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How SWOT transforms flood forecasting

SWOT satellite in orbit

The Surface Water and Ocean Topography, or SWOT, satellite is designed to conduct the first-ever global survey of Earth’s surface water and will collect detailed measurements of how water bodies on Earth change over time. Credit: NASA/JPL-Caltech

The SWOT satellite, a partnership between NASA and CNES are revolutionizing flood forecasting by providing comprehensive data on water heights on nearly all bodies of water on Earth.

Rivers, lakes and reservoirs act as the arteries of our planet, transporting life-sustaining water through vast, interconnected networks. When Earth’s water cycle runs too fast, flooding can result, endangering lives and property. That risk increases as climate change alters precipitation patterns and more and more people worldwide live in flood-prone areas.

Scientists and water managers use many types of data to predict floods. This year they have a new instrument at their disposal: freshwater data from the Surface Water and Ocean Topography (SWOT) satellite. The observatory, a collaboration between NASA and the French space agency CNES (Centre National d’Études Spatiales), measures the height of almost all water surfaces on Earth. SWOT is designed to measure any major river wider than about 100 meters, and preliminary results suggest it may be able to sense many smaller rivers.

Souris River Flood in North Dakota

Floods in the Souris River inundated this North Dakota community in 2011. The U.S.-French SWOT satellite gives scientists and water managers a new tool to view flooding in 3D, information that can improve predictions about where and how often flooding will occur. Credit: North Dakota State Water Commission

Current meters can accurately measure water levels in rivers, but only at individual locations, often far apart. Many rivers have no flow meters at all, especially in countries without the means to maintain and monitor them. Meters can also be disabled by flooding and are unreliable when water overflows the river bank and flows into areas they cannot measure.

SWOT provides a more comprehensive 3D view of floods, measuring their height, width and slope. Scientists can use this data to better track how floods flow through a landscape, improving predictions about where flooding will occur and how often.

Floods caused by monsoon rains in Northeast Bangladesh

Floods caused by monsoon rains cover much of northeastern Bangladesh in this image from October 8, 2023, with data from SWOT. The US-French satellite is the first to provide timely, accurate water surface elevation information over entire regions at high resolution, enabling better flood forecasting. Credit: NASA/JPL-Caltech/UNC-Chapel Hill/Google Earth

Building a better flood model

An effort to incorporate SWOT data into flood models is being led by J. Toby Minear of the Cooperative Institute for Research in Environmental Sciences (CIRES) in Boulder, Colorado. Minear is exploring how SWOT data can be incorporated into the National Oceanic and Atmospheric Administration’s National Water Model, which predicts the potential for flooding and its timing along U.S. rivers. SWOT freshwater data will fill the spatial gaps between meters and help scientists like Minear determine the water levels (heights) at which flooding occurs at specific locations along rivers.

SWOT data on the Sacramento River slope in California

SWOT data on river slopes – such as shown here for California’s Sacramento River – can improve predictions about how quickly water flows through rivers and out of landscapes. To calculate the slope, scientists subtract the lower water height (right) from the higher one (left) and divide it by the segment length. Credit: NASA/JPL-Caltech/UNC-Chapel Hill/Google Earth

He expects SWOT to improve the National Water Model data in several ways. For example, it will provide more accurate estimates of river slopes and how they change with river flow. In general, the steeper the slope of a river, the faster the water flows. Hydrological modelers use slope data to predict the speed that water moves through a river and out of a landscape.

SWOT will also help scientists and water managers quantify how much water lakes and reservoirs can store. Although there are approximately 90,000 relatively large U.S. reservoirs, only a few thousand of them have water level data included in the National Water Model. This limits scientists’ ability to know how reservoir levels compare to surrounding land heights and potential flooding. SWOT measures tens of thousands of U.S. reservoirs, along with almost all natural U.S. lakes that are larger than about two football fields combined.

Some countries, including the US, have made significant investments in river monitoring networks and detailed local flood models. But in Africa, South Asia, parts of South America and the Arctic, there is little data on lakes and rivers. In such places, flood risk assessments are often based on rough estimates. Part of SWOT’s potential is that it will allow hydrologists to fill these gaps, providing information about where water is stored in landscapes and how much flows through rivers.

Marissa Hughes Levels Tripod

UNC-Chapel Hill doctoral candidate Marissa Hughes levels a tripod to install a GPS unit to accurately measure the height of the water surface of a section of New Zealand’s Waimakariri River. The measurements were used to calibrate and validate data from the US-French SWOT satellite. Credit: Alyssa LaFaro/UNC Research

Tamlin Pavelsky, chief of SWOT freshwater science at NASA and a researcher at the University of North Carolina at Chapel Hill, says SWOT can help address the growing threat of flooding from extreme storms fueled by climate change. “Think of Houston and Hurricane Harvey in 2017,” he said. “It is very unlikely that without climate change we would have seen 60 inches of rain from one storm. Societies will need to update engineering design standards and floodplain maps as heavy rainfall becomes more common.”

Pavelsky says these changes in Earth’s water cycle are changing society’s assumptions about flooding and what a floodplain is. “Hundreds of million people around the world will be at greater risk of flooding in the future as rainfall intensifies and population growth occurs in flood-prone areas,” he added.

SWOT flood data will have other practical applications. For example, insurers can use models based on SWOT data to improve flood hazard maps so they can better assess an area’s potential damage and loss risks. A major reinsurance company, FM Global, is among forty current early adopters of SWOT – a global community of organizations committed to integrating SWOT data into their decision-making activities.

“Companies like FM Global and government agencies like the US Federal Emergency Management Agency can refine their flood models by comparing them to SWOT data,” Pavelsky said. “Those better models will give us a more accurate picture of where and how often flooding is likely to occur.”

More about the mission

SWOT, launched on December 16, 2022 from Vandenberg Space Force Base in central California, is now in its operational phase and is collecting data that will be used for research and other purposes.

SWOT was developed jointly by NASA and CNES, with contributions from the Canadian Space Agency (CSA) and the UK Space Agency. NASA’s Jet Propulsion Laboratory, managed for the agency by Caltech in Pasadena, California, is leading the U.S. component of the project. For the flight system payload, NASA provided the KaRIn instrument, a GPS scientific receiver, a laser retroreflector, a two-beam microwave radiometer, and NASA instrument operations. CNES provided the Doppler Orbitography and Radioposition Integrated by Satellite (DORIS) system, the dual-frequency Poseidon altimeter (developed by Thales Alenia Space), the KaRIn radio frequency subsystem (jointly with Thales Alenia Space and with support from the UK Space Agency) , the satellite platform and ground operations. CSA supplied the high-power KaRIn transmitter construction. NASA provided the launch vehicle and the agency’s Launch Services Program, based at the Kennedy Space Center, and managed associated launch services.

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