Canopy-Scale Model Based on Optimization of Stomatal Conductance to Simulate Water and Carbon Fluxes in Crops.

Plants adjust themselves to fluctuating environmental conditions by regulation of stomatal conductance (g_s), the process that controls diffusion of CO2 from the atmosphere into the leaf and water loss through transpiration. In most canopy-scale vegetation models, the description of this process is based on empirical parameters, which have to be calibrated to the specific site conditions where the model applied. The hypothesis that stomata adapt optimally to its environment to maximize assimilation (A) for a given amount of water loss through transpiration (T) was introduced by Cowan and Farquhar (1977). This theory provides a framework for modeling the interactions between vegetation dynamics and soil moisture that does not rely on empirical calibration as long as photosynthetic canopy properties and total amount of water available for transpiration are known. The current study introduces a new approach to implement optimization theory of stomatal conductance into a canopy gas exchange model. The adequacy of the new approach was tested in a case study by comparing predicted diurnal cycles of assimilation and transpiration rates with observations on different crop species from Katharinenthalerhof in southwest Germany (2009 -2011). Transpiration and assimilation rates were derived from eddy-covariance measurements of latent heat flux (ET = E+T) and net ecosystem exchange (NEE = A+R). Partitioning between T and soil evaporation (E) and between ecosystem respiration (R) and A was estimated by the approaches proposed by ShuttleworthWallace (1985) and Lloyd and Taylor model (1994), respectively. In our model, g_s is derived from the maximization of L = A(g_s) – λT(g_s), where daily values of the marginal water use efficiency, λ (= ∂A / ∂E) were determined using the formalism of Lagrangian multipliers. The present study shows that the diurnal cycles of transpiration and assimilation can be delineated from optimal water-use hypothesis for different crop species and fluctuating soil moisture availability.