# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 3 Results
### Canopy Surface and Stomatal Conductances  
Canopy surface G\({}_{\rm SFC}\) of wheat was calculated from E\({}_{\rm can}\) and D\({}_{\rm S}\) using Equation (8) (green line; Figure 8A; 26-day-old; [CO\({}_{2}\)] = 400 \(\mu\)mol mol\({}^{-1}\); PPF = 1600 mol m\({}^{-2}\) s\({}^{-1}\); T\({}_{\rm air}\) = 21 \({}^{\circ}\)C; RH = 68%). Once g\({}_{\rm A}\) was determined, G\({}_{\rm SFC}\) was used for estimating canopy G\({}_{\rm S}\) using Equation (9) (green line; Figure 8B). Assuming that canopy G\({}_{\rm S}\) equals G\({}_{\rm SFC}\), that is, without taking g\({}_{\rm A}\) into account (green lines in Figure 8A,B) underestimates G\({}_{\rm S}\) by 40% in wheat.  
Figure 7: Plot of H versus \(\Delta\)T\({}_{\rm IR}\) (red line) and the offset from a 25-day-old wheat canopy exposed to changing CO\({}_{2}\) concentration.  
The sensitivity of G\({}_{\rm SFC}\) (Equation (8)) to errors from using \(\Delta\)T\({}_{\rm IR}\) instead of \(\Delta\)T\({}_{\rm A}\) was also explored (Figure 8). Surface G\({}_{\rm SFC}\) of wheat (Figure 8A) was only slightly greater when calculated from radiometric T\({}_{\rm R}\) instead of aerodynamic T\({}_{\rm Aero}\). The average surface G\({}_{\rm SFC}\) at the radiometric T\({}_{\rm R}\) was 1.6 mol m\({}^{-2}\) s\({}^{-1}\) (red line; Figure 8A) and 1.5 mol m\({}^{-2}\) s\({}^{-1}\) (green line; Figure 8A) at the aerodynamic T\({}_{\rm Aero}\). Therefore, neglecting the offset correction between \(\Delta\)T\({}_{\rm IR}\) and \(\Delta\)T\({}_{\rm A}\) in wheat resulted in only a -6% error in surface G\({}_{\rm SFC}\). The difference between the average radiometric G\({}_{\rm S}\) (2.3 mol m\({}^{-2}\) s\({}^{-1}\) or 18.0 s m\({}^{-1}\)) and aerodynamic G\({}_{\rm S}\) (2.1 mol m\({}^{-2}\) s\({}^{-1}\) or 19.7 s m\({}^{-1}\)) was also small (red vs. green line; Figure 8B). Thus, canopy G\({}_{\rm S}\) of wheat computed using the observed T\({}_{\rm R}\) instead of T\({}_{\rm Aero}\) was only 8% higher.  
In soybean at 400 \(\mu\)mol mol\({}^{-1}\) of CO\({}_{2}\), the surface radiometric G\({}_{\rm SFC}\) was 0.56 mol m\({}^{-2}\) s\({}^{-1}\) and aerodynamic G\({}_{\rm SFC}\) was 0.75 mol m\({}^{-2}\) s\({}^{-1}\), thus using T\({}_{\rm R}\) instead of T\({}_{\rm Aero}\) to estimate surface G\({}_{\rm SFC}\) of soybean resulted in a larger (\(-\)34%) error. The corresponding radiometric and aerodynamic values of canopy G\({}_{\rm S}\) were 0.7 mol m\({}^{-2}\) s\({}^{-1}\) and 1.1 mol m\({}^{-2}\) s\({}^{-1}\), a difference of 49%.  
In wheat, the sensitivity of G\({}_{\rm SFC}\) to errors in g\({}_{\rm A}\) was examined by comparing the measured G\({}_{\rm SFC}\) with the G\({}_{\rm SFC}\) obtained from the observed G\({}_{\rm S}\) and a typical value of field g\({}_{\rm A}\) reported in the literature (g\({}_{\rm A}\) = 2 mol m\({}^{-2}\) s\({}^{-1}\); [44]). This field value of g\({}_{\rm A}\) corresponds to less turbulent conditions (a smaller g\({}_{\rm A}\)) than were observed for wheat in the chamber, and it is closer to the g\({}_{\rm A}\) of soybean. The G\({}_{\rm SFC}\) calculated by inverting Equation (9) using measured G\({}_{\rm S}\) and field g\({}_{\rm A}\) (Figure 8A; dashed line) underestimates the measured G\({}_{\rm SFC}\) (Figure 8A; green line) by 33%. This analysis shows that large differences in surface G\({}_{\rm SFC}\) exist between field settings and controlled-environment chambers and that these occur because of differences in turbulence that can be accounted for only when g\({}_{\rm A}\) is known.  
The canopy g\({}_{\rm A}\) and G\({}_{\rm S}\) for wheat (18-day-old; [CO\({}_{2}\)] = 400 \(\mu\)mol mol\({}^{-1}\); PPF = 1600 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\); T\({}_{\rm air}\) = 21 \({}^{\circ}\)C; RH = 68%) and soybean (25-day-old; [CO\({}_{2}\)] = 400 \(\mu\)mol mol\({}^{-1}\); PPF = 750 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\); T\({}_{\rm air}\) = 21 \({}^{\circ}\)C; RH = 64%) canopies are reported in Table 2.