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How do I model a system where there is intermittent demand or where demand is being cut off from it's water source at some point by closing a value?
There are two ways to simulate this situation.
1) Set up a hydraulic pattern on your demand nodes then at the point the demands are being cut off from the system give the pattern a 0 multiplier. See screen shot below for an example.
2) If you are using a throttle control valve (TCV) to cut off the flow from the system by closing the valve using a relative closure pattern then be sure to apply pressure dependent demands to the area of your model where the demand is dropping to 0. You can find information for how to setup pressure dependent demands from this wiki:
The model from the link below will help convey the setup for each of these situations.
Once the model file is downloaded you can read over the project properties (File > Project Properties...) for some details about each scenario.
If you are simulating a case where pipes may not be flowing full all the time, Bentley SewerGEMS may be a better solution, as it can account for filling and draining and partly full pipes. The numerical solver used in WaterCAD and WaterGEMS is based on pipes flowing full. In SewerGEMS, you would place an inflow at the source and model the users as outfalls, usually at the elevation of their rooftop tanks. Try this approach on a small part of the system to test the work flow before trying to run a big model.