly local self-supply abstraction or mine de-watering; very little (and sparse) data|
|South Australian Gulf||Flow regimes and groundwater levels strongly affected by high levels of development and flow regulation|
|South-east Coast||High levels of development in areas serving Sydney and Melbourne, and in areas east of Melbourne; less change in flow regime due to development in East Gippsland and north New South Wales coast|
|South-west Coast||Flow regimes greatly affected by high level of development and river regulation, and prolonged drought (changed climate); highly developed groundwater systems have widely lowered watertables beyond the reach of many dependent ecosystems on Swan Coastal Plain|
|South-western Plateau||Very low level of water resource development, with mostly local self-supply abstraction or mine de-watering; very little data|
|Tasmania||Low level of development relative to water availability; flow regimes changed by regulation of flows associated with hydro-electricity generation in places|
|Timor Sea||Apart from Ord River and Darwin water supply, very low levels of development, leaving flow regimes largely unchanged|
|Recent trends||Improving||Stable||Confidence||Adequate high-quality evidence and high level of consensus|
|Deteriorating||Unclear||Limited evidence or limited consensus|
|Evidence and consensus too low to make an assessment|
|Grades||Very good: There are no significant changes in long-term water flows or levels as a result of human activities|
|Good: Some flows or levels have changed in some areas, but not to the extent that the changes are significantly affecting ecosystem function|
|Poor: Some flows or levels have changed substantially in some areas, to the extent that ecosystem function is significantly affected in some areas|
|Very poor: Flows or levels have changed substantially and over a wide area. Ecosystem function is seriously affected|