The best available projections for Australia’s future climate are presented in Chapter 2 of this report. These projections indicate that warming of the continent is highly likely. Rainfall projections are less certain, but the ensemble results of the global climate models and emissions scenarios in the fourth assessment report of the Intergovernmental Panel on Climate Change suggest that the southern part of the continent is likely to be drier than the past. The whole continent is expected to warm.
CSIRO projected the likely future yields of surface water and groundwater systems for large and important regions of Australia, based on the same climate scenarios (see Box 4.2). These analyses characterised the impacts of water diversions on flow regimes under the historical climate, and the likely future impacts on flow regimes under conditions of planned developments and future climate. The key findings, by region, are outlined below.
- The impacts of climate change by 2030 are uncertain; however, surface water availability across the entire division is more likely to decline than to increase (Figure 4.22). A decline in the south is more likely than in the north. In the south, a very substantial decline is possible. In the north, significant increases are possible. The median decline among climate projections for the entire division is 11% (9% in the north of the Murray–Darling Basin and 13% in the south).
- The median water availability decline would reduce total surface water use by 4% under current water-sharing arrangements. It would reduce flow at the Murray mouth by 24%, to be 30% of the total pre-development outflow. In volumetric terms, most of the impact of climate change would be borne by the environment rather than by consumptive water users.
- The relative impact of climate change on surface water use would be much greater in dry years. Under the median 2030 climate, diversions in driest years would fall by more than 10% in most New South Wales regions, around 20% in the Murrumbidgee and Murray regions, and from around 35% to more than 50% in the Victorian regions. Under the dry extreme 2030 climate, diversions in driest years would fall by more than 20% in the Condamine–Balonne, around 40–50% in New South Wales regions (except the Lachlan), more than 70% in the Murray, and 80–90% in the major Victorian regions.
- Climate modelling indicates that future rainfall will be similar to historical averages. Evapotranspiration (transfer of water to the atmosphere from evaporation and plant transpiration) may be slightly higher.
- Under the median modelled future climate, rainfall would increase by 1% under the wet extreme and decrease by 3% and 7% under the median and dry extremes, respectively. Run-off would increase by 1% under the wet extreme and decrease by 5% and 10% under the median and dry extremes, respectively.
- Climate change between now and 2030 is expected to have only a very minor impact on groundwater levels.
- About 1–2% of the area’s subcatchments and 6–15 of the 150 key ecological sites would potentially be impacted under future climate and current levels of development.
- The appropriate historical climate baseline for future projections is the period 1975–2007 because a climate shift occurred in the region in the mid-1970s—rainfalls after this time have been 10–15% lower than the long-term mean going back to 1900 (associated with a 50% decrease in run-off into local supply dams). The post-1975 rainfall is currently used by Western Australian water managers and suppliers for planning.
- Almost all combinations of global climate models and emission scenarios indicate at least some reduction in rainfall for the area by 2030. However, current models account for only about half of the drying experienced in the region since 1975, and water yield projections may therefore be conservative.
- Future mean annual surface water yields in the region are likely to be 24% lower, on average, by 2030 (with a possible range of 4–49%), compared with 1975–2007.
- Groundwater demonstrates greater resilience to climate change where watertables are now within a few metres of the surface. As these watertables fall, evaporative and drainage losses are reduced, which results in increased net recharge. Groundwater yields are projected to fall significantly under areas of native vegetation. Yields would decrease by a third to a half in places such as the Gnangara Mound, which provides Perth with most of its drinking water. Groundwater levels are expected to continue to rise under dryland agriculture, which covers more than 56% of the Perth Basin. Future groundwater yields in the region are likely to be 2% lower, on average, by 2030, with a range of +2% to −7%. Many groundwater-dependent ecosystems have already been severely impacted by the drying climate since 1975.
- The environmental impacts are most significant for ecological river functions that require high river flows, and where falling groundwater levels affect wetlands that are dependent on groundwater levels being at or above the land surface.
There have been equivalent studies of the water supply catchments of Sydney, Melbourne and Adelaide, based on similar climate modelling inputs and local hydrological modelling. These form crucial inputs into metropolitan supply planning for those regions.
(2011). Inland water: Climate change . In: Australia state of the environment 2011, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/science/soe/2011-report/4-inland-water/6-risks/6-1-climate-change, DOI 10.4226/94/58b656cfc28d1