We can combine the use of the time function with the sketched graph. This is very useful when you have a historical record for some external factor, e.g. rainfall or temperature: the graph you sketch is a copy of the historical record.

For example, to use a sketch of how rainfall varies throughout the year in your model, you first create a variable called “rainfall”. Double-click on this variable to set its properties, and click on the “Graph” button. This opens the sketch pad window.

Enter the range on the X-axis, from a minimum of zero to a maximum of 12 which corresponds to one year, if the time unit is one month.(in general, the maximum value on the X-axis will correspond to the length of time you want the simulation to run for) Enter the range of the Y-axis from, for example, zero to 30, depending on how rainy your locality is.

Sketch a curve with perhaps two well-defined peaks, to enable you to see the response clearly when you run the model. Use the on-line help if you're unsure of how to draw the curve.

Click “OK” and return to the equation dialogue box. You’ll notice that the function name *graph( )* has been entered in the equation. To complete the expression, use the *time( )* function as its argument. The expression should read “graph(time())”.

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