Discover surface water
Very little of the Earth’s water is fresh water (about 2.5%) and less than 2% of the fresh water is surface water. Just under 70% of fresh water on earth is contained in glaciers and ice caps, and about 30% is groundwater.
In the Murray–Darling Basin, one of the flattest catchments on Earth, surface water exists against the odds. On average, the Basin receives about 530,000 GL of water as rainfall. Of this amount, 94% evaporates from waterways and floodplains or is transpired by plants (ie. used and released back into the atmosphere).
The average annual amount of water that flows in the Basin’s waterways is about 32,500 GL. But given Australia’s variable climate, this amount can vary from under 7,000 GL (in 2006) to almost 118,000 GL (in 1956)
Influence of climate
There is considerable variation in surface water runoff from 1 region of the Basin to another. This is related primarily to different climates but also to the extent of seasonal variation within a region from year to year.
There is a strong east-west rainfall gradient and a strong north-west to south-east temperature gradient across the Basin. Rainfall is summer dominant in the north and winter dominant in the south.
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. For example, the upper Murray, Murrumbidgee and Goulburn–Broken rivers catchments account for around 45% of the Basin's total annual runoff from 11% of its area. The upper Murray catchment alone accounts for 17% of runoff from 1.4% of the land area. By contrast, the group of rivers flowing into the Darling River contribute 32% of the Basin's annual runoff from 60% the area. The Darling River catchment itself accounts for 11% per cent of the Basin's area, but less than 0.5% of annual runoff.
Many of the Basin’s waterways are ephemeral, which means they flow for parts of the year or only in seasons of high rainfall. Overall, some 86% of the Basin contributes virtually no runoff to the river system, except in times of flood.
During very wet periods, water flows over the banks of the waterways and out onto wide, flat floodplains. Water evaporates quickly from the shallow expanses of water.
The Murray–Darling system therefore carries one of the world's smallest volumes of surface water for its size. Mean annual discharge (the water that flows past a certain point) is 0.4 megalitres per second (ML/s). By comparison, the Amazon is 290 ML/s and the Ganges–Brahmaputra is 38 ML/s.
In the highly variable Australian climate, water storage and controlled release have been necessary to ensure that almost 3.5 million people who depend on the Basin’s water have access to drinking water in the driest of years and in periods of drought.
Historically, the natural flow of surface water was altered with the construction of dams to support and foster large irrigated agriculture schemes such as the Riverland in South Australia that was developed in the 1880s and the Murrumbidgee Irrigation Area in southern New South Wales that was developed in the 1910s.
Dams were also constructed to minimise the impact of flooding on property and towns that had established along the rivers.
The total volume of publicly-managed water storage in the Basin is just over 34,000 GL — that is more than the surface water run-off from an average year's rainfall. This magnitude of river regulation and consumption has reduced water flow through the river system and out through the Murray Mouth.
Computer modelling during the Basin Plan's development showed that before water resources were developed by European settlers, an average of 12,500 GL/y would flow out to sea (flushing out salt and providing many riverine environments with life-giving water). In 2009, flow though the river mouth had reduced to a long-term average of 5,100 GL/y. The Basin Plan strives to set sustainable diversion limits of water used by Basin communities and industries, in order to improve flows in the rivers of the Basin and to maintain adequate flows through the Murray Mouth.
Surface water use
The water resources of the Basin are highly developed. Long-term averages (1895–2009) indicate that approximately 42% of total surface water runoff is diverted from the river systems. Surface water is extracted from the rivers in a number of ways. It may be pumped directly from the river onto a property alongside the river or it may be diverted by weirs into the extensive channel system of an irrigation scheme and transferred to properties tens or hundreds of kilometres from the diversion point. Surface water or floodwater may also be 'harvested' from overland flow, as is practiced in southern Queensland and northern New South Wales.
A long-term average of almost 11,000 GL of water per year is diverted for household, urban or agricultural use. An additional 2,700 GL is intercepted by on-farm storages and forestry plantations.
In developing the Basin Plan, we determined that the long-term average environmentally sustainable level of take for surface water is 10,873 gigalitres per year (GL/y). To achieve this level of take, the Basin Plan determined that 2,750 GL/y would need to be recovered from the 2009 baseline diversion level.
Depending on the season and water allocations, somewhere between 80 and 90% of water diverted from the river systems, directly or via irrigation schemes, is used for irrigated agriculture. After agriculture, the water supply industry is the largest user, which is water that is lost or unaccounted for as a result of transmission to the end user. Households, mining, manufacturing and other industries each only use a few per cent of consumptive water.
The Bureau of Meteorology produces an annual National Water Account. The Basin is one of the regions for which a report is prepared. The account documents information about water stores and flows, water rights and water use. It also reports on the volumes of water traded, extracted and managed for economic, social, cultural and environmental benefit. The account reports on surface water and groundwater.