eddycovariance

Eddy covariance is a micrometeorological technique used to measure the exchange of gases, heat, and moisture between the land surface and the atmosphere. It infers turbulent fluxes from the covariance between fluctuations in vertical wind speed and fluctuations in a scalar of interest, typically sampled at high frequency over time.

The method relies on fast-response sensors to capture rapid air movements and scalar concentrations. A three-dimensional, high-frequency anemometer (often a sonic anemometer) measures wind components, while a fast gas or infrared analyzer monitors scalar concentrations such as carbon dioxide and water vapor. Fluxes are computed as the time-averaged covariance ⟨w' s'⟩, where w' is the fluctuation of vertical wind and s' is the fluctuation of the scalar. Adjustments are commonly applied to align the signals in time and to account for density and buoyancy effects, using corrections such as the Webb, Pearman, and Leuning (WPL) formulation.

Data are typically processed in 15- to 30-minute intervals, with steps including coordinate rotation to a steady mean flow, lag time adjustment between wind and scalar signals, despiking, detrending, and quality control. Footprint analysis estimates the contributing area, and gap-filling or modeling may be used to address missing data. Calibration, spectral corrections, and density-corrected terms are important for accuracy.

Applications include measuring net ecosystem exchange of CO2 (NEE), evapotranspiration through latent heat flux, and energy fluxes (sensible heat, latent heat). Eddy covariance is widely used in forests, croplands, wetlands, grasslands, and urban environments to study carbon budgets, water use, and responses to environmental change. Limitations include the need for relatively homogeneous terrain, sufficient turbulence, and careful calibration and data processing to account for various biases and uncertainties.