A systematic approach to the simulation of short‐term processes in the plant‐environment complex

. A conceptual framework is presented for modelling short‐term processes in the plant and its environment as an integrated system. Flows of water, water vapour, heat, momentum, CO 2 , soluble carbohydrate and phosphorus are all described by equations of the same general type, i.e. in terms of diffusivity‐type parameters, capacities and potential gradients. A representative volume of the crop is divided horizontally into layers and vertically between crop and environment for treatment by a finite‐difference method. Vertical flow occurs in the atmosphere, soil, stems and larger roots, andilateral flow between leaves and air, and between finer roots and soil. The interception of direct sunlight and the flux densities of downward and upward diffuse radiation within layers are calculated by a step‐wise procedure. The conversions of materials within the plant are treated as functions of appropriate state variables. Schemes for carbon and phosphorus provide for flow to and from the translocation system, and for photosynthesis, respiration and growth. A model of a fully‐established lucerne crop is described and the sensitivity of model performance to changes in a number of parameter values explored. Simulation runs under varying conditions indicate realistic prediction of diurnal trends.