updated January 2013
Unsustainable extraction of freshwater and other human interference with the water cycle are the immediate causes of water scarcity within a river basin.
Over-extraction has its most straightforward manifestation in the level of aquifers, underground reserves charged by the passage of water through soil and rocks. If withdrawals exceed the natural rate of recharge, the level of an aquifer will fall, eventually drying up altogether. In parts of India, the water table is believed to have fallen more than 300 metres, the lack of control exacerbated by a policy of allowing farmers unlimited and free access to water.
Human intervention which degrades the quantity and quality of the natural supply of freshwater occurs in three principal ways.
Firstly, there are 48,000 large dams in place around the world, with many more under construction. Dams alter the natural flow of a river, often improving water and energy security for some, at the expense of others.
Secondly, soil moisture is lost in land degradation caused by poor farming practices and deforestation. And thirdly, surface waters are polluted by run-off of chemicals used in farming and by untreated industrial and household wastewater in cities. This is an acute problem in less developed countries where environmental and sanitation regulations remain inadequate.
In many countries of sub-Saharan Africa, there is an additional category of “economic” water scarcity which is caused by too little human intervention. This occurs when natural supplies are sufficient to meet demand but fail to reach users due to shortcomings in distribution or storage infrastructure.