Instrumentation for Eddy Covariance
Figure 1. Basic Instruments Required for Measuring Flux:
CO2/H2O Analyzer and Sonic Anemometer
The eddy covariance method is a direct means for measuring turbulent fluxes in the atmospheric boundary layer, near the surface. In a typical setup, sensors are mounted on a tower for measuring fluxes above the atmosphere-vegetation interface and for within-vegetation measurements. The basic instruments include a CO2/H2O analyzer (open path and/or closed path), and a sonic anemometer.
Because of the fast vertical movement of the wind and small amounts of gas and water vapor carried by the upward and downward winds, eddy covariance measurements require very sophisticated (fast and precise) instrumentation.
To compute turbulent fluxes, measured scalar densities (e.g., temperature from a sonic anemometer, water vapor, CO2, and CH4 from gas analyzers) and 3-dimensional wind components (u, v, and w, from a sonic anemometer) are recorded in a data acquisition system. These data are subsequently processed using flux computation software.
In addition to the basic instrumentation, optional sensors may be used to collect ancillary data. These sensors include a net radiometer, soil heat flux plates, soil temperature sensors, and soil water content sensors.
The instrumentation shown in the image to the left is typical for an eddy covariance installation, with a 3-dimensional sonic anemometer and an open path gas analyzer. The horizontal separation between the sonic anemometer and other sensors should be kept to a minimum; preferably not exceeding 15 cm. Instrument arrangement should also minimize distortion of airflow into the sonic anemometer.