Simulation of Evapotranspiration and Its Response to Plant Water and CO2 Transfer Dynamics.

Evapotranspiration (ET) is controlled by atmospheric demand, plant and soil hydraulic constraints, and the plant physiological activities that determine canopy resistance. This paper introduces a new ET scheme developed for the Ecological Assimilation of Land and Climate Observations (EALCO) model that integrates these controls into one dynamic system. This scheme is based on solving the governing equation system that represents the coupled canopy energy–water–CO₂ transfer dynamics, where the canopy temperature T_c, plant water potential ψ_c, and leaf intercellular CO₂ concentrations C_i are simultaneously obtained and used in ET calculations. Modeled ET was compared with eddy correlation flux measurement at a boreal aspen forest. Results showed that the correlation coefficient (R) between modeled and measured daily ET was greater than 0.96. The average absolute error was approximately 0.3 mm day^-1. Modeled ET was generally higher than measured ET by 10%. This is consistent with the energy balance closure analyses from observations that showed that turbulent energy flux was frequently less than 90% of the total available energy. The effects of the plant CO₂ and water transfer dynamics on ET simulations were investigated by running the model in two additional settings. These were 1) static C_i—where the ratio of C_i to atmospheric CO₂ concentration was set to a constant value, and 2) static ψ_c—where the ψ_c was linearly related to soil water potential. The dynamic CO₂ transfer scheme and the static C_i scheme produced relatively small differences in ET that mainly occurred at a subdaily time scale. Differences in ET produced using the dynamic water transfer scheme and the static ψ_c scheme depended on ecosystem water conditions and were more significant when the plant was under water stress. Ignoring the dynamic water transfer process in the model decreased the correlation coefficient between modeled and measured ET more significantly in drier years. This implies that the dynamic water transfer scheme is of more importance for ET estimates in arid or semiarid ecosystems. [ABSTRACT FROM AUTHOR]