The range in climate across the Murray–Darling Basin reflects it size, of more than 1 million square kilometres, and its diverse geography ─ from rugged mountains to flat semi-arid plains.
The climate of the Basin is sub-tropical in the north, semi-arid in the west and mostly temperate in the south. Rainfall graduates from summer dominant to winter dominant, from north to south. The eastern side of the Basin has high average annual rainfall, up to 1,500 mm and in the south, snow falls for several months each winter on the peaks of the Great Dividing Range. The western side of the Basin is typically hot and dry, and average annual rainfall is generally less than 300 mm.
Evaporation rates in the Basin are high, with 94% of the rainfall that falls in the Basin being used by plants (transpired) or evaporating from the land and surface water. The low-lying topography of the Basin ─ warm to hot semi-arid conditions in most regions ─ and the meandering and slow-flowing nature of the creeks and rivers, all combine to make an environment characterised by high evaporation.
The catchments draining the Great Dividing Range on the south-east and southern margins of the Basin make the largest contributions to total runoff, despite their smaller size. Overall, around 86% of the Basin contributes almost no runoff to the river system, except in times of flood.
Long-term climate averages in the Basin
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Annual rainfall is 470 mm
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Potential evapotranspiration is 1,174 mm
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Daily temperature is about 25°C in January and 9°C in July.
The climate is highly variable and weather conditions from season to season can be influenced by many types of weather systems and their complex interactions. Climate variability is a feature of the Australian landscape and has been for hundreds of thousands of years, as the continent cycled in and out of glacial periods (ice ages).
The Bureau of Meteorology prepares an annual National Water Account for several regions of Australia, including the Murray–Darling Basin. In addition to a statement of water received, stored and supplied, the document presents annual weather records compared with long-term climate records.
Climate change
The climate and its impact on rainfall, water availability and water quality are expected to change in the future at a rate unprecedented in the experience of humankind.
Climate change studies predict that the Basin’s climate is likely to become drier in the future and more variable, due to the rising concentrations of greenhouse gases in the atmosphere. So, in addition to more extreme droughts, there may also be more extreme floods.
Climate models predict a reduction in winter rainfall for south-eastern Australia, translating to a considerable reduction in both winter and annual runoff. For example, if the global average temperature increases by 1°C, average annual rainfall in south eastern Australia is expected to decline by between 0% to 9%. Average annual runoff is expected to decline by between 2% to 22% for the southern section of the SEACI region (south of 33° latitude). Reductions in rainfall and runoff for a 2°C of global warming are projected to be approximately double these. Projected changes across the northern half of the SEACI region are less certain, with some models projecting an increase, and some a decrease in rainfall and therefore runoff.
Average surface water availability across the entire Basin for 2030 is projected to fall by 10%. The impact is expected to be greater in the southern Basin, and these predictions are also more reliable in the south.
Recent work by CSIRO and the Bureau of Meteorology under the Climate Change in Australia program provides comprehensive information about climate change projections in various regions of the Basin.The prospect of climate change means that many factors need to be considered when planning for the future management of water for people, agriculture and the environment. While it is impossible to ‘drought-proof’ the Basin, much can be done to build resilience and secure strong and vibrant communities and industries, and ensure there is enough water for a healthy environment.
Climate change considerations are embedded in the policy making, water resource planning, operations and asset management of the Murray─Darling Basin Authority. The Basin Plan is adaptive and designed to help us respond to anticipated climate change. The computer modelling that supports the Basin Plan incorporates 114 years of climate data from south-eastern Australia. This means that our planning and management have been designed around what is arguably one of the most variable climate systems in the world.
With new knowledge and information, as well as results of monitoring and evaluation, state and local governments, industry sectors and individuals can modify how they use water to ensure the long-term sustainability of the Basin.
