# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 3 Results
### Canopy-to-Air Temperature Difference  
The difference between aerodynamic \(\Delta\mathrm{T_{A}}\) and radiometric \(\Delta\mathrm{T_{IR}}\) is affected by two physical factors: the field of view of the IR transducers and the chamber wind speed. The field of view of the sensor with respect to the canopy surface influenced the magnitude of the radiometric \(\mathrm{T_{R}}\) measured by the infrared transducers. Differences in \(\mathrm{T_{IR}}\) and \(\mathrm{T_{Aero}}\) are probably due to differences in how well radiometric measurements truly represent the average canopy temperature profile. With constant \(\mathrm{T_{air}}\) and \(\mathrm{PPF_{0}}\) provided by the chamber, radiometric \(\mathrm{T_{R}}\) was affected by the vertical positioning of the infrared transducers above or within the canopy. Generally, \(\mathrm{T_{R}}\) was higher in the surface layers of foliage and became lower as the IR transducer was inserted into the canopy foliage. Once the IR transducers were positioned, the canopy-to-air temperature difference was compared to the canopy-to-air temperature difference obtained from H.  
The relation between H, \(\Delta\mathrm{T_{IR}}\), and \(\Delta\mathrm{T_{A}}\) was explored in soybean by changing the amount and quality of incident radiation (Figure 5; 45-day-old; [CO\({}_{2}\)] = 400 \(\mathrm{\SIUnitSymbolMicro mol^{-1}}\); PPF = 1050 \(\mathrm{\SIUnitSymbolMicro mol^{-2}}\) s\({}^{-1}\); \(\mathrm{T_{air}}\) = 22 \({}^{\circ}\)C; RH = 62%). In the dark, the energy balance components under each water filter were similar, yielding H \(\sim\) \(-\)75 W m\({}^{-2}\) (Figure 5, top), but radiometric \(\Delta\mathrm{T_{IR}}\) and aerodynamic \(\Delta\mathrm{T_{A}}\) differed by a nearly constant offset (Figure 5, bottom). The spikes in H observed at the beginning and at the end of the photoperiod are artifacts that occur when H is obtained by subtraction and chamber energy fluxes and temperatures equilibrate. Figure 5: Diurnal changes in canopy sensible heat, radiative, and aerodynamic canopy-to-air temperature differences, and the offset of soybean illuminated by lamps (**A**) without and (**B**) with a water filter to remove excess longwave radiation. During the photoperiod, removing the water filter under the HPS lamps increased \(R_{\mathrm{net}}\) by \(\sim\)50 W m\({}^{-2}\) due to a 30% greater PPF\({}_{\mathrm{o}}\) transmission and due to increased longwave radiation as the lamps heated the glass of the water filter. Without the water filter, the ratio of photosynthetic to non-photosynthetic shortwave radiation dropped from 83:17 to 66:34, and sensible heat flux increased up to approximately \(-\)95 W m\({}^{-2}\) (Figure 5A, top), from approximately \(-\)120 W m\({}^{-2}\) (Figure 5B, top) as the additional radiation from the lamps warmed the canopy. Radiometric \(\Delta\mathrm{T}_{\mathrm{IR}}\) was consistently higher than aerodynamic \(\Delta\mathrm{T}_{\mathrm{A}}\), and the offset was between 0.8 and 1.0 \({}^{\circ}\)C higher than it was in the dark. In fact, the sensible heat flux calculated from \(\Delta\mathrm{T}_{\mathrm{IR}}\) using Equation (6) often had an opposite sign to values of sensible heat flux calculated from the energy balance equation (Figure 5, middle). However, relative changes in the magnitude of \(\Delta\mathrm{T}_{\mathrm{IR}}\) as a function of time paralleled the relative changes in H and \(\Delta\mathrm{T}_{\mathrm{A}}\) (Figure 5, bottom), and the difference between the measured \(\Delta\mathrm{T}_{\mathrm{IR}}\) and \(\Delta\mathrm{T}_{\mathrm{A}}\) remained constant throughout the photoperiod. Wind speed determines canopy g\({}_{\mathrm{A}}\) and affects how the foliage warms as PPF\({}_{\mathrm{o}}\) is increased. In a wheat canopy (25-day-old; [CO\({}_{2}\)] = 1200 \(\mu\)mol mol\({}^{-1}\); T\({}_{\mathrm{air}}\) = 22 \({}^{\circ}\)C; RH = 68%; no water filter), changes in PPF\({}_{\mathrm{o}}\) at two chamber wind speeds were used to explore the offset between \(\Delta\mathrm{T}_{\mathrm{A}}\) and \(\Delta\mathrm{T}_{\mathrm{IR}}\) (Figure 6).