Tuesday, December 20, 2011

Soil Salinity in South Eastern Australia

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Soil Salinity in South Eastern Australia

Soil salinity is one of the nation’s greatest problems and the danger it poses to Australia’s agricultural industry is rising. Salinity occurs in two main forms; primary and secondary. The former occurs naturally throughout the world in arid climates. According to the Western Australian department of agriculture, million hectares of Australian land are effected by natural salinity. This is generally not a problem as environments are able to adapt to natural salinity occurring.

Secondary Salinity, on the other hand, is a result of a man-induced “change in the water balance, leading to more water in the soil and a rising watertable towards the surface”.

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In south-eastern Australia, soil salt exists as a consequence of two main geological occurrences. One, from the breakdown of parent rock; the process of which takes thousands of years. The other is in from wind blown salt, predominantly in the form of rain water from the ocean. The amount of salt deposited by wind and rain should not be undervalued. It is estimated that between 0 kg/ha (usually inland with low rainfall) to more than 00 kg/ha (coastal with high rainfall) are deposited every year. According to the Department of Agriculture, the salt then becomes “stored in the landscape through the balance of salt input (through rainfall) and loss through leaching or drainage from the catchment. In areas where potential evaporation is high and rainfall is low, salt falls on the landscape but is not flushed out. It therefore accumulates, usually below the root zone of original native vegetation”.

‘Recharge’ is the term given to the water which gets through to the watertable. If the recharge levels are high enough, the watertable rises bringing salt to the surface and hence causing major problems. In south-eastern Australia, one of the major causes of the salinity problem has been land clearing. Prior to European settlement the vast majority of the land was covered in thick bushland. This contained perennial plants with deep roots that penetrated deep into the ground and absorbed a large amount of water seeping down to the watertable; preventing it from rising. When the land was cleared however these trees were removed and replaced by farm crops which have only very small roots. While there is still the same amount of water going into the soil, the smaller roots are unable to absorb as much recharge water and consequently more water enters the watertable and it rises. Due to the lesser tolerance of salt in crops than in bushland the problem is increased because once the watertable rises, crops have a much greater difficulty dealing with the increased salt levels in the soil.

A particular problem in southeastern Australia (though prevalent in most other states) is irrigation which significantly contributes to recharge levels and increases the level of the watertable.

The department of agriculture states that “Salinity is usually noticed first when plants grow poorly, and yields of farm plants (crops and pastures) are reduced by more than 5 to 0%. In severe cases, bare or ‘weedy’ patches develop with salt obvious on the surface. These patches are known as ‘salt scalds’”.

Another problem more specific to South Eastern Australia is the salinisation of river systems, particularly the Murray. According to the South Australian Dryland Salinity Committee. “Irrigation areas were initially considered to be the major driver of rising stream salinity in South-eastern Australia. Enhanced recharge under intensive irrigated agriculture developments adjacent to the Murray River in South Australia over the last 100 years has resulted in increased discharge of saline groundwater into the river, resulting in rising river salt loads and salinities.” More recently, it has also been acknowledged that dryland farming itself, as a result of the clearance of native vegetation has contributed to saline groundwater running into the river system.

Aside from the environmental and agricultural problems that salinisation of the Murray brings, another added predicament is that Adelaide draws much of its water supply from this river.

While Salinity has serious agricultural effects, it also poses serious environmental issues such as a significant loss in biodiversity, contamination of water in river systems and loss of plant life. Perhaps the greatest problem however, is that once it occurs in a particular area, it is very difficult to restore it to its original state.

As the saying goes, prevention is far better than cure. Over the last few decades, the government has realized the gravity of the salinity problem and various efforts have been made to try and restrict irrigation as well as implementing a number of farming schemes. One of the key points in combating the salinity problem in South Eastern Australia is efficient water use. The most effective way in implementing this, according the South Australian Salinity Strategy Committee is in “remnant vegetation protection and re-vegetation, including farm forestry”. In South Eastern Australia over 18 000 ha of vegetation has been fenced in the last ten years and in Victoria “an additional 1 000 ha of commercial plantations were established in 1, building on a base of 50 000 ha across the state”.

Irrigation, one of the most contributing factors to soil salinity has had a number of strategies put in place to reduce its effect on recharge levels. In Victoria, the state government has been in the process of putting in water allocation systems for farmers, covering around 70% of Victoria’s diverted water resources. The Murray-Darling Basin Ministerial Commission cap has set a limit on water diversions across the whole Murray-Darling Basin.

Another, very different strategy used in Victoria has attempted to lessen the impact of salinity by adaptation, in areas that are too saline to be reclaimed, or other methods are not economically viable. Three main approaches, to varying degrees have been used in these cases. The first has been growing salt-tolerant/resistant crops or pasture. This method has been particularly used in areas too saline to be of any more use. The second has been investing in saline aquaculture, such as farming salt-water fish and lastly desalinization of some of the salty groundwater and using the desalinated water for stocks or crops.

In high rainfall areas of Victoria, they have attempted to change farming systems in order to combat salinity, for example from wool production to farm forestry

Despite methods to prevent or control salinity, approximately 500 ha are effected each year by salinity in Victoria. While irrigation controls have helped reduce the amount of potential damage, it is still a major contributor to the problem and only by further management can its effect on recharge levels be reduced. Another avenue is further research into more effective ways of irrigating crops and pastures.

The method of adapting salt-effected land by either growing salt-tolerant plants or using drainage systems is effective in reducing the environmental effect of an existing problem but it does nothing to prevent further spreading of salinity in other regions.

By far the most effective means of controlling salinity is re-vegetation. According to one source, up to 60% to 80% of land in some catchments in Victoria. Unfortunately, this is also very expensive, and in many instances not economically feasible. The re-vegetation that has occurred has been found to be extremely effective in reducing the watertable again and maintaining a stable environment, and enabling biodiversity to reach its original levels.

Despite these efforts, salinity is still a growing problem, and although it is being confronted, the momentous task of firstly stemming its increase and then reducing it is a major problem that must be addressed in the future.

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