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Study on the processing method of nighttime CO_2 eddy covariance flux data in chinaflux

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Science China Earth Sciences 2006年第S2期49卷 36-46页

作者： ZHU Zhilin, SUN Xiaomin, WEN Xuefa, ZHOU Yanlian, TIAN Jing & YUAN Guofu Key Laboratory of Ecosystem Network Observation and Model, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

At present, using Eddy Covariance (EC) method to estimate the "true value" of carbon sequestration in terrestrial ecosystem arrests more attention. However, one issue is how to solve the uncertainty of observations (especially the nighttime CO2 flux data) appearing in post-processing CO2 flux data. The ratio of effective and reliable nighttime EC CO2 flux data to all nighttime data is relatively low (commonly, less than 50%) for all the long-term and continuous observation stations in the world. Thus, the processing method of nighttime CO2 flux data and its effect analysis on estimating CO2 flux annual sums are very important. In this paper, the authors analyze and discuss the reasons for underestimating nighttime CO2 flux using EC method, and introduce the general theory and method for processing nighttime CO2 flux data. By analyzing the relationship between nighttime CO2 flux and air fraction velocity u., we present an alternate method, Average Values Test (AVT), to determine the thresholds of fraction velocity (u.c) for screening the effective nighttime CO2 flux data. Meanwhile, taking the data observed in Yucheng and Changbai Mountains stations for an example, we analyze and discuss the effects of different methods or parameters on nighttime CO2 flux estimations. Finally, based on the data of part chinaflux.stations and related literatures, empirical models of nighttime respiration at different sites in chinaflux.are summarized.

关键词： eddy covariance nighttime CO2 flux data correction chinaflux.

A preliminary study for spatial representiveness of flux observation at chinaflux.sites

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Science China Earth Sciences 2006年第S2期49卷 24-35页

作者： Ml Na, YU Guirui, WANG Panxing, WEN Xuefa & SUN Xiaomin Nanjing University of Information Science and Technology, Nanjing 210044, China Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China

The results of eddy covariance observation system could represent the physical process at certain area of the surface. Thus point-to-area representativeness was of primary interest in flux observation. This research presents a preliminary study for flux observation at chinaflux.sites by the use of observation data and Flux Source Area Model (FSAM). Results show that the footprint expands and is further away from flux tower when atmosphere becomes more stable, the observation height increases, or the surfaces become smoother. This suggests that the area represented by the flux observation becomes larger. The distances from the reference point to the maximum point Smax and the minimum point x1 of source weight function (Dmax and Dmin, respectively) can be influenced by atmosphere stability which becomes longer when atmosphere is more stable. For more rough surfaces and lower observation point Dmax and Dmin become shorter. This research gives the footprint at level P=90% at chinaflux.sites at different atmosphere stability. The preliminary results of spatial representiveness at chinaflux.sites were given based on the dominant wind direction and footprint response to various factors. The study also provides some theoretical basis for data quality control and evaluating data uncertainty.

关键词： eddy covariance method footprint source weight function chinaflux.

Recent progress and future directions of chinaflux

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Science China Earth Sciences 2006年第S2期49卷 1-23页

作者： YU Guirui , FU Yuling, SUN Xiaomin, WEN Xuefa & ZHANG Leiming Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China School of Earth Sciences, Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

The eddy covariance technique has emerged as an important tool to directly measure carbon dioxide, water vapor and heat fluxes between the terrestrial ecosystem and the atmosphere after a long history of fundamental research and technological developments. With the realization of regional networks of flux measurements in North American, European, Asia, Brazil, Australia and Africa, a global-scale network of micrometeorological flux measurement (FLUXNET) was established in 1998. FLUXNET has made great progresses in investigating the environmental mechanisms controlling carbon and water cycles, quantifying spatial-temporal patterns of carbon budget and seeking the "missing carbon sink" in global terrestrial ecosystems in the past ten years. The global-scale flux measurement also built a platform for international communication in the fields of resource, ecology and environment sciences. With the continuous development of flux research, FLUXNET will introduce and explore new techniques to extend the application fields of flux measurement and to answer questions in the fields of bio-geography, eco-hydrology, meteorology, climate change, remote sensing and modeling with eddy covariance flux data. As an important part of FLUXNET, chinaflux.has made significant progresses in the past three years on the methodology and technique of eddy covariance flux measurement, on the responses of CO2 and H2O exchange between the terrestrial ecosystem and the atmosphere to environmental change, and on flux modeling development. Results showed that the major forests on the North-South Transect of Eastern China (NSTEC) were all carbon sinks during 2003 to 2005, and the alpine meadows on the Tibet Plateau were also small carbon sinks. However, the reserved natural grassland, Leymus chinensis steppe in Inner Mongolia, was a carbon source. On a regional scale, temperature and precipitation are the primary climatic factors that determined the carbon balance in major terrestrial ecosystems in

