Documentation on the Terrestrial Observatory of Vielsalm (OTV)

Documentation on the Terrestrial Observatory of Vielsalm (OTV

Unexpected sensitivity of the annual net ecosystem exchange to the high frequency loss corrections in a grazed grassland site in Belgium

The eddy covariance technique is widely used to measure CO2 and other gas fluxes. However, eddy covariance fluxes are affected by systematic errors that must be corrected. Among them, high frequency loss corrections are particularly important in this regard, especially when using a closed-path infrared gas analyzer. In this study, we compared three approaches to do these corrections for CO2 fluxes and evaluated their impact on the carbon balance an intensively grazed grassland site in Belgium . In the first approach, the computation of correction factors was based on the measured sensible heat cospectra (‘local’ cospectra), whereas the other two were based on theoretical models (Kaimal et al., 1972). The correction approaches were validated by comparing the nighttime eddy covariance CO2 fluxes corrected with each approach and in situ soil respiration measurements. We found that the local cospectra differed from the Kaimal theoretical shape, although the site could not be considered ‘difficult’ (i.e., fairly flat, homogeneous, low vegetation, sufficient measurement height), appearing less peaked in the inertial subrange. This difference greatly affected the correction factor, especially for night fluxes. Night fluxes measured by eddy covariance were found to be in good accord with in situ soil respiration measurements when corrected with local cospectra and to be overestimated when corrected with Kaimal cospectra. As the difference between correction factors was larger in stable than unstable conditions, this acts as a selective systematic error and has an important impact on annual fluxes. On the basis of a 4-year average, at DTO the errors reach 71-150 g C m-2 y-1 for net ecosystem exchange (NEE), 280-562 g C m-2 y-1 for total ecosystem respiration (TER) and 209-412 g C m-2 y-1 for gross primary productivity (GPP), depending on the approach used. We finally encourage site PIs to check the cospectrum shape at their sites and, if necessary, compute frequency correction factors on the basis of local cospectra rather than on Kaimal cospectra.Bilan carboné d’une exploitation agricole wallonne pratiquant le système allaitant

Climatic and management drivers of CO2 exchanges by a production crop: Analysis over three successive 4-year crop rotation cycles

Carbon dioxide (CO2) exchanges between crops and the atmosphere are influenced by both climatic and crop management drivers. The investigated crop, situated at the Lonzée Terrestrial Observatory (LTO, candidate ICOS site) in Belgium and managed for more than 70 years using conventional farming practices, was monitored over three complete sugar beet/winter wheat/potato/winter wheat rotation cycles from 2004 to 2016. Continuous eddy-covariance measurements and regular biomass samplings were performed in order to obtain the daily and seasonal Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), Total Ecosystem Respiration (TER), Net Primary Productivity (NPP), and Net Biome Production (NBP). Meteorological data and crop management practices were also recorded. Over the 12 years, NEE was negative (-4.34 kg C m-2) but NBP was positive (1.05 kg C m-2), i.e. as soon as carbon exportation by harvest and carbon importation (manure, slimes) are included in the budget, the site behaves as a carbon source. At the crop rotation scale (4 years) it was quite remarkable to observe that NBP was very similar over the three rotations (0.30-0.36 kg C m-2), despite climatic and management differences between years. Crop type impacted carbon exchanges, with sugar beet and winter wheat crops leading to higher net carbon sequestration than seed potato crops. For one given crop, larger growth length and cumulated global radiation drove larger cumulated NEE. Net carbon emissions were observed during intercrops, but growing mustard during these periods reduced their rates and provided carbon residues to the soil. NBP values suggest that one sixth of the total soil organic carbon stock at LTO (6.23 ± 0.16 kg C m-2 in [0, 60] cm) would be lost in 12 years. Large uncertainties (mostly due to biomass measurements) affect NBP estimates, but still, this figure is huge and should encourage cultural practices returning carbon to the soil

