Validation of a minimum microclimate disturbance chamber for net ecosystem flux measurements

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 hea