Carbon dioxide in inland waters, including rivers, lakes, and streams, primarily forms as organic matter decomposes. When the concentration of CO2 is higher in the water than in the surrounding air, the water emits CO2 as a gas.
However, the specific sources and quantities of these emissions have remained unclear. "We need to know how much CO2 is being generated so we can predict how it will respond to climate change," said Matthew Winnick, assistant professor of Earth, Geographic, and Climate Sciences at UMass Amherst. Winnick is the lead author of a recent study published in 'AGU Advances'. "As temperature rises, we tend to think that a lot of the natural carbon cycle processes will respond to that and potentially amplify climate change."
Traditional CO2 estimates for streams and rivers have relied on averaging CO2 concentrations over large areas and then applying these averages across entire regions, an approach that lacks precision.
"When you have a really turbulent stream, it's going to de-gas a lot faster," Winnick explained. Small headwater streams, for example, receive more CO2 from groundwater than downstream waters, affecting their CO2 emission rates. "Aggregating these really steep mountain reaches in with low slope areas just misses a lot of information," he added.
In their study, the researchers developed an advanced model that simulates carbon emissions from each stream reach individually, producing more accurate emission estimates for these waters.
Winnick and co-author Brian Saccardi, a former UMass graduate student, initially tested this model in the East River watershed in Colorado's Rocky Mountains. They then expanded their approach across 22 million stream reaches in the U.S. in collaboration with Colin Gleason, Armstrong Professor of Civil and Environmental Engineering, and Craig Brinkerhoff, a former UMass doctoral student and co-lead author of the study.
The research team discovered that their model estimated total emissions at 120 million metric tons of carbon, in contrast to the 159 million metric tons estimated by standard methods - a difference of approximately 25%. "As a result, the whole budget shifts because these mountain areas play a really strong role in CO2 emissions at the continental scale," Winnick said.
Accurate estimates of CO2 emissions can influence carbon sequestration strategies. Winnick highlighted the potential of projects that add calcium carbonate minerals to streams to convert CO2 to a more stable form.
"If we want to know if these methods can work, we need to know how much CO2 is in these river systems," he said, noting that CO2 levels can vary dramatically over short distances along a stream. "So having ways to predict these dynamics will really help in evaluating whether and where carbon sequestration might be effective."
The research also explores a central debate in carbon emission studies: the origins of CO2 in these systems. Winnick's findings support the idea that CO2 primarily originates from near-stream sediments, where water exchanges with underground sources, rather than from groundwater further from the stream. However, he acknowledged this area requires further study. "We hope this study will spur more efforts to get a more precise budget for where this CO2 is coming from," he said.
Research Report:Toward Modeling Continental-Scale Inland Water Carbon Dioxide Emissions
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