Revising the high frequency response correction of scalar fluxes measured by closed-path eddy covariance systems

2021 ◽  
Author(s):  
Ivan Mammarella ◽  
Olli Peltola ◽  
Toprak Aslan ◽  
Andreas Ibrom ◽  
Eiko Nemitz ◽  
...  
Keyword(s):  
Transfer Function ◽  
Frequency Response ◽  
Eddy Covariance ◽  
High Frequency ◽  
Small Scale ◽  
Systematic Bias ◽  
Closed Path ◽  
Scalar Fluxes ◽  
Wide Range ◽  
Response Correction

<p>Eddy covariance (EC) scalar flux loss at high frequency is due to the incapability of the measurement system to detect small-scale variation of atmospheric turbulent signals. This systematic bias is particularly important for closed-path systems, and it is mainly related to inadequate sensor frequency response, sensor separation and the air sampling trough tubes and filters. Here, we investigate the limitations of current approaches, based on measured power spectra (PSA) or cospectra (CSA), to empirically estimated the spectral transfer function of the EC system needed for the frequency response correction of measured fluxes. We performed a systematic analysis by using EC data from a wetland and forest site for a wide range of attenuation levels and signal-to-noise ratio. We proposed a novel approach for PSA that uses simultaneously the noise and the turbulent signals present in the power spectrum, providing robust estimates of spectral transfer function for all conditions. We further theoretically derived a new transfer function to be used in the CSA approach which specifically accounts for the interaction between the low-pass filtering induced phase shift and the high frequency attenuation. We show that current CSA approaches neglect such effect, giving a non-negligible systematic bias to the estimated scalar fluxes from the studied sites. Based on these findings, we recommend that spectral correction methods, implemented in EC data processing algorithms, are revised accordingly.</p>

scholarly journals The high-frequency response correction of eddy covariance fluxes – Part 2: An experimental approach for analysing noisy measurements of small fluxes

2021 ◽  
Vol 14 (7) ◽  
pp. 5089-5106
Author(s):  
Toprak Aslan ◽  
Olli Peltola ◽  
Andreas Ibrom ◽  
Eiko Nemitz ◽  
Üllar Rannik ◽  
...  
Keyword(s):  
Time Series ◽  
Transfer Function ◽  
Frequency Response ◽  
Time Constant ◽  
Experimental Approach ◽  
Power Spectra ◽  
Closed Path ◽  
Time Constants ◽  
Low Pass ◽  
Response Correction

Abstract. Fluxes measured with the eddy covariance (EC) technique are subject to flux losses at high frequencies (low-pass filtering). If not properly corrected for, these result in systematically biased ecosystem–atmosphere gas exchange estimates. This loss is corrected using the system's transfer function which can be estimated with either theoretical or experimental approaches. In the experimental approach, commonly used for closed-path EC systems, the low-pass filter transfer function (H) can be derived from the comparison of either (i) the measured power spectra of sonic temperature and the target gas mixing ratio or (ii) the cospectra of both entities with vertical wind speed. In this study, we compare the power spectral approach (PSA) and cospectral approach (CSA) in the calculation of H for a range of attenuation levels and signal-to-noise ratios (SNRs). For a systematic analysis, we artificially generate a representative dataset from sonic temperature (T) by attenuating it with a first order filter and contaminating it with white noise, resulting in various combinations of time constants and SNRs. For PSA, we use two methods to account for the noise in the spectra: the first is the one introduced by Ibrom et al. (2007a) (PSAI07), in which the noise and H are fitted in different frequency ranges, and the noise is removed before estimating H. The second is a novel approach that uses the full power spectrum to fit both H and noise simultaneously (PSAA21). For CSA, we use a method utilizing the square root of the H with shifted vertical wind velocity time series via cross-covariance maximization (CSAH,sync). PSAI07 tends to overestimate the time constant when low-pass filtering is low, whilst the new PSAA21 and CSAH,sync successfully estimate the expected time constant regardless of the degree of attenuation and SNR. We further examine the effect of the time constant obtained with the different implementations of PSA and CSA on cumulative fluxes using estimated time constants in frequency response correction. For our example time series, the fluxes corrected using time constants derived by PSAI07 show a bias between 0.1 % and 1.4 %. PSAA21 showed almost no bias, while CSAH,sync showed bias of ±0.4 %. The accuracies of both PSA and CSA methods were not significantly affected by SNR level, instilling confidence in EC flux measurements and data processing in set-ups with low SNR. Overall we show that, when using power spectra for the empirical estimation of parameters of H for closed-path EC systems the new PSAA21 outperforms PSAI07, while when using cospectra the CSAH,sync approach provides accurate results. These findings are independent of the SNR value and attenuation level.

