# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 1 Introduction  
Understanding boundary layer and land surface feedbacks on canopy transpiration is essential for developing simpler and realistic climate change models and for improving the prediction of ecosystem-level water fluxes in response to climatic variables [1; 2]. Canopy stomatal conductance (G\({}_{5}\)), a key physiological factor controlling transpiration from plant stands, is an important component of land surface feedbacks because it regulates evapotranspiration and surface temperature changes in response to incident radiation, CO\({}_{2}\) concentration, and vapor pressure deficit (VPD). This regulatory function is reflected in canopy temperature, which in turn, determines the magnitude and directionof sensible heat exchange between the vegetation and its environment. At regional scales, stomata exert little control, and daily transpiration of well-watered vegetation is predominantly controlled by radiation and temperature [3; 4; 5], in part due to feedbacks that cannot be predicted from single leaf measurements alone [3]. Since canopy-scale transpiration is determined by the ratio between canopy aerodynamic conductance (g\({}_{\text{A}}\)) and G\({}_{\text{S}}\)[4; 6; 7], improved methods for measuring g\({}_{\text{A}}\), as well as measuring responses of G\({}_{\text{S}}\) to environmental variables (e.g., light, CO\({}_{2}\), VPD, soil moisture, and temperature), are needed for studying the processes controlling feedback and stomatal control of evaporation from regional land surfaces. In the field, g\({}_{\text{A}}\) is often approximated by the conductance to momentum transfer determined using the log-wind profile approximation, which requires at least 100 m of fetch and thus cannot be used in controlled-environment chambers [8]. In controlled environments, leaf boundary conductance has been estimated from measurements with wet filter paper analogs [9], from cooling curves of metal models of leaves [10], or from combined energy balance and temperature measurements using metal leaf models [11; 12]. However, [PERSON] and [PERSON] [3] argue that leaf level measurements of stomatal control of transpiration may not be applicable to plant canopies in the field because the amount of ventilation in leaf cuvettes and plant chambers typically prevents feedback between transpiration and VPD observed in the field. G\({}_{\text{S}}\) can be derived using energy balance approaches from canopy surface conductance to water vapor (G\({}_{\text{SFC}}\)), latent heat flux (LE), and the VPD at the leaf surface (Ds). Similarly, the single-layer or \"big leaf\" G\({}_{\text{S}}\) may be computed from G\({}_{\text{SFC}}\), provided the boundary layer conductance to water vapor (i.e., g\({}_{\text{A}}\)) and the mean aerodynamic canopy temperature (T\({}_{\text{Aero}}\)) are known [13; 14]. In the field, G\({}_{\text{SFC}}\) is calculated from canopy-level LE obtained using lysimeters, Bowen ratio, and eddy correlation systems [15], or by inverting the Penman-Monteith equation [13]. However, these approaches for measuring canopy-level LE do not permit partitioning of transpiration among individual species and often cannot distinguish between transpiration and evaporation from the soil or from wet leaf surfaces. Thus, G\({}_{\text{SFC}}\) is not always related to estimates of canopy G\({}_{\text{S}}\) derived from single leaf measurements because it often includes significant contributions from soil evaporation [13]. [PERSON] et al. [16] used canopy-level energy balance measurements to estimate sensible heat flux (H) of a wheat field from radiometric canopy temperature when canopy g\({}_{\text{A}}\) and LE were known. Their approach produced accurate estimates of hourly LE, which suggests that g\({}_{\text{A}}\) could be estimated if H and the canopy-to-air temperature difference are measured accurately.