Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. Two types of water scarcity have been defined: physical or economic water scarcity. Physical water scarcity is where there is not enough water to meet all demands, including that needed for ecosystems to function effectively. Arid areas (for example Central and West Asia, and North Africa) often suffer from physical water scarcity. On the other hand, economic water scarcity is caused by a lack of investment in infrastructure or technology to draw water from rivers, aquifers, or other water sources, or insufficient human capacity to satisfy the demand for water. Much of Sub-Saharan Africa is characterized by economic water scarcity. 

The essence of global water scarcity is the geographic and temporal mismatch between fresh water demand and availability. At the global level and on an annual basis, enough freshwater is available to meet such demand, but spatial and temporal variations of water demand and availability are large, leading to physical water scarcity in several parts of the world during specific times of the year. The main driving forces for the rising global demand for water are the increasing world population, improving living standards, changing consumption patterns (for example a dietary shift toward more animal products), and expansion of irrigated agriculture. Climate change, such as altered weather-patterns (including droughts or floods), deforestation, increased water pollution and wasteful use of water can also cause insufficient water supply. Scarcity varies over time as a result of natural hydrological variability, but varies even more so as a function of prevailing economic policy, planning and management approaches. Scarcity can be expected to intensify with most forms of economic development, but, if correctly identified, many of its causes can be predicted, avoided or mitigated.

Water scarcity assessments need to incorporate information on green water (soil moisture), water quality, environmental flow requirements, globalization, and virtual water trade. There is a need for collaboration between hydrological, water quality, aquatic ecosystem science and social science communities in water scarcity assessment. "Water stress" has been used as parameter to measure water scarcity, for example in the context of Sustainable Development Goal 6. Two-thirds of the global population (4 billion people) live under conditions of severe water scarcity at least one month of the year. Half a billion people in the world face severe water scarcity all year round. Half of the world's largest cities experience water scarcity.

Options for reducing water scarcity include: supply and demand side management, cooperation between countries, water conservation (including prevention of water pollution), expanding sources of usable water (through wastewater reuse or desalination) and virtual water trade.

 

Note:   The above text is excerpted from the Wikipedia article Water scarcity, which has been released under the GNU Free Documentation License.