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@ARTICLE{Graf:132926,
      author       = {Graf, Alexander and Werner, J. and Langensiepen, M. and van
                      de Boer, A. and Schmidt, Marius and Kupisch, M. and
                      Vereecken, H.},
      title        = {{V}alidation of a minimum microclimate disturbance chamber
                      for net ecosystem flux measurements},
      journal      = {Agricultural and forest meteorology},
      volume       = {174-175},
      issn         = {0168-1923},
      address      = {Amsterdam [u.a.]},
      publisher    = {Elsevier},
      reportid     = {FZJ-2013-01502},
      pages        = {1 - 14},
      year         = {2013},
      abstract     = {A minimum-disturbance chamber for canopy net CO2 and H2O
                      flux measurements is described. The system is a passively
                      (optionally actively) ventilated tunnel with large (similar
                      to 0.14 m2) in- and outlet cross sections covering a surface
                      area of approximately 1.6 m2. A differential, non-drying
                      closed-path gas analyzer is used to minimize the requirement
                      for concentration build-ups or drawdowns between the in- and
                      outlet, and 0.05 mm thick FEP (fluorinated ethylene
                      propylene) film is used as top and wall material to minimize
                      radiation modifications. First measurement results in
                      passively ventilated mode are presented from two different
                      sites: a wheat field at a lowland site in the westernmost
                      part of Germany, and differently cropped adjacent fields in
                      the Pyrenee foothills in France, including grassland and a
                      cereal-dominated crop mixture for fodder production. Flux
                      estimates derived from measurements over 20–30 min
                      deployments were compared to concurrent observations by
                      three eddy covariance (EC) stations. The system compared
                      well to EC measurements, with bias of $−6.6\%$ for latent
                      heat flux and $+9.0\%$ for CO2 flux (R2 = 0.78 and 0.74,
                      respectively). The presence of the chamber causes a
                      reduction of less than $4\%$ in incoming shortwave
                      radiation, and an increase of about $18\%$ in downward
                      longwave radiation. Near the outlet, CO2 concentration was
                      on average modified by $−3\%$ with respect to outside
                      conditions, water vapour concentration by $+22\%,$ and
                      temperature by +0.9 K, staying below published modifications
                      of a comparable non-steady-state chamber closed for 2 min.
                      Ventilation speed varied by less than $9\%$ across the inlet
                      cross section. Limitations include a minimum wind speed
                      requirement that can be set as low as 0.2 m s−1 for a raw
                      data logging frequency of 1 s−1 or higher, but would need
                      to be higher for slow-response gas analyzers. At the same
                      time, a measurement period of 10 min or more is recommended
                      to minimize random errors from storage term fluctuations. A
                      correction for the added water vapour volume by
                      evapotranspiration is derived and tested, which typically
                      affects H2O flux itself by less than $+2\%$ and CO2 flux
                      measurements by 1 μmol/J latent heat.},
      cin          = {IBG-3},
      ddc          = {630},
      cid          = {I:(DE-Juel1)IBG-3-20101118},
      pnm          = {246 - Modelling and Monitoring Terrestrial Systems: Methods
                      and Technologies (POF2-246) / DFG project 139819005 - Links
                      between local scale and catchment scale measurements and
                      modelling of gas exchange processes over land surfaces
                      (139819005)},
      pid          = {G:(DE-HGF)POF2-246 / G:(GEPRIS)139819005},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000318382900001},
      doi          = {10.1016/j.agrformet.2013.02.001},
      url          = {https://juser.fz-juelich.de/record/132926},
}