Eddy-covariance flux measurements

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Nov 3, 2013 (4 years and 1 month ago)

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Eddy
-
covariance flux measurements

Eddy covariance flux measurements for Net Carbon Exchange and for Latent
Heat flux from the global network of observation sites are used to constrain
ecosystem

physiology and fast

processes

from the synoptic variations
to the
seasonality of fluxes in ORCHIDEE. Note that

we do not consider the
sensible heat flux measurements, as the overall objectives of CARBONES
concern the carbon balance and not the energy balance.

1) Flux data assimilated in ORCHIDEE


We use harmonize
d

quality

checked and gap filled data (LEVEL4) from a
new global synthesis

called the L
aThuile

dataset. These new data are made
available

during the course of the project. For other sites the use of the data is

negotiated

with the PIs. This dataset

forms a

unique collection of 600 site
-
years of online hourly

measurements of CO2, water vapour, and heat fluxes.
Figure
1

shows the location of the different sites. In order to avoid

dealing

with the large error

correlations

both in the model and the measurements
, we
are using

daily

mean values of Net Ecosystem Exchange and Latent Heat flux
in the CCDAS. Note that

days

with

less

than 80% of half
-
hourly data left out
the assimilation.


Figure
1
: Localization of the different FLUXNET sites
that are available in the FLUXNET database (www.fluxdata.org)


2) Data uncertainties


Eddy covariance measurement

errors

consist of a random and systematic

error
component. The random

error

can

be

evaluated by using the high temporal
density of the dataset
, interpreting observations made under the same
conditions as repeated

measurements. Errors are largely

Gaussian

distributed

with standard deviations

which

increase

with the flux magnitude.
Autocorrelation of fluxes is

usually

low, below
a correlation coef
ficient of
0.6
at a lag of 0.5 h (Lasslop et al. 2008). Despite of the low

autocorrelation,
model parameter

optimizations

with flux data have shown

that the use of only

every second or every

third data point is

reasonable

because the low

autocorrelation

co
uld

be due to filling of data gaps (Lasslop et al. 2008).

S
ystematic

errors

in eddy covariance measurements are caused by advection,
low turbulence or variable footprints of the measurement station during day
-

and night
-
time. These systematic errors domina
te the overall uncertainty of
annual total carbon fluxes while uncertainties from the random error
component are negligible (Lasslop et al. 2010). Systematic errors
can

be

quantified

using

datasets

from multiple years and sites
covering a range of
plausible values of the wind
velocity

or by using only day
-
time observations
(Lasslop et al. 2010)
.

Nevertheless, overall relationships between estimated
carbon fluxes

3)
Data policy


In Carbones, data authorized under the Free
-
Fair
-
Use policy has been ut
ilized.
As documented at
www.fluxdata.org
, the data available in this database (a
subset of the LaThuile dataset) have been furnished by individual scientists
who encourage their use under an open data policy that em
phasizes the free
and open exchange of scientific information.

The data are made freely available to the public and the scientific community
in the belief that their wide dissemination will lead to greater understanding
and new scientific insi
ghts and that

global scientific
problems require
international cooperation.

Open

access means that data are freely distributed without charge; there may
be charges for the cost of reproduction and delivery when access is not web
-
b
ased. Data download is unrestricted and

requires only a free registration
needed for web security reasons.

The FLUXNET participants that decided to share these data openly rely on
the ethics and integrity of the users to assure that the data providers and
FLUXNET receive fair credit for their w
ork through inclusion of the text
provided below in the acknowledgment.

The data users must send accepted papers or links to them to the fluxdata.org
staff and PIs of the sites used in the paper. It is also recommended to contact
the site PIs prior to
publication to prevent potential misuse or
misinterpretation of the data; if the work is based on only a few sites, this
contact is strongly recommended.

Downloaded data cannot be redistributed to others and must not be
redistributed via other websites, da
tabases
o
r any other storage system to
prevent circulation of different versions of the datasets.


The following acknowledgment text has to be used with publications:

This work used eddy covariance data acquired by the FLUXNET community
and in particular b
y the following networks: AmeriFlux (U.S. Department of
Energy, Biological and Environmental Research, Terrestrial Carbon Program
(DE
-
FG02
-
04ER63917 and DE
-
FG02
-
04ER63911)), AfriFlux, AsiaFlux,
CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux,
Fluxnet
-
Canada (supported by CFCAS, NSERC, BIOCAP, Environment Canada, and
NRCan), GreenGrass, KoFlux, LBA, NECC, OzFlux, TCOS
-
Siberia,
USCCC. We acknowledge the financial support to the eddy covariance data
harmonization provided by CarboEuropeIP, FAO
-
GTO
S
-
TCO, iLEAPS, Max
Planck Institute for Biogeochemistry, National Science Foundation,
University of Tuscia, Université Laval and Environment Canada and US
Department of Energy and the database development and technical support
from Berkeley Water Center, L
awrence Berkeley National Laboratory,
Microsoft Research eScience, Oak Ridge National Labor
atory, University of
California
-
Berkeley, University of Virginia.



4) References


Lasslop, G., M. Reichstein, J. Kattge, and D. Papale. 2008. Influences of
observat
ion errors in eddy flux data on inverse model parameter
estimation. Biogeosciences

5:1311
-
1324.

Lasslop, G., M. Reichstein, D. Papale, A. D. Richardson, A. Arneth, A. Barr,
P. Stoy, and G. Wohlfahrt. 2010. Separation of net ecosystem exchange
into assimila
tion and respiration using a light response curve approach:
critical issues and global evaluation. Global Change Biology 16:187
-
208.

Papale, D., M. Reichstein, E. Canfora, M. Aubinet, C. Bernhofer, B.
L
ongdoz, W. Kutsch, S. Rambal, R. Valentini, T. Vesala,

and D. Yakir.
2006. Towards a more harmonized processing of eddy covariance CO2
fluxes: algorithms and uncertainty estimation. Biogeosciences
Discussions 3:

961
-
992.

Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet,
P. Berbigier
, C. Bernhofer, N. Buchmann, T. Gilmanov, A. Granier, T.
Grünwald, K. Havránková, D. Janous, A. Knohl, T. Laurela, A. Lohila,
D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.
-
M. Ourcival, S.
Rambal, E. Rotenberg, M. Sanz, G. Seufert, F. Vaccari, T. Ve
sala, and
D. Yakir. 2005. On the separation of net ecosystem exchange into
assimilation and ecosystem respiration: review and improved algorithm.
Global Change Biology 11:1424
-
1439.