Climate change

Since the 2016 state of the environment report, the impacts of climate change have continued to be observed through the record high temperatures recorded during 2019–20, and the 2019–20 bushfire season (BOM 2020e, Royal Commission 2020, van Oldenborgh et al. 2021) (see the Climate chapter).

The Climate Council of Australia, in its 2018 report Deluge and drought: Australia’s water security in a changing climate (Steffen et al. 2018), identified that the ongoing impacts of climate change will include a greater frequency of severe droughts, more intense extreme rainfall events, a continuing decrease in cool-season rainfall and an increase in the time spent in drought.

These changes mean that there will be less water available for agriculture, urban water supplies and ecosystems in coming decades, especially in southern Australia, including areas around Melbourne, Adelaide and Perth (see the Climate impacts on aquatic biodiversity section in the Biodiversity chapter).

Changes in hydrology

Australia is already experiencing changes in hydrology due to climate change – most notably, a shift towards drier conditions in the south-west and south-east regions of the country. The 2020 state of the climate report (BOM & CSIRO 2020) shows that, during the past 20 years (2000–19), rainfall during April to October has been very low in southern parts of Australia (Figure 33). In south-western Australia, since 1970, there has been a 16% decrease in rainfall in April–October and a 20% decrease in May–July.

In northern Australia, the wet season from October to April has become wetter (Figure 34). However, the 2018–19 and 2019–20 wet seasons were below average as a result of the persistence of a positive phase of the Indian Ocean Dipole into December.

The declining rainfall has led to reduced streamflow. Analysis of streamflow data from the Bureau of Meteorology’s Hydrologic Reference Stations has identified declining annual median streamflows in several drainage divisions: the Murray–Darling Basin, the South West Coast, the South Australia Gulf, the South East Coast (Victoria) and the South East Coast (New South Wales). Of the 467 Hydrologic Reference Stations in Australia, three-quarters of the sites showed a declining trend and one-quarter showed a statistically significant trend (BOM & CSIRO 2020).

On a state and territory basis, there are significant decreasing trends in annual streamflows in south-eastern Queensland, and at most gauging stations in New South Wales, Victoria, south-eastern South Australia, south-western Western Australian and north-western Tasmania (Zhang et al. 2016).

Declining rainfall and a subsequent decline in streamflow and inflows to headwater storages in south-west Western Australia have been observed since the 1970s (Petrone et al. 2010). It was recognised that the lower observed streamflows would need to be accounted for in future water resource management (Rodgers & Ruprecht 1999). In response, the Western Australian Department of Water and its successors have reduced the reliance on surface water and increased the amount of water supplied from climate-resilient water sources, including desalination, and from groundwater. In 2019–20, nearly all Perth’s water supply was sourced from groundwater and desalination plants, in approximately equal percentages (BOM 2021b).

Future changes

Recent simulations from the global climate models project that, over coming decades, Australia will see continued warming, with more extremely hot days; a decrease in cool-season rainfall in the south and east, leading to more drought periods; a longer fire season; and more intense short-duration heavy rainfall (BOM & CSIRO 2020).

These projected climate change impacts will affect Australia’s hydrology. For example, Chiew (2006) estimated that a 1% reduction in rainfall could lead to 2–3.5% reduction in streamflow. This would result in:

• lower inflows into dams in southern Australia, with a resulting decrease in water security
• greater reliance on climate-resistant water sources such as desalination and recycling
• an increased frequency and severity of droughts, which would increase demand for water, and have adverse effects on aquatic ecosystems and the flora and fauna that inhabit them
• an increase in the frequency and severity of floods, which, in addition to the impact on lives and livelihoods, may necessitate the upgrading of infrastructure such as dams, treatment plants and flood levees, at considerable cost.

The combination of decreasing rainfall and higher evapotranspiration will result in decreasing soil moisture. This will increase water demand and also lead to stress in plants, reduction of habitats and an increase in the occurrence of acid sulfate releases (see Acid sulfate releases) (Steffen et al. 2018). Decreasing soil moisture is also often coupled with increasing soil compaction, which can reduce the permeability of the soil and exacerbate run-off.

Climate change effects will also affect the quality of water: increases in the severity of floods and droughts will change sediment loading, chemical composition, total organic carbon and microbial quality of drinking water (WHO 2011).

Bushfires

The 2019–20 bushfire season in south-eastern Australia was described as ‘the worst fires in our history’, ‘exceptional in size and impact’, ‘unprecedented’ and ‘by far Australia’s costliest natural disaster’ (Huf & McLean 2020) (see the Extreme events chapter). The Bureau of Meteorology, in its Annual Climate Statement 2019 (BOM 2020a), stated that ‘The extensive and long-lived fires appear to be the largest in scale in the modern record in New South Wales, while the total area burned appears to be the largest in a single recorded fire season for eastern Australia’. By the end of February, the Australasian Fire and Emergency Service Authorities Council estimated that more than 170,000 square kilometres (km²) had been burned. Although fires in the Northern Territory in 1969–1970 and 1974–75 were larger – burning more than 450,000 km² – they did not cause the same damage in terms of loss of life and damage to property and the environment (Ellis et al. 2004).

The immediate impacts of the bushfires on water were to water treatment plants that lost the ability to continue to provide reliable, safe drinking water. The fact that the fires occurred after a sustained period of severe rainfall deficiency meant that many communities had insufficient water available for firefighting needs and were at risk of running short of water (Khan 2020).

