Anybody who knows anything about plumbing ...
... knows you don't plumb two water sources together. Ever. For lots of reasons.
A cousin of mine decided it would be nice to build a manifold so he could close a valve or two, open a couple others, and thus easily prime either of the two jet-pumps on his irrigation systems ... with the city water supply as the priming water.
Several thousand dollars in fines, and quite the tongue lashing from the raised-rural Judge later, and now he understands why most of us have a pickle/olive barrel full of water handy to the pump for priming purposes. Once the pump is running, top up the barrel for next time.