关键词： eddy covariance flux measurement carbon budget terrestrial ecosystem FLUXNET chinaflux.

Determination of averaging period parameter and its effects analysis for eddy covariance measurements

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Science China Earth Sciences 2005年第Z1期48卷 33-41页

作者： SUN Xiaomin, ZHU Zhilin, XU Jinping, YUAN Guofu , ZHOU Yanlian & ZHANG Renhua Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Institute of Geographic Sciences and Natural Resources Research Chinese Academy of Sciences Beijing 100101 China Graduate School of the Chinese Academy of Sciences Beijing 100039 China Institute of Geographic Sciences and Natural Resources Research Chinese Academy of Sciences Beijing 100101 China Graduate School of the Chinese Academy of Sciences Beijing 100039 China

It is more and more popular to estimate the exchange of water vapor, heat and CO2fluxes between the land surface and the atmosphere using the eddy covariance technique. To get believable fluxes, it is necessary to correct the observations based on the different surface conditions and to determine relevant techinical parameters. The raw 10 Hz eddy covariance data observed in the Yucheng and Changbai Mountains stations were recalculated by various averaging periods (from 1 to 720 min) respectively, and the recalculated results were compared with the results calculated by the averaging period of 30 mins. Meanwhile, the distinctions of fluxes calculated by different averaging periods were analyzed. The continuous 15 days observations over wheat fields in the Yucheng station were mainly analyzed. The results are shown that: (i) In the Yucheng station, compared with the observations by 30 min, when the averaging period changes from 10 to 60 min, the variations of the eddy-covariance estimates of fluxes were less than 2%; when the averaging period changes less than 10 min, the estimate of fluxes reduced obviously with the reduction of the averaging period (the max relative error was -12%); and when the averaging period exceeds 120 min, the eddy covariance estimates of fluxes will be increased and become unsteady (the max relative error is over 10%); (ii) the eddy covariance estimates of fluxes over wheat field in the Yucheng station suggusted that it is much better to take 10 min as an averaging period in studying diurnal change of fluxes, and take 30min for a long-term flux observation; and (iii) normalized ratio was put forward to determine the range of averaging period of eddy covariance measurements. By comparing the observations over farmlands and those over forests, it is indicated that the increase of eddy covariance estimates over tall forest was more than that over short vegetation when the averaging period increased.

关键词： eddy covariance, various surface conditions, averaging period, parameter, chinaflux.

Simulating the exchanges of carbon dioxide, water vapor and heat over Changbai Mountains temperate broad- leaved Korean pine mixed forest ecosystem

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Science China Earth Sciences 2005年第Z1期48卷 148-159页

A process-based ecosystem productivity model BEPS (Boreal Ecosystem Productivity Simulator) was updated to simulate half-hourly exchanges of carbon, water and energy between the atmosphere and terrestrial ecosystem at a temperate broad-leaved Korean pine forest in the Changbai Mountains, China. The BEPSh model is able to capture the diurnal and seasonal variability in carbon dioxide, water vapor and heat fluxes at this site in the growing season of 2003. The model validation showed that the simulated net ecosystem productivity (NEP), latent heat flux (LE), sensible heat flux (Hs) are in good agreement with eddy covariance measurements with an R2 value of 0.68, 0.86 and 0.72 for NEP, LE and Hs, respectively. The simulated annual NEP of this forest in 2003 was 300.5 gC/m2, and was very close to the observed value. Driving this model with different climate scenarios, we found that the NEP in the Changbai Mountains temperate broad-leaved Korean pine mixed forest ecosystem was sensitive to climate variability, and the current carbon sink will be weakened under the condition of global warming. Furthermore, as a process-based model, BEPSh was also sensitive to physiological parameters of plant, such as maximum Rubisco activity (Vcmax) and the maximum stomatal conductance (gmax), and needs to be carefully calibrated for other applications.

