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Water scarcity occurs when the amount of water withdrawn from lakes, rivers or groundwater is so great that water supplies are no longer adequate to satisfy all human or ecosystem requirements, bringing about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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The Aral Sea Basin is situated between 55°00’ E and 78°20’ E and between 33°45’ N and 51°45’ N.

The Aral Sea Basin has a total area of 2.7 million km2 and it is shared by seven countries: Afghanistan, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan and the Islamic Republic of Iran.

The Aral Sea was once the world’s fourth largest inland sea. Problems began in the 1960s and 1970s with the diversion of the inflowing Amu Dar’ya and Syr Dar’ya rivers in order to grow cotton on arid land in what was then Soviet Central Asia. Ninety-four water reservoirs and 24,000 km of channels were constructed on these two rivers to support the irrigation of 7 million ha of agricultural land.

In 1963, the surface of the Aral Sea measured 66,100 km2, with an average depth of 16 metres and a maximum depth of 68 metres. The salt content was 1%. By 1987, 27,000 km2 of former lake bottom had become dry land. About 60% of the Aral Sea’s volume had been lost, its depth had declined by 14 metres, and its salt concentration had doubled. By the 1990s it was receiving less than one-tenth of its previous flow — and sometimes no water at all.
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