# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 2 Materials and Methods
### Temperature Measurements  
The temperature sensor used to control chamber air temperature was situated 20 cm above the canopy and 10 cm below the lamps. This reference location was chosen because the lamps were found to heat the air in the top 5 cm of the chamber near the water filter. Mean air temperature (T\({}_{\mathrm{air}}\)) at the reference location was measured using a shielded and aspirated thermocouple (Type-E, 30 gauge). Vertical profiles of air temperature within the canopies were measured with an aspirated thermocouple manifold. The thermocouples were arranged in parallel within a manifold that held the thermocouples evenly spaced (10 cm apart) and were ventilated at about 1-2 m s\({}^{-1}\) by a single aspirator (a vacuum cleaner). The aspirated thermocouples were shielded from incident radiation by plastic tubing wrapped in aluminum foil. The vertical profiles in temperature were expressed as an air temperature difference from the reference air temperature above the canopy. Canopy temperature measurements made using infrared temperature sensors are described using the nomenclature and definitions of [PERSON] and [PERSON] [28]. Two nadir-viewing (e.g., perpendicular to the canopy) infrared sensors in each chamber (Model IRTS-P, Apogee Instruments, Logan, UT, USA) were used to measure canopy brightness temperature (T\({}_{\mathrm{canopy,IR}}\)), which is a directional temperature that depends on the angle of observation, the wavelength band of the infrared sensor, the sensor body temperature, and sensor position above the top of the canopy. The IRTS-P infrared sensors have a 90\({}^{\circ}\) field of view and an accuracy of \(\pm\)0.2 \({}^{\circ}\)C. The 8-14 \(\upmu\)m wavelength band was viewed. They were placed in the center of the canopy at a height of 10 cm above the foliage, where the chamber walls could not be seen. The calibration procedures, the field of view considerations, and the functions used to correct for sensor body temperature for these sensors are described in [PERSON] et al. [29]. Canopy T\({}_{\mathrm{Aero}}\), formally defined as the extrapolation of air temperature profile down to an effective height within the canopy at which the vegetation components of sensible and latent heat flux arise [18], is the mean canopy temperature felt by the air that solves the energy balance equation exactly. T\({}_{\mathrm{Aero}}\) cannot be measured directly. It can be obtained from H when T\({}_{\mathrm{air}}\) and g\({}_{\mathrm{A}}\) are known, but is typically approximated by T\({}_{\mathrm{R}}\), the canopy radiometric temperature [13; 27]. Canopy T\({}_{\mathrm{R}}\) was derived from T\({}_{\mathrm{canopy,IR}}\) after correcting for the sky irradiance (e.g., proportional to sky temperature, T\({}_{\mathrm{Sky}}\)) that is reflected by the canopy and by the chamber walls into the field of view of the infrared sensor [28]. In the controlled-environment chambers, sky irradiance is emitted by the warm chamber surface areas above the canopy, which were proportionally divided into a 20% chamber wall and an 80% glass water filter. The difference between \(T_{\mathrm{R}}\) and \(T_{\mathrm{canopy,IR}}\) depends on the canopy emissivity, \(\varepsilon_{\mathrm{c}}\) (Equation (1)):  
\[\sigma\,T^{4}_{\mathrm{canopy,IR}}=\varepsilon_{\mathrm{c}}{}^{\bullet}\sigma{ }^{\bullet}T^{4}_{\mathrm{R}}+(1-\varepsilon_{\mathrm{c}}){}^{\bullet}\sigma{ }^{\bullet}T^{4}_{\mathrm{sky}} \tag{1}\]  
where \(\sigma\) = Stefan-Boltzman constant (W m\({}^{-2}\) K\({}^{-4}\)), and \(T_{\mathrm{sky}}\) = temperature of the chamber surfaces above the canopy (K). In this paper, it was assumed that \(T_{\mathrm{R}}\approx T_{\mathrm{canopy,IR}}\) because the correction for canopy emissivity is small (\(\approx\) 0.2 \({}^{\circ}\)C).