HR: 0800h
AN: B51C-0965    [Abstracts]
TI: Downward CO$_2$ flux observed over a bare field by the eddy covariance method using an open-path infrared gas analyzer
AU: * Ono, K
EM: onok@niaes.affrc.go.jp
AF: National Institute for Agro-Environmental Sciences, 3-1-3, Kannondai, Tsukuba, 305-8604 Japan
AU: Miyata, A
EM: amiyat@niaes.affrc.go.jp
AF: National Institute for Agro-Environmental Sciences, 3-1-3, Kannondai, Tsukuba, 305-8604 Japan
AU: Yamada, T
EM: yamadatm@niaes.affrc.go.jp
AF: National Institute for Agro-Environmental Sciences, 3-1-3, Kannondai, Tsukuba, 305-8604 Japan
AU: Nagai, H
EM: iagan@niaes.affrc.go.jp
AF: National Institute for Agro-Environmental Sciences, 3-1-3, Kannondai, Tsukuba, 305-8604 Japan
AU: Yoshikoshi, H
EM: yosysuos@mbox.nc.kyushu-u.ac.jp
AF: Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Mano, M
EM: mmano@niaes.affrc.go.jp
AF: National Institute for Agro-Environmental Sciences, 3-1-3, Kannondai, Tsukuba, 305-8604 Japan
AU: Saito, M
EM: msaito@suiri.tsukuba.ac.jp
AF: University of Tsukuba, 1-1-1, Tennodai, Tsukuba, 305-8572 Japan
AB: Improvement of fast response infrared gas analyzers enabled us to carry out long-term measurement of CO$_2$ exchange between terrestrial ecosystem and the atmosphere. However, CO$_2$ flux measurement by the eddy covariance method still has several problems to be solved before we estimate annual net CO$_2$ exchange based on the method. One of the problems the authors are facing now is downward CO$_2$ flux often observed at a paddy field in dormant season. In order to understand why those unrealistic fluxes are observed, we made an intensive field experiment at a ploughed paddy field from March to April 2003. The study site was flat and homogeneous, and little vegetation was found during the experiment. Through-flow chamber measurement showed emission flux between 0.5 and 1.0 $\mu$molm$^{-2}$s$^{-1}$. CO$_2$ concentration measured at 1.1 m and 3.8 m also showed gradients indicating upward transport. However, CO$_2$ flux measured by the eddy covariance method using an open-path infrared gas analyzer showed downward transport in the daytime even after the density correction was applied when sensible heat flux was typically between 150 and 200 Wm$^{-2}$. Contrary, the eddy covariance method using a closed-path infrared gas analyzer showed emission from the soil and the magnitude of the flux was similar to that by the chamber measurement. If we assume mass balance of dry air, an increase of the observed sensible heat flux by 50% or pressure flux as much as -30 to -40 Pa ms$^{-1}$ is needed to compensate the differences between open- and closed-path eddy covariance measurements. The possibility of underestimation of sensible heat flux density measured with sonic thermometry can be excluded because it agreed well with the sensible heat flux measured with a fine-wire thermocouple. Although we did not measure pressure fluctuation directly, the pressure flux estimated from the turbulent kinetic energy budget equation as well as from literatures were smaller than the required in the correction by one order of magnitude. Those curious downward fluxes have significant influence on annual budget of CO$_2$ at the paddy field because during the dormant season about 20% of the observed fluxes by the open-path eddy covariance method were downward.
DE: 1694 Instruments and techniques
DE: 1704 Atmospheric sciences
DE: 1719 Hydrology
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting