Lower-cost eddy covariance for CO ₂ and H ₂ O 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 (University of Exeter). In the current study, we tested the performance of an 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 4 %–7 % (R2=0.91–0.95), and the latent heat (LE) fluxes were higher by 1 %–5 % in 2020 and 23 % in 2021 (R2=0.84–0.91). The large difference between latent heat fluxes in 2021 seemed 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. The stronger attenuation of the LC-EC led to larger cumulative differences between spectral methods of 0.15 %–38.8 % compared to 0.02 %–11.36 % of the CON-EC. 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. It was shown with the LC-EC and CON-EC systems that the agroforestry site had a 105.6 g C m−2 higher carbon uptake compared to the grassland site and 3.1–14.4 mm higher evapotranspiration when simultaneously measured for 1 month. Our results show that LC-EC has the potential to measure EC fluxes at a grassland and agroforestry system at approximately 25 % of the cost of a CON-EC system.