太空中计算河流流量

The frustrated attempts of a UCLA graduate student to quantify the amount of water draining from Greenland's melting ice sheet led him to devise a new way to measure river flows from outer space, he and his professor report in a new study. The new approach relies exclusively on the measurements of a river's width over time, which can be obtained from freely available satellite imagery.

Currently, hydrologists（水文学者） calculate a river's discharge -- the volume of water running through it at any given time -- by taking a series of measurements on the ground, including not just the river's width but also its depth at multiple points and the velocity（速率） of its flows. Politics, financial considerations and terrain often stand in the way of obtaining such measurements.

"Our new method doesn't require access to the country or getting in the river to safely take measurements in the field," said Colin Gleason, lead author of the study and a doctoral student in geography in UCLA's College of Letters and Science. "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 discovery is highlighted March 17 in the online edition of Proceedings of the National Academy of Sciences (PNAS).

With the potential to be applied anywhere in the world, the new approach is expected to provide information that will benefit agriculture, sanitation and flood preparation, especially in countries without the resources to physically measure and monitor their rivers.

Additionally, it holds promise for improving the accuracy of climate models, which track recycling of water between the atmosphere and land, and ensuring that countries with trans-boundary rivers comply 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 Laurence C. Smith, co-author of the study and chair of UCLA's geography department. "I'm very excited about this discovery."

A noted authority on climate change and its affect on the Arctic, Smith has been trying for the past 20 years to figure out a way to estimate river discharge on Earth from outer space. The closest he had been able to get to the goal was an approach that employed satellite imagery but still required on-site verification.

"You still needed measurements on the ground to calibrate the method, which defeated the purpose," he said. "But now we can estimate discharge with river width alone, and width we can see from space."

For the PNAS proof-of-concept study, Gleason and Smith used their new system and satellite imagery from the U.S. government's Landsat program to measure the discharge of three rivers -- the Mississippi, Canada's Athabasca and China's Yangtze. They then compared those results with discharge figures from respected government agencies that monitor those rivers and whose calculations are recognized as highly accurate. The estimates of the UCLA geographers came within 20 to 30 percent of those figures.

"That might sound like a big error, but right now, we have no idea about the flow rates of most rivers around the world," Smith said. "To get a number that is within even 30 percent of accurate is incredibly helpful."