The heavens opened above Sydney this week, but Chris Davis, the National Water Commissioner, believes that when this happens billions of litres of lost opportunities are sent tumbling into the sea.
Mr Davis, giving a keynote address to an international stormwater conference in Sydney, said 429 gigalitres of water, equivalent to almost the entire contents of Sydney Harbour, was going into the sea each year instead of being captured and used.
Across the metropolitan area, about two gigalitres of stormwater was collected each year, a fraction of what could be harvested. There were at least 70 to 100 initiatives across Sydney that captured and re-used stormwater, but this could be increased significantly.
Capturing stormwater was not easy and it was hard to store and treat. But there were real opportunities in the newly developed urban areas of the north-west and south-west, where urban water-capturing design could be built into the infrastructure.
Yesterday’s conference heard that there had been an evolution from the old 19th century concept that all city authorities had to do was to maintain a water supply. Other services, such as sewerage, had been introduced since, but the present ideal was ”water-sensitive” cities that had adapted to rainfall.
Mr Davis said Warragamba Dam supplied about 570 gigalitres and remained the principal water supply but the city need not be totally dependent on this supply.
If use of stormwater, capture of rainwater and recycling of treated sewage were employed, it was possible to supplement Sydney’s water supply by 70 gigalitres a year.
Mr Davis said the desalination plant had been the target of unfair criticism. ”I think it has put a real floor under the worst outcome from drought,” he said. ”When you run it, what it does is keep Warragamba Dam on an even keel.”
Dr Chris Walsh, a principal research fellow in resource management from the University of Melbourne, said that the present drainage system in cities whose aim was to get rid of water quickly was doing nothing for the urban environment.
”Ninety per cent of water in urban areas is washed down the drain and into the nearest stream, where 80 to 90 per cent of the water that falls on a forest doesn’t get to the stream at all,” he said.
”Virtually none of the water that gets to the stream from a forest gets there through overland flow: it all gets there slowly and cleanly through the soils.
”In contrast all the water that gets to the stream through the drainage system gets there directly through the pipes, picking up pollutants along the way. When it stops raining, urban streams are starved of water because they are no longer receiving flows through the soils that once filtered into the ground now covered by roofs and roads.
”Addressing this problem, by changing the way we are managing stormwater to keep it in our catchments and use most of it, will not only protect our streams, but will provide our cities with plentiful water that can reduce our demand on imported water, cool our cities, make them greener, healthier places and reduce the risks of summer flash floods.”
