In rural Ky, cisterns are just a way of life. Every downspout has a diverter a few feet off the ground. Sort of like an exhaust cutout valve. One way goes in the the cistern and the other way goes to the ground. You keep the valve set to waste the water. After it has been raining for 15-20 minutes, you flip the valve to save the water in the cistern. This way the rain washes and garbage off the roof onto the ground. Then when the roof is cleaner, you keep that water. Couple of factors come into play. First is how much rain do you get a year? Second is square footage of your roof. Third is how many gallons does your cistern hold? I know a guy who had a 20k gallon one built when he built his house 30-40 years ago. If you already have city water, you are getting in a different area. Water departments don't like it when you try to cut their revenue.