Skip to main content

Lund University Publications

LUND UNIVERSITY LIBRARIES

Two-level eddy covariance measurements reduce bias in land-atmosphere exchange estimates over a heterogeneous boreal forest landscape

Klosterhalfen, Anne ; Chi, Jinshu ; Kljun, Natascha LU orcid ; Lindroth, Anders LU orcid ; Laudon, Hjalmar ; Nilsson, Mats B. LU and Peichl, Matthias (2023) In Agricultural and Forest Meteorology 339.
Abstract

Estimates of land-atmosphere exchanges of carbon, energy, water vapor, and other greenhouse gases based on the eddy covariance (EC) technique rely on the fundamental assumption that the flux footprint area is homogeneous. We investigated the impact of source area heterogeneity on flux estimates in single-level EC measurements over a managed boreal forest landscape. For this purpose, we compared single-level measurements with those from a two-level approach consisting of concurrent EC measurements at 60 and 85 m above the ground. This two-level set-up provided a unique opportunity to obtain nearly congruent diel footprint areas by combining data from the higher and lower levels during day- and nighttime, respectively. We found that the... (More)

Estimates of land-atmosphere exchanges of carbon, energy, water vapor, and other greenhouse gases based on the eddy covariance (EC) technique rely on the fundamental assumption that the flux footprint area is homogeneous. We investigated the impact of source area heterogeneity on flux estimates in single-level EC measurements over a managed boreal forest landscape. For this purpose, we compared single-level measurements with those from a two-level approach consisting of concurrent EC measurements at 60 and 85 m above the ground. This two-level set-up provided a unique opportunity to obtain nearly congruent diel footprint areas by combining data from the higher and lower levels during day- and nighttime, respectively. We found that the variation in the averaged footprint area between day- and nighttime was reduced by up to 89% in the two-level approach compared to the single-level data at the higher level (85 m). Considering spring, summer, and fall months, the resulting relative potential bias in flux observations due to landscape heterogeneity was highest at short time steps (≤ daily) ranging between 35% and 325% for half-hourly data. During winter months, when stable atmospheric regimes prevailed during day and night, the footprints within the diel course nearly overlapped also at a given single level and hence no improvement of flux estimates was found. The absolute cumulated sums for the study period (excluding winter months) of gross primary production, ecosystem respiration, latent heat, and sensible heat flux were underestimated by about 28%, 52%, 5%, and 3%, respectively, whereas that of net ecosystem CO2 exchange was overestimated by about 109% in the single-level approach. Overall this study suggests that footprint heterogeneity may introduce considerable bias in single-level flux estimates — particularly at short time scales — with large implications for model-data fusion studies, site comparisons, and up- or downscaling of land-atmosphere exchange processes.

