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.
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.
Wetlands include a wide variety of habitats such as marshes, peatlands, floodplains, rivers and lakes, and coastal areas such as saltmarshes, mangroves, and seagrass beds, but also coral reefs and other marine areas no deeper than six metres at low tide, as well as human-made wetlands such as waste-water treatment ponds and reservoirs.