I do believe some things could be done along the lines of what you mentioned. I know it sounds unrealistic, but there are ways we could catch rain runoff on properties all over the southland, and save it in cisterns.
Water needs could be met for a month or two by using this water, and in good years, perhaps three months of water could be saved and used.
This water could be used for watering lawns, washing cars, and other things that might not require completely fresh water.
At my folks house in the foothills, they got over 45 inches of rain during the winter. You start thinking about the volume of water that represents over square miles of landscape.
I always thought that a folding system of plastic film could set up a natural runoff of water into a cistern and save literally thousands of gallons of water on just one property.
My folks have about half an acre. That should have worked out to about 2 acre feet of water on their property that year. If some way were devised to purify that rain water on their property, they could use it for all their needs. We might be talking about six months or more supply of water there.
There was a thread just a few months ago about the water commission here in CO preventing private land owners from capturing water from their roofs. According to water law the water belongs to the water conservation district downstream.