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

rounding these most fundamental of measurements. In this paper, we examine the performance of open-path Eddy covariance is the most direct technique for measuring water, eddy covariance, a flux measurement technique that was C, and energy fluxes above crops and managed ecosystems. When first used in the 1970s (e.g., Desjardins and Lemon, using open-path gas analyzers, corrections for air density fluctuations 1974). It saw limited use at first because of technical 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 problems—Desjardins and Lemon used a propeller anebalance closure, a common problem when using eddy covariance, mometer to sense vertical wind speed. However, as new implies that CO2 fluxes often are underestimated. Research was consonic anemometers and improved gas analyzers became ducted to evaluate performance of the density corrections by making available, the popularity of the technique increased rapeddy covariance measurements above a large parking lot where CO2 idly. However, like many measurements, eddy covariand water vapor fluxes were almost zero. Uncorrected and corrected ance can be fraught with pitfalls, and data often require flux measurements were compared to the “known” values to deterextensive postprocessing to remove sources of error. In mine accuracy. Data also were collected from a tallgrass prairie and this paper, we examine some of the most important a cedar forest to examine how density corrections and adjustments for corrections that must be applied to flux data obtained energy balance closure affected daily C balances. Raw measurements using eddy covariance. from the parking lot showed apparent, density-induced, downward CO2 fluxes (i.e., apparent photosynthesis) of approximately-0.4 mg Eddy covariance is the most direct technique for meam 2 s 1 that were correlated with sensible heat. On average, the daily suring heat and mass fluxes between the surface and uncorrected CO2 flux was-12.7 g m 2 d 1, but the density correction the atmosphere (Baldocchi et al., 1988). Historically, the reduced and changed the direction of the flux to 1.8 g m 2 d 1, which complexity and cost of eddy covariance instrumentation was very close to independent chamber measurements of 2.8 g m 2 has limited its use. However, as sonic anemometers and d 1. Density corrections in the forest and prairie changed average open-path gas analyzers become less expensive and daily CO2 fluxes by 20 to 80%. Energy balance closure averaged 80 more reliable, it is probable that researchers from a and 95% in the prairie and forest, respectively. Corrections based on wide range of disciplines will use this technique. Eddy energy balance closure changed daily C balances by 16 to 35%. A covariance instruments are well suited for continuous plethora of post-measurement corrections, coupled with lack of energy long-term monitoring of field-scale processes. Agronobalance closure, signals the need for additional research before eddy covariance can be routinely applied in agronomic research. mists, range scientists, and others conducting research on managed systems may select the technique to evaluate how management decisions and interannual variations in climate affect field-scale water and C balances. T movement of energy, water, and C between the Furthermore, as new types of gas analyzers are develfield surface and the atmosphere is one of the most oped, it will be possible to estimate the flux of many fundamental processes in the soil–plant–atmosphere different trace gases (e.g., N2O), isotopes, aerosols, and continuum. Crop production, in essence, is the act of organic compounds. Thus, eddy covariance could bemining C from the atmosphere and refining and packagcome a useful tool in examining agriculture’s impact on ing CO2 molecules into a valuable commodity (i.e., grain air and water quality. or forage). As pointed out by Campbell and Norman Eddy covariance is often touted as the best available (1998), turbulent mixing and transport of air is essential method for flux measurement because it does not refor replenishing the supply of C and removing water quire simplifying assumptions about the physics of the vapor surrounding the crop canopy. Turbulent transport boundary layer, unlike flux profile or Bowen ratio (B) in the surface boundary layer also affects sensible (H) approaches. Unfortunately, raw eddy covariance meaand latent ( E) heat fluxes, which along with the radiasurements must be corrected to account for a host of tion balance, govern evapotranspiration and canopy factors, many of which result from limitations in the temperature. Measuring the movement of energy and instruments or non-ideal boundary-layer conditions mass in the surface boundary layer is one of the most (i.e., advection, non-simple terrain). Corrections include essential tasks associated with micrometeorology. Unadjustment for density variations caused by fluxes of fortunately, there are gaps and unresolved issues surheat and water vapor (Webb et al., 1980), adjustment to account for the separation distance between the aneJ.M. Ham, Dep. of Agron., Kansas State Univ., Manhattan, KS 66506; mometer and gas analyzer (Moore, 1986), adjustments and J.L. Heilman, Dep. of Soil and Crop Sci., Texas A&M Univ., to account for frequency response of the sensors (e.g., College Station, TX 77843. Contribution no. 03-83-J from the Kansas Horst, 1999; Massman, 2000; Massman and Lee, 2002), Agric. Exp. Stn. Received 24 Aug. 2002. *Corresponding author ([email protected]). coordinate rotation to account for the slope of the terPublished in Agron. J. 95:1393–1403 (2003).  American Society of Agronomy Abbreviations: B, Bowen ratio(s); DOY, day of year; G, soil heat flux; H, sensible heat flux; Rn, net radiation; E, latent heat flux. 677 S. Segoe Rd., Madison, WI 53711 USA