Open-path Eddy Covariance System Research Articles

Estimating crop coefficients and water use of a full-bearing mango orchard in north-eastern South Africa using the fraction of vegetation cover and a dual source evapotranspiration model

In semi-arid countries like South Africa, commercially produced mango (Mangifera Indica L.) are grown entirely under irrigation. However, there is little accurate quantitative information on the water use of mango orchards, and few accurate water use models currently exist. In this study, we evaluated the performance of the FAO 56 method of estimating crop evapotranspiration (ETc) and a dual source water use model against measurements of actual consumptive water use of a commercial mango orchard. The FAO approach calculates ETc as the product of a crop coefficient (Kc) and the reference evapotranspiration (ETo). We derived Kc for well-watered orchards from readily available data including the fraction of vegetation cover, average tree height, and a stomatal control coefficient. The dual-source model calculated orchard evapotranspiration as the sum of tree transpiration and orchard floor evaporation derived from the modified Shuttleworth and Wallace model. Actual evapotranspiration was measured using an open path eddy covariance system, while tree transpiration was measured using the heat ratio method of monitoring sap flow. Data were collected in a mature ‘Tommy Atkins’ mango orchard grown with micro-sprinkler irrigation in north-eastern South Africa. Results show that the reference evapotranspiration explained most of the variation in orchard transpiration (R2 ∼ 0.78) compared to the solar radiation (R2 ∼ 0.66) and the vapour pressure deficit of the air (R2 ∼ 0.66). Compared with field measured water use data, the FAO method gave relatively accurate estimates of transpiration (R2 = 0.74, NRMSE = ±18.00 %, NMAE = ±15.00 %, NSE = 0.70) and evapotranspiration (R2 = 0.58, NRMSE = ±19.00 %, NMAE = ±16.00 %, NSE = 0.55). NSE is the Nash-Sutcliffe Efficiency. The dual-source model yielded slightly less accurate estimates of transpiration (R2 = 0.61, NRMSE = ±22.00 %, NMAE = ±20.00 %, NSE = 0.39) and evapotranspiration (R2 = 0.52, NRMSE = ±23.00 %, NMAE = ±25.00 %, NSE = 0.51). The basal crop coefficient (Kcb) showed small seasonal fluctuations, varying between 0.40 and 0.60 throughout the year. In contrast, Kc showed clear seasonality varying between 0.59–0.64 at flowering and fruit set and rising to 0.81–0.90 during the fruit growing phase. Estimates of the annual total transpiration (686 mm) from the FAO method were within 5 % of the actual measured values (677 mm) while the dual source model estimated 612 mm of transpiration, which was almost 10 % lower than the measured values. The results highlight that even though the FAO method requires fewer input parameters, it is potentially more accurate in estimating water use in mango orchards than the dual-source model.

A dataset of carbon and water fluxes in a Phragmites australis wetland in the Yellow River Delta during 2011–2018

Wetland ecosystems play an important role in mitigating global climate change. The long-term monitoring of carbon and water fluxes based on eddy covariance (EC) would be helpful to better protect and utilize ecosystem services of wetlands. However, related data and studies focusing on carbon and water fluxes in wetland ecosystems are still limited. Hence, there is an urgent need to provide more observational data for researchers. Yellow River Delta Ecological Research Station of Coastal Wetland, Chinese Academy of Sciences, has established an open-path EC system in a Phragmites australis wetland in 2010 and obtained large amounts of flux data to date. In this paper, the EC-based dataset consisted ecosystem-scale carbon and water fluxes at the study site from 2011 to 2018. Moreover, all the data were shown at four time scales, i.e., half-hourly, daily, monthly, and yearly. This dataset is of great importance for estimating carbon and water fluxes in wetland ecosystems and examining the flux variations at different time scales.

