Validation of a minimum microclimate disturbance chamber for net ecosystem flux measurements
Validation of a minimum microclimate disturbance chamber for net ecosystem flux measurements
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DOI：
10.1016/j.agrformet.2013.02.001
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发表时间：
2013-06
影响因子：
6.2
通讯作者：
A. Graf;J. Werner;M. Langensiepen;A. Boer;M. Schmidt;Moritz Kupisch;H. Vereecken
中科院分区：
文献类型：
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作者：
A. Graf;J. Werner;M. Langensiepen;A. Boer;M. Schmidt;Moritz Kupisch;H. Vereecken
描述了一种用于测量冠层净CO2和H2O通量的最小扰动室。该系统是一个被动（可选主动）通风隧道，具有大（类似于0.14 m2）的入口和出口横截面，覆盖约1.6 m2的表面积。使用差分、非干燥闭合路径气体分析仪来最大限度地减少入口和出口之间的浓度累积或下降要求，并且使用0.05 mm厚的FEP（氟化乙烯丙烯）膜作为顶部和壁材料来最大限度地减少辐射修改。被动通风模式的第一次测量结果，从两个不同的网站：麦田在低地网站在最西部的部分德国，和不同的种植相邻领域的Pyrenee山麓在法国，包括草地和谷物为主的作物混合饲料生产。通过20- 30分钟部署的测量得出的通量估计值与三个涡度协方差（EC）站的同时观测值进行了比较。该系统与EC测量结果进行了很好的比较，潜热通量的偏差为-6.6%，CO2通量的偏差为+9.0%（R2分别为0.78和0.74）。电离室的存在导致入射短波辐射减少不到4%，而向下的长波辐射增加约18%。在出口附近，相对于外部条件，CO2浓度平均修改了−3%，水蒸气浓度修改了+22%，温度修改了+0.9K，保持低于可比的非稳态腔室关闭2分钟的公开修改。通风速度在入口横截面上的变化小于9%。限制包括最低风速要求，对于1 s-1或更高的原始数据记录频率，最低风速要求可以设置为0.2ms-1，但对于慢响应气体分析仪，最低风速要求需要更高。同时，建议测量时间为10分钟或更长，以最大限度地减少储存期波动造成的随机误差。对蒸散增加的水汽量进行了修正，其对H2O通量的影响小于+2%，对CO2通量的影响小于1μmol/J。
A minimum-disturbance chamber for canopy net CO2and H2O flux measurements is described. The system is a passively (optionally actively) ventilated tunnel with large (similar to 0.14m2) in- and outlet cross sections covering a surface area of approximately 1.6m2. 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 0.05mm 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 20–30min 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 CO2flux (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, CO2concentration was on average modified by −3% with respect to outside conditions, water vapour concentration by +22%, and temperature by +0.9K, staying below published modifications of a comparable non-steady-state chamber closed for 2min. 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 0.2ms−1for a raw data logging frequency of 1s−1or higher, but would need to be higher for slow-response gas analyzers. At the same time, a measurement period of 10min 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 CO2flux measurements by 1μmol/J latent heat.