Assessing the state of inland water environments in 2011 is difficult, due to the effects of recent droughts. Long, dry periods are a natural feature of the continent. We have seen the millennium drought that south-eastern Australia experienced between late 2000 and 2010, as well as the much longer drying period in south-western Australia from 1975 to the present. The Bureau of Meteorology has summarised the modern history of major droughts since the late 19th century.43
To what degree can we attribute the changes in inland water environments to the ‘natural’ process of drought, and to what degree are recent droughts themselves ‘natural’?
There is no absolute answer to the first question. There are, however, reasons that a river or wetland might be made more vulnerable to a drought than it would naturally have been before European settlement. In ‘regulated’ river systems (like the Murray–Darling Basin, the Ord River or the Onkaparinga River), where dams and other control structures are managed to provide long-term water security, the rules of operation often do not favour maintaining environmental flows over water security during times of water scarcity. This was highlighted in the CSIRO Sustainable Yields assessment of the Murray–Darling Basin; under current arrangements, although allocations drop in times of drought, environmental flows drop relatively more in all valleys except the Gwydir.
Even unregulated inland systems may be made more vulnerable to drought through the cumulative impacts of other stresses on the system—such as sedimentation, eutrophication (excessive nutrients in a body of water), pests, weeds and the removal of streamside vegetation—and damage to the connectivity of river and wetland systems, which might otherwise provide regional refuges for aquatic species during dry times.
The other question of drought versus human-induced climate change is considered in Chapter 3: Atmosphere. The possible effects of emerging pressures resulting from climate change are discussed in Section 6 of this chapter.
By late 2000, it was clear that much of south-eastern Australia was in drought. In early 2008, the Bureau of Meteorology stated that the drought that was devastating south-eastern Australia was ‘very severe and without historical precedent’, with rainfall totals at record lows in many regions, including many critical to the Murray River. At that point in the drought, rainfall figures were similar to the severe drought that lasted from 1939 to 1945, and the Federation drought, which ran from 1895 to 1903, although average daily temperatures were higher by about 1 °C.
The drought eventually broke, dramatically, with widespread flooding in the Murray–Darling, southern Victoria and south-east Queensland by late 2010; 2010 was Australia’s wettest year since 2000 (690 millimetres of rainfall compared with the long-term average of 450 millimetres; Figure 4.15). Exemplifying how fast the water resource situation turned around, in late December 2009, total water in storage in the Murray–Darling was at 26.1% of capacity (6575 gigalitres); at the same time a year later, it was at 80.8% (20 358 gigalitres).
CSIRO44 concluded that the drought in at least the northern part of the affected region (south-east Queensland) was probably associated with decadal variability rather than climate change. However, further south, there is evidence that observed changes in the large-scale circulation affecting south-eastern Australia are associated with global warming, which is therefore likely to have contributed to the drought.45