Aerosol interference with open-path eddy covariance measurement in a lake environment

Open-path eddy covariance (OPEC) is widely used for measuring trace gas fluxes between the surface and the atmosphere. At a lake in Eastern China (Lake Taihu), the CO2 flux measured with OPEC was often negative at night (with values as low as −22.1 μmol m−2 s−1) and was coherent across the whole lake, as if the lake were a large sink of atmospheric CO2. The purpose of this study is to investigate the cause of this negative flux phenomenon. In addition to OPEC, we also used closed-path eddy covariance (CPEC) and the transfer coefficient (TC) method to measure the flux. The results show that the persistent negative CO2 flux phenomenon was observed with OPEC but not with CPEC or TC. Because air drawn into the CPEC analyzer was filtered but the OPEC analyzer was influenced by aerosol contamination, the most logical explanation was that particles deposited on the optical lens of the OPEC analyzer changed its cross-sensitivity to water vapor. The direct evidence of this interference was a strong positive correlation between the OPEC analyzer's signal strength and the CO2 mixing ratio observed at 10 Hz. We suggest that it is possible to perform post-field correction to this negative flux bias using the 10 Hz signal strength data. In comparison, an OPEC system at a nearby land site did not experience aerosol interference due to low water vapor flux at night and lack of hygroscopic growth of particles on the optical lens in low humidity conditions. The type of aerosol interference reported here may also occur in high humidity and high pollution conditions elsewhere.

Assessing the energy and water balance dynamics of rain-fed rooibos tea crops (Aspalathus linearis) under changing Mediterranean climatic conditions

Rooibos (Aspalathus linearis), a herbal tea crop and a member of the plant family Fabaceae, grows in a narrow ecological range under Mediterranean conditions in South Africa's fynbos biome. There is growing uncertainty on the sustainability of rooibos production under rain-fed conditions in a region which is expected to get warmer and drier in future. No accurate quantitative information currently exists on how the water use, growth and yield of this crop is affected by environmental factors, especially climate and soils. This paucity of data reduces the ability of farmers to develop effective strategies to cope and adapt to the negative effects of climate variability and change. The aim of this study was to investigate the seasonal changes in the energy and water balance of rooibos fields and to establish how this relates to crop yield. An open path eddy covariance system was used to quantify the actual evapotranspiration (ETa) and energy balance components over two growing seasons while sap flow was monitored using stem heat balance sap flow gauges. These data were used to develop and test a simple ETa and yield model which was applied to estimate changes in crop water use and yield under the predicted future climatic conditions. More than 36% of the annual total ETa (∼ 338 mm) was consumed during the first three months of the growing season (September to November) driven by residual soil moisture from the winter rains. Crop transpiration accounted for only 16% of ETa early in the growing season rising to almost 100% during the summer dry months. Daily peak transpiration at maximum canopy cover barely reached 1.0 mm suggesting that this crop uses very little water. Six climate change models were used to simulate crop water use and yield for the far future under the RCP 8.5 emission scenario. All the models predicted increases in both the annual ETa and yield by between 8% and 16% in the period 1961–2099. This study reveals that implementing water conservation and weed management practices early in the growing season can save substantial amounts of soil moisture which can sustain rooibos production under low rainfall conditions.

Contribution of understorey vegetation to evapotranspiration partitioning in apple orchards under Mediterranean climatic conditions in South Africa

Orchard evapotranspiration (ET) is a complex flux which has been the subject of many studies. It often includes transpiration from the trees, cover crops and weeds, evaporation from the soil, mulches and other orchard artefacts. In this study we investigated the contribution of the orchard floor evaporative fluxes to whole orchard ET focusing on the transpiration dynamics of understorey vegetation which is currently not well known. Data on the partitioning of ET into its constituent components were collected in apple (Malus Domestica Bork) orchards with varying fractional canopy cover. The study orchards were in the prime apple growing regions in South Africa. The orchards were planted to the Golden Delicious/Reinders and the red cultivars (i.e. Cripps’ Pink/ Royal Gala/Fuji). Tree transpiration was quantified using the heat ratio method and the thermal dissipation sap flow techniques. Understorey transpiration was measured at selected intervals using micro stem heat balance sap flow gauges calibrated against infrared gas analyser readings. Orchard ET was measured using an open path eddy covariance system while the microclimate, radiation interception, and soil evaporation were also monitored. Orchard floor evaporative fluxes accounted for as much as 80% of the measured ET in young orchards with dense understorey vegetation that covered most of the orchard floor. In these orchards the understorey transpiration was of the same order of magnitude as the bare moist soil evaporation suggesting that water use by the understorey vegetation was substantial. However, in mature orchards with a high canopy cover (>55% fractional cover), orchard floor water losses were less than 30% of the measured ET. Understorey transpiration rates were much lower, contributing less than 10% of the whole orchard ET. Significant volumes of water can be saved, especially in young orchards, by keeping the orchard floor vegetation short, reducing the area occupied by understorey vegetation, and by reducing the wetted ground surface area.

