4.2

Eddy covariance and Bowen ratio Estimatesof water vapor and CO₂ fluxes above a crested wheatgrass

Larwrence E. Hipps, Utah State University, Logan, UT; and S. Ivans, S. Ivans, D. A. Johnson, and N. Z. Saliendra

Because of interest in the global carbon cycle, measurements of CO₂ fluxes have received increasing attention the last several years. However, few studies have made direct comparisons of CO₂ fluxes measured by eddy covariance (EC) and Bowen ratio (BR) systems. This is of interest because USDA-ARS currently has a network of sites estimating CO₂ and water vapor fluxes on rangeland ecosystems in the western USA using the BR approach, while the larger Ameriflux network uses the EC technique. We established EC and BR systems above a pasture dominated by crested wheatgrass (Agropyron spp.) in central Utah and obtained measurements of CO₂ and water vapor fluxes between March and November 2001. The pasture surface is rather uniform and optimal for the BR approach.

The BR system is made by CSI in Logan, Utah, and measures water vapor and CO₂ concentration at two heights. The EC system is also made by CSI and consists of a 3-dimensional sonic anemometer and LI-7500 open-path humidity and CO₂ sensor. Comparisons of the fluxes between the two systems showed that both systems gave very similar estimates of water vapor fluxes. The agreement for CO₂ fluxes was not quite as strong, but still quite good, with agreement varying at different times during the season. Agreement was improved when energy balance closure was forced for the EC fluxes. Both systems exhibited very low and unrealistic CO₂ fluxes during some nights. This is likely the result of very low turbulence and decoupling of the surface and atmosphere. This issue is being further investigated. However, both systems report reasonably similar magnitudes of fluxes and seasonal trends.

  Extended Abstract (148K)

Session 4, ARS Network
Tuesday, 21 May 2002, 8:30 AM-11:15 AM

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