“Rainfall was highest on record for October for large parts of the Murray–Darling Basin in NSW and Victoria,” the Bureau of Meteorology said in its monthly statement, while noting the national rainfall total was two-and-a-half times the average level.

At the same time, Australia’s national ecosystem mapping network, known as TERN, was monitoring an accumulation of saturated soil in the Murray-Darling region. The system, which uses remote sensors to map the topsoil water content of the whole continent in one-kilometre-square grids, showed deep blue patches indicating the highest levels of moisture in areas that have now become flood zones.

The Horton overland flow

All the extra rainfall couldn’t be absorbed by the ground, so rather than sinking vertically, it had to move horizontally – a process known to scientists as Horton overland flow. It overloaded rivers and poured into full dams, such as Wyangala Dam near Cowra in the state’s Central West.

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The dam was forced to release huge volumes of extra water on Monday night, with a peak of 230,000 megalitres a day – half the total volume of Sydney Harbour – pouring out downstream into an already-flooded Lachlan River. This scale of release has never happened before.

How much water the ground can soak up depends on the composition of the soil.

“Sandy soils will not hold much water, but clayey soils, of the type seen around Moree and Narrabri where they grow cotton, do absorb a great deal more,” said Professor Brajesh Singh, a soil scientist at the University of Western Sydney.

The clay soils in the state’s north and Central West – the current epicentre of the flood zone – hold three to four times as much water as coastal soils, but as a result they take much longer to dry out.

Dipole in decline, La Nina to linger

“If you have a real gap between rainfall events, then the soil will have a capacity to absorb again,” Singh said. “The holding capacity will depend on the frequency of the rain. But most of the soils in these regions are already saturated. I think we have already reached that and we don’t have any more soil capacity.”

What happens next will depend on falls over the next month. The Indian Ocean Dipole (IOD), which has been driving some of the heavy rain, seems to be in decline, but the persistent La Nina means the year of flood disasters isn’t over yet.

“The IOD should start to decay in the next month or so, but the La Nina might hang around until the end of summer, so we can expect more of these events,” Reid said.

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