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Use of change-point detection for friction–velocity threshold evaluation in eddy-covariance studies

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https://ir.library.oregonstate.edu/concern/articles/w3763738j

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  • The eddy-covariance method often underestimates fluxes under stable, low-wind conditions at night when turbulence is not well developed. The most common approach to resolve the problem of nighttime flux underestimation is to identify and remove the deficit periods using friction-velocity (u*) threshold filters (u*[superscript Th]). This study modifies an accepted method for u*[superscript Th] evaluation by incorporating change-point-detection techniques. The original and modified methods are evaluated at 38 sites as part of the North American Carbon Program (NACP) site-level synthesis. At most sites, the modified method produced u*[superscript Th] estimates that were higher and less variable than the original method. It also provided an objective method to identify sites that lacked a u*[superscript Th] response. The modified u*[superscript Th] estimates were robust and comparable among years. Inter-annual u*[superscript Th] differences were small, so that a single u*[superscript Th] value was warranted at most sites. No variation in the u*[superscript Th] was observed by time of day (dusk versus mid or late night), however, a few sites showed significant u*[superscript Th] variation with time of year. Among-site variation in the u*[superscript Th] was strongly related to canopy height and the mean annual nighttime u*. The modified u*[superscript Th] estimates excluded a high fraction of nighttime data - 61% on average. However, the negative impact of the high exclusion rate on annual net ecosystem production (NEP) was small compared to the larger impact of underestimating the u*[superscript Th]. Compared to the original method, the higher u*[superscript Th] estimates from the modified method caused a mean 8% reduction in annual NEP across all site-years, and a mean 7% increase in total ecosystem respiration (R[subscript e]). The modified method also reduced the u*[superscript Th]-related uncertainties in annual NEP and R-e by more than 50%. These results support the use of u*[superscript Th] filters as a pragmatic solution to a complex problem. (C) 2012 A.G. Barr. Published by Elsevier B.V. All rights reserved.
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  • Barr, A., Gu, L., Law, B., Margolis, H., McCaughey, J., Munger, J., . . . Fischer, M. (2013). Use of change-point detection for friction-velocity threshold evaluation in eddy-covariance studies. Agricultural and Forest Meteorology, 171, 31-45. doi: 10.1016/j.agrformet.2012.11.023
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  • 171
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  • We also thank the funding agencies that have supported these long-term, fluxtowernetworks. AGB acknowledges support from the Climate Research Division of Environment Canada and thanks Xiaolin Wang for advice on change-point detection techniques and two anonymous reviewers for their constructive comments. ADR and DYE acknowledge support from Office of Science (BER), US Department of Energy, through the Terrestrial Carbon Program under Interagency Agreement No. DE-AI02-07ER64355 and through the Northeastern Regional Center of the National Institute for Climatic Change Research. DP thanks the GHG-Europe and CarboExtremeEuropean Research projects for their support. Effort by MLF andcollection of US-ARM site data were supported at LBNL by theU.S. Department of Energy, Office of Biological and Environmental Research, under contract number DE-AC02-05CH1123.
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