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Demand distribution

If i distribute demands proportianately among nodes both the upstream node and the downstream node will get a proportional percentage of that demand. But in a system which is branched meaning that there are no loops and water only flows in one direction  (downstream) shouldn't all demands be assigned to the upstream node  ? And so how do I make sure that all demands are assigned to the upstream node?

  • So a requirement for the model I am creating is that the model retains a 1 to 1 relationship with the GIS map of the network. Now in the GIS map there are branches connected to pipes but the pipes in GIS have not been snapped at the point where the branch connects to the pipe (see image below). Now what I want to do is simply replace the branch with a lateral with the same demand as the entire branch. However the question that arises is, at what point (at how much demand) does it impact the accuracy of your model when distributing among nodes. Or in other words how much demand can you distribute among nodes before it significantly affects the accuracy of your model?

  • It would still make sense to divide the demand proportionally by pipe length. Imagine a pipe from point A to point C with an intermediate demand at point b halfway along the pipe with water flowing in the direction of point A to point C. To model this exactly you would need to approach the pipe as 2 separate pipes, one from point A to point b and one from point b to point C, because the pressure loss from point A to point b would differ from the pressure loss from point b to point C. However, this would mean splitting each pipe at each tap/customer connection point and would result in very large models with a lot of short pipes. To simplify this you could indeed assign all demands to the upstream node (point A in this case), but then you would neglect the additional pressure loss caused by the demand at point b for pipe section A-b. If you assign the demand proportionally by pipe length, in the case of point b being halfway along the pipe this would come down to 50% at point A and 50% at point C, you apply an extra pressure loss to pipe A-C corresponding to half the demand of point b which is roughly equivalent to the extra pressure loss when you would model the pipe as 2 separate pipes with all the demand applied at point b.