In this study, an environmental variable model for estimating stomatal conductance of maize (Zea mays L.) leaves over a long term is developed, based on research results concerning responses of stomatal conductance to environmental variables. This model is actually a combination of optimized formulae for potential stomatal conductance during daytime, PSC, and for the relative degree of stomatal opening during daytime, RDO, with a basic form expressed as a multiplication of inter-day and intra-day factors. We call this environmental variable model as the combination model. Compared with other models presented in the past, this combination model is convenient in practical use because it can estimate not only maximum and average values but also instantaneous values of stomatal conductance during daytime. Also, the theoretical description of this model has been clarified by introducing into it the concept of environmental stress. As compared with Jarvis-type multiplication environmental variable models, we think this combination model is a refinement and development of the Jarvis-type models. (C) 1998 Elsevier Science B.V. All rights reserved.
