HYDROLOGIC PROCESSES, SCALE, CLIMATE VARIABILITY, AND WATER RESOURCES FOR SEMIARID WATERSHED MANAGEMENT
Location: Southwest Watershed Research
Title: Calculating CO2 and H2O eddy covariance fluxes from an enclosed gas analyzer using an instantaneous mixing ratio
| Burba, G. - |
| Schmidt, A. - |
| Nakai, T. - |
| Kathilankal, J. - |
| Fratini, G. - |
| Hanson, C. - |
| Law, B. - |
| Mcdermitt, D.K. - |
| Eckles, R. - |
| Furtaw, M. - |
| Velgersdyk, M. - |
Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 15, 2011
Publication Date: January 3, 2012
Citation: 5. Burba, G., Schmidt, A., Scott, R.L., Nakai, T., Kathilankal, J., Fratini, G., Hanson, C., Law, B., Mcdermitt, D., Eckles, R., Furtaw, M., Velgersdyk, M. 2012. Calculating CO2 and H2O eddy covariance fluxes from an enclosed gas analyzer using an instantaneous mixing ratio. Global Change Biology. 18:385-399.
Interpretive Summary: Measurements of the evaporation and carbon dioxide exchange between the earth’s surface and atmosphere are critical for determining local, regional and global budgets and improved understanding of land and ocean exchanges important to the earth’s hydrological and carbon cycles. There is a continual need to improve the accuracy and reliability of the sophisticated instruments that are used to measure these exchanges. This study evaluates a new instrument that is used to measures carbon dioxide and water vapor. As this instrument can be used to calculate these exchanges in new and potentially more accurate ways, it was necessary to compare it with existing instruments and techniques. The results suggest that with proper temperature, water vapor, and pressure measurements in the instrument body, water vapor and carbon dioxide fluxes can be computed confidently with the new instrument. This has important implications for future measurements, because avoiding previously needed corrections could have the advantages of increasing measurement quality, reducing the magnitude of minimum detectable flux, unifying data processing steps, and assuring better intercomparisons between different sites and networks.
Eddy covariance flux research has relied on open- or closed-path gas analyzers for producing estimates of net ecosystem exchange of carbon dioxide (CO2) and water vapor (H2O). The two instruments have had different challenges that have led to development of an enclosed design that is intended to maximize strengths and minimize weaknesses of both traditional designs. Similar to the closed-path analyzer, the enclosed design leads to minimal data loss during precipitation events and icing, and it does not have surface heating issues. Similar to the open-path design, the enclosed design has good frequency response due to small flux attenuation loss in the short intake tube, does not need frequent calibration, has minimal maintenance requirements, and can be used in a very low power configuration. Another important feature of such a design is the ability to output instantaneous mixing ratio, or dry mole fraction, so that instantaneous thermal and pressure-related expansion and contraction, and water dilution of the
sampled air have been accounted for. Thus, no density corrections should be required to compute fluxes during postprocessing. Calculations of CO2 and H2O fluxes via instantaneous mixing ratio from the new enclosed CO2/H2O gas analyzer were tested in nine field experiments during 2009–2010 in a wide range of ecosystems and setups. Fluxes computed via a mixing ratio output from the instrument without applying density corrections were compared to
those computed the traditional way using density corrections. The results suggest that with proper temperature, water vapor, and pressure measurements in the cell, gas fluxes can be computed confidently from raw covariance of mixing ratio and vertical wind speed, multiplied by a frequency response correction. This has important implications for future flux measurements, because avoiding hourly density corrections could have the advantages of increasing flux measurement quality and temporal resolution, reducing the magnitude of minimum detectable flux, unifying data processing steps, and assuring better intercomparison between different sites and networks.