The longer-term impacts are on the quantity and quality of water from dam catchments that have experienced extensive destruction from bushfires. These include (White et al. 2006, Alexandra & Finlayson 2020):

• mobilisation of sediment, ash, nutrients and other contaminants during rainfall and flash floods
• increased turbidity, nutrient loads and contamination, and low dissolved oxygen levels
• increased occurrence of algal blooms, hypoxic black water and fish deaths.

The bushfires resulted in the destruction of 30% of the Warragamba Dam catchment, with impacts on Sydney’s water supply; high-intensity burning of 39% of the Corin Dam catchment in the Australian Capital Territory (Icon Water 2020); and burning of 57% of the Hume Dam catchment in the southern Murray–Darling Basin.

A consequence of the burning of the Warragamba Dam catchment was seen in February 2020. A significant rainfall event saw run-off from the catchment increase the volume in Warragamba Dam from 41% to 75% capacity in a matter of days. However, the run-off also brought large quantities of sediment, ash and debris into the storage; as a consequence, Warragamba Dam was not used as a source of water supply for Sydney for more than a week. As late as September 2020, when Warragamba Dam reached full supply capacity, concerns about the quality of supply were such that Sydney’s desalination plant was still running at 20% capacity as a contingency in case it was needed to respond to water quality issues related to the bushfires.

The longer-term effects on run-off from the burned catchments and inflows to the dams are yet to be seen, but the deforestation and regrowth of catchment vegetation (Figure 35) will result in changes to the catchment yield and water security over coming years and decades (Langford 1976, Kuczera 1987, Jayasuriya et al. 1993, Brookhouse et al. 2013).

Heavy rainfall and flooding

Periodic significant flooding is an inherent feature of Australia’s environment, a consequence of the continent’s variable climate. From 2016 to 2021, widespread and major flooding occurred on several occasions in various parts of Australia.

Before the shift to drier conditions in 2017, there were 2 major occurrences of widespread flooding in eastern Australia. Following the autumn 2016 breakdown of the 2015–16 El Niño climate pattern, very heavy rainfall occurred across much of the east coast and Tasmania in June 2016. Flooding occurred in 32 river systems extending from south-east Queensland to Tasmania. All river basins in northern Tasmania experienced major flooding, with the South Esk River in Launceston experiencing its most significant flood in 90 years, and the Mersey River setting a new flood level record (BOM 2016).

The wet conditions continued later into 2016, and resulted in the wettest September on record for New South Wales and the Murray–Darling Basin, and the second wettest for Victoria and South Australia. With catchments already wet from rainfall earlier in the year, the heavy rainfall led to major flooding across central New South Wales, western Victoria, parts of western Queensland, and areas around Adelaide in South Australia. The Lachlan River at Forbes Iron Bridge recorded its highest peak since 1952, and an evacuation order was issued for parts of the town (BOM 2017).

In January–February 2019, an intense and very slow-moving monsoon low over northern Queensland brought very heavy rainfall to tropical Queensland. Around Townsville, many new records were set for multiday rainfall, with 12-day totals of more than 2,000 millimetres (mm) at some higher-elevation sites. The rain led to major flooding in coastal communities between Daintree and Mackay, including in the Burdekin, Ross, Bohle, Haughton, Herbert and Black rivers. Ross River Dam near Townsville reached more than 200% capacity, and its spillway gates were fully opened on 3 September. Extensive and persistent major flooding also occurred in the Gulf Country, including in the Cloncurry, Leichardt, Flinders and Norman rivers. The floodwaters spread across an area of more than 1.5 million hectares, causing infrastructure damage and extensive stock losses (Figure 36) (BOM 2019a). The period of flooding in the Gulf Country was extended with the arrival of ex-tropical cyclone Trevor, bringing more heavy rainfall to the region at the end of March (BOM 2020a).

In early 2020, wet conditions returned to south-eastern Australia. In late January–February 2020, widespread heavy rainfall occurred over coastal New South Wales, inland southern Queensland and south-east Queensland. This led to significant flooding in a number of Queensland catchments, including the Georgina–Eyre, Logan–Albert, Condamine–Balonne and Warrego catchments, as well as major flooding in the Orara, Hawkesbury-Nepean and Georges rivers in New South Wales. Run-off from the event almost doubled Sydney’s accessible water in storage, with Warragamba Dam, Sydney’s largest storage, increasing from 42% to 81% capacity in only 10 days, an increase of 800 gigalitres, or enough to fill Sydney Harbour one and a half times. Reduced vegetation cover due to the 2019–20 bushfires over a large proportion of the catchment may have increased the speed and volume of run-off (BOM 2021g).

In late March 2021, unusually extensive and persistent rainfall returned to New South Wales, and heavy and extensive rainfall occurred in both coastal and inland areas. Numerous locations on the north coast had more than 600 mm of rainfall over 4 days. The New South Wales coastal region experienced its wettest week on record, and New South Wales as a whole had its second wettest day on record (BOM 2021h).

Unlike the previous year, this heavy rainfall encountered catchments that were already wet, leading to more widespread and severe flooding across much of the state. Along the coast, major flooding occurred in the Clarence, Nambucca, Manning and Gloucester, Hastings, Camden Haven, Paterson and Williams, and Hawkesbury-Nepean rivers, and Wollombi Brook, with record flood heights recorded at several locations. Sydney’s Hawkesbury-Nepean catchment experienced its most significant flooding in more than 30 years. In inland areas, major flooding also occurred near the New South Wales – Queensland border in the Gwydir, Mehi, Macintyre and Condamine rivers, moving downstream into the Baaka/Barka – Darling catchment in April 2021. Storage levels in the northern Murray–Darling Basin catchments had 2 of their 3 largest single-day increases since 1993 (BOM 2021h).