Much as I dislike bunny-huggers and tree-****ers, I have to admit that here, in Aridzona, most of the push to get rainwater collection systems to be common-place is from the more leftist leaning bunch. I know of whole subdivisions in some areas of the state, without accesable aquifers for well water, which require each home to have rainwater collection systems.
They make sense, no mater where you live. Sorry about those states with antique water laws. Of course, there are several ongoing litigations over water rights here in State and Federal courts. Ah me oh my. What a place.
By the way, thanks to whomever is responsible for my upgrade. I just noticed I'm a "journeyman".

Here in the UK I store water in a cistern in the roof space.
This holds about 200 litres, and is filled with city water automaticly by a float valve as used in a lavatory flush cistern.

An outlet is fitted near the top of the cistern, and this water is used normally for laundry and toilet flushing.
Any interuption in the water supply, would result in the water level in the cistern dropping below the upper outlet, thereby preventing any more water being used for low priority uses.

A second outlet is fitted near the bottom of the cistern, and is connected via microbore copper tube to a water tap in a conclealed place.
The concealement prevents anyone from borrowing, useing, stealing, or requistioning the water.

The reserve capacity is about 150 litres, or enough for drinking for a month or two.
The cistern is kept vlean and has a tight fitting lid, therefore the water should be fit to drink, though I keep chlorine tablets as a second line of defense.

They are generally to protect the water supply for places downstream.
If Colarado carefully collected all it's rainwater, sprayed it onto golf course and let it evaporate then California would be in trouble.

It always cracks me up to hear about water "usage". Nobody takes water. We just borrow it for a little while...

I had a salesman try to sell me on a very expensive washing machine that only used 2 oz of water for a full load. Ok, maybe a slight exaggeration... But my point was that my washer is on the same drain line as a toilet, far away from the "central" line, so I wanted one that used a lot of water to keep my line clean, just in case. He wasn't sure what to say about that.

I also told him that the quicker we used it, the quicker we returned it to the hydrological cycle, and the quicker it would come back to us. Circle of life kind of thing... I like playing with salesmen...

There's the same amount of water now as there has ever been for the most part, it's just in different places and under different people's control and that's the crux of the matter.

For the most part, this is true. The amount of water fluctuates regularly as it is ionized by various methods (including ambient UV in daylight) into H2 and O2, then recombined in combustion and other catylistic processes. I'd estimate the fluctuation to be somewhere around +/- 100,000 gallons per 24 hour period. But I could be off by an order of magnitude.