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Eddy covariance technique to measure CO2, water and energy fluxes between biosphere and atmosphere is widely spread and used in various regional networks. Currently more that 250 eddy covariance sites are active around the world measuring carbon exchange at high temporal resolution for different biomes and climatic conditions. These data are usually acquired using the same method but they need a set of corrections that are often differently applied to each site and in a subjective way. In this paper a new standardized set of corrections are proposed and the uncertainties introduced by these corrections are assessed for 8 different forest sites in Europe with a total of 12 yearly datasets. The uncertainties introduced on the two components GPP (Gross Primary Production) and TER (Terrestrial Ecosystem Respiration) are also discussed and a quantitative analysis presented . The results show that a standardized data processing is needed for an effective comparison across biomes and for underpinning inter-annual variability. The methodology presented in this paper has also been integrated in the European database of the eddy covariance measurements.

Eddy covariance technique to measure CO2, water and energy fluxes between biosphere and atmosphere is widely spread and used in various regional networks. Currently more than 250 eddy covariance sites are active around the world measuring carbon exchange at high temporal resolution for different biomes and climatic conditions. In this paper a new standardized set of corrections is introduced and the uncertainties associated with these corrections are assessed for eight different forest sites in Europe with a total of 12 yearly datasets. The uncertainties introduced on the two components GPP (Gross Primary Production) and TER (Terrestrial Ecosystem Respiration) are also discussed and a quantitative analysis presented. Through a factorial analysis we find that generally, uncertainties by different corrections are additive without interactions and that the heuristic u(*)-correction introduces the largest uncertainty. The results show that a standardized data processing is needed for an effective comparison across biomes and for underpinning interannual variability. The methodology presented in this paper has also been integrated in the European database of the eddy covariance measurements. Eddy covariance technique to measure CO2, water and energy fluxes between biosphere and atmosphere is widely spread and used in various regional networks. Currently more than 250 eddy covariance sites are active around the world measuring carbon exchange at high temporal resolution for different biomes and climatic conditions. In this paper a new standardized set of corrections is introduced and the uncertainties associated with these corrections are assessed for eight different forest sites in Europe with a total of 12 yearly datasets. The uncertainties introduced on the two components GPP (Gross Primary Production) and TER (Terrestrial Ecosystem Respiration) are also discussed and a quantitative analysis presented. Through a factorial analysis we find that generally, uncertainties by different corrections are additive without interactions and that the heuristic u(*)-correction introduces the largest uncertainty. The results show that a standardized data processing is needed for an effective comparison across biomes and for underpinning interannual variability. The methodology presented in this paper has also been integrated in the European database of the eddy covariance measurements. Eddy covariance technique to measure CO2, water and energy fluxes between biosphere and atmosphere is widely spread and used in various regional networks. Currently more than 250 eddy covariance sites are active around the world measuring carbon exchange at high temporal resolution for different biomes and climatic conditions. In this paper a new standardized set of corrections is introduced and the uncertainties associated with these corrections are assessed for eight different forest sites in Europe with a total of 12 yearly datasets. The uncertainties introduced on the two components GPP (Gross Primary Production) and TER (Terrestrial Ecosystem Respiration) are also discussed and a quantitative analysis presented. Through a factorial analysis we find that generally, uncertainties by different corrections are additive without interactions and that the heuristic u(*)-correction introduces the largest uncertainty. The results show that a standardized data processing is needed for an effective comparison across biomes and for underpinning interannual variability. The methodology presented in this paper has also been integrated in the European database of the eddy covariance measurements.

The Basel UrBan Boundary Layer Experiment (BUBBLE) was a year-long experimental effort to investigate in detail the boundary layer structure in the City of Basel, Switzerland. At several sites over different surface types (urban, sub-urban and rural reference) towers up to at least twice the main obstacle height provided turbulence observations at many levels. In addition, a Wind Profiler and a Lidar near the city center were profiling the entire lower troposphere. During an intensive observation period (IOP) of one month duration, several sub-studies on street canyon energetics and satellite ground truth, as well as on urban turbulence and profiling (sodar, RASS, tethered balloon) were performed. Also tracer experiments with near-roof-level release and sampling were performed. In parallel to the experimental activities within BUBBLE, a meso-scale numerical atmospheric model, which contains a surface exchange parameterization, especially designed for urban areas was evaluated and further developed. Finally, the area of the full-scale tracer experiment which also contains several sites of other special projects during the IOP (street canyon energetics, satellite ground truth) is modeled using a very detailed physical scale-model in a wind tunnel. In the present paper details of all these activities are presented together with first results.

International audience; This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems. We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long-term temperature sensitivity exceeds the short-term sensitivity. Thus, in those ecosystems, the application of a long-term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from halfhourly to annual time-scales, which can reach 425% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long-term temperature sensitivity is lower than the short-term temperature sensitivity resulting in underestimation of annual sums of respiration. We introduce a new generic algorithm that derives a short-term temperature sensitivity of Reco from eddy covariance data that applies this to the extrapolation from night- to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and Reco, we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.

