Doyle, Geoffrey (USDA-ARS, Grazinglands Research Laboratory, 7207 W. Cheyenne, El Reno, OK, 73036; Phone: 4052625291; Fax: 4052620133; Email: gdoyle@grl.ars.usda.gov)

Direct Comparison Of Two Systems For The Measurement Of Carbon, Water And Energy Fluxes In Tallgrass Prairie

G.L. Doyle *, W. Dugas, H. Mayeux

Micrometrological literature reflects a lack of extensive research on grassland mass and energy flux, and there remains a lacks of a standard method/measurements system. The objective of this study was to compare mass and energy fluxes of sensible heat flux density (H), latent heat flux density (LE) and carbon dioxide (CO₂) from CSI-manufactured Eddy Covariance (EC) and Bowen Ratio/Energy Balance (BREB) instrumentation. The study site was a tallgrass prairie located in central Oklahoma, with above ground biomass of 8 Mg ha^-1, and mean annual precipitation of 840 mm y^-1. Results from co-located EC and BREB units over more than 36 month period indicated that both systems demonstrated similar seasonal patterns, yet the EC measured 30% lower values for H and LE when compared to the BREB. The sum of EC H and LE fluxes were 20-70% less than what was required to close energy balance. On days with flux greater than 0, BREB CO₂flux was 2 - 10% more positive (more loss) when compared to the EC. The tallgrass prairie site typically was a source of CO₂for up to 8 months each year, and a sink during the other 4 months - where the uptake rates were sufficiently high enough to produce average annual uptake. Average daily CO₂ flux for the entire period was approximately -0.4 g CO₂ m^-2 d^-1small sink).