Lower-cost eddy covariance for CO₂ and H₂O fluxes over grassland and agroforestry

Abstract. Eddy covariance (EC) measurements can provide direct and non-invasive ecosystem measurements of the exchange of energy, water (H₂O) and carbon dioxide (CO₂). However, conventional eddy covariance (CON-EC) setups (ultrasonic anemometer and infrared gas analyser) can be expensive, which recently led to the development of lower-cost eddy covariance (LC-EC) setups. In the current study we test the performance of a LC-EC setup for CO₂ and H₂O flux measurements at an agroforestry and adjacent grassland site in a temperate ecosystem in northern Germany. The closed-path LC-EC setup was compared with a CON-EC setup using an enclosed-path gas analyser (LI-7200, LI-COR Inc., Lincoln, NE, USA). The LC-EC CO₂ fluxes were lower compared to CON-EC by 7–13 % (R² = 0.91–0.95) and the latent heat fluxes were higher by 2–3 % in 2020 and 23 % in 2021 (R² = 0.84–0.90). The large difference between latent heat fluxes in 2021, seems to be a consequence of the lower LE fluxes measured by the CON-EC. Due to the slower response sensors of the LC-EC setup, the (co)spectra of the LC-EC were more attenuated in the high-frequency range compared to the CON-EC. This stronger attenuation of the LC-EC requires a larger spectral correction and as a consequence larger differences between spectral correction factors of different spectral correction methods. At the agroforestry site where the flux tower was taller compared to the grassland, the attenuation was lower, because the cospectrum peak and energy-containing eddies shift to lower frequencies which the LC-EC can measure. With the LC-EC and CON-EC systems was shown that the agroforestry site had a 2.3 times higher carbon uptake compared to the grassland site and both had an equal evapotranspiration when simultaneously measured for one month. Our results show that LC-EC has the potential to measure EC fluxes at various land-use systems for approximately 25 % of the costs of a CON-EC system.