http://www.epa.gov/OST/BASINS/
http://wwwbrr.cr.usgs.gov/mms/
To address the problems of model selection, application, and analysis, a set of modular modeling tools, termed the Modular Modeling System (MMS) is being developed by the NRP Precipitation-Runoff Modeling Project. The approach being applied in developing MMS is to enable a user to selectively couple the most appropriate process algorithms from applicable models to create an "optimal" model for the desired application. Where existing algorithms are not appropriate, new algorithms can be developed and easily added to the system. This modular approach to model development and application provides a flexible method for identifying the most appropriate modeling approaches given a specific set of user needs and constraints.
MMS is an integrated system of computer software developed to (1) provide the research and operational framework needed to enhance development, testing, and evaluation of physical-process algorithms; (2) facilitate integration of user-selected algorithms into operational physical-process models; and (3) provide a common framework in which to apply historic or new models and analyze their results. MMS uses a module library that contains modules for simulating a variety physical processes. In the context of current development work, these are water, energy, chemical, and biological processes. A model is created by selectively coupling appropriate modules from the library to create a suitable model for a desired application. When existing modules do not provide appropriate process algorithms, new modules can be developed.
MMS has three major components: pre-process, model, and post-process (fig. 1.1). The pre-process component includes tools used to input, analyze, and prepare spatial and time-series data for use in model applications. The model component includes the tools to develop and apply models. The post-process component provides tools to display and analyze model results, and to pass results to management models or other types of software. A system supervisor, in the form of an X-window graphical user interface (GUI), provides users access to all components and features of MMS. MMS was developed for UNIX-based workstations and uses X-windows and Motif for the GUI.
By definition, a module is a set of computer source code used to simulate one, or more, water, energy, chemical, and biological processes. A given process, or combination of processes, can have several modules in the library, each representing an alternative conceptualization or approach to simulating the process(es). Each module typically needs selected inputs to drive it and computes selected outputs that could be used as inputs to other modules.
http://wwwbrr.cr.usgs.gov/mms/html/chap3.frame.new.html#HDR1
For a researcher a model is a tool for understanding the system. By plugging together a number of black boxes, which specifics and behavior are obscure and hardly understood, we do not significantly increase our knowledge about the system. From a complex natural system we go to another system that may be also too complex to deal with. The results generated are difficult to interpret, when there is not enough understanding of the processes that are actually modeled. This is especially true for ecological and socio-economic systems, which are hard to structure and decompose. The decomposition of such systems requires careful analysis of spatial and temporal scales of processes considered and is closely related to specific goals of the model built. In this context the modular approach is useful, if the focus is shifted from reusability and "plug-and-play" to transparency, analysis and hierarchical description of various processes and system components. With the modules being transparent and open for experiment and analysis, the researcher can better understand the specifics of the model formalism that is inherited. It is easier to decide whether the modules provided are suitable or if they should be modified to match the specifics and goals of the concrete study. It is mostly for this reason that when possible, LHEM offers modules in STELLA. We recommend the Spatial Modeling Environment (SME) to put modules together and to run them locally or spatially in conjuncture with the "user code" modules.