[1] Data from a network of eddy covariance stations in Europe and North America (FLUXNET) were analyzed to examine the diurnal patterns of surface energy and carbon fluxes during the summer period across a range of ecosystems and climates. Diurnal trends were quantified by assessing the time of day surface fluxes and meteorological variable reached peak values, using the “diurnal centroid” method; the diurnal centroid enabled us to discern whether the peak activity of the variable of interest is weighted more toward the morning or afternoon. In this paper, diurnal centroid estimates were used to diagnose which atmospheric and physiological processes controlled carbon dioxide, water vapor, and sensible heat fluxes across different ecosystems and climates. Sensitivity tests suggested that the diurnal centroids for latent (LE) and sensible (H) heat flux depend on atmospheric resistance, static stability in the free atmosphere, stomatal response to vapor pressure deficit, and advection. With respect to diurnal trends of surface energy fluxes at FLUXNET sites, maximum LE occurred later in the day relative to H at most tall forests with continental climates. The lag between LE and H was reduced or reversed at sites that were influenced by advection or by afternoon stomatal closure. The time of peak carbon uptake of temperate forests occurred earlier relative to the temporal peak of photosynthetically active radiation, as compared to boreal forests. The timing of this peak occurred earlier during periods with low soil water content, as it did during the summer in Mediterranean climates. In this case, the diurnal centroid for the CO2 flux was influenced by the response of respiration and photosynthesis to increasing afternoon temperature and by afternoon stomatal closure.

Forests play an important role in the water and energy balance of the land surface. It has been known since the early studies of Horton (1919) and Rutter (1975) that the water use of forests can be considerably higher than that of vegetation of different structure and height. Subsequent work has elucidated the main factors influencing this behavior (e.g., Shuttleworth and Calder 1979; Shuttleworth 1989). They showed that the combination of a high aerodynamic roughness, with a relative low and strongly controlled surface resistance, was the main cause for high evaporation rates from wet canopies and somewhat low transpiration rates from dry canopies. They also suggested that care must be given to the separate modeling of dry and wet canopy evaporation, and that total evaporation could not simply be derived from equations relating total evaporation to a net radiation estimate. Building on that work, numerous modeling studies at regional and global scale have provided evidence that the interaction of the forests with the atmosphere is a major component in shaping regional to global climate and weather (Nobre et al. 1991; Blyth et al. 1994). Forests not only use more water by evaporating more, they also influence the rainfall patterns and magnitude at regional and global scales by increasing the low level moisture convergence (supply of moisture through horizontal advection in the lower layers of the atmosphere).

International audience; As length and timing of the growing season are major factors explaining differences in carbon exchange of ecosystems. we analyzed seasonal patterns of net ecosystem carbon exchange (F-NEE) using eddy covariance data of the FLUXNET data base (http://www-eosdis.ornl.gov/FLUXNET). The study included boreal and temperate. deciduous and coniferous forests, Mediterranean evergreen systems, rainforest, native and managed temperate grasslands, tundra, and C-3 and C-4 crops. Generalization of seasonal patterns are useful for identifying functional vegetation types for global dynamic vegetation models, as well as for global inversion studies, and can help improve phenological modules in SVAT or biogeochemical models. The results of this study have important validation potential for global carbon cycle modeling. The phasing of respiratory and assimilatory capacity differed within forest types: for temperate coniferous forests seasonal uptake and release capacities are in phase, for temperate deciduous and boreal coniferous forests, release was delayed compared to uptake. According to seasonal pattern of maximum nighttime release (evaluated over 15-day periods. F-max) the study sites can be grouped in four classes: (1) boreal and high altitude conifers and grasslands: (2) temperate deciduous and temperate conifers; (3) tundra and crops; (4) evergreen Mediterranean and tropical forest,,, Similar results are found for maximum daytime uptake (F-min) and the integral net carbon flux, but temperate deciduous forests fall into class 1. For forests, seasonal amplitudes of F-max and F-min increased in the order tropical < Mediterranean and temperate coniferous < temperate deciduous and boreal forests, and the pattern seems relatively stable for these groups. The seasonal amplitudes of F-max and F-min are largest for managed grasslands and crops. Largest observed values of F-min varied between -48 and -2 mumol m(-2) s(-1), decreasing in the order C-4-crops > C-3-crops > temperate deciduous forests > temperate conifers > boreal conifers > tundra ecosystems. Due to data restrictions, our analysis centered mainly on Northern Hemisphere temperate and boreal forest ecosystems. Grasslands, crops, Mediterranean ecosystems, and rainforests are under-represented. as are savanna systems, wooded grassland, shrubland, or year-round measurements in tundra systems. For regional or global estimates of carbon sequestration potentials, future investigations of eddy covariance should expand in these systems.

