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New map details shifting flood risks in the United States

"If a river basin is getting wetter, in the Midwest for example, your flood risk is also probably increasing," said researcher Louise Slater.

Brooks Hays 
A new map highlights the increasing flood risks in the northern half of the country and the decreasing risks in the West, South and Southwest. Photo by University of Iowa/American Geophysical Union
As rainfall patterns shift and water tables change, regional flood risks are being reshaped. In a new study, scientists warn of increased flood risks in the northern half of the United States. Meanwhile, risks are on the decline in the West, South and Southwest.
The map and accompanying study, published this week in the journal Geophysical Research Letters, highlight the larger amounts of water being store in the ground in the northern half of the United States, especially in the middle of the country. Satellite surveys and stream gauges corroborate the data. Elsewhere, in the South and West, droughts have dried out the ground and shrunk the water table.
"It's almost like a separation where generally flood risk is increasing in the upper half of the U.S. and decreasing in the lower half," study author Gabriele Villarini, an associate professor in civil and environmental engineering at the University of Iowa, said in a news release. "It's not a uniform pattern, and we want to understand why we see this difference."
Over the last half-century, most U.S. rainfall has been absorbed by the Midwest and the Plains states. As a result, there's more ground water.
"The river basins have a memory," said co-author Louise Slater. "So, if a river basin is getting wetter, in the Midwest for example, your flood risk is also probably increasing because there's more water in the system."
Slater and Villarini used their new research as an opportunity to change how flood risks our communicated. Instead of focusing on stream flow or gauge numbers, the researchers labeled risk the same way most people do, as a level of danger to people and property.
"The concept is simple," said Villarini. "We're measuring what people really care about."

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