Natural climate variability over the past century has emerged as the main trigger of soil salinity problems in south-eastern Australia, according to the surprise outcome of a major new groundwater study conducted by the University of New South Wales. The finding overturns decades of accepted wisdom by revealing that land-clearing - which has long been attributed with the major role - has only a secondary part to play in the development of dryland salinity in south-eastern Australia.
Until now, land-clearing and the removal of deep-rooted perennial native vegetation was thought to have been the only significant cause of dryland soil salinity in eastern New South Wales, with rainfall being essentially constant.
Likewise, the solution to the problem was thought to lie in large-scale revegetation programs.
"We got the dryland salinity concept distorted because we did not take account of rainfall variation," says one of the study's authors, Professor Ian Acworth.
"Cutting down trees turns out to be a secondary factor to rainfall, which we now know has followed a 100-year cycle.
"It turns out that after an extended dry period from about 1895 to 1946, there was a marked shift to wetter conditions after 1947 and the alarming increase in the observed area of farmland subsequently affected by dryland salinity in the 1970s and '80s was a result of that shift.
"The good news, for the salinity issue at least, is that the balance has now shifted back again. That extended wet period ended about 2000 and we have returned to dry conditions since 2001 and groundwater levels are now falling. If the same pattern is followed over the next 40 years or so there would seem to be little to worry about until mid-century, when salinity could return as a major problem."
"The lack of groundwater monitoring data in the past is mainly responsible for the incorrect conceptual model of the causes of dryland salinity," said Professor Acworth of the NSW Department of Environment and Climate Change.
"Textbooks on the subject will now have to be rewritten. It's clear that we need to monitor groundwater levels far more comprehensively in future."
While there is no doubting this is good news in that land clearing is not the driving force behind soil salinity, it does not address other issues it causes such as destruction of habitat for native species, soil erosion and impacts on biodiversity – There's no green light on widespread land-clearing. As Professor Acworth suggested, things must be monitored more comprehensively.
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