Flooded With Information

 

The real explosion in satellite-based flood information came in 2000, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite began collecting 250-meter-spatial-resolution imagery of almost the entire globe every day. A second MODIS instrument was launched on NASA’s Aqua satellite in 2002. Although MODIS wasn’t specifically designed to observe floods, Brakenridge says it’s turned out to be the perfect tool.

“Before the era of satellite remote sensing, and particularly before MODIS, there was no way to map flooding over large regions,” he says. The limitation is due to the inescapable trade off between the level of detail a satellite can provide and the size of the area it can observe at one time. As the level of detail goes up, the area covered goes down. For large floods, says Brakenridge, “A satellite with high spatial resolution isn’t very useful if it means you need 30 scenes to get a complete image of the flooded area.”

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  Wide-scale satellite image of floods in China, with area covered by high-resolution sensor for comparison
 

MODIS provides the right balance of coverage and detail, showing large floods as they are happening all over the world. Brakenridge uses the imagery provided by the MODIS Rapid Response Team, a group at NASA’s Goddard Space Flight Center that processes and distributes global MODIS imagery and customized products in near-real time. He uses natural-color imagery that looks like a digital photo and a kind of false-color imagery that adds color to non-visible wavelengths of energy detected by MODIS, such as infrared. Standing water absorbs infrared wavelengths of energy, so in the imagery it appears almost black.

Sometimes even those kinds of image enhancements aren’t enough. After all, where there are floods, there is rain, and where there is rain, there are clouds. Son Nghiem of NASA’s Jet Propulsion Laboratory at California Institute of Technology helps Brakenridge accommodate those unwelcome visitors in many flood-related images. He processes radar data collected by NASA’s QuikSCAT satellite, which sends out microwave pulses that can penetrate clouds. The timing and strength of the return signal reveal the surface beneath. Brakenridge combines those observations with the images from MODIS and other sensors to map floods even when the clouds linger for days or weeks.

Even when they can see through the clouds to see how big an area is underwater during a flood, Brakenridge points out that they are still missing a key piece of information: depth. Recently Brakenridge has been investigating how to use topographic information collected by radars on the Space Shuttles to get an idea of how deep standing water might be. Brakenridge has been combining the high-quality topographic maps produced by the Shuttle Radar Topography Mission (SRTM) with the flood imagery from MODIS and other sensors. The SRTM maps tell him the altitude of various features on the surface, and he can gauge how high the water is by what features have been covered up.

 

The wide area seen by sensors like MODIS allows flood monitoring over the entire globe in near-real time, while high-resolution satellites such as Landsat can zoom in to a specific area. This partial MODIS image shows flooding along China’s Huai River on July 23, 2003. The white outline shows the smaller area covered by a single Landsat scene. (Image by Jesse Allen, based on data provided by the MODIS Rapid Response Team)

  Topographic map of the Tondano River, with flood extent  

Brakenridge and his colleagues determine the depth of flood waters by matching maps of flooded areas (blue) with topographic data (gray shades, lighter is higher elevation) that reveals the elevation at the water’s surface. The graph below the image shows the topography (dark blue line) and depth of flooding (light blue fill) along the cross section of terrain marked with a white line in the top image. Scientists derive flood depth by matching the water extent visible in recent satellite images with the topography along the edges of the flooded area. (Image courtesy Dartmouth Flood Observatory)

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