Cities concentrate population, activities and then resource utilization. Urban resources can be ranked into four main categories that are energy, material, food, and water. This last one is of vital interest for living forms but also in processing and carrying a great part of the fluxes induced by the constructed urban metabolism. The question of its sustainability in front of the increasing urban population but also of a more erratic climate is addressed. Urban water resources drivers are here analyzed in the light of recent urban metabolisms representations and also emerging alternative management possibilities. A balance equation is proposed and confronted to the natural water resource functioning to highline main differences and terms of interest. This is a first step to improve urban water resource utilization and perhaps detect its limits.