Scientific computer models are most often implemented in FORTRAN, Pascal, and C, which are general computer languages. When a researcher other than the model author sets out to use a model, he or she is often faced with problems in understanding how that code works. These problems include:

- Implicit physical units; unit conversions mixed in with equations
- Lack of documentation on procedures and functions, variables, scaling or fudge factors, and equations
- Limited descriptive ability of variable names
- Unstated or implicit assumptions about the scale and domain
- Unclear differences between scientific (i.e. theoretical equations) and implementation details (i.e. control and data structures)

Ecosystem modellers need better technology to facilitate building, sharing and reusing their models. The Scientists' Intelligent Graphical Modelling Assistant (SIGMA) creates an environment for model building, sharing and reuse which provides an alternative to more conventional approaches which too often yield poorly documented, awkwardly structured model code.

Dr. Richard Keller and colleagues in the
Computational Sciences Division
have built SIGMA to address the abovementioned problems with scientific model implementation.
The SIGMA interface provides a list of model quantities which can
be selected for computation. Equations to calculate the model
quantities may be chosen from an existing
**library**
of ecosystem modelling equations, or built using a specialized
**equation editor**
Inputs for these equations may be supplied by data or by calculation
from other equations. Each variable and equation is expressed using
ecological terminology and scientific units, and is documented with
explanatory descriptions and optional literature citations. Automatic
scientific
**unit conversion**
is supported and only physically-consistent
equations are accepted by the system. The system uses knowledge-based
semantic conditions to decide which equations in its library make sense
to apply in a given situation, and supplies these to the user for
selection. The equations and variables are graphically represented as a
**data flow diagram**
which provides a complete summary of the model.

**Forest-BGC**
(Running and Coughlan 1988), a stand-level model that
simulates photosynthesis and evapotranspiration for conifer canopies,
was originally implemented in Fortran and subsequently reimplemented
using SIGMA. The SIGMA version reproduces daily results and also
provides a knowledge base which greatly facilitates inspection,
modification and extension of Forest-BGC.

Contact:

Jennifer Dungan, JCWS, Inc.

**Email**: jdungan@gaia.arc.nasa.gov

**USmail**:
Mail Stop 242-4

NASA Ames Research Center

Moffett Field, CA 94035-1000

**Phone**: (650) 604-3618

**FAX**: (650) 604-4680

Dr. Richard Keller is the principal investigator on the project.

A fuller description of the SIGMA system, including technical reports, a tutorial and other applications can be reached from the SIGMA home page

SIGMA is funded by NASA's Applied Information Systems Research and Mission Operations programs.

Ames Research Center Home Page ECOSAT Home Page

Information last updated: 7/26/95