State and local municipalities own the water, and code enforcement officers who are always driving around town inspecting properties for code violations usually do the enforcement. The consequences are confiscation of equipment and a fine.

It also depends on water demands in an area. Even though the Douglas County model demonstrated non-significance, I am sure if there was another region in the state that could demonstrate otherwise and wanted to push the issue, the regulation would be back on the books, at least for part of the state. In Queensland AU, it was a pretty hefty fine if you were caught sequestering rain water, and enforcement at the time was pretty diligent. They were in a dire situation there, though. Idaho might be convinced to drop the regulation, but as long as you've got commie liberals running the show in Washington, I wouldn't ever expect the law there to get repealed.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)