a country is prone to experiencing physical water scarcity. this metric has its faults in that it is an average; precipitation delivers water unevenly across the planet each year and annual renewable water resources vary from year to year. this metric also does not describe the accessibility of water to individuals, households, industries, or the government. lastly, as this metric is a description of a whole country, it does not accurately portray whether a country is experiencing water scarcity. for example, canada and brazil both have very high levels of available water supply but still, experience various water-related problems. also, some tropical countries in asia and africa have low availability of freshwater resources. more sophisticated indicators average ecological water scarcity at the provincial level in china 2016-2019. water scarcity assessments need to incorporate information on green water (soil moisture), water quality, environmental flow requirements, globalization, and virtual water trade. since the beginning of the 2000s, water scarcity assessments have applied more sophisticated models which are supported with spatial analytical tools. they include: green-blue water scarcity, water footprint-based water scarcity assessment, cumulative abstraction to demand ratio—considering temporal variations, lca-based water stress indicators (life cycle assessments), integrated water quantity–quality environment flow in the water scarcity assessment. since the early 2010s assessments have combined both quantity- and quality-induced water scarcity. another indicator, called ecological water scarcity has been proposed that considers water quantity, water quality, and environmental flow requirements. for example, results from a modelling study in 2022 show that northern china suffered more severe ecological water scarcity than southern china. the main driving factor of ecological water scarcity in most provinces was water pollution rather than human water use. overall, there is a need for collaboration between hydrological, water quality, aquatic ecosystem science and social science communities in water scarcity assessment. available water children fetch water from a muddy stream in a rural area during dry season. the water is taken back home and undergoes filtration and other treatments before usage. global use of freshwater, 2016 fao datamain articles: water resources, fresh water, and water supply the united nations (un) estimates that, of 1.4 billion cubic kilometers (1 quadrillion acre-feet) of water on earth, just 200,000 cubic kilometers (162.1 billion acre-feet) represent freshwater available for human consumption. a mere 0.014% of all water on earth is both fresh and easily accessible. of the remaining water, 97% is saline, and a little less than 3% is difficult to access. the fresh water available to us on the planet is around 1% of the total water on earth. the total amount of easily accessible freshwater on earth, in the form of surface water (rivers and lakes) or groundwater (in aquifers, for example), is 14,000 cubic kilometers (nearly 3359 cubic miles). of this total amount, 'just' 5,000 cubic kilometers are being used and reused by humanity. technically, there is a sufficient amount of freshwater on a global scale. hence, in theory, there is more than enough freshwater available to meet the demands of the current world population of more than 7 billion people, and even support population growth to 9 billion or more. due to the unequal geographical distribution and especially the unequal consumption of water, however, it is a scarce resource in some parts of the world and for some parts of the population. apart from the conventional surface water sources of freshwater such as rivers and lakes, other resources of freshwater such as groundwater and glaciers have become more developed sources of freshwater, becoming the main source of clean water. groundwater is water that has pooled below the surface of the earth and can provide a usable quantity of water through springs or wells. these areas where groundwater is collected are also known as aquifers. more and more of these sources are being drawn upon as conventional sources' usability decreases due to factors such as pollution or disappearance due to climate changes. human population growth is a significant contributing factor in the increasing use of these types of water resources. scale current estimates water scarcity was listed in 2019 by the world economic forum as one of the largest global risks in terms of potential impact over the next decade. it is manifested by partial or no satisfaction of expressed demand, economic competition for water quantity or quality, disputes between users, irreversible depletion of groundwater, and negative impacts on the environment. about half of the world's population currently experience severe water scarcity for at least some part 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. almost two billion people do not currently have access to clean drinking water. a study in 2016 calculated that globally, the population under water scarcity increased from 0.24 billion (14% of global population) in the 1900s to 3.8 billion (58%) in the 2000s. this study analyzed water scarcity using the fundamental concepts of shortage (impacts due to low availability per capita) and stress (impacts due to high consumption relative to availability). future predictions girls of squatter settlement in dharan collect water from riverin the 20th century, water use has been growing at more than twice the rate of the population increase. specifically, water withdrawals are predicted to increase by 50 percent by 2025 in developing countries, and 18 per cent in developed countries. one continent, for example, africa, has been predicted to have 75 to 250 million inhabitants lacking access to fresh water. by 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity, and two-thirds of the world population could be under stress conditions. by 2050, more than half of the world's population will live in water-stressed areas, and another billion may lack sufficient water, mit researchers find. with the increase in global temperatures and in an increase in water demand, six out of ten people are at risk of being water-stressed. the drying out of wetlands globally, at around 67%, was a direct cause of a large number of people at risk of water stress. as the global demand for water increases and as climate temperatures rise, it is estimated that two-thirds of the population, in 2025, will live under water stress.: 191 according to a projection by the united nations, by 2040, there can be about 4.5 billion people affected by a water crisis (or water scarcity). additionally, with the increase in population, there will be a demand for food, for the food output to match the population growth, there would be an increased demand for water to irrigate crops. the world economic forum estimate that global water demand will surpass global supply 40% by 2030. increasing the demand for water as well as increasing the population results in a water crisis where there is not enough water to share in healthy levels. the crises are not only due to quantity but quality also matters. a study found that of ~39 million groundwater wells 6-20% are at high risk of running dry if local groundwater levels decline by a few meters, or – as with many areas and possibly more than half of major aquifers – continue to decline. impacts there are several impacts and symptoms of water scarcity: serious restrictions on water use, "growing conflict between users and competition for water, declining standards of reliability and service, harvest failures and food insecurity".: 6 specific examples include: food insecurity in the middle east and north africa region inadequate access to safe drinking water for about 885 million people groundwater overdrafting (excessive use) leading to diminished agricultural yields overuse and pollution of water resources harming ecosystems and biodiversity regional conflicts over scarce water resources sometimes resulting in warfare. water supply shortages