The total quantity of fresh water available on the planet is constant. The concept of water rarity is meaningless except in a local or regional context and depends on the relation between the variable availability of resources (due to natural or climatic causes) and changes to quantities drawn off.

There are three possible types of actions to satisfy increasing needs resulting from urban demography; firstly reduce individual and collective waste, secondly accelerate the natural resource renewal cycle and thirdly "create" new resources by desalination and recycling of previously treated waste water.