Rivers and streams cover more of the Earth than previously thought, and therefore have a larger impact on concentrations of greenhouse gases in the atmosphere, according to a study led by a Texas A&M hydrologist.
George Allen, assistant professor of geography in the College of Geosciences at Texas A&M, alongside Tamlin Pavelsky at the University of North Carolina at Chapel Hill, used satellite images from NASA and built on previous research to develop a comprehensive global map of river lengths and widths. Their database includes more than 2 million kilometers of rivers wider than 30 meters.
The study, published last week in Science magazine, found that Earth is covered by just under 300,000 square miles of rivers and streams, not including glacier-covered areas.
"We added up all the rivers and streams around the world [and] got an area larger than the size of Texas. The estimate was significantly larger than the previous best estimate, particularly in the Arctic," Allen said.
Allen said two previous studies to determine spatial estimates on Earth for rivers and streams were more "theoretical," with one of the studies providing estimates by looking at river elevation, water runoff and "estimate of flow." The latest study, Allen said, directly measured large rivers from satellite imagery. For the rivers that were too narrow to accurately measure from the satellite images, the researchers applied a statistical technique that utilized the fractal characteristics of river networks.
"There are many more narrow rivers than wide rivers, and we found that the relationship between river length and width is actually pretty predictable," Allen said. Allen began the research that became this study in 2012 while earning his doctorate at UNC.
The study's abstract reads in part, "We found that rivers and streams likely play a greater role in controlling land-atmosphere fluxes than currently represented in global carbon budgets."
The researchers also found that the surface area of rivers and streams in many developed parts of the world was less than expected. Allen cautioned Monday that extrapolating too much from that finding would be speculative.
Rivers and streams emit significant amounts of greenhouse gases to the atmosphere.
"A greater global total surface area implies that rivers and streams contribute more gases to the atmosphere like carbon dioxide and methane than currently represented by carbon cycling models," Allen said.
Allen called the database "a first step to a lot more work," adding Monday that the work was already being used for at least one other study, with several others forthcoming. The database is being used to measure change in the time of year when certain rivers have iced and thawed over the past 30 years, Allen said. The database will also be used as a core dataset for an upcoming NASA satellite mission that will measure changes in river and lake water height globally.