# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 1 Introduction
However, canopy g\({}_{\text{A}}\) determined from changes in radiometric canopy temperature differs from g\({}_{\text{A}}\) determined using the log-wind profile approximation because it includes the conductance to heat and water vapor across leaf boundary layers, as well as the turbulent conductance caused by the movement of air eddies between the canopy and the atmosphere [17; 18]. Canopy G\({}_{\text{S}}\) obtained from energy balance approaches may contain considerable errors because Ds and LE are estimated using measurements of canopy radiometric temperature (T\({}_{\text{R}}\)) to approximate the aerodynamic canopy temperature [19; 20; 21]. In the field, estimating T\({}_{\text{Aero}}\) from infrared measurements is complicated because radiometric measurements depend on the view angle of the sensor, sun angle, degree of crop cover, spatial variability of canopy emissivity, and atmospheric attenuation, and they often include significant temperature contributions from soil surfaces [22; 23; 24; 20; 25]. A systematic difference of \(-1\)\({}^{\circ}\)C was measured between radiometric and aerodynamic temperatures by [PERSON] and [PERSON] [26], although differences ranging from 2 to 6 \({}^{\circ}\)C have also been observed [13]. The difference between T\({}_{\text{Aero}}\) and T\({}_{\text{R}}\) can be very small in dense canopies, but it can exceed 10 \({}^{\circ}\)C in sparse vegetation because of contributions from soil temperature ([18; 27]. These differences are significant because small errors of \(-1\)\({}^{\circ}\)C in the surface-to-air temperature difference can represent an uncertainty in latent heat fluxes of \(\sim\)40 W m\({}^{-2}\)[13]. Many complicating factors that affect infrared measurements of canopy temperature in field settings can be minimized in controlled environments by using high planting density canopies grown under constant lighting. In dense canopies, canopy brightness temperature measured with infrared sensors approximates canopy radiometric temperature [28], but errors due to radiation reflected into the sensor and artifacts caused by fluctuating sensor body temperatures remain [29]. The purpose of this study was to develop a radiometric method for measuring canopy \(\mathrm{G_{S}}\) of well-watered plant canopies in controlled environments. The hypothesis tested was that a radiometric method utilizing canopy-level energy balance measurements provides more accurate estimates of canopy stomatal conductance than bottom-up methods scaling leaf-level to canopy-level conductance or top-down methods that estimate canopy surface conductance from field data. Bottom-up methods require that leaf area index be known and must integrate the responses of leaf stomatal conductance to vertical gradients in radiation, temperature, and humidity. Conductances from top-down methods using field data typically include significant contributions of soil evaporation, and field radiometric data include soil surface temperatures that cause significant differences between radiometric and aerodynamic temperatures [13]. Simultaneous measurements of energy balance, gas fluxes, and canopy temperature at constant environmental conditions were used to compute canopy \(\mathrm{G_{S}}\) from surface \(\mathrm{G_{SFC}}\) and canopy \(\mathrm{g_{A}}\) (Figure 1). The relation between radiometric and aerodynamic temperatures was studied by varying incident radiation and wind speed. Canopy \(\mathrm{G_{S}}\) and \(\mathrm{g_{A}}\) of high planting density wheat (_Trticulum aestivum_ L. cv. USU Apogee) and soybean (_Glycine max_ L. cv. Hoyt) canopies were measured at 400 umol mol\({}^{-1}\) CO\({}_{2}\). The radiometric method was used to explore the effects of rising CO\({}_{2}\) concentration on canopy \(\mathrm{G_{S}}\) and to describe stomatal feedbacks to transpiration using the canopy-scale decoupling coefficient.