(Less)
Please use this url to cite or link to this publication:
author
; ; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Footprint model FFP, Heterogeneous landscape, Latent heat flux, Net ecosystem CO exchange, Sensible heat flux, Tall tower eddy covariance
in
Agricultural and Forest Meteorology
volume
339
article number
109523
publisher
Elsevier
external identifiers
  • scopus:85163533360
ISSN
0168-1923
DOI
10.1016/j.agrformet.2023.109523
language
English
LU publication?
yes
additional info
Funding Information: This research was supported by the Kempe Foundation [Grant #JCK-1815 ], by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) [Grant #942-2015-49 ], the Swedish Research Council, and contributing research institutes to the Swedish Integrated Carbon Observation System (ICOS-Sweden) Research Infrastructure and the Swedish Infrastructure for Ecosystem Science (SITES) . This research contributes also to the Strategic Research Area: ModElling the Regional and Global Earth system, MERGE, funded by the Swedish government . Further, JC acknowledges funding from Kempe Foundation [Grant #SMK-1743 ] and Center for Ocean Research in Hong Kong and Macau (CORE) . We thank the staff from the Svartberget Field Station, especially Per Marklund, Paul Smith, and Giuseppe De Simon, for their continuous support in data acquisition and instrument maintenance. We also thank Jutta Holst (INES, Lund University) for her ICOS data support. Finally, we thank the reviewers for their helpful and constructive comments that improved the quality of this paper. Funding Information: This research was supported by the Kempe Foundation [Grant #JCK-1815], by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) [Grant #942-2015-49], the Swedish Research Council, and contributing research institutes to the Swedish Integrated Carbon Observation System (ICOS-Sweden) Research Infrastructure and the Swedish Infrastructure for Ecosystem Science (SITES). This research contributes also to the Strategic Research Area: ModElling the Regional and Global Earth system, MERGE, funded by the Swedish government. Further, JC acknowledges funding from Kempe Foundation [Grant #SMK-1743] and Center for Ocean Research in Hong Kong and Macau (CORE). We thank the staff from the Svartberget Field Station, especially Per Marklund, Paul Smith, and Giuseppe De Simon, for their continuous support in data acquisition and instrument maintenance. We also thank Jutta Holst (INES, Lund University) for her ICOS data support. Finally, we thank the reviewers for their helpful and constructive comments that improved the quality of this paper. Publisher Copyright: © 2023 The Author(s)
id
1dc47f12-0149-4e6d-b415-97044692b231
date added to LUP
2023-07-16 22:16:27
date last changed
2024-06-29 05:52:53
@article{1dc47f12-0149-4e6d-b415-97044692b231,
  abstract     = {{<p>Estimates of land-atmosphere exchanges of carbon, energy, water vapor, and other greenhouse gases based on the eddy covariance (EC) technique rely on the fundamental assumption that the flux footprint area is homogeneous. We investigated the impact of source area heterogeneity on flux estimates in single-level EC measurements over a managed boreal forest landscape. For this purpose, we compared single-level measurements with those from a two-level approach consisting of concurrent EC measurements at 60 and 85 m above the ground. This two-level set-up provided a unique opportunity to obtain nearly congruent diel footprint areas by combining data from the higher and lower levels during day- and nighttime, respectively. We found that the variation in the averaged footprint area between day- and nighttime was reduced by up to 89% in the two-level approach compared to the single-level data at the higher level (85 m). Considering spring, summer, and fall months, the resulting relative potential bias in flux observations due to landscape heterogeneity was highest at short time steps (≤ daily) ranging between 35% and 325% for half-hourly data. During winter months, when stable atmospheric regimes prevailed during day and night, the footprints within the diel course nearly overlapped also at a given single level and hence no improvement of flux estimates was found. The absolute cumulated sums for the study period (excluding winter months) of gross primary production, ecosystem respiration, latent heat, and sensible heat flux were underestimated by about 28%, 52%, 5%, and 3%, respectively, whereas that of net ecosystem CO<sub>2</sub> exchange was overestimated by about 109% in the single-level approach. Overall this study suggests that footprint heterogeneity may introduce considerable bias in single-level flux estimates — particularly at short time scales — with large implications for model-data fusion studies, site comparisons, and up- or downscaling of land-atmosphere exchange processes.</p>}},
  author       = {{Klosterhalfen, Anne and Chi, Jinshu and Kljun, Natascha and Lindroth, Anders and Laudon, Hjalmar and Nilsson, Mats B. and Peichl, Matthias}},
  issn         = {{0168-1923}},
  keywords     = {{Footprint model FFP; Heterogeneous landscape; Latent heat flux; Net ecosystem CO exchange; Sensible heat flux; Tall tower eddy covariance}},
  language     = {{eng}},
  month        = {{08}},
  publisher    = {{Elsevier}},
  series       = {{Agricultural and Forest Meteorology}},
  title        = {{Two-level eddy covariance measurements reduce bias in land-atmosphere exchange estimates over a heterogeneous boreal forest landscape}},
  url          = {{http://dx.doi.org/10.1016/j.agrformet.2023.109523}},
  doi          = {{10.1016/j.agrformet.2023.109523}},
  volume       = {{339}},
  year         = {{2023}},
}