scholarly journals Bias Errors due to Leakage Effects When Estimating Frequency Response Functions

2012 ◽  
Vol 19 (6) ◽  
pp. 1257-1266 ◽  
Author(s):  
Andreas Josefsson ◽  
Kjell Ahlin ◽  
Göran Broman
Keyword(s):  
Frequency Response ◽  
Measurement Data ◽  
Approximate Expression ◽  
Bias Error ◽  
Systematic Bias ◽  
Response Functions ◽  
Random Errors ◽  
Frequency Response Functions ◽  
Rectangular Window ◽  
Wide Range

Frequency response functions are often utilized to characterize a system's dynamic response. For a wide range of engineering applications, it is desirable to determine frequency response functions for a system under stochastic excitation. In practice, the measurement data is contaminated by noise and some form of averaging is needed in order to obtain a consistent estimator. With Welch's method, the discrete Fourier transform is used and the data is segmented into smaller blocks so that averaging can be performed when estimating the spectrum. However, this segmentation introduces leakage effects. As a result, the estimated frequency response function suffers from both systematic (bias) and random errors due to leakage. In this paper the bias error in theH1andH2-estimate is studied and a new method is proposed to derive an approximate expression for the relative bias error at the resonance frequency with different window functions. The method is based on using a sum of real exponentials to describe the window's deterministic autocorrelation function. Simple expressions are derived for a rectangular window and a Hanning window. The theoretical expressions are verified with numerical simulations and a very good agreement is found between the results from the proposed bias expressions and the empirical results.

scholarly journals InnFLUX – an open-source code for conventional and disjunct eddy covariance analysis of trace gas measurements: an urban test case

2020 ◽  
Vol 13 (3) ◽  
pp. 1447-1465 ◽  
Author(s):  
Marcus Striednig ◽  
Martin Graus ◽  
Tilmann D. Märk ◽  
Thomas G. Karl
Keyword(s):  
Open Source ◽  
Eddy Covariance ◽  
High Frequency ◽  
Urban Canopy ◽  
Lag Time ◽  
Trace Gas ◽  
High Temporal Resolution ◽  
Eddy Covariance Method ◽  
Wide Range ◽  
Gas Measurements

Abstract. We describe and test a new versatile software tool for processing eddy covariance and disjunct eddy covariance flux data. We present an evaluation based on urban non-methane volatile organic compound (NMVOC) measurements using a proton transfer reaction quadrupole interface time-of-flight mass spectrometer (PTR-QiTOF-MS) at the Innsbruck Atmospheric Observatory. The code is based on MATLAB® and can be easily configured to process high-frequency, low-frequency and disjunct data. It can be applied to a wide range of analytical setups for NMVOC and other trace gas measurements, and is tailored towards the application of noisy data, where lag time corrections become challenging. Several corrections and quality control routines are implemented to obtain the most reliable results. The software is open source, so it can be extended and adjusted to specific purposes. We demonstrate the capabilities of the code based on a large urban dataset collected in Innsbruck, Austria, where three-dimensional winds and ambient concentrations of NMVOCs and auxiliary trace gases were sampled with high temporal resolution above an urban canopy. Concomitant measurements of 12C and 13C isotopic NMVOC fluxes allow testing algorithms used for determination of flux limits of detection (LOD) and lag time analysis. We use the high-frequency NMVOC dataset to generate a set of disjunct data and compare these results with the true eddy covariance method. The presented analysis allows testing the theory of disjunct eddy covariance (DEC) in an urban environment. Our findings confirm that the disjunct eddy covariance method can be a reliable tool, even in complex urban environments when fast sensors are not available, but that the increase in random error impedes the ability to detect small fluxes due to higher flux LODs.

scholarly journals An eddy-covariance system with an innovative vortex intake for measuring carbon dioxide and water fluxes of ecosystems

