The bit where people are worrying that roof catchments would reduce input to salmon streams sounds a bit hysterical.
Agreed. Fortunately, I believe the people working for the water resources agency were astute enough to see through that argument. I believe that the rules were changed mostly for western WA, which get a lot of rain. Much of that run off needs to pass through water treatment plants before being dumped into the lakes and Puget Sound. If they could get every homeowner to capture their rainwater and use it in the garden, the western communities could save million$ on the treatment process. Perhaps if
every home did this, it
could affect the amount of water entering those streams. It would take a lot of roofs to make a measurable difference.
For the eastern half of the state, in the rain shadow of the Cascades, every drop that gets captured and used in the gardens is another drop that the municipal systems do not need to treat and distribute.
Many western regions could help solve their water problems if the tax assessor would visit all homes in August. Measure how many square feet of
lawn was still alive after (no rain) summer. Add it to the 'value' of the home, and tax accordingly. Lawns would become a thing of the past.