Carbon budget estimation based on different methods of CO2 storage flux in forest ecosystems

Accurate measurement of CO2 storage flux (Fs) in forest ecosystems is of great significance for estimating ecosystem carbon budget by eddy covariance (EC). The errors in the estimation of ecosystem carbon budget caused by different methods for calculating Fs has yet not been comprehensively assessed. Using data from an open-path EC system and an eight-level CO2/H2O profile system (AP100, Campbell Scientific Inc., USA) in a broadleaved deciduous forest at the Maoer-shan in 2018, we evaluated the methodological effect of Fs[2-min mean profile (P2 min), 30-min mean profile (P30 min) and 30-min mean EC single point (Ps)] on the estimation of net ecosystem exchange (NEE), ecosystem respiration (Re), and gross primary productivity (GPP). The results showed that the impact of Fs methods on forest carbon flux generally increased with the increases of time scale, indicating that gap-filling of flux data would further amplify the impacts of Fs estimation methods. At the annual scale, NEE based on P2 min and Ps methods were 36.3% and 29.4% lower than that based on P30 min, while Re based on P2 min was higher than that based on P30 min and Ps by 8.7%. The GPP based on P2 min was 5.4% higher, while that based on Ps was 2.1% lower than that based on P30 min. The traditional P30 min ignored the instantaneous changes in CO2 concentration, Ps missed the changes of CO2 concentration within canopy, and thus both underestimated the actual Re. The approximately instantaneous profile (2-min mean profile) had higher temporal and spatial resolution and could more accurately estimate forest carbon budget with non-flat terrain and complex canopy structure. Our findings had great implications for solving the underestimation of forest Re and GPP as well as the overestimation of net carbon sink on complex conditions with the EC method.

Estimating evapotranspiration and determining crop coefficients of irrigated sweet potato (Ipomoea batatas) grown in a semi-arid climate

Accurate quantification of crop water use or evapotranspiration (ET) is crucial in agriculture for improved irrigation scheduling and water resource planning across a wide range of farming conditions. The aim of this study was to quantify ET dynamics for irrigated sweet potato (Ipomoea batatas) in order to determine the FAO-type single crop coefficients (Kc) that can be used in models to estimate crop water use under a wider range of semi-arid climatic conditions. An open-path eddy-covariance system containing energy balance sensors was installed in a 1.3 ha field to estimate ET of orange-fleshed sweet potato. The study was conducted during the 2014/15 and 2015/16 growing seasons. Daily ET varied between 0.5 and 5.5 mm (linked closely to canopy cover and weather conditions), with total seasonal measured ET of 361 and 347 mm for the 2014/15 and 2015/16 seasons, respectively. Time-averaged Kc values of 0.47 for the initial stage, 0.97 for the mid-season stage and 0.44 for the late growth stage were derived from this study. In addition, a heat unit based Growing Degree Days - Kc equation was successfully developed and validated in this study to adjust Kc values of sweet potato to specific climates. These Kc values can be used in combination with the FAO-56 reference evapotranspiration (ETo) to estimate site-specific sweet potato ET, which is vital for improving water management of irrigated sweet potato cropping systems growing under semi-arid conditions.

