March 19, 2014
Satellite-Based Imagery Could Make It Easier To Monitor River Discharge
redOrbit Staff & Wire Reports - Your Universe Online
After growing frustrated attempting to measure the amount of water draining from Greenland’s melting ice sheet, one UCLA graduate student has developed a new method of measuring the width of a river over an extended period of time using freely available imagery from orbiting satellites.
Currently, the volume of water running through a river at any given time is determined using a series of ground-based measurements. Hydrologists calculate this measurement, which is also known as the river’s discharge, using not just its width but also its depth at multiple locations and the velocity of its flows.
However, writing in the March 17 online edition of the Proceedings of the National Academy of Sciences (PNAS), lead author Colin Gleason describes how using observations from space could help eliminate some of the political or financial concerns that can make obtaining those measurements more difficult.
“Our new method doesn't require access to the country or getting in the river to safely take measurements in the field,” Gleason, a doctoral student in geography in UCLA's College of Letters and Science, explained in a statement Monday. “As long as we can get multiple pictures of a river and apply this method to them, we can tell you how much water was flowing in the river at the time the images were taken.”
The method developed by Gleason and his professor, UCLA geography department chair Laurence C. Smith, could potentially be used anywhere in the world and is expected to provide data that will prove beneficial in the fields of agriculture, sanitation and flood preparation – especially in countries lacking the resources to physically monitor and measure their rivers.
Furthermore, the study authors said that using satellite imagery to measure water drainage could improve the accuracy of climate models, which have to account for the balance of atmospheric water. It could also help make sure nations that have trans-boundary rivers are in compliance with international water-sharing agreements.
“This work represents a breakthrough in our ability to address one of the most pressing environmental challenges of the 21st century, which is ensuring access to sufficient water supply for human beings and ecosystems,” said Smith, who has been working on a way to estimate river discharge from space for the past two decades.
“I'm very excited about this discovery,” he added. In the past, Smith was able to develop an approach that used satellite imagery, but still required on-site verification, which “defeated the purpose.” With this new system, however, he said that scientists “can estimate discharge with river width alone, and width we can see from space.”
For their proof-of concept study, he and Gleason used their new method and images obtained from the Landsat program to measure the discharge of the Mississippi River in the US, the Athabasca River in Canada and the Yangtze River in China. They then compared those results from highly-accurate discharge figures from government agencies, and found that their results came to within 20 to 30 percent of those measurements.
“That might sound like a big error, but right now, we have no idea about the flow rates of most rivers around the world,” explained Smith. “To get a number that is within even 30 percent of accurate is incredibly helpful.”
The Greenland ice sheet is the world’s second largest body of ice, behind only the Antarctic ice sheet. It originally covered 660,000 square miles, but has been melting rapidly due to global warming. Its runoff is expected to drastically elevate the worldwide sea levels, and Smith has been attempting to use field measurements to verify the current computer simulation-generated projections of that increase, according to the university.
However, terrain such as fast-moving water filled with ice chunks and riverbanks that behave like quicksand have impeded his work, which encouraged them to develop their new streamlined method – which requires only one form of input, width, instead of three. Since submitting their paper for publication, Gleason and Smith tested their technique on another 19 rivers, attaining the same level of accuracy as reported in the original trial with one exception.
“The geographers found that the approach doesn't work on rivers with vertical concrete sides, like the river that runs through Los Angeles,” the university said. “Ultimately, the geographers plan to test the method on a total of more than 30 rivers around the globe. With at least four per continent, the test rivers are being selected for the diversity of conditions that they represent.”