关键词： carbon cycle, water cycle, BEPSh model, eddy covariance measurement, chinaflux.

CO_2 flux evaluation over the evergreen coniferous and broad-leaved mixed forest in Dinghushan, China

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Science China Earth Sciences 2006年第S2期49卷 127-138页

作者： WANG Chunlin, YU Guirui, ZHOU Guoyi, YAN Junhua, ZHANG Leiming, WANG Xu, TANG Xuli & SUN Xiaomin South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China Climate and Agrometeorology Center of Guangdong Province, China Meteorological Administration, Guangzhou 510080, China Institute of Geographic Science and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China

The Dinghushan flux observation site, as one of the four forest sites of chinaflux. aims to acquire long-term measurements of CO2 flux over a typical southern subtropical evergreen coniferous and broad-leaved mixed forest ecosystem using the open path eddy covariance method. Based on two years of data from 2003 to 2004, the characteristics of temporal variation in CO2 flux and its response to environmental factors in the forest ecosystem are analyzed. Provided two-dimensional coordinate rotation, WPL correction and quality control, poor energy-balance and underestimation of ecosystem respiration during nighttime implied that there could be a CO2 leak during the nighttime at the site. Using daytime (PAR > 1.0μmol-1·m-2·s-1) flux data during windy conditions (u* > 0.2 m·s-1), monthly ecosystem respiration (Reco) was derived through the Michaelis-Menten equation modeling the relationship between net ecosystem CO2 exchange (NEE) and photosynthetically active radiation (PAR). Exponential function was employed to describe the relationship between Reco and soil temperature at 5 cm depth (Ts05), then Reco of both daytime and nighttime was calculated respectively by the function. The major results are: (i) Derived from the Michaelis-Menten equation, the apparent quantum yield (α) was 0.0027±0.0011 mgCO2·μmol-1 photons, and the maximum photosynthetic assimilation rate (Amax) was 1.102±0.288 mgCO2·m-2·s-1. Indistinctive seasonal variation of o or Amax was consistent with weak seasonal dynamics of leaf area index (LAI) in such a lower subtropical evergreen mixed forest. (ii) Monthly accumulated Reco was estimated as 95.3±21.1 gC·m-2 mon-1, accounting for about 68% of the gross primary product (GPP). Monthly accumulated NEE was estimated as -43.2±29.6 gC·m-2·mon-1. The forest ecosystem acted as carbon sink all year round without any seasonal carbon efflux period. Annual NEE of 2003 and 2004 was estimated as -563.0 and -441.2 gC·m-2·a-1 respectively, accounting for about 32% of G

关键词： carbon dioxide flux eddy covariance Dinghushan southern subtropical region evergreen coniferous and broad-leaved mixed forest chinaflux.

Study of a model for correcting the effects of horizontal advection on surface fluxes measurement based on remote sensing

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Science China Earth Sciences 2006年第S2期49卷 273-280页

作者： TIAN Jing, ZHANG Renhua, SUN Xiaomin, ZHU Zhilin & ZHOU Yanlian Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

As well known, the methods of remote sensing and Bowen Ratio for retrieving surface flux are based on energy balance closure; however, in most cases, surface energy observed in experiment is lack of closure. There are two main causes for this: one is from the errors of the observation devices and the differences of their observational scale; the other lies in the effect of horizontal advection on the surface flux measurement. Therefore, it is very important to estimate the effects of horizontal advection quantitatively. Based on the local advection theory and the surface experiment, a model has been proposed for correcting the effect of horizontal advection on surface flux measurement, in which the relationship between the fetch of the measurement and pixel size for remote sensed data was considered. By means of numerical simulations, the sensitivities of the main parameters in the model and the scaling problems of horizontal advection were analyzed. At last, by using the observational data acquired in agricultural field with relatively homogeneous surface, the model was validated.

关键词： fetch horizontal advection energy balance pixel size chinaflux.