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August 2007

Before the flood

NCAR wades into forecasting dangerous waters

satelitte image

The Ganges and Brahmaputra rivers pass through Bangladesh, converging and forming the largest intertidal delta in the world. The rivers and their tributaries deposit massive amounts of silt and clay that create a maze of waterways and islands in the Bay of Bengal, as seen in this satellite image. (Image courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center.)

Epic floods around the world this summer have forced thousands from their homes in Britain, spurred emergency evacuations in Texas, and killed hundreds in China. But perhaps no place in the world is as vulnerable to floods as Bangladesh, where more than 40% of the land area is within about 30 feet (10 meters) of sea level. As of early August, millions of Bangladeshis had been marooned or displaced by floodwaters, with the death toll at more than 1,000.

What’s more, such catastrophes are a recurring event in this densely populated, low-lying nation. Two great rivers that cut through the country—the Ganges and Brahmaputra—can adversely affect millions of people when they overflow their banks every few years. Farmers and fishers can easily lose a year’s worth of income during a single flood. And with climate change expected to cause a greater number of heavy rainfall events in the future, scientists are studying the likelihood of more severe flood scenarios in Bangladesh and other low-lying places.

But thanks to a collaboration between RAL, Georgia Institute of Technology, and the Asian Disaster Preparedness Center, now Bangladeshis in flood-prone areas can at least expect more advance warning of rising waters. The team has developed a high-tech system for predicting floods in the country. This summer, its forecasts began reaching the people who need them most.

“Our goal is that long-range flood forecasts, for the first time, will consistently reach many rural individuals in Bangladesh who are in jeopardy of losing their homes, businesses, and possibly their lives,” says Tom Hopson, a postdoctoral fellow in RAL who is NCAR’s leader on the project. “It’s hugely satisfying to see these forecasts being used to help people.”

The system uses a combination of weather forecasts, satellite observations, river gauges, and hydrologic modeling techniques to predict when major rivers will crest in selected regions of the country during the monsoon season, which runs approximately June–October. The 1- to 10-day forecasts are then distributed to more than 100,000 residents of districts adjacent to the Ganges and Brahmaputra. Because few households in this extremely poor region have radios or even electricity, the nonprofit Asian Disaster Preparedness Center disseminates forecasts by activating a network of governmental and nongovernmental organizations, as well as volunteers, to spread the word in person.

The forecasts will provide Bangladeshis with more advance notice of floods than they’ve had in the past, giving farmers a chance to either postpone planting or harvest at least a portion of their crops, move livestock to safety, encircle fish ponds with nets to prevent fish from escaping, and stock food and other supplies. Over the next year or two, the system will begin providing 20-day forecasts, to be eventually followed by seasonal forecasts.

tom hopson

Tom Hopson.

“The goal here is to help very local, grassroots economies,” Tom says. “The forecasts can also alert relief agencies to prepare to bring in drinking water, cholera tablets, and other essentials in case of a major flood.”

The system is based on an ensemble of weather predictions for South Asia generated by the European Centre for Medium-Range Weather Forecasts. It incorporates estimates of precipitation from satellite observations provided by NASA and NOAA, along with discharge measurements from Bangladesh’s rivers. Updated daily with new information, the system emphasizes modeling and satellite data to compensate for a lack of river gauge and radar data upstream of Bangladesh.

“This is cutting-edge technology, with which we analyze information from a number of sources to generate forecasts of the probability of major flooding,” Tom says.

The team, which includes Georgia Tech’s Peter Webster, who is the principal investigator, and Ramasamy Selvaraju of the Asian Disaster Preparedness Center, hopes to expand the system to encompass more of Bangladesh over the next few years, since past floods have submerged as much as two-thirds of the nation. The team may also study the feasibility of applying the forecasting system technology in other countries that are vulnerable to floods, such as Cambodia and Ghana.

Front Range flash floods

Closer to home, RAL’s Dave Gochis and David Yates are developing a flash flood prediction system for the Front Range’s South Platte River Basin, spanning the area from Cheyenne to the Palmer Divide north of Colorado Springs.

The system will provide forecasters, emergency managers, and the public with flash flood forecasts that are more targeted than those currently available, giving up to a few hours’ notice of potential flash floods in specific drainages.

The system combines data from meteorology and hydrology, disciplines that scientists often struggle to link. On the weather side, it utilizes radar data, short-term forecasts and a high-resolution version of the Weather Research and Forecasting model (WRF), while on the hydrological side it incorporates rain gauge and stream flow data and physically-based hydrology models.

“This project is about integrating science and technology from two scientific disciplines that have evolved faster individually than at their interface,” Dave Gochis says.

The two plan to perform case studies this summer by simulating past Front Range flash floods, including the deadly Fort Collins flood of 1997. The system will undergo operational testing on a trial basis next summer when forecasts are shared with the Denver Urban Drainage and Flood Control District and the Denver/Boulder National Weather Service.


In this issue...

Before the flood

Storm World author comes to Center Green

Another successful year for leadership programs

Mary Marlino to head NCAR Library, e-Science

Random profile: Jennifer Boehnert

CISL cultivates the next generation

Double trouble on the storm front

Delphi Question

Just One Look


 

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