Why water is a global resource?

In most river basins in the world, the available water resources are partly used to produce goods that are exported from the basin for consumption elsewhere. International trade in commodities implies long-distance transfers of water in virtual form. Knowledge about the virtual-water flows entering and leaving a country or a river basin can cast a completely new light on the actual water situation of the country or river basin concerned. Import of water-intensive commodities reduces national water demand; on the contrary, export of water-intensive commodities raises the demand

http://www.waterfootprint.org/?page=files/GlobalWaterFootprint