Predicting transpiration rates of hydroponically grown plant communities in controlled environments

Canopy transpiration is a major factor determining crop evapotranspiration and energy budgets. Unfortunately the development of robust models of canopy transpiration is hindered by a lack of reliable data due to the difficulties of making canopy-scale measurements. However, measurements of canopy water vapor and carbon fluxes via gas exchange techniques are possible in controlled environments. Simultaneous measurements of transpiration, photosynthesis, and canopy temperature were made in wheat and soybean communities. These data were used to calculate chamber aerodynamic and canopy stomatal conductances, and to model the response of canopy transpiration to CO₂ concentration and vapor pressure deficit. Canopy stomatal conductance was found to decrease diurnally by 20–30% in well-watered crops grown under constant environmental conditions. The magnitude of this diurnal decrease in the canopy stomatal conductance of wheat and soybean decreased with increasing ambient CO ₂ concentrations. Eight models describing how canopy stomatal conductance responds to environmental changes were incorporated into a canopy transpiration model. The results and methods developed in this study will allow future physiologically-based canopy transpiration models to incorporate these models for predicting the response of transpiration rates in controlled environments.