Fabric curtains stretch across the huge Warragamba Dam to trap ash and sediment expected to wash off wildfire-scorched slopes and into the reservoir that holds 80 percent of untreated drinking water for the Greater Sydney area.
In Australia’s national capital of Canberra, where a state of emergency was declared on Friday because of an out-of-control forest fire to its south, authorities are hoping a new water treatment plant and other measures will prevent a repeat of water quality problems and disruption that followed wildfires 17 years ago.
There have not yet been major impacts on drinking water systems in southeast Australia from the intense fires that have burned more than 40,000 square miles since September. But authorities know from experience that the biggest risks will come with repeated rains over many months or years while the damaged watersheds, or catchment areas, recover. Because of the size and intensity of the fires, the potential impacts are not yet clear.
“The forest area burned in Australia within a single fire season is just staggering,” said Stefan Doerr, a professor at Swansea University in England who studies the effects of forest fires on sediment and ash runoff. “We haven’t seen anything like it in recorded history.”
The situation in Australia illustrates a growing global concern: Forests, grasslands and other areas that supply drinking water to hundreds of millions of people are increasingly vulnerable to fire due in large part to hotter, drier weather that has extended fire seasons, and more people moving into those areas, where they can accidentally set fires.
More than 60 percent of the water supply for the world’s 100 largest cities originates in fire-prone watersheds — and countless smaller communities also rely on surface water in vulnerable areas, researchers say.
When rain does fall, it can be intense, dumping a lot of water in a short time, which can erode denuded slopes and wash ash, sediment and debris into waterways and reservoirs. Besides reducing the amount of water, the runoff can introduce pollutants, and nutrients that create algae blooms.
The area that burns each year in many forest ecosystems has increased in recent decades, and that expansion likely will continue through the century because of a warmer climate, experts say.
Most of the more than 25,000 square miles that have burned in Victoria and New South Wales have been forest, including rainforests, according to scientists in New South Wales and the Victorian government. Some believe that high temperatures, drought and more frequent fires may make it impossible for some areas to be fully restored.
Very hot fires burn organic matter and topsoil needed for trees and other vegetation to regenerate, leaving nothing to absorb water. The heat also can seal and harden the ground, causing water to run off quickly, carrying everything in its path.
That in turn can clog streams, killing fish, plants and other aquatic life necessary for high-quality water before it reaches reservoirs. Already, thunderstorms in southeast Australia in recent weeks have caused debris flows and fish kills in some rivers, though fires continue to burn.
