Water scarcity

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. One is physical. The other is economic water scarcity.^[2]: 560 Physical water scarcity is where there is not enough water to meet all demands. This includes water needed for ecosystems to function. Regions with a desert climate often face physical water scarcity.^[3] Central Asia, West Asia, and North Africa are examples of arid areas. Economic water scarcity results from a lack of investment in infrastructure or technology to draw water from rivers, aquifers, or other water sources. It also results from weak human capacity to meet water demand.^[2]: 560 Many people in Sub-Saharan Africa are living with economic water scarcity.^[4]: 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.^[5] One of the main causes of the increase in global water demand is the increase in the number of people that has resulted in the human overpopulation driving climate change, mass species extinction and many of not most of the other serious problems currently facing humanity.^[6] Others are the rise in living conditions, changing diets (to more animal products),^[7] and expansion of irrigated agriculture.^[8][9] Climate change (including droughts or floods), deforestation, water pollution and wasteful use of water can also mean there is not enough water.^[10] These variations in scarcity may also be a function of prevailing economic policy and planning approaches.

Water scarcity assessments look at many types of information. They include green water (soil moisture), water quality, environmental flow requirements, and virtual water trade.^[7] Water stress is one parameter to measure water scarcity. It is useful in the context of Sustainable Development Goal 6.^[11] Half a billion people live in areas with severe water scarcity throughout the year,^[5][7] and around four billion people face severe water scarcity at least one month per year.^[5][12] Half of the world's largest cities experience water scarcity.^[12] There are 2.3 billion people who reside in nations with water scarcities (meaning less than 1700 m³ of water per person per year).^[13][14][15]

There are different ways to reduce water scarcity. It can be done through supply and demand side management, cooperation between countries and water conservation. Expanding sources of usable water can help. Reusing wastewater and desalination are ways to do this. Others are reducing water pollution and changes to the virtual water trade.