Wastewater has been defined as the water discharged from a community after it has been fouled by various uses and containing waste, i.e. liquid or solid matter. It may be a combination of the liquid or water-carried domestic, municipal and industrial wastes, together with such groundwater, surface water and storm water as may be present.

Population growth, rapid urbanization, and increasing water supply and sanitation provision will all generate increased problems from wastewater pollution.

It has been estimated that the total global volume of wastewater produced in 1995 was in excess of 1,500 km3.

There is the understanding that each litre of wastewater pollutes at least 8 litres of freshwater, so that on this basis some 12,000 km3 of the globe’s water resources is not available for use each year. If this figure keeps pace with population growth, then with an anticipated population of 9 billion by 2050, the world’s water resources would be reduced by some 18,000 km3 annually.
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At the start of the 21st century unclean water is the world’s second biggest cause of death for children.

Every year some 1.8 million children die as a result of diarrhoea and other diseases caused by unsafe water and poor sanitation. This corresponds to 4,900 deaths each day or an under-five population equivalent in size to that of London and New York combined.

The diseases and conditions of ill-health directly associated with water, sanitation and hygiene include infectious diarrhoea (which, in turn, includes cholera, salmonellosis, shigellosis, amoebiasis and a number of other protozoal and viral infections), typhoid and paratyphoid fevers, acute hepatitis A, E and F, fluorosis, arsenicosis, legionellosis, methaemoglobinaemia, schistosomiasis, trachoma, intestinal helminth infections (including ascariasis, trichuriasis and hookworm infection), dracunculiasis, scabies, dengue, the filariases (including lymphatic filariasis and onchocerciasis), malaria, Japanese encephalitis, West Nile virus infection, yellow fever and impetigo.

The ill health associated with deficits in water and sanitation undermines productivity and economic growth, reinforcing the deep inequalities that characterize current patterns of globalization and trapping vulnerable households in cycles of poverty.
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Water scarcity occurs when the amount of water withdrawn from lakes, rivers or groundwater is so great that water supplies are no longer adequate to satisfy all human or ecosystem requirements, bringing about increased competition among potential demands.

Water scarcity has also been defined as a situation where water availability in a country or in a region is below 1000 m3 per person per year. However, many regions in the world experience much more severe scarcity, living with less than 500 m3 per person per year

Water scarcity is among the main problems to be faced by many societies and the World in the XXIst century. Water use has been growing at more than twice the rate of population increase in the last century, and, although there is no global water scarcity as such, an increasing number of regions are chronically short of water.
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Precipitation is defined as any of all of the forms of water particles, whether liquid or solid, that falls from the atmosphere and reach the ground. The forms of precipitation are: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail, and ice pellets.

Countries’ precipitation ranges from 100 mm/yr in arid, desert-like climates to over 3,400 mm/yr in tropical and highly mountainous terrains.

About 40% of the precipitation that falls on land comes from ocean-derived vapour. The remaining 60% comes from land-based sources.

The monsoon, tropical cyclones and mid-latitude frontal and convective storm systems are important mechanisms controlling precipitation, while orographic lifting is another.

Towards the poles and with increasing altitude, a greater proportion of the precipitation occurs as snow. The annual snowfall over the earth is estimated to be about 1.7×1013 tons, covering an area that varies from year to year between 100 and 126 million km2.
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Evapotranspiration is the process of water loss in vapour form from a unit surface of land both directly by evaporation from the ground and by transpiration through leaf surfaces during a specific period of time. Soil moisture is defined as the water stored in or at the continental surface and available for evaporation.

The processes of evaporation and transpiration (evapotranspiration) are closely linked to the water found in soil moisture; these processes act as driving forces on water transferred in the hydrological cycle.

Soil moisture storage is dependent on a number of factors in addition to precipitation and evaporation, such as soil type, soil depth, vegetation cover and slope.

Movement through soil and vegetation is large and accounts for 62% of annual globally renewable freshwater.
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