A dynamic model of crop growth and partitioning of biomass

A model is presented of growth and partitioning to leaves, stems and roots in herbaceous, vegetative crops in response to atmospheric conditions and water supply. It comprises 12 state variables and 33 parameters (including four functional relationships), all of direct physiological significance. The important characteristic of the model is the simultaneous consideration of crop assimilate and water balances achieved by calculations made at short time steps (1 h or less) in order to capture the physiological responses of crop growth and water use as they respond to diurnal environmental patterns.In the model, root-zone water content decreases with transpiration and soil evaporation, and increases with rainfall, irrigation and deepening of the root zone as the crop develops. Photosynthesis depends upon intercepted radiation and temperature and also on canopy conductance determined from crop water status. Respiration of organs is calculated as separate requirements for maintenance and growth. Transpiration proceeds with photosynthesis but in response to evaporative demand, reducing crop water content, which is in turn replenished from the root zone based on its water content and the root length that explores it. Partitioning of assimilate to leaves, stems, and roots depends upon diurnal oscillations in assimilate supply, temperature, and crop water content within limits set by phenological development. Phenological development, here the initiation and expansion of leaves and the maturity and senescence of canopy and root systems, is determined by temperature. Examples, and trends, of model performance are compared with measured physiological and agronomic responses of sunflower to strategies of irrigation.