Given that clouds continue to form, and rain repeatedly falls, one could be forgiven for thinking that water is a limitless renewable resource. However, there are few – if any – examples of cities which have been designed to live utterly within the means of local rainfall resources. Rainwater harvesting from roofs produces relatively modest quantities of water: even in quite wet regions, where rainfall exceeds 1,500 millimeters per year (e.g. Seattle or Glasgow), each person would need about 10 square meters of roof to supply typical domestic consumption.
Even in those wet areas, it does not rain every day, so storage tanks (perhaps as much as 10 m3 per person, depending on annual weather patterns ) would also be needed to ensure that water was available for use all the time. And even then there are further considerations: bird droppings on roofs can contaminate the water, and storing water in tanks for a long period runs the risk of Legionnaires’ Disease, due to bacterial colonization.
So even in parts of the world with relatively abundant water resources, it is usual for water to be delivered to cities from large rural catchments. The catchment areas needed to supply sufficient water for a large urban population can be considerable.