Our system of rainwater collection uses a number of different sources.  The roof on the main facility drains into a gutter system that runs around the building, then to the retention tanks through an aqueduct.  The roof of the art gallery is called a butterfly roof, where all the water drains to the center the entire length of the roof.  Again this water then moves down the aqueduct system to the retention tanks.  We have no need to run pipes from the city for water: totally self sufficient.  Keep an eye out for seminars on how you can set up your own water collection system!