As forests burn around the world, drinking water is at risk

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% 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 deadly 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 (104,000 square kilometers) 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.

And because of the size and intensity of the fires, the potential impacts are not clear yet.

“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% 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,...