2017 ◽  
Vol 10 (3) ◽  
pp. 1259-1267 ◽  
Author(s):  
Jingyong Ma ◽  
Tianshan Zha ◽  
Xin Jia ◽  
Steve Sargent ◽  
Rex Burgon ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Frequency Response ◽  
Eddy Covariance ◽  
Urban Forest ◽  
Vortex Chamber ◽  
Optical Signal ◽  
Dust Particles ◽  
Closed Path ◽  
Sample Cell ◽  
Airborne Dust

Abstract. Closed-path eddy-covariance (EC) systems are used to monitor exchanges of trace gases (e.g., carbon dioxide [CO2], water vapor [H2O], nitrous oxide and methane) between the atmosphere and biosphere. Traditional EC-intake systems are equipped with inline filters to prevent airborne dust particulate from contaminating the optical windows of the sample cell which causes measurement degradation. The inline filter should have a fine pore size (1 to 20 µm is common) to adequately protect the optics and a large filtration surface area to extend the time before it clogs. However, the filter must also have minimal internal volume to preserve good frequency response. This paper reports test results of the field performance of an EC system (EC155, Campbell Scientific, Inc., Logan Utah, USA) with a prototype vortex intake replacing the inline filter of a traditional EC system. The vortex-intake design is based on fluid dynamics theory. An air sample is drawn into the vortex chamber, where it spins in a vortex flow. The initially homogenous flow is separated when particle momentum forces heavier particles to the periphery of the chamber, leaving a much cleaner airstream at the center. Clean air (75 % of total flow) is drawn from the center of the vortex chamber, through a tube, to the sample cell where it is exposed to the optical windows of the gas analyzer. The remaining 25 % of the flow carries the heavier dust particles away through a separate bypass tube. An EC155 system measured CO2 and H2O fluxes in two urban-forest ecosystems in the megalopolis of Beijing, China. These sites present a challenge for EC measurements because of the generally poor air quality which has high concentrations of suspended particulate. The closed-path EC system with vortex intake significantly reduced maintenance requirements by preserving optical signal strength and sample-cell pressure within acceptable ranges for much longer periods. The system with vortex intake also maintained an excellent frequency response. For example, at the Badaling site, the amount of system downtime attributed solely to clogged filters was reduced from 26 % with traditional inline filters to 0 % with the prototype vortex intake. The use of a vortex intake could extend the geographical applicability of the EC technique in ecology and allow investigators to acquire more accurate and continuous measurements of trace-gas fluxes in a wider range of ecosystems.

Identifying a Transfer Function From a Frequency Response

2007 ◽  
Author(s):  
Manuel Duarte Ortigueira ◽  
Duarte Vale´rio ◽  
Jose´ Sa´ da Costa
Keyword(s):  
System Identification ◽  
Transfer Function ◽  
Frequency Response ◽  
Transfer Functions ◽  
Sea Waves ◽  
Low Frequencies ◽  
Wide Range ◽  
Simulation Results ◽  
New Formulation ◽  
Pseudo Inverse

In this paper, the classic Levy identification method is reviewed and reformulated using a complex representation. This new formulation is able to solve the well known bias of the classic method at low frequencies. The formulation is generic, addressing both integer order and fractional order transfer functions. A new algorithm based on a stacked matrix and its pseudo-inverse is proposed to accommodate the data over a wide range of frequencies. Several simulation results are presented, together with a real system identification. This system is the Archimedes Wave Swing, a prototype of a device to convert the energy of sea waves into electricity.

scholarly journals Evaluation of a lower-powered analyser and sampling system for eddy-covariance measurements of nitrous oxide fluxes

2017 ◽  
Author(s):  
Shannon E. Brown ◽  
Steve Sargent ◽  
Claudia Wagner-Riddle
Keyword(s):  
Frequency Response ◽  
Diode Laser ◽  
Eddy Covariance ◽  
Tunable Diode Laser ◽  
Closed Path ◽  
Laser Absorption ◽  
Eddy Covariance System ◽  
Diode Laser Absorption ◽  
New System ◽  
Absorption Spectrometer

