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Experimental Test of Density and Energy-Balance Corrections on Carbon Dioxide Flux as Measured Using Open-Path Eddy Covariance

^a Dep. of Agron., Kansas State Univ., Manhattan, KS 66506
^b Dep. of Soil and Crop Sci., Texas A&M Univ., College Station, TX 77843

^* Corresponding author (jayham{at}ksu.edu).

Received for publication August 24, 2002. Eddy covariance is the most direct technique for measuring water, C, and energy fluxes above crops and managed ecosystems. When using open-path gas analyzers, corrections for air density fluctuations due to heat and water vapor flux must be applied, and these corrections are often larger in magnitude than the CO₂ fluxes. Lack of energy balance closure, a common problem when using eddy covariance, implies that CO₂ fluxes often are underestimated. Research was conducted to evaluate performance of the density corrections by making eddy covariance measurements above a large parking lot where CO₂ and water vapor fluxes were almost zero. Uncorrected and corrected flux measurements were compared to the "known" values to determine accuracy. Data also were collected from a tallgrass prairie and a cedar forest to examine how density corrections and adjustments for energy balance closure affected daily C balances. Raw measurements from the parking lot showed apparent, density-induced, downward CO₂ fluxes (i.e., apparent photosynthesis) of approximately-0.4 mg m^-2 s^-1 that were correlated with sensible heat. On average, the daily uncorrected CO₂ flux was-12.7 g m^-2 d^-1, but the density correction reduced and changed the direction of the flux to 1.8 g m^-2 d^-1, which was very close to independent chamber measurements of 2.8 g m^-2 d^-1. Density corrections in the forest and prairie changed average daily CO₂ fluxes by 20 to 80%. Energy balance closure averaged 80 and 95% in the prairie and forest, respectively. Corrections based on energy balance closure changed daily C balances by 16 to 35%. A plethora of post-measurement corrections, coupled with lack of energy balance closure, signals the need for additional research before eddy covariance can be routinely applied in agronomic research.

Abbreviations: B, Bowen ratio(s) • DOY, day of year • G, soil heat flux • H, sensible heat flux • Rn, net radiation • E, latent heat flux