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And while the global population grew by 18.6% from 2000 to 2015, the population in these regions grew by 34.1% over the same period. This means that between 58 million and 86 million more people were flooded in these places over 15 years.

“The increase in flooding is not particularly surprising,” he says. Beth Tellmanco-founder of flood mapping initiative From Cloud to Street and lead author of the study. “But what struck me was that people were moving to places where we have observed flooding in the past.”

Researchers have looked at more than 3,000 events around the world. Dartmouth Flood Observatory Database that records floods reported in the media. They matched events with location data to satellite images. MODILARAn instrument mounted on two NASA satellites that has been capturing daily images of Earth since 2000.

The researchers used an algorithm to map where the flooding occurred by ranking which pixels were covered with water and which were not. They then added population data to see how trends in flooded areas changed over time.

Low- and middle-income countries have witnessed the fastest population growth in flood-prone regions over the past two decades, with the highest growth rates in Sub-Saharan Africa and South Asia.

Tellman says socioeconomic factors may explain some of the movement. Vulnerable groups may have no choice but to settle in flood zones where land may be cheaper and more accessible.

Using satellite imagery, the researchers were able to more accurately describe the effects of actual flooding than with conventional models. Models can capture some types of flooding, such as those that occur around rivers and along coasts. However, for others caused by heavy rainfall or random events (such as dams breaking or a storm surge lined up with high tide), satellite images provide a clearer picture.

The 913 mapped floods are just a fraction of the tens of thousands of floods that occur globally each year. “This is just the tip of the iceberg,” Tellman says.

MODIS takes images with a resolution of 250 meters, approximately two football fields long. That means researchers won’t be able to map smaller floods or those in most cities. Clouds also interfered with the image processing algorithm and missed brief floods, as satellites only passed a certain point on Earth once or twice a day.

Newer instruments have much higher resolution and can see beyond clouds, he says. Bessie Schwarz, co-founder and CEO of Cloud to Street. These tools, together with artificial intelligence, can now map floods even more accurately. But to systematically map flooding over time, the researchers had to rely on images from a single source using longer-existing technology.

This effort gives scientists a clearer picture of the scale and human impact of recent floods than any other source. The results will be particularly useful for modelers trying to predict risk, he says. Philip WardReviewing flood risk assessment at Vrije Universiteit Amsterdam and not included in the study.