When rivers overflow their banks, they flush freshwater, nutrients, and sediment onto surrounding lowlands. These nutrient-dense floodplains attract agriculture and development, but when humans encroach on these areas, the risk of floods and damage to wetland ecosystems rises. In a new study published in Scientific Data, researchers found that the world has lost 600,000 square kilometers of floodplains in 27 years.
“As development and growth happen, intentional and unintentional encroachments onto floodplains happen,” said Raghu Murtugudde, an Earth systems scientist at the Indian Institute of Technology Bombay and the University of Maryland, who was not involved in the study.
To figure out how much area has been lost, a team of scientists developed a new global data set of floodplain development that occurred between 1992 and 2019. The data set included information culled from a map of floodplains (GFPLAIN250m) created from NASA’s Shuttle Radar Topography Mission, land use maps from the European Space Agency, and maps of major river basin extents. With a resolution of 250 meters, the data illuminate how individual floodplains were altered.
The researchers found that people have converted 460,000 square kilometers of floodplain to agricultural land and developed an additional 140,000 square kilometers. That’s a land area larger than Madagascar.
“It’s like losing a whole country,” said Adnan Rajib, a civil engineer at the University of Texas at Arlington and lead author of the study.
Floodplains have historically been used for human settlement and food production, and these uses compromise the ability of floodplains to provide other ecosystem services such as clean water and flood control, said Ellen Wohl, a fluvial geomorphologist at Colorado State University who was not involved in the study.
Earlier studies had documented a rise in population and development in floodplains around the world. However, scientists didn’t know how much had occurred relative to areas outside of floodplains. The current study showed that floodplains were 1.75 times more affected than areas outside floodplains, according to Rajib. The data underscore that floodplains should be considered separately from other landscapes in policy decisions, he opined.
“The study is a nice global mapping of the loss of floodplains, which is very useful for adaptation and land use planning,” Murtugudde said, adding that the scale of the study gave him greater confidence in the findings.
The rate of floodplain loss was highest in Asia. Policy frameworks and floodplain protection initiatives are either relatively underdeveloped or nonexistent in many Asian countries, Rajib said. Large populations and the corresponding demand for food, shelter, and jobs lead to unplanned development in floodplains, he added. Floods in Delhi, India, this past summer—the worst the city has seen in 4 decades—were caused by severe encroachment on floodplains, for example.
One third of floodplain wetland losses occurred in North America, and nearly 60% of the urbanization in global floodplains occurred in Europe, Rajib added. “The importance of this study is that it calls attention to the ubiquity of floodplain losses on a global scale, as well as providing region-specific information,” Wohl said.
Mapping Loss
The authors took a closer look at the dominant alterations in six major basins and found that land use changes varied. In the Great Lakes Basin in North America, for example, forested floodplains were urbanized, whereas in the Nile Basin, grasslands were converted to agricultural land. In the Danube River Basin in Europe and the Yangtze River Basin in Asia, agriculture land was developed.
“The rapid loss of naturally functioning floodplains in some regions of the world should provide the impetus to protect existing floodplains in these regions and restore floodplain functions where possible.”
Different land use types provide different habitats and ecosystem services, so having this detail is important, Wohl said. Land use conversions can also alter the effects of floods and change a floodplain’s carbon storage potential.
Policymakers and those concerned about United Nations Sustainable Development Goals can use this information to see where and how change is happening at a granular level and make policy decisions, Rajib said. “They can see what the driver of the change is—is it agricultural land becoming cities, or forests becoming agriculture?” The researchers developed and made public tools for other researchers and stakeholders to explore and analyze individual basins. The data can inform policies to prevent or reverse floodplain alteration and invest in restoration to reduce risk to lives, livelihoods, and the environment, Rajib said.
The findings emphasized and reinforced previous work, Wohl said. “The rapid loss of naturally functioning floodplains in some regions of the world should provide the impetus to protect existing floodplains in these regions and restore floodplain functions where possible.”
—Deepa Padmanaban (@deepa_padma), Science Writer