This conundrum will sound hypothetical, but it represents a real-world problem: You have a pump sucking water through the same diameter line from the same level in 2 different places in a pool. FWIW, the pump is a Hayward Super Pump Series Model SP2607X102S: http://www.hayward-pool.com/prd/In-Ground-Pool-Pumps-Super-Pump-_10201_10551_13004_-1_14002__I.htm One suction line is ~30' long, while the other is ~70' long. The lines meet at a Jandy diversion valve just upstream of the pump which allows the vacuum to be accurately measured by isolating each line. The steady state vacuum would not be expected to be equal in each suction line but which line should give the higher reading? It's been many years since I studied physics but the following competing theories emerge in my mind: Theory 1. The shorter line should provide the greater vacuum reading because it's, well, shorter and thus less resistance (greater flow/greater suction). Theory 2. The longer line should provide the greater vacuum reading because, since it's longer, it provides the greater resistance and therefore the pump must suck harder to move the water. Before I announce what vacuum differential I measured, I'd like to see if anyone can please confirm Theory 1, Theory 2, or perhaps some other one. Thanks in advance for your help.