Rainwater Harvesting

We have several buildings which we are seeking to harvest rainwater to an earth dam for use. Basically we have several gutters and downpipes. The Downpipes will in turn be connected to collection pipes which eventually flow into the earth dam. The water should basically flow into the earth dam through gravity. The objective here is to establish the furthest building we can collect water from and size the collection pipes accordingly.

So far I have done the following:

  1. Modelled the network in WaterGEMS
  2. Calculated the volume of rainfall that will be collected at each downpipe. The Volume is then added to the junction as a negative value to the downpipe. FYI the Junction is basically at the Tee (Connection between downpipe and Collector)

Am i in the right direction so far? Is WaterGEMS the right modelling solution?

I currently seem to have one problem though. Negative Junction Pressures, with reasonably high velocities, in areas where logically there shouldn't be none. Am i missing something.