APSIM: a novel software system for model development, model testing and simulation in agricultural systems research

APSIM (Agricultural Production Systems Simulator) is a software system which allows (a) models of crop and pasture production, residue decomposition, soil water and nutrient flow, and erosion to be readily re-configured to simulate various production systems and (b) soil and crop management to be dynamically simulated using conditional rules. A key innovation is change from a core concept of a crop responding to resource supplies to that of a soil responding to weather, management and crops. While this achieves a sound logical structure for improved simulation of soil management and long-term change in the soil resource, it does so without loss of sensitivity in simulating crop yields. This concept is implemented using a program structure in which all modules (e.g. growth of specific crops, soil water, soil N, erosion) communicate with each other only by messages passed via a central ‘engine’. Using a standard interface design, this design enables easy removal, replacement, or exchange of modules without disruption to the operation of the system. Simulation of crop sequences and multiple crops are achieved by managing connection of crop growth modules to the engine. A shell of software tools has been developed within a WINDOWS environment which includes user-installed editor, linker, compiler, testbed generator, graphics, database and version control software. While the engine and modules are coded in FORTRAN, the Shell is in C++. The resulting product is one in which the functions are coded in the language most familiar to the developers of scientific modules but provides many of the features of object oriented programming. The Shell is written to be aware of UNIX operating systems and be capable of using the processor on UNIX workstations.