SIMULATION

MEDECS, the mediterranean ecosystem simulator, was developed to model the dynamics of mediterranean- type vegetation in southern California and central Chile. MEDECS is a difference/differential-equation model which simulates the seasonal patterns of resource use and plant response in one-day time steps. It uses submodels of plant growth, decompo sition, and inorganic nutrient release to describe the effects of physiological processes, nutrient availability, and environmental parameters. MEDECS is a compartmental model of the producer-decomposer system. In use, it simulates the seasonal progres sion of changes in each of the compartments. It is being used in on-going research to examine hypotheses about single-species and multiple-species (competi tive) communities of plants.

After reviewing existing approaches to the general stochastic programming problem, an improved experi mental method is proposed. This method uses a va riety of mathematical programming algorithms and any desired pattern of parameter variation. Statistical analysis of the results allows decision-makers to make probabilistic statements about the values of the decision variables and of the objective function. Illustrative examples are given.

This paper reports an analogue-logical simulation study of the performance of a hydraulic servo posi tion-control system. The system has positional limits at which the velocity of the servo output becomes zero. Pairs of logic-controlled integrators are used to simulate these limits without generating a velocity transient. Minimised logic expressions control the integrators. Test results provide examples of system performance.