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## Running a simulation

Figure 2.14: Model Menu

We are now entering the Model menu. This menu covers two further dialogs on entering model specific parameters.

The Model Selection allows you to select the complexity modes for the three submodels: sewer, WWTP and river. The text field on the right provides some hints on the selected mode. If you change complexity modes, it is usually necessary to go back to earlier data dialogs to enter additional parameters. E.g. if you need in-sewer removal to be considered, you have to go back to the substance data dialog to enter a removal rate for sewers. Please note that in-sewer removal is the only higher model in which only one parameter has to be entered. All other submodels require a somewhat larger data set.

As mentioned earlier, GREAT-ER uses the Monte-Carlo method to set a stochastic approach on top of a deterministic model. The number of Monte-Carlo shots performed during a simulation also has to be entered within the Model Selection dialog. For a sample application take thousand shots.

The Model Parameters dialog allows you to edit some model-specific parameters related to the environment in general and to sewer, WWTP and river in detail. The dialog structure and functionality is similar to the substance dialog. One difference is the "Default" button. Whenever substance data is very specific and no usual values can be given, some of the model-related parameters are inconvenient but can be seen as general for preliminary overview. To make the use of higher model complexity modes somewhat easier, default values can be loaded from the GREAT-ER databases. None of these parameters are needed for simulations on complexity mode one. Please note, that each default value has to be set explicitly. If you want a set of all default values to be used for some scenarios this is currently only possible by a workaround: Create a generic scenario without substance or catchment. All further scenarios can be created from this generic one by saving it to a new scenario (use Save As).

Figure 2.15: Model Selection Dialog

Background Concentrations are included to consider additional loads within the river network which might be monitored upstream from the first known discharge. Note that the given value is used as it is and is only added to the model results after the simulation. The background concentration is not considered within the elimination processes.

Figure 2.16: Background Concentration Dialog

You are now able to start a simulation. First all data needed for the simulation are exported. Depending on your computer system, this could take a while. Finally, a new window appears, displaying the progress of the simulation:

Figure 2.17: Running simulation on Aire catchment

Due to some system timings it is not possible to change to another window during this initial phase. After a short delay, window control becomes available again and you may change back to the ARCVIEW window, running the simulation in the background. A remark `[running]` in the window title bar identifies that a simulation is running for the active scenario. You may now proceed to work with another scenario or even on the scenario view currently under simulation. But obviously you should not modify scenario parameters while a simulation is running. You should also keep in mind that working on a view is a very power-consuming task and will therefore increase the simulation running time.

After the simulation has finished, the results will be displayed on the scenario's view: The river stretches on the scenario's view will be coloured depending on the model results. A new theme is visible in the view's table of contents entitled Csim mean [unit]. Csim stands for simulated concentration (in contrast to monitored concentration), mean identifies that the mean value of the concentration distribution per stretch, resulting from the Monte-Carlo method, is displayed. More specifically, the mean values of the C sim, internal are displayed. Please refer to the model description to learn more about the various result outputs of the GREAT-ER model system.

Figure 2.18: Visualisation of simulation results for the Aire catchment

If you need to abort a simulation for some reason, click on the simulation progress window. A dialog appears, asking you if the simulation should be continued.

Figure 2.19: Process of simulation abortion

If you then select "No", the simulation will be aborted and a report will inform you about the message sent back from the simulation system to the GREAT-ER user interface:

Figure 2.20: Process of simulation abortion

The meaning of the menu item Stop Simulation is not the same. This item is a kind of a safety guard: simualtions may crash (identified for example, by an error message titled `rsxnt`) for some reasons, e.g. a very large number of Monte-Carlo shots or an unknown system instability To proceed with GREAT-ER to save your open scenarios before closing (which is strongly recommended) you should select the Stop Simulation menu item. This will remove the `[running]` remark and will change some settings behind the scenes of the user interface. Again, after selecting the item you should close GREAT-ER and restart.

- Save a scenario
  - Save
  - Save As

Next: Save a scenario Up: Tutorial Previous: Editing catchment data