中国亚热带-暖温带过渡区锐齿栎林净生态系统碳交换特征

在2017年1月1日-2017年12月31日期间，采用涡度相关法对位于亚热带-暖温带气候过渡区的河南宝天曼国家级自然保护区的65年生锐齿栎（Quercus aliena）天然次生林的碳通量进行了连续观测。结果表明：在观测期间，该森林生态系统在生长季5-10月份为碳汇，非生长季各月为碳源，净碳吸收量与释放量分别在7月和4月达到最大。净生态系统生产力为569.4 g C m^-2a^-1，生态系统呼吸为529.9 g C m^-2a^-1，总生态系统生产力为1099.3 g C m^-2a^-1。30min尺度上夜间净生态系统碳交换量与5cm深度土壤温度的关系可用指数方程表示（R²=0.21，P < 0.001），其温度敏感性系数（Temperature sensitivity coefficient，Q₁₀）为2.2。如果排除夜间通量观测的误差，处在海拔较高地区的夜间低温和非生长季的低温抑制了生态系统呼吸排放，可能导致全年生态系统呼吸量较低。在生长季5-10月份，各月的白天净生态系统碳交换量对光合有效辐射的响应符合直角双曲线模型，初始光能利用效率、平均最大光合速率和白天平均生态系统呼吸强度呈明显的季节变化，范围分别是0.06-0.12 μmol CO₂ μmol^-1 photon、0.44-1.47 mg CO₂ m^-2s^-1和0.07-0.19 mg CO₂ m^-2s^-1。夏季7、8月份，较高的饱和水汽压差对白天锐齿栎林的碳吸收有明显的抑制作用；生长季末期9月份较高的土壤含水量对白天锐齿栎林的碳吸收也产生了抑制作用，表明生长末期降水过多影响森林的碳吸收。;Oak forests comprise the largest forest area in central China and are the potential carbon sink, while we know little about the carbon dioxide flux of oak forests in the transitional zone from subtropics to warm temperate, China. Using an open-path eddy covariance system and micro-climate instruments, the CO₂ flux, photosynthetic active radiation (PAR), air temperature, soil temperature and precipitation were simultaneously observed in a natural oak (Quercus aliena) forest at Baotianman National Nature Reserve. Based on the data sets during January to December 2017, dynamic change of CO₂ flux at different temporal scales and its underlying mechanism were analyzed. The results indicated that the diurnal and seasonal variations of CO₂ fluxes showed an obvious single peak pattern. The oak forest ecosystem was a carbon sink during the growing season (May-October), while a carbon source occurred during the non-growing season. Net carbon sequestration and emissions peaked in July and April, respectively. Mean annual net ecosystem productivity (NEP), ecosystem respiration (Re) and gross ecosystem productivity (GEP) were 569.4, 529.9 and 1099.3 g C m^-2 year^-1, respectively. The relationship between net ecosystem carbon exchange (NEE) measured at half-hour interval during night and soil temperature at depth of 5 cm can be expressed by an exponential equation (R²=0.21, P < 0.001), with its temperature sensitivity coefficient (Q₁₀) of 2.2. A low temperature at night and in the non-growing season at high elevation resulted in lower Re throughout the year. The relationship between NEE and PAR at daytime could be well expressed by a rectangular hyperbolic equation during growing seasons. Monthly initial light use efficiency, maximum photosynthetic capacity and daytime Re were 0.06-0.12 μmol CO₂ μmol^-1 photon, 0.44-1.47 mg CO₂ m^-2s^-1 and 0.07-0.19 mg CO₂ m^-2s^-1, respectively. Both higher vapour pressure deficit in July and August and higher soil moisture at the end of the growing season inhibited carbon uptake of the oak forest, indicating a negative effect of increased precipitation on carbon sequestration at the end of the growing season.

Remote sensing estimation of urban surface evapotranspiration based on a modified Penman–Monteith model

To date, remote sensing-based algorithms for inferring urban surface evapotranspiration (ET) remain little studied. Based on the modifications of the remote sensing Penman–Monteith (RS-PM) model, we propose an urban RS-PM model for estimating urban surface ET. Compared with the traditional RS-PM model, our urban RS-PM model is specifically developed for urban areas and is characterized by the following improvements: (1) excluding the interference of impervious surface components in urban areas by replacing the vegetation cover fraction index with land surface component fraction parameters inversed through linear spectral mixture analysis for calculating the area proportions of vegetation and soil; (2) considering the effect of the component fractions of vegetation or soil on all energy components of the surface energy balance by applying the modified multisource parallel model for estimating the component latent heat flux; and (3) optimizing the calculation of the component net radiation flux by considering the component surface characteristics. This urban RS-PM model was tested on an urban area of Xuzhou in the eastern Chinese province of Jiangsu. Landsat 8 operational land imager and thermal infrared sensor satellite images acquired between 2014 and 2016, together with their corresponding meteorological data and flux observation data, were used for estimating the ET of the study area for eight dates with the model. The results were validated by the latent heat flux data observed by an open path eddy covariance system. Validation shows the goodness of fit (R2), the root-mean-square error, the mean relative error, and the correlation coefficient (r) between estimated ET and observed ET for the eight dates were 0.8965, 24.14 W · m − 2, 18.5%, and 0.9546, respectively. The results prove that the urban RS-PM model is effective in estimating ET of urban areas with an acceptable accuracy.

Greenhouse gas emissions and energy exchange in wet and dry season rice: eddy covariance-based approach.

