water scarcity 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: 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 map of global water stress (a symptom of water scarcity) in 2019. water stress is the ratio of water use relative to water availability and is therefore a demand-driven scarcity. 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. definitions global physical and economic water scarcitywater scarcity has been defined as the "volumetric abundance, or lack thereof, of freshwater resources" and it is thought to be "human-driven".: 4 this can also be called "physical water scarcity". there are two types of water scarcity: physical water scarcity and economic water scarcity.: 560 environmental water requirements are sometimes included in water scarcity determinations but the approach to this varies from one organization to another.: 4 global water consumption 1900–2025, by region, in billions m3 per year related concepts there are several definitions of "water scarcity", "water stress", and "water risk" provided in the literature, and therefore a harmonization has been proposed by the ceo water mandate in 2014.: 2 in their discussion paper they state that these three terms should not be used interchangeably.: 3 water stress some organizations define "water stress" as a broader concept. under that definition, it would include aspects of water availability, water quality and the accessibility of water. the latter is related to existing infrastructure and whether customers can afford to pay for the water.: 4 this is termed by others as "economic water scarcity". fao defines water stress as the "symptoms of water scarcity or shortage". such symptoms could be "growing conflict between users, and competition for water, declining standards of reliability and service, harvest failures and food insecurity".: 6 this is measured with a range of water stress indices. another definition for water stress is as follows: "water stress refers to the impact of high water use (either withdrawals or consumption) relative to water availability." water stress is therefore regarded as a "demand-driven scarcity". types two types of water scarcity have been defined: physical and economic water scarcity. these terms were first defined in a wide-ranging 2007 study on the use of water in agriculture over the previous 50 years of practitioners, researchers and policymakers, overseen by the international water management institute, with the aim of finding out if the world had sufficient water resources to produce food for the growing population in the future.: 1 physical water scarcity physical water scarcity occurs when natural water resources are insufficient to meet all demands, including that needed for ecosystems to function effectively. arid regions frequently suffer from physical water scarcity. human influence on climate has led to increased water scarcity in areas where water was previously hard to come by. it also occurs where water seems abundant but where resources are over-committed, such as when there is overdevelopment of hydraulic infrastructure, often for irrigation or energy generation. symptoms of physical water scarcity include "severe environmental degradation, declining groundwater and water allocations that favor some groups over others".: 6 another indicator, called ecological water scarcity has been proposed that considers water quantity, water quality, and environmental flow requirements. water is physically scarce in densely populated arid areas (for example central and west asia, and north africa), with projected availabilities of less than 1000 cubic meters per capita per year. a study in 2007 found that more than 1.2 billion people live in areas of physical water scarcity. this water scarcity relates to water available for food production, rather than for drinking water which is a much smaller amount. some academics have advocated for expanding the two dimensions of water scarcity (i.e. the physical and economic dimensions) to a third dimension which would be called ecological water scarcity. this dimension would focus on the water demand of ecosystems: it would refer to the minimum quantity and quality of water discharge that are required to maintain sustainable and functional ecosystems. however other publications state that this aspect is simply part of the definition of physical water scarcity. economic water scarcity people collect clean drinking water from a tapstand in the town of ghari kharo, in western sindh province in pakistan. 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 meet the demand for water.: 560 it causes people without reliable access to water to have to travel long distances to fetch water, which is often contaminated from rivers for domestic and agricultural uses (irrigation). according to the united nations development programme, economic water scarcity is the commonest cause of countries or regions experiencing water scarcity, as most countries or regions have enough water to meet household, industrial, agricultural, and environmental needs, but lack the means to provide it in an accessible manner. around one-fifth of the world's population currently live in regions affected by physical water scarcity. one-quarter of the world's population is affected by economic water scarcity. much of sub-saharan africa is characterized by economic water scarcity.: 11 developing water infrastructure there could therefore help to reduce poverty. investing in water retention and irrigation infrastructure would also help to increase food production, especially in developing countries that largely rely on low-yield agriculture. being able to provide a community with water that is adequate for consumption would also greatly benefit the people's health. overcoming this type of scarcity, however, can require more than just new infrastructure; it requires socio-economic and socio-political types of intervention that address poverty and socio-inequality but because there is a lack of funding, much planning must come into play. although much emphasis is put on improving water sources for drinking and domestic purposes, we know that much more water is used for other uses such as bathing, laundry, livestock and cleaning than for drinking and cooking alone. this observation suggests that putting too much emphasis on drinking water needs addresses an insignificant part of the problem of water resources and therefore limits the range of solutions available. related concepts water security this section is an excerpt from water security. a society with a high level of water security makes the most of water's benefits for humans and ecosystems, and limits the risk of destructive impacts associated with water. the risks include too much water (flood), too little water (drought and water scarcity) or poor quality (polluted) water. a more detailed definition of water security is: "the reliable availability of an acceptable quantity and quality of water for health, livelihoods and production, coupled with an acceptable level of water-related risks". access to wash services (water, sanitation and hygiene) is one component of achieving water security. some organizations use the term water security more narrowly for water supply aspects only. a typical dry lakebed is seen in california, which is experiencing its worst megadrought in 1,200 years, precipitated by climate change, and is therefore water rationing. water scarcity and lack of water security present an existential threat. water risk this section is an excerpt from water security § water risk. water risk refers to the possibility of problems to do with water. examples are water scarcity, water stress, flooding, infrastructure decay and drought.: 4 water risk is inversely related to water security. this means as water risk increases, water security decreases. water risk is complex and multidimensional. it includes risks from natural and human-made disasters such as flooding and drought. these can lead to infrastructure failure and worsen hunger. when these disasters take place, they result in water scarcity or other problems. the potential economic effects of water risk are significant. water risks threaten entire industries. examples are the food and beverage sector, agriculture, oil and gas, utilities and semiconductor manufacturing. agriculture uses 69% of global freshwater. so this industry is very vulnerable to water stress. assessments and indicators lake chad has shrunk by 90% since the 1960s. simple indicators relatively simply indicators include: the water use to availability ratio (or criticality ratio), physical and economic water scarcity—the iwmi indicator, water poverty index. "water stress" has been used as parameter to measure water scarcity, for example in the context of sustainable development goal 6. in this context, a report by fao in 2018 has defined water stress as: "the ratio between total freshwater withdrawn (tfww) by all major sectors and total renewable freshwater resources (trwr), after taking into account environmental flow requirements (efr)". this means that the value for tfww is divided by the difference between trwr minus efr.: xii environmental flows are water flows required to sustain freshwater and estuarine ecosystems. previously, a definition used for millennium development goal 7, target 7.a was simply the proportion of total water resources used, without taking into consideration efr.: 28 with this definition, water stress is defined by the following categories: <10% is low stress; 10-20% is low-to-medium; 20-40% medium-to-high; 40-80% high; >80% extremely high. indicators are used to measure the extent of the water scarcity. one approach to measuring water scarcity is to calculate the amount of annual water resources available per person. for example, according to the "falkenmark water stress indicator" (developed by malin falkenmark), a country or region is said to experience "water stress" when annual water supplies drop below 1,700 cubic meters per person per year. at levels between 1,700 and 1,000 cubic meters per person per year, periodic or limited water shortages can be expected. when water supplies drop below 1,000 cubic meters per person per year, the country faces "water scarcity". however, the falkenmark water stress indicator does not help to explain the true nature of water scarcity. renewable freshwater resources main article: list of countries by total renewable water resources renewable freshwater supply is a metric often used in conjunction when evaluating water scarcity. this metric is informative because it can describe the total available water resource each country contains. by knowing the total available water source, an idea can be gained about whether