An Open System for Measuring Canopy Gas Exchange.

Three portable, CETA (Canopy Evapo-Transpiration and Assimilation) chamber systems were built and evaluated in 2006.  This chamber system is an open or flow-through system that, once deployed in the field, can operate unattended for extended periods (e.g. overnight for example). The CETA chamber consists of an aluminum framework, 1 m x 0.75 m in cross-section and 1 m tall covered with transparent film.  Differentials between incoming and outgoing atmospheric water vapor and carbon dioxide concentrations are used to calculate evapotranspiration and canopy photosynthesis at 10 s intervals using solenoid valve actuated sample lines connected to an infrared gas analyzer. A programmable data logger controls fan speed and air flow rate in order to control chamber air temperature to within 0.5 ºC of ambient air temperature using a feedback control algorithm.  In order to validate the mass balance equations used to calculate canopy evapotranspiration, the CETA chamber was placed over potted plants sitting atop a mini-lysimeter. A wide variety of crop canopies and soil water content were created with greenhouse-grown plants. Data analysis indicates good agreement between CETA evapotranspiration measurements and the mini-lysimeter over wide ranges of soil moisture contents and canopy leaf area.