Lowland tropical rice-rice system has a unique micrometrological characteristic that affects both energy component and net ecosystem energy. Periodic and seasonal variations of methane (CH4), carbon dioxide (CO2), and energy exchange from irrigated lowland rice-rice ecosystem were studied using open-path eddy covariance (EC) system during the dry (DS) and wet (WS) seasons in 2015. Concurrently, the manual chamber method was employed in nitrous oxide (N2O) measurement efflux. Cumulative net ecosystem carbon exchange (NEE) was observed highest (- 232.55g C m-2) during the WS and lowest (- 14.81g C m-2) during wet fallow (WF). Similarly, the cumulative net ecosystem methane exchange (NEME) was found highest (13,456.5mg CH4 m-2) during the WS and lowest (2014.3mg CH4 m-2) during the WF. Surface energy fluxes, i.e., sensible (Hs) and latent heat (LE) fluxes, showed a similar trend. With the advancement of time, the ratio of ecosystem respiration (Re) and gross primary production (GPP) increased. The cumulative global warming potential (GWP) for the two cropping seasons including two fallows was 13,224.1kg CO2 equivalent ha-1. The GWP and NEME showed a similar trend as soil enzymes and labile carbon pools in both seasons (except GWP at the harvesting stage in the wet season). The mean NEE exhibited a more negative value with decrease in labile pools from panicle initiation to harvesting stage in the WS. Soil labile C and soil enzymes can be used as an indicator of NEE, NEME, and GWP in lowland rice ecology. Graphical abstract Schematic presentation of GHG emission and energy exchange in lowland rice.

Quantifying and reducing the differences in forest CO2-fluxes estimated by eddy covariance, biometric and chamber methods: A global synthesis

Carbon dioxide (CO2) fluxes between terrestrial ecosystems and the atmosphere are primarily measured with eddy covariance (EC), biometric, and chamber methods However, it is unclear why the estimates of the CO2-fluxes, when measured using different methods, converge at some sites but diverge at others. We synthesized a novel global dataset of forest CO2-fluxes to evaluate the consistency between EC and biometric or chamber methods for quantifying the CO2 budget in forest ecosystems. The EC approach, compared with the other two methods, overestimated net ecosystem production (NEP) by 25% (0.52MgCha−1yr−1), and underestimated ecosystem respiration (Re) by 10% (1.39MgCha−1yr−1) and gross primary production by 3% (0.48MgCha−1yr−1). The differences between EC and the other methods were greater at the sites with complex topography and dense canopy than at the sites with flat topography and open canopy. Forest age also influenced the differences mainly through changes in leaf area index. Open-path EC system induced large positive bias in the NEP estimated by EC, presumably due to its surface-heating effect. These results suggest that EC method likely produce biased estimates of NEP and Re in forest ecosystems. A global extrapolation suggests that the differences in the forest CO2-fluxes measured with different methods be consistent with the global overestimation of NEP and underestimation of Re by EC method. Accounting for these differences would substantially improve our estimates of the forest carbon budget. The uncertainties involved in each method were also discussed. To reduce uncertainty in quantifying both local and global carbon budgets, we recommend cross-validation of forest CO2-fluxes measured by different methods with more accurate measurements and careful data processing strategies.

Cloudiness regulates gross primary productivity of a poplar plantation under different environmental conditions

Cloud cover regulates the gross primary productivity (GPP) of forest ecosystems by changing the radiation component and other environmental factors. In this study, we used an open-path eddy covariance system and microclimate sensors installed over a poplar plantation in northern China to measure the carbon exchange and climate variables during the mid-growing seasons (June to August) in 2014 and 2015. The results indicated that the GPP of the plantation peaked when the clearness index (CI) was between 0.45 and 0.65, at which point diffuse photosynthetically active radiation (PARdif) had reached its maximum. Cloudy skies increased the maximum ecosystem photosynthetic capacity (Pmax) by 28% compared with clear skies. PARdif and soil moisture were the most and the least crucial drivers for photosynthetic productivity of the plantation under cloudy skies, respectively. The ecosystem photosynthetic potential was higher under lower vapor pressure deficit (VPD &lt; 1.5 kPa), lower air temperature (Ta &lt; 30 °C), and nonstressed conditions (REW &gt; 0.4) for cloudy skies due to effects of Ta and VPD on stoma. Overall, our research highlighted the importance of cloud-induced radiation component change and environmental variation in quantifying the GPP of forest ecosystems.

