I looked at the model and animated the profile. At times the flow to the lower tanks is so great that the HGL drops below the ground. This is an elevation problem. The HGL at the reservoir is at roughly the same elevation as the ground. with any head loss at all, you will get negative pressure. Bottom line is that if your source HGL is 1902 ft and the ground downstream is at 1902 ft, you can't avoid negative pressure if you have flow. There are a few solutions: 1. Raise the source HGL and possibly fill the tanks through a pressure sustaining valve (PSV) so that you don't have much head loss. 2. If it's not drinking water, you may want to consider building an open channel across the plateau and then only converting to a pipe on the downhill section. 3. Build a tunnel instead of cut-and-cover pipe with horizontal directional drilling. 4. Pump across the plateau. 3 and 4 are expensive and 2 is only for non-potable water. What options do you have to raise the source HGL?
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