Australia: The Land Where Time Began
This basin includes 26 major catchments covering a total area of more than 1/7 of the Australian continent. It is Australia's biggest river system, but even in good times, and including all the water taken by farming along its routs, its annual flow is less than 1 day's flow of the Amazon. This situation is not unexpected, considering the fact that it is on the world's driest inhabited continent. It continues to flow throughout the year, but only because some of its flow is from relatively well watered regions, most of its drainage being from arid areas with erratic, occasional rain. It is essentially inward-flowing. Some of the tributaries that combine to form the Darling River arise in the Great Divide, but they cross some very arid country before reaching the Murray River, so they lose a lot of their water to evaporation in the hot dry areas they pass through. They contribute 12 % to the Murray flow.
Tributaries of the Murray in New South Wales flow westward from the Great Divide, those in Victoria drain a part of the Great Divide where it curves westwards, so they flow northwards. 13 % of the Murray flow is contributed by the Murrumbidgee system. 75 % is contributed by tributaries of the Murray River upstream of the point where it is joined by the Murrumbidgee.
The Darling system receives most runoff from summer rain, while the Murray system depends more on winter rainfall. The extremes of the climate means that the flow of the Murray-Darling System is also very variable, depending on the highs and lows of the ENSO weather system.
Because the Murray-Darling system passes though Australia's major food bowl, most of the water coming down the Murray is diverted for irrigation long before it reaches the sea. The amount reaching the sea before the present extremely dry time was only about 20 % of its full potential. Now it is danger of stopping flowing completely, as the farmers along various sections of its flow are reluctant to give up their irrigation water to keep the river flowing. The result is that the system is dying. It can no longer supply sufficient water to irrigate crops and still continue to flow to its mouth.
One of the biggest problems for the continued flow of the Murray River in dry times is the growing of cotton at several places along its tributaries. Cubbie Station, the largest irrigated property in the Southern Hemisphere, is one of the biggest users of water from the system, with its water storage the size of Sydney Harbour, using between 200,000 and 500,000 megalitres of water to irrigate its crops with a total area of 130 km2, of which cotton has a large water requirement, made even more environmentally unfriendly by the necessity to apply large amounts of pesticides and herbicides. Some say crops such as cotton and rice should not be grown in Australia because of their large water requirements.
In New South Wales, other cotton producers withdraw large amounts of water from the Lachlan River, though on a smaller scale than Cubbie Station, but enough for the pumps to have caused the river to flow 'backwards' towards the pumps in dry times.
There are 26 sub-catchments in the Murray-Darling Basin - Avoca, Benanee, Border, Broken, Campaspe, Castlereagh, Condamine-Culgoa, Darling, Goulburn, Gwydir, Kiewa, Lachlan, Lake George, Loddon, Lower Murray, Macquarie-Bogan, Mallee, Moonie, Murray Riverina, Murrumbidgee, Namoi, Ovens, Paroo, Upper Murray, Warrego and Wimera-Avon.
Rivers and floodplain wetlands
According to the authors3 within the approximately 1,061,000 km2 of the Murray-Darling Basin catchment are 17 large, complex floodplain wetland systems, each associated with relatively large rivers that are either perennial or intermittent. The sources of most of these rivers are mainly in temperate uplands, more than 500 m above mean sealevel, on the margins of the Basin in the south and east, flowing inland towards the lowlands of less than 300 m above mean sea level across areas of decreasing elevation to depositional settings that are in semiarid to arid (dryland) areas (Thoms & Sheldon, 2000; Ward et al., 2002; Warner, 1986), the result being that these lowland-dryland rivers are allogenic, rarely receiving tributaries along their middle and lower reaches, their water typically being supplied by the headwater catchments. Because of the lack of tributaries downstream from their headwaters these rivers are unable to replenish the water lost to evaporation, infiltration and distributaries on the dryland plains they flow across.
Along these lowland reaches of these inland-flowing rivers they are surrounded by dryland environments. Floodplain wetlands that are intermittent and semi-permanent along these arid reaches of the rivers include marshes that are lotic (flowing water), swamps that are lentic (standing water), as well as riparian woodlands that are maintained by river floods, their anabranches and their distributary channels that are outflowing and floodouts (places where water flows across the land following the breakdown of channels. Examples of this are:
(Kingsford, 2003; Kingsford & Thomas, 2004; MDBC, 2006). The extensive floodplain wetlands comprise about 6 % of the Murray-Darling Basin. When the lakes are included the area is 6.5 %, that makes up more than 95 % of all wetland areas in the catchments of the inland (Kingsford et al., 2004).
In the Murray-Darling Basin the lowland-dryland and floodplain wetlands can be severely affected by climatic swings and reduced water supplies as they are dependent on the water flowing from the middle and upper catchments of the system for their very survival. The natural variability of the climate of the area, changes occurring over years to decades, impacts the rivers and wetlands, such as changes in direct rainfall and maximum and minimum temperatures, as well as affecting the hydrology of the catchment. In the Pacific, Indian and Southern Oceans large scale temperature fluctuations influence the air pressure and circulation patterns, as well as weather and rainfall.
According to the authors3 as the floodplain wetlands and aquatic ecosystems have adapted to the erratically variable climate it is difficult to generalise or simplify the requirements and preferences of these ecosystems in regard to water flow and flood regimes.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|