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol1. Several studies suggest that the terrestrial biosphere is gaining carbon2,3,4,5,6,7,8, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha-1 yr-1, with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Publisher Summary The chapter has described the measurement system and the procedure followed for the computation of the fluxes and the procedure of flux summation, including data gap filling strategy, night flux corrections and error estimation. It begins with the introduction of estimates of the annual net carbon and water exchange of forests using the EUROFLUX methodology. The chapter then provides us with the theory and moves on to discuss the eddy covariance system and its sonic anemometer, temperature fluctuation measurements, infrared gas analyser, air transport system, and tower instrumentation. Additional measurements are also given in the chapter. Data acquisition and its computation and correction is discussed next in the chapter by giving its general procedure, half-hourly means (co-)variances and uncorrected fluxes, intercomparison of software, and correction for frequency response losses. The chapter has also discussed about quality control and four criteria are investigated here for the same. Spatial representativeness of measured fluxes and summation procedure are reviewed. The chapter then moves on to the discussion of data gap filling through interpolation and parameterization and neural networks. Corrections to night-time data and error estimation are also explored in the chapter. Finally, the chapter closes with conclusions.

The warm season (mid-June through late August) partitioning between sensible (H) and latent (LE) heat flux, or the Bowen ratio (beta=H/LE), was investigated at 27 sites over 66 site years within the international network of eddy covariance sites (FLUXNET). Variability in beta across ecosystems and climates was analyzed by quantifying general climatic and surface characteristics that control flux partitioning. The climatic control on beta was quantified using the climatological resistance (R-i), which is proportional to the ratio of vapor pressure deficit (difference between saturation vapor pressure and atmospheric vapor pressure) to net radiation (large values of R-i decrease beta). The control of flux partitioning by the vegetation and underlying surface was quantified by computing the surface resistance to water vapor transport (R-c, with large values tending to increase beta). There was a considerable range in flux partitioning characteristics (R-c, R-i and beta) among sites, but it was possible to define some general differences between vegetation types and climates. Deciduous forest sites and the agricultural site had the lowest values of R-c and beta (0.25-0.50). Coniferous forests typically had a larger R-c and higher beta (typically between 0.50 and 1.00 but also much larger). However, there was notable variability in R-c and R-i between coniferous site years, most notably differences between oceanic and continental climates and sites with a distinct dry summer season (Mediterranean climate). Sites with Mediterranean climates generally had the highest net radiation, R-c, R-i, and beta. There was considerable variability in beta between grassland site years, primarily the result of interannual differences in soil water content and R-c

International audience; This paper examines long-term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under nonwater- stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light-class carboxylase-based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process-based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange

Results from four successful tracer experiments carried out in a dense forest canopy are presented. For each experiment, sulphur hexafluoride SF 6 was released from a point source in trunk space and air samples were collected at 19 horizontally distributed locations in the canopy. Meteorological measurements include fast-response turbulence data recorded at several locations in and above the forest. Measured mean concentrations of SF 6 are compared against concentrations calculated by an analytical solution to the equation of diffusion. For near neutral and weak unstable conditions the model performs surprisingly well, even though the wind profile and turbulence statistics deviate substantially from surface layer characteristics. Still if more than an estimate of the concentration field is needed a more sophisticated model should be applied.

In semi-arid regions of Central Asia, water shortage results from low annual precipitation (P) with high interannual variability. Evapotranspiration (ET) dominates water balance losses entirely. Previous studies showed large differences between individual grassland sites in the partitioning of ET into evaporation and transpiration, but only little difference in the evaporative ratio ET/P. The hydrological model BROOK90 was applied to the Xilin river catchment in Inner Mongolia (China) in combination with data from moderate resolution imaging spectroradiometer (MODIS) and ET measurements. The ET part of the model was parameterised using several years of eddy covariance (EC) measurements over grasslands differing in grazing intensity and leaf area index (LAI). Using a relatively well-established relationship of LAI and P as well as LAI and temperature derived from MODIS, the water balance components of a 16 km² sample area in the catchment were modelled with a 1 km² resolution for the vegetation period of 2006. All pixels were modelled assuming a similar ET control as at the EC sites. Spatial variation in ET as well as in the partitioning of ET between transpiration and evaporation could be identified. The results indicate the potential of using MODIS data and BROOK90 to upscale ET of semi-arid grasslands from site to larger grass dominated catchments.

We applied a site evaluation approach combining Lagrangian Stochastic footprint modeling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.

During the International Biological Program (IBP), systematic investigations of carbon flow through ecosystems and the structuring of carbon pools within ecosystems were carried out for the first time across the existing spectrum of biome types. Since then, concerted efforts have been made to understand the changes occurring in ecosystem carbon pools and carbon flow during aging and maturation of the system, in response to disturbances, and as symptomatic of ecosystem status or “health” (see review by Reiners 1983, Odum 1985). In other words, thorough knowledge of ecosystem carbon pools and carbon flow for a wide variety of ecosystem types and states is viewed as a key means by which better understanding of ecosystem response, resilience, and sustainable function maybe achieved.