Any model we run on a computer comes as a piece of software. So in some cases to solve a particular modeling task we may try to find an appropriate model that has been developed previously for a similar case, and see if this software package, if available, can be adapted to the needs of this project. Some models are distributed for a price; others are available for free. The Register of Ecological Models (REM - http://eco.wiz.uni-kassel.de/) is a meta-database for models in ecology. It can be a good starting point if one is looking for a particular model. In some cases one will be able to download the executables from the website, otherwise one will have to contact the authors. For the vast majority of models the source code is unlikely to be available and we can never be sure what actually goes on inside the processor. We can only look at the output, at the documentation, run scenarios, analyze trends, but we ultimately have to trust the model developers in that the model is programmed properly. Also, we can make no changes to this kind of model.
The fact that models come as software black boxes may be one of the reasons that model reuse is not very common. It may take a long time to learn and understand an off-the-shelf model, and it may be quite frustrating if after this investment we find out that the model is not quite applicable to our case. It certainly helps when models are well documented and have good user guides, tutorials, and come with nice graphic user interfaces (GUIs). Most of the models that are commercially distributed have very slick GUIs that help set up these tools for particular applications. For example, the WEAP (Water Evaluation and Planning system - http:// www.weap21.org) is a user-friendly software tool that helps with an integrated approach to water resources planning. The core of the model is a water balance model that calculates the dynamics of supply and demand in a river system. To set up the model the user is guided through a series of screens, which start from a river schematic that can be arranged on top of an ArcView map, and then takes care of data input with a series of dialogue boxes for water use, loss and reuse, demand management, priorities, etc. The results are then displayed in the same GUI in charts and tables and on the schematic of the river system. Scenarios that describe different demand and supply measures are driving the system, and are connected with the various results.
These user interfaces certainly help in the use of the models; however, extending the model capabilities is not a straightforward task, if at all possible. Especially when
General purpose Computer Languages and Libraries
Swarm, Repast, MASON, Cormas OpenMI, SME, SAMT
DYNAMO, CSMP, SYSL NetLogo, StarLogo
Excel, Lotus123, OpenOffice
Extend, Goldsim, SimuLink SIMSAB, SONCHES Simile
Powersim, Madonna, Stella, Vensim, ModelMaker
Glumso, BALSECT, SimCity
Figure 1 Software tools span the whole continuum between conceptual models and computer code. In most cases, the higher up this diagram you get the harder it is to master the software, the less user friendliness you find, and the more flexibility, versatility, and computing power you get.
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