The real cause of salinity

Deforestation? No — scientists now believe the main trigger is changes in rainfall patterns

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.

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, who holds the Gary Johnston Chair of Water Management of the UNSW Connected Waters Initiative.

“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.”

Professor Acworth and Aleksandra Rančić, of the NSW Department of Environment and Climate Change, uncovered this new insight by examining thousands of old records of groundwater levels in bores sunk in many parts of NSW, going back more than 100 years.

By correlating these figures to rainfall records and other climate and historical data, they were able to confirm that groundwater levels within the bores had risen and fallen in line with the variations in rainfall, with extensive land-clearing only becoming significant during wet periods.

“The lack of groundwater monitoring data in the past is mainly responsible for the incorrect conceptual model of the causes of dryland salinity,” Professor Acworth says. “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.”

Tree clearing and the switch to shorter-rooting crops by the beginning of the 20th Century helped alleviate the impact of lower rainfall at that time, by increasing groundwater recharge. But when above-average rainfall began in the second half of the 20th Century, the excess water available at the surface led to the increased recharge of shallow groundwater. In turn, that brought damaging salts to the surface, scalding crops and other plants life and reducing farm productivity.

The Federal Government has recently given a substantial funding boost to groundwater studies through the new $30 million National Centre for Groundwater Research and Training (NCGRT) co-funded through the Australian Research Council and the National Water Commission (see earlier story here, with another $15 million announced in the recent Budget through the Super Science Initiative to be spent on groundwater equipment, infrastructure, monitoring and long-term environmental research sites.



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