Researchers at the University of Massachusetts Amherst have utilized an advanced modeling technique to assess carbon dioxide emissions from inland waters across 22 million lakes, rivers, and reservoirs in the U.S. This innovative approach is the first of its kind to be implemented on a continental scale, indicating that earlier methods might have overestimated CO2 emissions by as much as 25%.
Carbon dioxide in rivers, lakes, and streams primarily arises from the decomposition of organic materials. If the concentration of CO2 in the water surpasses that in the air, the water will release it as a gas. However, the exact amount of CO2 emitted and the sources contributing to it remain uncertain.
According to Matthew Winnick, assistant professor of Earth, Geographic, and Climate Sciences and lead author on the study published in AGU Advances, understanding CO2 production is essential in order to forecast its reaction to climate change. “With rising temperatures, we expect that many natural carbon cycle processes will respond accordingly and possibly intensify climate change,” he notes.
Previously, estimates were created by averaging CO2 concentrations in streams across large regions and applying this average to nearby waters. This method, however, does not accurately reflect the variations in CO2 emissions in different environments.
“In rapidly flowing streams, CO2 escapes much more quickly,” Winnick explains. Moreover, smaller streams closer to their sources derive more CO2 from groundwater than those further downstream, which also affects the amount of CO2 emitted by that section of the river. “Combining very steep mountain streams with flatter regions overlooks significant data,” he adds.
In contrast, the researchers’ new model assesses carbon movement in each stream segment independently, yielding more accurate estimates of CO2 emissions from these aquatic systems.
Winnick and Brian Saccardi, a former graduate student in the Earth, Geographic, and Climate Sciences Department and co-lead author of the paper, initially verified their method in the East River watershed of Colorado’s Rocky Mountains. Collaborating with Colin Gleason, an Armstrong Professor of civil and environmental engineering, and Craig Brinkerhoff, then a UMass doctoral student and co-lead author of the study, they successfully implemented their model on 22 million distinct stream segments nationwide.
“After testing in the Colorado mountains, we found that Colin and Craig were already conducting large-scale river network modeling. It was a natural progression,” Winnick notes.
The findings indicated that their modelled emissions totaled 120 million metric tons of carbon, compared to the traditional aggregate estimate of 159 million metric tons, representing a 25% discrepancy. “This has significant implications for the overall carbon budget, as these mountainous regions are crucial contributors to CO2 emissions on a continental scale,” Winnick asserts.
Accurate carbon emissions estimates can impact efforts in carbon sequestration, which might involve adding calcium carbonate minerals to streams to convert CO2 into a more stable form.
“For us to evaluate the effectiveness of such methods, it’s vital to understand the CO2 levels in these rivers,” he emphasizes, noting the variability of CO2 across different sections of a stream. “CO2 can fluctuate dramatically over short distances. Predicting these variations can significantly aid in determining the potential effectiveness of carbon sequestration projects.”
Another ongoing discussion in this field revolves around the source of CO2 — whether from groundwater or stream corridors. Accurate predictions of how carbon emissions may shift due to climate change depend on understanding these sources, as distinct environments respond differently.
“If CO2 originates from near-stream sediments where there is active water exchange between the stream and underground, its response to temperature and precipitation changes will differ compared to that in the hillslope groundwater system,” Winnick explains.
His research leans towards identifying stream corridors, which incorporate water in streams and adjacent sediments as the primary source, though he admits this area remains a topic of ongoing exploration. “We hope this study encourages further research to refine our understanding of the origins of CO2,” he concludes.
This research was made possible through funding from the U.S. National Science Foundation.
