A new, mechanistic, modular computer model of cotton (Gossypium hirsutum L.) physiology, development, growth, and productivity is being constructed as a component of the USDOE program of research into the effects of increasing CO₂ on vegetation. For such a model to be successful, and for the uses of the model to be consistent with the capabilities of the model, explicit model objectives must be defined. These objectives are (1) the prediction of cotton crop responses (e.g., changes in growth or water use) to increased atmospheric CO₂ concentration per se, and (2) the prediction of cotton responses to climate changes that may occur as a result of the so-called greenhouse effect associated with the annually increasing CO₂ concentration of the atmosphere. Consequently, the model has been named COTCO₂ (cotton response to CO₂).

The objectives of this project imply that the COTCO₂ model must extrapolate beyond the data available for model development, because plants are not now growing under the CO₂ conditions of interest. The fundamental requirement of such models is the use of explicit physiological mechanisms, so as not to rely on empirical formulas that may not apply to the (unknown) future environment. Therefore, the COTCO₂ model is based on physiologically mechanistic principles wherever possible. There are constraints with such an approach, but the explicit goal of this project is the development of a biological (mechanistic), as opposed to a statistical (empirical), model of cotton physiology and growth. Although the model is one of cotton, its general structure and many specific components will be applicable to other crops and plants in general.

A working version of the model, coded in FORTRAN, now exists which gives excellent results. Future model development will be concerned with incorporating more physiological detail, and flexibility, into the model. Finally, more extensive comparisons of model solutions and field and glasshouse observations will be made.