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The eddy covariance method (Dyer, 1961; Tanner and Greene, 1989) is a one-dimensional (vertical) approach for measuring the exchange of gases within the atmospheric surface layer (Campbell and Norman, 1998). Key instrumentation includes sonic anemometers that rapidly (10-Hz) measure wind velocity and gas analyzers that rapidly measure heat/gas concentrations in the atmosphere. The covariance between vertical wind velocities and heat/gas concentrations determines the net exchange of these quantities between the ecosystem and atmosphere. Additional instrumentation was installed at each site to measure net radiation, soil-heat flux, soil temperatures, air temperature and relative humidity, and distance of water above or below land surface (pressure transducers). Pressure transducers were corrected for drift using depth-to-water measurements from the top of well casings to the water surface. Monthly site visits were made to download data, perform sensor inspections and complete other site maintenance. All instrumentation was visually inspected, leveled, cleaned, or replaced as necessary.
Raw, 10-Hz, vertical wind speed, virtual temperature, and gas concentration data were processed to half-hourly fluxes using EddyPro software (version 4.0.0) following Express protocols that included spiking filters, double coordinate rotations, blocked-average detrending, statistical filters, air density and oxygen corrections (Tanner and Thurtell , 1969; Baldocchi et al., 1988; Webb et al., 1980; Tanner et al. 1993), and high-pass filtering.
Processed data yielded half-hourly mean values of NEE, methane, sensible and latent heat fluxes that were filtered to remove periods with (1) unrealistic fluxes (latent and sensible heat fluxes >400 and <-50 watts m-2, NEE >10 and <-10 µmol m-2 s-1 at the Dwarf Cypress site, NEE >25 and <-2 µmol m-2 s-1 at the Cypress Swamp site , NEE >20 and <-20 µmol m-2 s-1 at the Pine Upland site , and (2) friction velocity (u*) <0.05 m s-1.
For more details on the methodology, please see the journal article associated with this data.
Baldocchi, D.D., Hicks, B.B., and Meyers, T.P.: Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods: Ecology, v. 69, no. 5, p. 1331-1340, 1988.
Campbell, G.S., and Norman, J.M.: An introduction to environmental biophysics: New York, Springer, 286 p., 1998.
Dyer, A.J.: Measurements of evaporation and heat transfer in the lower atmosphere by an automatic eddy covariance technique: Quarterly Journal of the Royal Meteorological Society, v. 87, p. 401-412, 1961.
Tanner, B.D. and Greene J.P.: Measurement of sensible heat and water vapor fluxes using eddy correlation methods: Final report prepared for U.S. Army Dugway Proving Grounds, Dugway, Utah, 1989.
Tanner, C.B., and Thurtell G.W.: Anemoclinometer measurements of Reynolds stress and heat transport in then atmospheric surface layer: University of Wisconsin Technical Report ECOM-66- G22-F, 82 p., 1969.
Tanner, B.D., Swiatek, E., and Greene J.P.: Density fluctuations and use of the krypton hygrometer in surface flux measurements: Management of irrigation and drainage systems: Irrigation and Drainage Division, American Society of Civil Engineers, July 21-23, 1993, Park City, Utah, p. 945-952, 1993.
Webb, E.K., Pearman, G.I., Leuning R.: Correction of flux measurements for density effects due to heat and water vapour transfer. Quarterly Journal of the Royal Meteorological Society 106, 85-100, 1980.
U.S. Department of the Interior, U.S. Geological Survey
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