Researchers at São Paulo State University’s Departments of Cartography and Mathematics & Computer Science (FCT-UNESP) in Presidente Prudente, São Paulo State, Brazil, have demonstrated that earth observation satellites can help estimate the carbon content in inland waters such as lakes, rivers and reservoirs, thereby contributing to a better understanding of the carbon cycle in aquatic environments. The carbon cycle is the biogeochemical process by which this vital element moves between land, atmosphere and oceans.
The results of their research, conducted with FAPESP’s support and coordinated by Professor Enner Herênio de Alcântara, were published in an article in the journal Remote Sensing Letters and presented to the fall meeting of the American Geophysical Union (AGU), held in December 2016 in San Francisco, California, USA.
The researchers used Landsat 8, the eighth satellite in NASA’s Landsat Program and the seventh to reach orbit successfully, to map the colored dissolved organic matter (CDOM) absorption coefficient in Barra Bonita Reservoir, São Paulo State.
“It’s the first time anyone has used images from Landsat 8’s OLI sensor to map the CDOM absorption coefficient in inland waters in Brazil,” Alcântara told Agência FAPESP.
Also known as ‘yellow substance’ or Gelbstoff, CDOM is a photoactive component of dissolved organic matter that strongly absorbs short-wavelength light ranging from blue to ultraviolet and that plays a key role in the carbon cycle. In addition, it can be used to estimate the dissolved organic carbon in aquatic systems.
This type of organic matter affects water quality, hinders penetration by sunlight and alters the aquatic system’s thermal properties. It is called colored because it contains high concentrations of humic and fulvic acids, dark-brown compounds originating from the chemically and biologically decomposed remains of plants and animals.
Optical sensors coupled to satellites, such as Landsat 8’s Operational Land Imager (OLI), can measure the radiance (energy flow per unit area of surface and per solid angle) from CDOM as it interacts with the Sun’s electromagnetic radiation, Alcântara explained.
“The Sun’s electromagnetic radiation can only interact with CDOM because during the interaction, this fraction of organic matter absorbs the shortest wavelengths,” he said.
Based on this finding, the researchers decided to see whether Landsat 8 could estimate with acceptable accuracy the CDOM absorption coefficient at 440 nanometers (nm) in inland waters. This is the first step toward establishing a carbon content ratio in an aquatic environment.
To do this, they developed an empirical model to estimate the CDOM absorption coefficient at 440 nm on a regional scale using remote sensing data and water samples, and they then applied it to a series of images of Barra Bonita Reservoir captured by Landsat 8’s OLI.
Application of the model to the images enabled them to obtain maps of the spatial distribution of the CDOM absorption coefficient at 440 nm in Barra Bonita Reservoir with a low error rate, according to Alcântara.
“The CDOM absorption coefficient is considered an indicator to estimate the concentration of dissolved organic matter,” he said. “We expect that in the near future, it will be possible to estimate the carbon content of inland waters in Brazil using satellite images and thereby to make it possible to reach a better understanding of the carbon balance in aquatic environments.”
Moreover, he added, the Landsat Program has been recording images of Earth’s surface since the 1970s, so it is possible to plot time series of the concentration of dissolved carbon in inland waters for the past four decades.
“That’s one of the reasons we chose to use Landsat,” he said. “It enables us to reconstitute the history of an environment and find out whether the concentration of carbon there has increased, and if so, why.”
Carbon concentrations in inland waters typically increase due to changes in land use and plant cover near the aquatic system, he explained.