Assessing the surface heating effect of the open-path analyzer of an eddy covariance system with fine-wire thermocouples

The open-path CO2/H2O analyzer that is currently widely used in the eddy covariance (EC) system possesses a surface heating effect, which may influence the accuracy of the eddy-flux measurements. Taking the open-path EC system in the temperate deciduous forest of the Maoershan Ecosystem Research Station as a case, we assessed the heating effect of the open-path CO2/H2O ana-lyzer (Li-7500) with fine-wire thermocouples and tested the applicability of Burba's equations. The results indicated that between April 23 and May 28 in 2016, the daytime air temperature in the mid-optical-path of the Li-7500 was on average 0.2 ℃ higher than the ambient air temperature, while the nocturnal temperature was almost equal, with the peak of the heating often occurring in the morning transition period and at noon or in the early afternoon. The diurnal variation in the heating effect on the sensible heat flux (HS,HE) estimated from the simple linear-fitting models in the fourth method by Burba (BurbaLF) was overall a rectangle-like wave, whereas that from the multiple regression models (BurbaMR) showed a weak mono-peak shape. Compared with direct mea-surements (K079) with the K-type fine-wire thermocouples (with a 0.079 mm diameter of single bare wire) and the HS,HE modeled with two fine-wire thermocouples (K079Model), the estimated HS,HE with the BurbaLF and BurbaMR varied little in the daytime with a lower but wider daytime-peak (20 W·m-2), a sharp transition between daytime and nighttime, and a typical nocturnal va-lue of ＜ 5 W·m-2. The peak values of HS,HE estimated from the K079 and K079Model could be higher than 40 W·m-2, and the values at night fluctuated around zero. The sensible heat flux in the optical-path of the Li-7500 was increased by 13.6% during the daytime. The value of the daytime heating effect on the CO2 turbulent flux (Fc,HC) estimated from the K079 and K079Model was averaged about 0.5 mg CO2·m-2·s-1, twice of the value estimated previously. Compared with the direct measurements, the Burba equations underestimated the Fc,HC around the noon due to the inadequate HS,HE, but overestimated the Fc,HC in the morning, the late afternoon and the night because of over-corrections of HS,HE, resulting in an overall overestimate of Fc,HC. These findings validated that both K079 and K079Model methods can be used to estimate the Fc,HC.

Performance Evaluation of an Integrated Open-Path Eddy Covariance System in a Cold Desert Environment

AbstractPerformance evaluation of an integrated eddy covariance (EC) instrument called the IRGASON, with a separated EC for reference, was conducted in a desert riparian Populus euphratica stand in the lower Tarim River basin in northwestern China. The separated EC consisted of an open-path gas analyzer and a sonic anemometer separated by 20 cm. The IRGASON integrates an open-path gas analyzer and a sonic anemometer into the same sensing volume, thus eliminating sensor separation in comparison to the traditional open-path EC setup. Integrating the infrared gas analyzer’s sensing head into the sensing volume of the sonic anemometer had negligible effects on wind speed and friction velocity observations of the IRGASON. Physiologically unreasonable daytime CO2 uptake was observed by both systems during the cold winter season (mean air temperature of −6.7°C), when the trees were dormant without any photosynthetic activities. The mean midday CO2 flux was −1.65 and −1.61 μmol m−2 s−1 for the IRGASON and the separated EC setup, respectively. No evidence was found for sensor self-heating as the cause of the apparent uptake CO2 flux. Instead, the uptake CO2 flux appeared to be an artifact of the spectroscopic effect of the IRGASON’s gas analyzer. After adjusting for this spectroscopic effect using a relationship with the sensible heat flux, the wintertime IRGASON CO2 flux became physiologically reasonable (mean value of −0.04 μmol m−2 s−1).

