Sure, maybe relieve pressure on the municipal supply, but that doesn’t mean a thing to the farmers, ranchers and municipalities downstream.
Just because he’s using less water locally doesn’t mean that the water he’s not using is placed back into the streams or aquafers. For the most part, that’s water that’s not going downstream. Multiply that by 10,000, 100,000 or even a million as you move upstream. It has an effect, ESPECIALLY during a drought. People used to KILL eachother over this stuff.
And be SURE that water rights is some of the oldest and most litigated law in the US. If you can think of it, it’s already been litigated and decided long ago. Going to trial over this is a 99.99999% loser for the homeowner with the barrels.