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Title: Concerning the Measurement and Magnitude of Heat, Water, Vapor, and Carbon Dioxide Exchange from a Semiarid Grassland

item Morgan, Jack
item Smith, David

Submitted to: Journal of Applied Meteorology and Climatology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2008
Publication Date: 5/1/2009
Citation: Alfieri, J., Blanken, P., Morgan, J.A., Smith, D.P. 2009. Concerning the Measurement and Magnitude of Heat, Water, Vapor, and Carbon Dioxide Exchange from a Semiarid Grassland. Journal of Applied Meteorology and Climatology 48(5):982-996.

Interpretive Summary: Rising atmospheric carbon dioxide (CO2) concentrations are an increasing concern due to the effects on global climate change. While we understand that all ecosystems exchange CO2 with the atmosphere, there is still considerable uncertainty about the dynamics of that process. Photosynthesis in green plants controls the uptake of CO2 during daylight periods in growing seasons, while respiration of plants and soil organisms results in CO2 losses. The balance between these processes determines whether major ecosystems accumulate or sequester carbon (C) which has important implications for understanding the role of various ecosystems in the global C cycle, and our ability to manage those ecosystems in ways to encourage C sequestration. Storage of C is a potentially important activity to reduce the amount of greenhouse gases accumulating in Earth’s atmosphere and reduce potential impacts on global climate change. In order to develop management practices to store C, we must first be able to accurately measure the land atmosphere exchange of CO2 to observe how it responds to environmental variables, including management. This experiment examines two popular methodologies used to study the land atmosphere exchange of CO2, and points out that the results of the two different systems can be quite different, affecting the interpretation of the results. Suggestions are given on how to correct these two measurement technologies so that the results can be compared between studies which use the two differing technologies.

Technical Abstract: Grassland environments constitute a significant proportion of the earth’s terrestrial surface; therefore, they play a key role in a number of processes linking the land surface with the atmosphere. In order to investigate and better understand these linkages, a variety of techniques including field and modeling studies, are required. This study, which utilizes a 42-day dataset collected at the Central Plains Experimental Range in northeastern Colorado, compares two popular methods of measuring the turbulent fluxes of latent heat, sensible heat, and carbon dioxide between the surface and atmosphere, namely, the Eddy Covariance (EC) and the Bowen Ratio Energy Balance (BREB) methods. It was found that while the two methods yielded similar results, important differences did exist. These differences could be directly linked to both the practical limitations and theoretical considerations of each of the methods. The BREB method tended to overestimate the magnitude of the sensible heat, latent and carbon dioxide fluxes by approximately 28%, 25%, and 95%, respectively, compared to the EC method. This overestimate was in large part related to the requirements of energy balance closure and similarity theory for eddy diffusivities that underpin the BREB method. The eddy diffusivities for all scalars were a function of atmospheric stability and were between 25% and 40% greater for the BREB method than for the EC method. Overall, the results suggest that when these discrepancies are accounted for, BREB can give reliable estimates of the surface energy and carbon fluxes over shortgrass steppe rangelands.