Measurements of methane emissions from a beef cattle feedlot using the eddy covariance technique

The eddy covariance (EC) technique has been extensively used in several sites around the world to measure energy fluxes and CO2 exchange at the ecosystem scale. Recent advances in optical sensors have allowed the use of the EC approach to measure other trace gases (e.g. CH4, NH3 and N2O), which has expanded the use of eddy covariance for other applications, including measuring gas emissions from livestock production systems. The main objectives of this study were to assess the performance of a closed-path EC system for measuring CH4, CO2, and H2O fluxes in a beef cattle feedlot and to investigate the spatial variability of eddy covariance fluxes measured above the surface of a feedlot using an analytical flux footprint analysis. A closed-path EC system was used to measure CH4, CO2, and H2O fluxes. To evaluate the performance of this closed-path system, an open-path EC system was also deployed on the flux tower to measure CO2 and H2O exchange. The performance assessment of the closed-path EC system showed that this system was suitable for EC measurements. The frequency attenuations, observed for the closed-path system CO2 and CH4 cospectra in this study, are in agreement with results from previous instrument comparison studies. For the water vapor closed-path cospectra, larger attenuations were likely caused by water vapor molecule interaction with the sampling tube walls. Values of R2 for the relationship between H2O and CO2 fluxes, measured by open-path and closed-path systems, were 0.94–0.98, respectively. The closed-path EC system overestimated the CO2 by approximately 5% and underestimated the latent heat fluxes by about 10% when compared with the open-path system measurements. Measured CH4 and CO2 fluxes during the study period from the feedlot averaged 2.63μmolm−2s−1 and 103.8μmolm−2s−1, respectively. Flux values were quite variable during the field experiment and the footprint analysis was useful to interpret flux temporal and spatial variation. This study shows indication that consideration of atmospheric stability condition, wind direction and animal movement are important to improve estimates of CH4 emissions per pen surface or per head of cattle.

Comparative measurements of water vapor fluxes over a tall forest using open- and closed-path eddy covariance system

Abstract. Eddy covariance using infrared gas analyzes has been a useful tool for gas exchange measurements between soil, vegetation and the atmosphere. So far, comparisons between the open- and closed-path eddy covariance (CP) system have been extensively made on CO2 flux estimations, while lacking in the comparison of water vapor flux estimations. In this study, the specific performance of water vapor flux measurements of an open-path eddy covariance (OP) system was compared against a CP system over a tall temperate forest in northeastern China. The results show that the fluxes from the OP system (LEop) were generally greater than the LEcp though the two systems shared one sonic anemometer. The tube delay of closed-path analyzer depended on relative humidity, and the fixed median time lag contributed to a significant underestimation of LEcp between the forest and atmosphere, while slight systematic overestimation was also found for covariance maximization method with single broad time lag search window. After the optimized time lag compensation was made, the average difference between the 30 min LEop and LEcp was generally within 6.0 %. Integrated over the annual cycle, the CP system yielded a 5.1 % underestimation of forest evapotranspiration as compared to the OP system measurements (493 vs. 469 mm yr−1). This study indicates the importance to estimate the sampling tube delay accurately for water vapor flux calculations with closed-path analyzers, and it also suggests that some of the imbalance of the surface energy budget in flux sites is possibly caused by the systematic underestimation of water vapor fluxes measured with closed-path eddy covariance systems.

Tropical low land rice ecosystem is a net carbon sink

A yearlong field study was undertaken to investigate the carbon dioxide (CO2) and methane (CH4) exchange in relation to ecosystem carbon (C) balance in tropical low land rice by employing open path eddy covariance (OPEC) system. The study was conducted on annual basis, included both wet and dry seasons crop. This is the first report from India on net ecosystem CH4 exchange (NEME) from low land flooded rice ecology through eddy covariance techniques. On annual basis the cumulative net ecosystem CO2 exchange (NEE) was −457gCm−2 and NEME was +262.62kgCH4ha−1, respectively. On the other hand, the annual gross primary production (GPP) was +1340gCm−2 and ecosystem respiration (RE) was +883gCm−2. On seasonal basis the rate of NEE was in the range of −3.29 and −2.74gCm−2 day−1, respectively and NEME was +2.80 and +4.34mgm−2h−1 in wet and dry cropping seasons, respectively. As a whole, the integrated NEE in dry and wet seasons was −355 and −341gCm−2, whereas, the integrated NEME in the two seasons was +84 and +129kgha−1, respectively. Considering the net ecosystem production (NEP or −NEE: +695gCm−2), rhizodeposition, algal biomass, root, stubble and compost added as ecosystem C input and NEME (+213kgha−1), C removal through harvest and dissolved organic C as ecosystem C output, the tropical rice acts as net C sink both seasonally and annually. Although CH4 is a source of C loss from low land flooded rice ecosystem, considering all the components of C balance on system basis, this ecology has a good potential to store considerable amount of C.

