La Trobe University ground water expert, Dr John Webb, says that following recent drought-breaking floods, it will be critical to refocus on this issue – and for farmers in previously salt-affected areas to take remedial action as rapidly as possible.
Initially the floods performed a useful service, washing salt from our rivers. At the same time they also began to replenish our supplies of underground water. In salinity-prone areas, especially near creeks and low-lying farmland, this is a very mixed blessing.
‘One effect of the long drought was to lower the water table,’ says Dr Webb. ‘Salinity is strongly related to how close the ground water is the surface. Ground water levels in much of Victoria dropped to a depth of four metres or more during the drought.’
He says the floods and recent heavy rain will again raise the ground water table, bringing it very close to the surface in many parts of Victoria. ‘This makes it imperative that action to lower water tables – such as planting trees near creek beds and low-lying farmland – starts as soon as possible after the floods pass.
Once ground water rises again to within two metres of the surface, possibly within a few years, evaporation will concentrate the water, making it more saline.
Ideal opportunity to strike roots
‘It’s difficult to predict exactly how long this will take,’ says Dr Webb. ‘A lot depends on how much rain we get between now and then. But the floods have provided an ideal opportunity for trees to strike roots and help lower the water table.
‘The longer we leave this the more difficult it will get, and the more damage there will be to farm land.’ He says famers need to remember the areas that went saline before the drought.
Areas that were very badly affected by salt, and where it is difficult to grow trees, should be fenced off so they are not eroded by stock. They can then be regenerated with special salt-tolerant vegetation which helps suck up some of the water.
The La Trobe University Environmental Geoscience group headed by Dr Webb has been studying dryland salinisation in western and central Victoria for more than eight years. The group conducts studies into floods, the impact of farming practices and of commercial forestry plantations on ground water, and aspects of climate change.
Dr Webb is also a member of the National Centre for Groundwater Research and Training. He says while salinity is well understood at a general level, it can vary greatly at local level, even from paddock to paddock.
La Trobe researchers measure ground water levels through a network of data loggers installed in bores. These provide scientists with readings at intervals which can vary from 30 minutes to monthly.
‘With climate change and global warming one theory is that overall rainfall might decrease but individual rainfall events might become larger and more intense, and we might see more and bigger floods. So how much impact does a large flood have on ground water? Could it balance out the effect of less rainfall?’ says Dr Webb.
Dependent on local geology
That question is being tested by PhD researcher Sanjeeva Manamperi. He is working in the Loddon and Campaspe area on a joint project with Victorian Department of Sustainability and Environment examining the response of the water tables. So the recent floods have been perfect for his project.
‘Initial results show that in some places a big rainfall event will put a lot of ground water into the ground quite quickly. In other places it makes almost no difference,’ says Dr Webb.
‘It all depends on the geology and the nature of different soils rather than vegetation cover. And, as you’d expect, shallow bores tend to respond more quickly than the deeper ones.
‘Volcanoes allow the water to seep in really quickly,’ he says. Once you get out on the plains it’s much slower.’ So around Clunes, Daylesford and Ballarat – where around two million years ago volcanoes created deep vents in the earth’s crust – ground water is likely to replenish more quickly than on the northern plains near Rochester and Kerang.