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Appendix 1. The Modelica, MODCOM and IMA modelling
frameworks
A1.1 Modelica
Modelica [see URLs] began life in an ESPRIT project "Simulation in Europe, Basic Research
Working Group" (SiE-WG). It involved a number of developers of object-oriented modelling
software collaborating in the development for a common specification for physical systems
modelling (e.g. electrical, mechanical and hydraulic systems). It supports the idea of hierarchical
composition: a component can be specified in terms of basic components (e.g. resistor, capacitor),
and that component can then itself be used as a building block. Mathematically, Modelica
models represent sets of differential-algebraic equations, extended to allow for discrete events
(such as a relay switching off or on).
It is important to note that Modelica is a specification for a representational language: it is not a
software product. Instead, particular software developers will develop tools capable of handling
models specified in the Modelica language. These tools can include diagramming tools to
develop a model by selecting icons representing basic components and linking them together,
tools for reporting on the model in various ways, and tools for actually undertaking simulations.
The Modelica language itself and many of the component libraries are free; a number of software
houses have developed commercial modelling environments that comply with the Modelica
specification. Modelica is capable of specifying models with hundreds of thousands of equations.
It is also capable of handling array data structures, enabling disaggregated models to be handled.
Modelica essentially operates at two levels. Most users will build models by selecting and
linking pre-defined components, as described above, setting parameter values appropriate to that
component (e.g. resistance for a resistor). This is an intuitive method of working, matching
closely to the physical act of building an engineered object by selecting physical components with
the appropriate attributes. At a lower level, defining the mathematical properties of the
component is a specialised activity, and not one that most modellers would engage in.
It is tempting to consider Modelica as a standard, off-the-shelf solution to the needs of ecosystem
modelling. Many ecosystem models also consist of DAEs, with array-style disaggregation. The
concept of composition applies equally well to ecosystems, and support of re-use of pre-built
components should prevent duplication of effort between various groups.
However, my understanding of Modelica is that it is inappropriate for ecosystem modelling, for
several reasons. First, specifying equations for relationships between the model variables is much
more part of the modelling process in ecosystem modelling. We cannot expect modellers to work
with just the set of equations entered into a library, no matter how comprehensive this might be.
Second, the notion of discrete, basic building blocks is much more appropriate for modelling
engineered systems than for natural systems. Third, Modelica currently has no mechanism for
specifying dynamically-varying populations of objects - an important requirement for ecosystem
modelling.
A1.2 MODCOM
MODCOM [see URLs] is being developed jointly by workers in Oregon State University (USA)
and Plant Research International at Wageningen (Netherlands) as a framework for ecosystem
modelling. It is described thus by its developers:
"MODCOM is a tool to assist modellers in creating a broad range of object-oriented simulation
models by either creating reusable components, or assembling pre-existing components into