Calibration of soil moisture sensors for a long-term field experiment

In the framework of the ICOS RI network, a field site in Lonzée, Belgium, is equipped to provide long-term data on greenhouse gas emissions from an agricultural field and the associated environmental variables. Soil moisture is one of the state variables which are monitored with high temporal resolution and with several repetitions in the field to take into account soil heterogeneity. In order to facilitate field installation in combination with agricultural practices, Sentek Enviroscan sensors, a collection of FDR sensors at different depths on a stick, were chosen to measure soil moisture. In this contribution, we will discuss the results of a detailed calibration experiment we performed for this sensor type and compare it to the results we got from a different FDR sensor: the ML3 Thetaprobe. We calibrated the probes for the different soil horizons at 3 different locations in the field using big reconstructed soil columns which were brought to defined soil moisture levels in the lab. The results showed that the universal calibration relationship of the sensors gave quite similar results as the soil-specific calibration up till a moisture content of 40%. We also observed that the higher the soil moisture content becomes, the more difficult it is to obtain a homogeneous distribution of the water in the calibration column which might have an impact on the sensor readings

Altered energy partitioning across terrestrial ecosystems in the European drought year 2018

Drought and heat events, such as the 2018 European drought, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO(2)exchange. Each of these quantities may aggravate or mitigate the drought, heat, their side effects on productivity, water scarcity and global warming. We used measurements of 56 eddy covariance sites across Europe to examine the response of fluxes to extreme drought prevailing most of the year 2018 and how the response differed across various ecosystem types (forests, grasslands, croplands and peatlands). Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004-2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought affected. These received on average 9% more solar radiation and released 32% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased net CO(2)uptake by 17.8%, but did not significantly change net evapotranspiration. The response of these fluxes differed characteristically between ecosystems; in particular, the general increase in the evaporative index was strongest in peatlands and weakest in croplands. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'

ECOSTRESS: NASA's next generation mission to measure evapotranspiration from the International Space Station

The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station ECOSTRESS) was launched to the International Space Station on June 29, 2018. The primary science focus of ECOSTRESS is centered on evapotranspiration (ET), which is produced as level‐3 (L3) latent heat flux (LE) data products. These data are generated from the level‐2 land surface temperature and emissivity product (L2_LSTE), in conjunction with ancillary surface and atmospheric data. Here, we provide the first validation (Stage 1, preliminary) of the global ECOSTRESS clear‐sky ET product (L3_ET_PT‐JPL, version 6.0) against LE measurements at 82 eddy covariance sites around the world. Overall, the ECOSTRESS ET product performs well against the site measurements (clear‐sky instantaneous/time of overpass: r2 = 0.88; overall bias = 8%; normalized RMSE = 6%). ET uncertainty was generally consistent across climate zones, biome types, and times of day (ECOSTRESS samples the diurnal cycle), though temperate sites are over‐represented. The 70 m high spatial resolution of ECOSTRESS improved correlations by 85%, and RMSE by 62%, relative to 1 km pixels. This paper serves as a reference for the ECOSTRESS L3 ET accuracy and Stage 1 validation status for subsequent science that follows using these data

Towards long-term standardised carbon and greenhouse gas observations for monitoring Europe's terrestrial ecosystems : a review

Research infrastructures play a key role in launching a new generation of integrated long-term, geographically distributed observation programmes designed to monitor climate change, better understand its impacts on global ecosystems, and evaluate possible mitigation and adaptation strategies. The pan-European Integrated Carbon Observation System combines carbon and greenhouse gas (GHG; CO2, CH4, N2O, H2O) observations within the atmosphere, terrestrial ecosystems and oceans. High-precision measurements are obtained using standardised methodologies, are centrally processed and openly available in a traceable and verifiable fashion in combination with detailed metadata. The Integrated Carbon Observation System ecosystem station network aims to sample climate and land-cover variability across Europe. In addition to GHG flux measurements, a large set of complementary data (including management practices, vegetation and soil characteristics) is collected to support the interpretation, spatial upscaling and modelling of observed ecosystem carbon and GHG dynamics. The applied sampling design was developed and formulated in protocols by the scientific community, representing a trade-off between an ideal dataset and practical feasibility. The use of open-access, high-quality and multi-level data products by different user communities is crucial for the Integrated Carbon Observation System in order to achieve its scientific potential and societal value.Peer reviewe

The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.Peer reviewe