September 20, 2016

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Evaluating Coupled Carbon and Water Vapor Exchange with Carbon Isotopes in the Community Land Model (CLM4.5)

Stable carbon isotopes allow for the calibration and improvement of land surface models.

The Science

Researchers used continuous observations of stable carbon isotopes that are exchanged between the land and atmosphere to better understand how a forest in the Colorado Rocky Mountains responded to stressful growing conditions.

The Impact

Stable carbon isotopes provide a useful and independent constraint upon stomatal conductance, an important ecosystem parameter that controls carbon and energy balance at the land surface. Isotopes also can help guide improvements in how nitrogen limitation is represented within the land model component of a climate model.


Researchers used stable carbon isotopes of carbon dioxide (CO2) to improve the performance of a land surface model, a component within Earth system climate models. They found that isotope observations can provide important information related to the exchange of carbon and water from vegetation driven by environmental stress from low atmospheric moisture and rate of carbon assimilation (photosynthetic rate). This information provided by isotope observations can go beyond what has traditionally been provided by land surface exchange of carbon, heat, and water measured from towers. Unexpectedly, the study also found that isotope observations provided guidance on how nitrogen limitation should be represented within models. Therefore, the study concludes that isotopes have a unique potential to improve model performance and provide insight into land surface model development.

Principal Investigator

David R. Bowling
University of Utah
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]


This work was supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.


Raczka, B., Duarte, H. F., Koven, C. D., and D. Ricciuto, et al. "An observational constraint on stomatal function in forests: Evaluating coupled carbon and water vapor exchange with carbon isotopes in the Community Land Model (CLM4.5)." Biogeosciences 13 5183–5204  (2016).