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There's rain in the forecast for the east coast of Australia, and that's good news for firefighters still fighting blazes near some of the country's largest cities. But rain after fires can also create problems, can even be dangerous. NPR's Rebecca Hersher has this report.
REBECCA HERSHER, BYLINE: When I called Petter Nyman this week, he was in Melbourne, walking outside. And I could hear the wind through the phone.
PETTER NYMAN: It's very smoky. We are expecting thunderstorms, so it's a lot more humid.
HERSHER: Nyman is a forest hydrologist. He studies how drought and wildfires can affect drinking water supplies. And while the rain in the Australian forecast is a good thing, the thunderstorms worry him.
NYMAN: If there's a lot of rain spread out over six months, that's a good outcome. If there are short intense bursts of rainfall, mainly through thunderstorms, those are the ones that are problematic.
HERSHER: Problematic because thunderstorms are more likely to drop a lot of rain in a short amount of time. And areas that burned can't absorb a lot of water for a couple reasons.
NYMAN: First, wildfire removes vegetation, which means that the rainfall hits the soil directly. Second, the wildfire heats the soil, which causes something called water repellency or hydrophobicity, which means that the water doesn't absorb into the soil. So most of the rain becomes surface runoff.
HERSHER: That runoff can sweep the soil and ash downhill and into streams and lakes. If a lot of sediment gets into reservoirs, the water gets really cloudy and it can't be sterilized, which means people can't drink it. Right now, the biggest drinking water concerns are for the areas around Canberra and Sydney. Water officials have set up filters at the reservoir that provide Sydney with the majority of its water to trap sediment.
Intense rain after fires can also be dangerous. A prime example, Nyman says, is what's happened in California in recent years. John McNeil is the assistant fire chief for Ventura County, Calif., an area that has a similar landscape to many of the places that are burning in Australia. In 2013, a wildfire started in the hills there.
JOHN MCNEIL: Yeah, it was called the Springs Fire.
HERSHER: Wind caused the fire to burn hot and fast, removing most of the vegetation and leaving the soil hard and ashy. At the base of the burned hills were about 200 to 300 homes.
MCNEIL: There's a small kind of retirement enclave of single-family dwellings below the area that burned.
HERSHER: And the year after the fire, there was a rainstorm.
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UNIDENTIFIED REPORTER: Voluntary evacuations tonight in Camarillo for people living near the hillside that burned in last year's Spring Fire.
HERSHER: The rain fell on hard soil. There were no plants to hold the dirt in place. Mud and rocks came loose and sped downhill.
MCNEIL: The high-impact areas - it just completely leveled the homes. There would be some walls standing, and then it was just completely sheared off from the debris and pretty sizable rocks that were coming down.
HERSHER: Luckily, people had evacuated in time. No one was killed. The risk of debris flows is rising in parts of California and Australia because of climate change and because of housing development patterns. Hotter weather is drying out plants and soil, exacerbating drought and making intense wildfires more likely. People are also building homes in wildfire-prone areas. And as the Earth gets hotter, extreme rain is getting more common.
In Australia, the forest hydrologist, Petter Nyman, has found that debris flows are already getting more frequent and severe in the southeastern part of the country. That's where this year's fires are burning.
NYMAN: The rainfall over the next six months is going to be critical. If there's a big storm, then there might be really widespread issues across the whole east coast of Australia.
HERSHER: All of this requires a quick pivot for emergency officials from fighting the fires to protecting homes and water supplies from the quenching rains.
Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.