Effects of different eddy covariance correction schemes on energy balance closure and comparisons with the modified Bowen ratio system

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Abstract

Eddy covariance (EC) and modified Bowen ratio (MBR) systems have been shown to yield subtly different estimates of sensible heat (H), latent heat (LE), and CO2 fluxes (Fc). Our study analyzed the discrepancies between these two systems by first considering the role of the data processing algorithm used to estimate fluxes using EC and later examining the effects of atmospheric stability on discrepancies between EC and MBR. We found that EC correction algorithms disproportionately increase the magnitude of LE and Fc, and consequently have a strong effect on the Bowen ratio measured by EC. Two corrections not universally employed were each found to account for up to 20% of LE and Fc: the correction for signal asynchrony by adjusting the lag between the IRGA and the sonic anemometer, and the frequency domain correction for path-length averaging in both instruments. Comparison of fluxes between two EC systems 10 m apart showed that LE and Fc are inherently more variable than H, highlighting the role of field heterogeneity in determining canopy gas exchange at very small spatial scales.

When all relevant corrections were applied, there was no bias for H and LE between EC and MBR. An examination of discrepancies between EC and MBR showed that the discrepancies were highest during neutral periods when shear drove vertical mixing much more than buoyancy. During these neutral periods, the correlation between T, H2O, and CO2 was much lower on average, which violates the similarity assumption exploited in the MBR technique. The largest discrepancies in Fc in both systems were clearly visible when plotted against a light–response curve, which creates the possibility of using a screening technique to exclude data that depart markedly from other recently collected data. The EC system tended to have the largest departures from the expected value at low light and during the night, while the MBR system tended to show occasionally spikes of high photosynthetic uptake.

Introduction

Bowen ratio-energy balance (BREB) is a widely used technique for measuring surface water and heat exchange (e.g., Gao et al., 1998, Hope and Evans, 1992), and owes much of its success to its ease of use, simple theoretical basis, and relatively modest instrumental requirements. The term “modified Bowen ratio” (MBR) has been adopted for systems which use the BREB technique to measure additional traces gases, in particular CO2 (Hall and Claiborn, 1997, Lindberg et al., 1995, Meyers et al., 1996, Muller et al., 1993, Schween et al., 1997). Eddy covariance (EC) is a more recent technique that has several distinct advantages, particularly for observations over tall canopies (Baldocchi et al., 2001), but takes considerably more effort to maintain, process, and quality assure results for long-term measurements (Aubinet et al., 2000, Papale et al., 2006). Considerable interest exists in using these systems to measure net ecosystem CO2 and H2O exchange across long time periods where integrals of measurement errors can become large (Moncrieff et al., 1996). As a result, there is a compelling need to ensure that the inferences of the magnitude of the source/sink strength in different ecosystems does not depend on the way CO2 and H2O fluxes are measured (Brotzge and Crawford, 2003, Liu and Foken, 2001). Agreement of EC and MBR measurements during the daytime rests on two factors. The foremost factor is the accuracy of measuring energy fluxes into the surface, including net radiation (Rn), ground heat flux (G), and ground storage (S), which directly determine the magnitude of H + LE for the BR system, and which constitutes a measure of energy balance closure in EC studies. A second factor is the different bias of the two systems in partitioning available energy toward either H or LE.

Although Massman and Lee (2002) discussed many EC data processing issues, few intercomparisons have focused on the role of data processing schemes as a source of measurement differences among research groups, or between direct comparisons between EC and MBR. Networks of flux towers require intercomparability among systems (Baldocchi et al., 2001), which is difficult to attain when both the instrumentation and correction schemes vary (Loescher et al., 2006). While standardization in data processing remains a goal for research using eddy covariance stations, considerable variation exists in data collection and archiving procedures, preprocessing of high frequency data, and postprocessing of covariance data (Twine et al., 2000).

This paper does not focus on the theory of the corrections themselves, but instead examines the consequences of different correction procedures on a dataset of CO2 and energy fluxes above a grassland. We hypothesize that a major cause of lack of energy balance closure and incomparability between EC and MBR is the correction algorithm used to process EC data.

Section snippets

Methods

Some of the major facets of EC correction are briefly outlined below for reference.

Coordinate rotation: Covariances should be rotated to a natural coordinate system (Lee et al., 2004) to partially fulfill a key assumption of the EC technique, namely that the flux measurement represents the flux perpendicular to the surface out of a nominal control volume defined at its top by the sensor and neglecting horizontal divergence so that the system becomes one-dimensional (Moncrieff et al., 2004). The

Instrument comparison

Comparisons between sensors are summarized in Table 1. There was overall close agreement between the net radiometers, although at the highest light levels a discrepancy of up to 50 W m−2 was observed. Other meteorological measurements (PAR, T, RH, U) were likewise very consistent. The poorest agreement was found among the soil measurements (Tsoil, G, and VSMC). Note that the G measurements using the two sets of REBS plates were permanently installed to accompany the MBR system, whereas the

Discussion

The motivation for this study was to determine whether EC and MBR gave similar estimates of surface fluxes for use in parameterizing land surface models for carbon, water, and energy balance studies. Although other studies have addressed this topic previously, none have examined the inherent uncertainty in EC measurements by using two colocated systems, and none have addressed how EC data processing affected parity between the two measurement systems. In addition, the use of two distinct,

Conclusion

Eddy covariance and modified Bowen ratio systems have been shown to yield subtly different estimates of H, LE, and Fc. Our study analyzed the discrepancies between these two systems by first considering the role of the data processing algorithm used to estimate fluxes using EC and later examining the effects of atmospheric stability on discrepancies between EC and MBR. We found that EC correction algorithms disproportionately increase the magnitude of LE and Fc, and consequently have a strong

Acknowledgements

This publication was made possible through support provided by US Universities, host country institutions and the Office of Agriculture and Food Security, Global Bureau, US Agency for International Development, under Grant No. PCE-G-98-00036-00. The opinions expressed herein are those of the author(s) and do not necessarily reflect the views of USAID.

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