The water cycle makes available only so much water each year in a given location. That means supplies per person, a broad indicator of water security, drop as population grows. Thus per capita water supplies worldwide are a third lower in 1992 than in 1970 due to the 1.8 billion people added to the planet since then. One of the clearest signs of water scarcity is the increasing number of countries in which population has surpassed the level that can be sustained comfortably by the water available. As a rule of thumb, hydrologists designate water-stressed countries as those with annual supplies of 1,000-2,000 cubic metres per person. When the figure drops below 1,000 cubic metres, nations are considered water-scarce -- that is, lack of water becomes a severe constraint on food production, economic development and protection of natural systems.
Almost 50 countries have more than three-quarters of their land in international river basins; 214 river basins around the world are international in the sense that they are shared. While many resource scarcities tend to threaten internal stability, water shortages in some regions threaten international conflict.
In 1992, 26 countries, collectively home to some 230 million people, fell into the water-scarce category. Many of them have very high population growth rates, so their water problems are deepening fast. Africa has the largest number of water-scarce countries, 11 in all (Algeria, Botswana, Burundi, Egypt, Kenya, Libya, Mauritania, Rwanda and Tunisia); by the end of this decade, four others will be added. By then the total number of Africans living in water-scarce countries will climb to 300 million -- a third of the continent's projected population. Nine out of the 14 countries in the Middle East already face water-scarce conditions, making it the most concentration region of water scarcity in the world. Industrialized countries that are water-scarce are Belgium, Hungary, Netherlands and Singapore. China, with 22% of the world's population and only 8% of its fresh water, faces obvious water constraints, most severe in and around Beijing and other portions of the North China Plain that yields one quarter of the country's grain.
The water scarcity problematique was illustrated at an early stage by the Segura Basin in Spain, where interbasin solidarity over water use and transfer became part of the cultural heritage from the 8th century - and has turned out to be difficult to abandon. Increasing consumption of groundwater has now reduced the volume of the Tagus River Water Transfer, so that the Tagus aqueduct carries only one third of the capacity.
Symptoms of water stress exist around the world -- not only just in water-scarce countries, but in parts of water-wealthy ones as well.
In Rio Grande, shared between Mexico and USA, water scarcity was a hot issue that is causing large concern and special water management challenges, especially during the current drought when the river went empty in the downstream end. What is now primarily considered is an intersectoral water transfer from agriculture to industry.