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