for National Geographic News
Six years ago Mozambique was hit by a succession of hurricanes that produced catastrophic flooding in the Limpopo Valley.
At the time it was difficult to warn of the approaching floodwaters because the southern African nation had only nine flood gauges on all of its rivers (Mozambique map and profile).
Today, however, scientists can spot flood dangers from space and are on the brink of being able to issue warningsjust as they may soon be able to spot the risk of mudslides, such as one that killed 1,500 people on February 17, in Leyte, Philippines.
Floods and landslides are Earth's deadliest natural disasters, Robert Adler, a scientist with NASA's Tropical Rainfall Measuring Mission, said yesterday, at a meeting of the American Geophysical Union in Baltimore, Maryland.
The satellite-generated warnings might take two forms.
One is under development by Robert Brakenridge and colleagues at Dartmouth College's Dartmouth Flood Observatory in Massachusetts. Brakenridge's system uses satellite instruments originally designed to monitor soil moisture and atmospheric water vapor. These moisture-detecting instruments, he says, can also see rivers overflowing their bankspotentially valuable information to people living downstream.
At present, this process can accurately assess water levels in 53 rivers worldwide. But hundreds more can easily be added to the system, once their space-based flood signatures are calibrated against traditional river-flow data.
In the U.S., satellite monitoring isn't likely to provide information that's not already available from traditional flood gauges. But in parts of the world without adequate gauges, that might not be the case.
For example, Kwabena Asante, of the U.S. Geological Survey's Sioux Falls South Dakota office, says that his state of South Dakota alone has 40 times as many gauges as the entire country of Mozambique.
"Remote sensing gives the opportunity to fill in the gaps," he said.
In addition, river gauges can easily be knocked out of commission.
"Every time there's a flood, the gauges get wiped out," Asante said.
Flood monitoring, of course, isn't the same thing as giving flood warnings.
"It's what the weather people call nowcasting, showing what's happening presently," Dartmouth's Brakenridge said.
The USGS's Asante hopes to carry the process backward a step, to predict floods from rainfall measurements collected by satellites that orbit the globe every few hours.
This rainfall data, he says, can be combined with global maps of topography, vegetation, and soil moisture (compiled and frequently updated by other satellites) to calculate how the rainwater would flow into river channels downstream.
Similar data can be used to determine the likelihood that waterlogged hillsides will produce deadly mudflows, says Yang Hong of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
In this case, maps made from space would be used to create a landslide-hazard map for the entire globe.
When high-hazard areas receive heavy rainfall, they could then be flagged as areas where landslides are likely to occur.
Before the deadly landslide in Leyte, for example, ten days of rain had dumped up to 19.5 inches (500 millimeters) of rain in high-hazard locations.
Unfortunately, no warning was possible at that time, because the landslide warning system wasand still isunder development.
"The techniques are in an experimental stage," said Adler, of NASA's Tropical Rainfall Measuring Mission. "We need to be very careful and validate them before they're put to use."
For example, he says, if the system winds up with too many false alarms, it won't be of much use.
"We're still in an early stage," Adler said.
If it works though, landslide warnings might someday be issued around the globe the way that tornado warnings are issued in the U.S. today.
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