Abstract. Nitrous oxide (N2O) fluxes measured using the eddy-covariance method capture the spatial and temporal heterogeneity of N2O emissions. Most closed-path trace-gas analyzers for eddy-covariance measurements have large-volume, multi-pass absorption cells that necessitate high flow rates for ample frequency response, thus requiring high-power sample pumps. Other sampling system components, including rain caps, filters, dryers, and tubing can also degrade system frequency response. This field trial tested the performance of a closed-path eddy-covariance system for N2O flux measurements with improvements to use less power while maintaining the frequency response. The new system consists of a thermoelectrically cooled tunable diode laser absorption spectrometer configured to measure both N2O and carbon dioxide (CO2). The system features a relatively small, single- pass sample cell (200 ml) that provides good frequency response with a lower-powered pump (~ 250 W). A new filterless intake removes particulates from the sample air stream with no additional mixing volume that could degrade frequency response. A single-tube dryer removes water vapor from the sample to avoid the need for density or spectroscopic corrections, while maintaining frequency response. This eddy-covariance system was collocated with a previous tunable diode laser absorption spectrometer model to compare N2O and CO2 flux measurements for two full growing seasons (May 2015 to October 2016) in a fertilized cornfield in Southern Ontario, Canada. Both spectrometers were placed outdoors at the base of the sampling tower demonstrating ruggedness for a range of environmental conditions (minimum to maximum daily temperature range: −26.1 to 31.6 °C). The new system rarely required maintenance. An in situ frequency response test demonstrated that the cutoff frequency of the new system was better than the old system (3.5 Hz compared to 2.30 Hz), and similar to that of a closed-path CO2 eddy- covariance system (4.05 Hz) using shorter tubing and no dryer that was also collocated at the site. Values of the N2O fluxes were similar between the two spectrometer systems (slope = 1.01, r2 = 0.96); CO2 fluxes as measured by the short-tubed eddy-covariance system and the two spectrometer systems correlated well (slope = 1.03, r2 = 0.998). The new lower-powered tunable diode laser absorption spectrometer configuration with the filterless intake and single-tube dryer showed promise for deployment in remote areas.

scholarly journals Evaluation of a lower-powered analyzer and sampling system for eddy-covariance measurements of nitrous oxide fluxes

2018 ◽  
Vol 11 (3) ◽  
pp. 1583-1597 ◽  
Author(s):  
Shannon E. Brown ◽  
Steve Sargent ◽  
Claudia Wagner-Riddle
Keyword(s):  
Frequency Response ◽  
Diode Laser ◽  
Eddy Covariance ◽  
Tunable Diode Laser ◽  
Closed Path ◽  
Laser Absorption ◽  
Eddy Covariance System ◽  
Diode Laser Absorption ◽  
New System ◽  
Absorption Spectrometer

Abstract. Nitrous oxide (N2O) fluxes measured using the eddy-covariance method capture the spatial and temporal heterogeneity of N2O emissions. Most closed-path trace-gas analyzers for eddy-covariance measurements have large-volume, multi-pass absorption cells that necessitate high flow rates for ample frequency response, thus requiring high-power sample pumps. Other sampling system components, including rain caps, filters, dryers, and tubing, can also degrade system frequency response. This field trial tested the performance of a closed-path eddy-covariance system for N2O flux measurements with improvements to use less power while maintaining the frequency response. The new system consists of a thermoelectrically cooled tunable diode laser absorption spectrometer configured to measure both N2O and carbon dioxide (CO2). The system features a relatively small, single-pass sample cell (200 mL) that provides good frequency response with a lower-powered pump ( ∼  250 W). A new filterless intake removes particulates from the sample air stream with no additional mixing volume that could degrade frequency response. A single-tube dryer removes water vapour from the sample to avoid the need for density or spectroscopic corrections, while maintaining frequency response. This eddy-covariance system was collocated with a previous tunable diode laser absorption spectrometer model to compare N2O and CO2 flux measurements for two full growing seasons (May 2015 to October 2016) in a fertilized cornfield in Southern Ontario, Canada. Both spectrometers were placed outdoors at the base of the sampling tower, demonstrating ruggedness for a range of environmental conditions (minimum to maximum daily temperature range: −26.1 to 31.6 °C). The new system rarely required maintenance. An in situ frequency-response test demonstrated that the cutoff frequency of the new system was better than the old system (3.5 Hz compared to 2.30 Hz) and similar to that of a closed-path CO2 eddy-covariance system (4.05 Hz), using shorter tubing and no dryer, that was also collocated at the site. Values of the N2O fluxes were similar between the two spectrometer systems (slope  =  1.01, r2 =  0.96); CO2 fluxes as measured by the short-tubed eddy-covariance system and the two spectrometer systems correlated well (slope  =  1.03, r2 =  0.998). The new lower-powered tunable diode laser absorption spectrometer configuration with the filterless intake and single-tube dryer showed promise for deployment in remote areas.

