Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. There are two types of water scarcity: physical water scarcity and economic water scarcity.: 560 Physical water scarcity is where there is not enough water to meet all demands, including that needed for ecosystems to function. 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 the result of a lack of investment in infrastructure or technology to draw water from rivers, aquifers, or other water sources, or insufficient human capacity to meet the demand for water.: 560 Much of Sub-Saharan Africa has economic water scarcity.: 11
There is enough freshwater available globally and averaged over the year to meet demand. As such, water scarcity is caused by a mismatch between when and where people need water, and when and where it is available. 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 (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.
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. Half a billion people live in areas with severe water scarcity throughout the year, and around four billion people face severe water scarcity at least one month per year. Half of the world's largest cities experience water scarcity. There are 2.3 billion people who reside in nations with water scarcities, which means that each individual receives less than 1 700 m3 of water annually. However, 380 billion m3 of municipal wastewater are produced globally each year.
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.