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@INPROCEEDINGS{Graf:139210,
      author       = {Graf, Alexander and van de Boer, Anneke and Werner, Julius
                      and Langensiepen, Mathias and Schmidt, Marius and
                      Schüttemeyer, Dirk and Vereecken, Harry},
      title        = {{A} tunnel-shaped flow-through chamber for minimum
                      disturbance net ecosystem flux measurements},
      reportid     = {FZJ-2013-05213},
      year         = {2012},
      abstract     = {Eddy covariance measurements are the method of choice to
                      determine net land surface – atmosphere fluxes of energy
                      and matter, but in some cases cannot be applied in a
                      representative way. In particular, requirements on the
                      measurement height and the principles underlying the fetch
                      or footprint of the sensor inhibit straightforward
                      application in small (<< 100 m upwind) ecosystems. Small
                      chambers as used on the leaf and soil level, on the other
                      hand, considerably disturb the ecosystem and require
                      numerous repetitions in space, if robust area-average net
                      fluxes are aimed at. Within the framework of a project also
                      including the upscaling of chamber- and downscaling of
                      micrometeorological measurements, we here present a third
                      approach: A chamber with strengths and weaknesses that are
                      expected to be in an intermediate range between traditional
                      chamber and micrometeorological methods. In order to ensure
                      comparability to micrometeorologically determined fluxes,
                      the chamber was tested on three different fields of
                      sufficient size against eddy covariance stations. The
                      chamber is a flow-through type and covers a comparatively
                      large (1.7 m²) ground surface. Measures to minimize
                      ecosystem disturbance include a thin FEP foil ceiling, a
                      wide in- and outlet, and ventilation at the order of
                      magnitude of outside wind speed. Here, we focus on
                      experiments with passive ventilation, where the in- and
                      outlet are aligned into the mean wind and longitudinal
                      matter advection is measured while suppressing any vertical
                      and crosswind turbulent or advective flux. In order to
                      comply with the small resulting concentration differences
                      between in- and outlet, a differential closed-path CO2 and
                      H2O analyzer is used. Comparisons to eddy covariance
                      measurements show a good agreement and slight negative bias
                      for evapotranspiration (latent heat flux), and a somewhat
                      larger scatter and bias for CO2 flux. The scatter of the CO2
                      flux is hypothetically attributed to spatial variability
                      between the footprint of both methods. The bias indicates a
                      possible slight to intermediate underestimation of fluxes,
                      depending on the hypothetical reasons of energy balance
                      non-closure of the eddy covariance measurement. Possible
                      reasons of such an underestimation are discussed, including
                      remaining microclimate modifications as well as the
                      longitudinal turbulent flux, and further tests of or
                      modifications to the system are discussed.},
      month         = {May},
      date          = {2012-05-29},
      organization  = {30th Conference on Agricultural and
                       Forest Meteorology/First Conference on
                       Atmospheric Biogeosciences, Boston
                       (U.S.A.), 29 May 2012 - 1 Jun 2012},
      subtyp        = {Other},
      cin          = {IBG-3},
      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)6},
      url          = {https://juser.fz-juelich.de/record/139210},
}