Models a fluid reservoir system using the current version of BIOMOD, an interactive computer-graphics system for biological modeling. The BIOMOD system features interaction with immediate feedback, hierarchical model structuring, and user-oriented model-definition languages. It operates on an interactive graphics console comprising a CRT screen, a RAND Tablet, and keyboard. The user may draw block diagrams, handprint or type text, push displayed "buttons" to initiate certain actions, and move labels. He may represent a model by a block diagram, each component of which may be defined by another block diagram. This facilitates devising complex models one portion at a time. When BIOMOD is completed, the user may define model components by chemical equations, differential equations, or data curves, rather than having to translate his model description into a conventional simulation language. During model simulation, the user may display curves for different variables, change scales, or alter simulation parameters. 41 pp. Ref.