Australia struggles against salination
The war on salt will be long, it will be hard - but there must be no withdrawal, according to the latest scientific research into dryland salinity, which concludes it may take generations to restore some salt-affected landscapes to fertile condition, while others may never recover.
A report by Dr Tom Hatton and Dr Ramsis Salama of Commonwealth Scientific and Industrial Research Organisation (CSIRO) Land and Water indicates that arresting the salinity crisis over large tracts of the Australian wheatbelt will be “akin to trying to turn round a supertanker”.
“But that is no reason for us not to try,” Dr Hatton says. “Ethically, we have to go ahead, planting trees and taking whatever steps we can to reduce the salt burden on the landscape.”
For farmers and land managers who have pinned their hopes on tree planting to quickly stem the saline onslaught, the news is grim. The volumes of salt beneath the soils of the Western Australia wheatbelt and some parts of the eastern states are so immense and drainage so slow that trees will make little headway in the short run.
“Against the background of vast stores of salt, the low hydraulic gradients and transmissivities, we conclude it is unlikely that complete restoration of these systems within normal human timescales is achievable,” Dr Hatton and Dr Salama say in a paper released this week.
Revegetation on up to 80 per cent of some catchments would be required, and response times of salinity control will be long. According to the most pessimistic analysts, this means not until the wheatbelt undergoes major geologic change or the climate alters dramatically. “We may have to accept that some changes may be, in practical terms, irreversible,” Dr Hatton explains.
Pumping can have very local impacts on water levels – if it is maintained in perpetuity and you can dispose of the saline water somewhere else. Pumping can also restore the freshness of streams if maintained for many decades, but disposal of the salt remains a problem.
Salinisation of rivers draining the WA wheatbelt is already causing massive loss of biodiversity, farm production and asset values. At present 10 per cent of the landscape is affected, but this is forecast to grow rapidly, to engulf 30-40 per cent of the land over the coming half century.
“At the moment we’re losing an area equal to one football oval an hour – or about 85 typical suburban blocks,” Dr Hatton says. “In WA, eighty per cent of the remnant native vegetation on farms and 50 per cent on public lands is at risk. In other words we stand to lose a large fraction of our native biodiversity. … Eighty per cent of the beds and banks of rivers and streams in the WA wheatbelt are seriously degraded. Wetlands are badly affected and riverine systems are largely degraded already. Below this landscape lies stores of salt as high as 10,000 tonnes per hectare.”
The problem, say Hatton and Salama, is that the rate at which groundwaters can recede following tree planting or improved agricultural practices is too slow to impede the release of huge tonnages of salt to our rivers. This is because, in the short term, stands of trees really only affect the water table within 10-30 metres. Thus for much of southern Australia, only in country with extensive tree cover, will trees have a significant impact on regional salinity control.
Over time, however, it will be different story. Trees, or an agriculture which uses water more effectively, will remain the front-line answer to salinity, and large-scale adoption is urgent, the researchers conclude.
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