scholarly journals An innovative eddy-covariance system with vortex intake for measuring carbon dioxide and water fluxes of ecosystems

2016 ◽  
Author(s):  
Jingyong Ma ◽  
Tianshan Zha ◽  
Xin Jia ◽  
Steve Sargent ◽  
Rex Burgon ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Frequency Response ◽  
Eddy Covariance ◽  
Urban Forest ◽  
Vortex Chamber ◽  
Optical Signal ◽  
Dust Particles ◽  
Closed Path ◽  
Sample Cell ◽  
Airborne Dust

Abstract. Closed-path eddy-covariance (EC) systems are used to monitor exchanges of carbon dioxide (CO2) and water vapor (H2O) between the atmosphere and biosphere. Traditional EC intake systems are equipped with in-line filters to prevent airborne dust particulates from contaminating the optical windows of the sample cell which degrades measurements. In order to preserve fast-frequency response, the in-line filter should be small, but small filters plug quickly and require frequent replacement. This paper reports the test results of a field-performance of an innovative EC system (EC155, Campbell Scientific, Inc.) with a prototype vortex intake replacing the in-line filter of a traditional EC system. The vortex intake design is based on fluid dynamics theory. An air sample is drawn into the vortex chamber, where it spins in a vortex flow. The initially homogenous flow is separated when particle momentum forces heavier particles to the periphery of the chamber, leaving a much cleaner air stream at the center. Clean air (75 % of total flow) is drawn from the center of the vortex chamber, through a tube to the sample cell with optical windows. The remaining 25 % of the flow carries the heavier dust particles away in a separate bypass tube. An EC155-system measured CO2 and H2O fluxes in two urban forest ecosystems in the megalopolis of Beijing, China. These sites present a challenge for EC measurements because of the generally poor air quality with high concentrations of suspended particulate. The closed-path EC system with vortex intake significantly reduced maintenance requirements by preserving optical signal strength and sample cell pressure within acceptable ranges for much longer periods. The system with vortex intake also maintained excellent high-frequency response. For example at the Badaling site, percentage system downtime due to plugged filters was reduced from 26 % with traditional in-line filters to 0 % with the prototype vortex intake. The use of vortex intake could extent the geographical applicability of the EC technique in ecology and allow investigators to acquire more accurate and continuous measurements of CO2 and H2O fluxes in a wider range of ecosystems.

scholarly journals Optimization of a gas sampling system for measuring eddy-covariance fluxes of H<sub>2</sub>O and CO<sub>2</sub>

2015 ◽  
Vol 8 (10) ◽  
pp. 10983-11028 ◽  
Author(s):  
S. Metzger ◽  
G. Burba ◽  
S. P. Burns ◽  
P. D. Blanken ◽  
J. Li ◽  
...  
Keyword(s):  
Stainless Steel ◽  
Relative Humidity ◽  
Data Processing ◽  
Frequency Response ◽  
Eddy Covariance ◽  
High Frequency ◽  
Field Tests ◽  
Particulate Filter ◽  
Sampling System ◽  
Gas Sampling

Abstract. Several initiatives are currently emerging to observe the exchange of energy and matter between the earth's surface and atmosphere standardized over larger space and time domains. For example, the National Ecological Observatory Network (NEON) and the Integrated Carbon Observing System (ICOS) will provide the ability of unbiased ecological inference across eco-climatic zones and decades by deploying highly scalable and robust instruments and data processing. In the construction of these observatories, enclosed infrared gas analysers are widely employed for eddy-covariance applications. While these sensors represent a substantial improvement compared to their open- and closed-path predecessors, remaining high-frequency attenuation varies with site properties, and requires correction. Here, we show that the gas sampling system substantially contributes to high-frequency attenuation, which can be minimized by careful design. From laboratory tests we determine the frequency at which signal attenuation reaches 50 % for individual parts of the gas sampling system. For different models of rain caps and particulate filters, this frequency falls into ranges of 2.5–16.5 Hz for CO2, 2.4–14.3 Hz for H2O, and 8.3–21.8 Hz for CO2, 1.4–19.9 Hz for H2O, respectively. A short and thin stainless steel intake tube was found to not limit frequency response, with 50 % attenuation occurring at frequencies well above 10 Hz for both H2O and CO2. From field tests we found that heating the intake tube and particulate filter continuously with 4 W was effective, and reduced the occurrence of problematic relative humidity levels (RH > 60 %) by 50 % in the infrared gas analyser cell. No further improvement of H2O frequency response was found for heating in excess of 4 W. These laboratory and field tests were reconciled using resistor-capacitor theory, and NEON's final gas sampling system was developed on this basis. The design consists of the stainless steel intake tube, a pleated mesh particulate filter, and a low-volume rain cap in combination with 4 W of heating and insulation. In comparison to the original design, this reduced the high-frequency attenuation for H2O by &amp;approx; 3/4, and the remaining cospectral correction did not exceed 3 %, even at a very high relative humidity (95 %). This standardized design can be used across a wide range of eco-climates and site layouts, and maximizes practicability due to minimal flow resistance and maintenance needs. Furthermore, due to minimal high-frequency spectral loss, it supports the routine application of adaptive correction procedures, and enables more automated data processing across sites.