Analysis of rainfed alfalfa evapotranspiration measured by an eddy covariance system

The aim of this study was to quantify the evapotranspiration (ETec) of a rainfed alfalfa crop using the eddy covariance technique. The study was carried out during the alfalfa growing seasons (April- August 2009, April-August 2010) at the experimental farm of the University of Perugia. In central Italy alfalfa is grown for 3 to 4 years continuously, with at least 3 cutting cycles for year (usually between April and August) and a dormant period in winter. For the quantification of ETec an open-path eddy covariance system (EC) was used. The derivation of water and energy fluxes starting from raw wind, temperature and gas concentration data by means of the EC technique implies a remarkably long sequence of operations including calibration, corrections and statistical tests for assessing data quality. These operations were carried out by the EddyPro® software. After that, the output data were used for the flux-partitioning and all original data, flagged with a quality indicator with non-turbulent conditions, were dismissed. Then the gap-filling of the EC and meteorological data was performed to obtain reliable values. Furthermore the test of the energy balance closure gave satisfactory results. The ETec dynamics were consistent with the growth stages and the cuttings during both 2009 and 2010. Furthermore the comparison between the tabulated crop coefficients (Kc) and the ratio of ETec to reference evapotranspiration (ET0) was performed. This analysis showed a good agreement during the 2nd cutting cycle (May-June) for both 2009 and 2010, whilst during the 3rd cutting cycle (July-August) the ratio ETec/ET0 was considerably lower than Kc for both years. The reason of this behavior was found in the presence of water stress conditions during the last cutting cycle. This fact was confirmed by the application of a bucket soil water model, used as an exploratory, not confirmatory, tool to analyze the soil water availability dynamics during the growing season. Additional measurement campaigns will be carried out in order to deepen the knowledge about the Kc dynamics in rainfed crops and to assess the productivity of water under various meteorological and agricultural conditions.

Analysis of rainfed alfalfa evapotranspiration measured by an eddy covariance system

The aim of this study was to quantify the evapotranspiration (ETec) of a rainfed alfalfa crop using the eddy covariance technique. The study was carried out during the alfalfa growing seasons (April- August 2009, April-August 2010) at the experimental farm of the University of Perugia. In central Italy alfalfa is grown for 3 to 4 years continuously, with at least 3 cutting cycles for year (usually between April and August) and a dormant period in winter. For the quantification of ETec an open-path eddy covariance system (EC) was used. The derivation of water and energy fluxes starting from raw wind, temperature and gas concentration data by means of the EC technique implies a remarkably long sequence of operations including calibration, corrections and statistical tests for assessing data quality. These operations were carried out by the EddyPro® software. After that, the output data were used for the flux-partitioning and all original data, flagged with a quality indicator with non-turbulent conditions, were dismissed. Then the gap-filling of the EC and meteorological data was performed to obtain reliable values. Furthermore the test of the energy balance closure gave satisfactory results. The ETec dynamics were consistent with the growth stages and the cuttings during both 2009 and 2010. Furthermore the comparison between the tabulated crop coefficients (Kc) and the ratio of ETec to reference evapotranspiration (ET0) was performed. This analysis showed a good agreement during the 2nd cutting cycle (May-June) for both 2009 and 2010, whilst during the 3rd cutting cycle (July-August) the ratio ETec/ET0 was considerably lower than Kc for both years. The reason of this behavior was found in the presence of water stress conditions during the last cutting cycle. This fact was confirmed by the application of a bucket soil water model, used as an exploratory, not confirmatory, tool to analyze the soil water availability dynamics during the growing season. Additional measurement campaigns will be carried out in order to deepen the knowledge about the Kc dynamics in rainfed crops and to assess the productivity of water under various meteorological and agricultural conditions.

The Study of Carbon Dioxide Flux on Xilamuren Grassland Based on the Eddy Covariance Technique

The eddy covariance is a micrometeorological technique to observe and investigate ecosystem flux nondestructively, as well as the main method the eddy covariance may help to research carbon dioxide exchange between global vegetations and the atmosphere. Carbon dioxide flux has been monitored on Xilamuren grassland of Wulanchabu city by Open Path Eddy Covariance System (OP-2). Carbon dioxide flux shows that carbon absorption is greater than its release, which has an evident carbon sink functions and obvious seasonal variety on the basis of experimental data from June 2009 to July 2011.