Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25-year-old, leaf area index 11 m 2 m À2 ) during two successive 7-day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO 2 to PPFD. NEE and stomatal conductance at the canopy level (G canopy ) was estimated from halfhourly eddy-covariance measurements of CO 2 and H 2 O fluxes. In addition, daily courses of CO 2 assimilation rate (A N ) and stomatal conductance (G s ) at shoot level were measured using a gas-exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer.We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun-acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves.Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels.

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The eddy covariance method was used for continuous measurement of the seasonal courses of the following parameters of the carbon cycle in a sedgegrass marsh type of wetland ecosystem (49°01 0 29 00 N, 14°46 0 13 00 E, South Bohemia, Czech Republic, Central Europe): gross ecosystem production (GEP), net ecosystem production (NEP) and ecosystem respiration. During a 3-year series of measurements, we recorded marked fluctuations of the water table, which affected the overall water regime of the wetland studied. Between-year differences in the water regime strongly influenced the total annual carbon sequestration. The lowest annual GEP and NEP of 996 and 152 g m -2 of carbon, respectively, were recorded in 2006, a year with two large floods, one in the spring, the other in the summer. By contrast, in the dry year of 2007, with no flood, the highest annual GEP and NEP were recorded: 1,328 and 274 g m -2 , respectively. Significant differences were found in the efficiency of solar energy use for GEP [gross radiation use efficiency, GRUE = GEP/PhAR (photosynthetically active radiation), i.e., amount of carbon gained per energy unit]. The highest GRUE was recorded immediately after the 2006 summer flood. In 2007, the GRUE decreased linearly with rising water table. A variable water regime thus markedly affects the processes of carbon accumulation and the efficiency of solar energy use for organic matter production in freshwater wetlands of the sedge-grass marsh type.

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We examined the net-ecosystem-exchange (NEE)-based annual carbon-balance estimates obtained from eddy-covariance (EC) measurements at an unmanaged sedge-grass marsh ecosystem (Třeboň, Czech Republic, 49°1′ N, 14°46′ E), seeking methods to improve the EC measurements in inhomogeneous environment. The data filtering procedure was developed using three thresholds: (a) a stationarity test; (b) a stability $${u}_{*}$$ u ∗ -threshold; and (c) a high relative humidity RH-threshold. This procedure was tested in 2014, a year without significant floods and drought events led to a stable water table, reducing the effect of soil respiration on the EC measurements. Estimates of annual carbon-balance were reduced from 182 to 234 ± 12 gC m−2 year−1 for the initial data to 39–44 ± 8 gC m−2 year−1 after the $$RH$$ RH ≤ 95% filtering and to 24–26 ± 7 gC m−2 year−1 after the further $${u}_{*}$$ u ∗ ≥ 0.1 m s−1 filtering. Applying the precipitation/fog threshold reduced this balance to 10–12 ± 7 gC m−2 year−1, closer to carbon neutrality. Up to 9.5% of this identified shift occurred during apparent nocturnal downslope katabatic drainage flows or plume descent coming from the nearby town of Třeboň. High-RH conditions account for up to 27% of this shift. Moreover, both conditions together account for an additional 67% of the identified carbon-balance change. Removing these non-ecosystem-related processes brings EC measurements closer to values of an unmanaged-ecosystem productivity, providing a better NEE-based estimate for the net ecosystem production. The presented procedure is applicable to EC measurements conducted at different wetlands or terrestrial ecosystems with similar conditions.

<p>In order to assure the quality of operational satellite products, there is a strong demand for timely available in-situ flux data. Typically, the Earth Observation Community has to rely on publicly available data processed and distributed by flux networks such as the European Fluxes Database Cluster, AmeriFlux Network, and other major networks globally.</p> <p>While the centralized processing systems employed by the major networks provide exceptional advantages for long-term data quality, reproducibility and comparability, to date these result in 1-5 year delays between the time of the actual in-situ flux measurement and the publicly online availability of processed and quality controlled data, especially for derived parameters such as Gross Primary Production (GPP) often used by Earth Observation experts. Such delays hamper the use of in-situ fluxes for timely (and ultimately near-real-time) operational satellite product monitoring, envisioned and often required by the Earth Observation Community.</p> <p>Within the European Copernicus Global Land Service (CGLOPS), a validation protocol is in place for each publicly available satellite product. One of the elements is the yearly Scientific Quality Evaluation (SQE), where data of the most recent calendar year are quality checked within the three months after the end of the year. This implies that in-situ data should be available within this timespan in order to be included in the operational quality monitoring. Recently, a set of new tools to collect, process, analyze, partition, time- and space- allocate and share time-synchronized flux data from multiple flux stations were developed and deployed globally. These new tools can be effective in solving the time delay issues listed above without sacrificing quality, reproducibility and comparability of the in-situ flux data.</p> <p>The fully automated remotely-accessible microcomputer, SmartFlux, utilizes EddyPro software  to calculate fully-processed fluxes in near-real-time, alongside supporting data and flux footprints. All data are merged into a single quality-controlled file timed using GPS-driven PTP time protocol to assure a microseconds-scale time synch between  the instruments within each station and between different stations.</p> <p>The flux data analysis software, Tovi, can seamlessly ingest the data from the SmartFlux stations to allow a non-micrometeorologist analyze and interpret the flux data. Specifically, it allows rapid execution of the QC/QA and data analysis steps using interactive GUI, including advanced QC and gap fill schemes, footprint calculations and flux apportioning, NEE (Net Ecosystem Exchange) flux partitioning, automated generation of specific lists of references for each workflow, etc. All processing routines and analysis steps are reproducibile and intercomparable to other SmartFlux stations across the globe.</p> <p>Based upon the timely needs for the in-situ flux data and the newly available technical tools, a pilot initiative was set-up to test the viability of using 2019 data generated by multiple SmartFlux stations and Tovi analysis software to quality control, gap fill, and partition NEE into GPP product to support the quality assurance analysis of the global Copernicus Dry Matter Productivity (DMP) product. This presentation will show the actual established workflow, and demonstrate the detailed post-processing of in-situ flux data for timely operational satellite product monitoring.</p>