Improved eddy covariance flux based transpiration estimates at high relative humidity and comparison to sap flux

2021 ◽  
Author(s):  
Weijie Zhang ◽  
Jacob A. Nelson ◽  
Rafael Poyatos ◽  
Diego Miralles ◽  
Mirco Migliavacca ◽  
...  
Keyword(s):  
Water Vapor ◽  
Relative Humidity ◽  
Eddy Covariance ◽  
High Frequency ◽  
High Relative Humidity ◽  
Closed Path ◽  
Sap Flux ◽  
Water Vapor Flux ◽  
Vapor Flux ◽  
Flux Measurements

&lt;p&gt;Eddy covariance (EC) directly measures evapotranspiration (ET), which consists of transpiration and evaporation (E) from the soil and other surfaces. For process understanding it is pivotal to separate ET into its components. Yet, its computation is highly sensitive to the methodology used to estimate T. Among the multiple methods proposed in recent years, T has been estimated from EC via the Transpiration Estimation Algorithm (TEA, Nelson et al., 2020), and from the sap flux measurement network SAPFLUXNET (Poyatos et al., 2020). These methods are applicable to a large number of measurement sites worldwide, and can help constrain the global estimates of the ratio of T to ET, T/ET. While EC measures water and carbon fluxes across ecosystems globally, water vapor flux measurements can be underestimated at high relative humidity (Ibrom et al., 2007; Mammarella et al., 2009) causing errors in the measured ET and propagating into the predicted T.&lt;/p&gt;&lt;p&gt;Here we report a method to detect and correct the high relative humidity error caused by attenuation of high frequency in water vapor measurements of a closed-path EC system. Our results of the comparison between present water use efficiency (WUE) with previous TEA-based WUE show that the corrected WUE is lower at high relative humidity than that derived from previous TEA at the sub-daily scale. Besides, we compare the corrected T estimates from EC to concurrent SAPFLUXNET sites to show an improved relationship between sap flux and EC based T, T/ET, and WUE. Finally, we explore the main abiotic factors, such as vapor pressure deficit, air temperature, and precipitation, influencing WUE estimated from different T estimation methodologies. These results provide an improved data-driven approach to the ongoing research on ET partitioning and the factors influencing the WUE across ecosystems globally.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Ibrom, A. et al. (2007) &amp;#8216;Strong low-pass filtering effects on water vapour flux measurements with closed-path eddy correlation systems&amp;#8217;, Agricultural and Forest Meteorology. doi.org/10.1016/j.agrformet.2007.07.007.&lt;/p&gt;&lt;p&gt;Mammarella, I. et al. (2009) &amp;#8216;Relative humidity effect on the high-frequency attenuation of water vapor flux measured by a closed-path eddy covariance system&amp;#8217;, Journal of Atmospheric and Oceanic Technology. doi.org/10.1175/2009JTECHA1179.1.&lt;/p&gt;&lt;p&gt;Nelson, J. A. et al. (2020) &amp;#8216;Ecosystem transpiration and evaporation: Insights from three water flux partitioning methods across FLUXNET sites&amp;#8217;, Global Change Biology. doi: 10.1111/gcb.15314.&lt;/p&gt;&lt;p&gt;Poyatos, R. et al. (2020) &amp;#8216;Global transpiration data from sap flow measurements: the SAPFLUXNET database&amp;#8217;, Earth System Science Data. doi:10.5194/essd-2020-227.&lt;/p&gt;

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