Adding Where a parameter cannot be defined using a function using the main user interface, adding usages manually allows you use a function to define almost any parameter in the model that doesn't specifically allow for it in the main user interface. For example you can make otherwise static parameters change over time like rainfall runoff parameters.
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Suppose you want to calibrate the storage routing exponent (m) on one of the reaches in a Source model. Initially, identify which link or node in the model you want to place the expression in. Then add the function usage manually to allocate a parameter to the variable (steps outlined in Figure 1). You You can now use the command line to the Command Line Runner to run the model many times iteratively while varying the parameter:Command Line Runner. This is also used for setting up a project to run with Insight: Objective optimisation.