Development of drainage system in the Aral Sea basin
In Central Asia 5,2 from 7,956 mln ha irrigated lands require artificial drainage because of soil secondary salinization and waterlogging.
These areas are spread all over Central Asia: on cones of withdrawal, central parts of inter-mountain bowls, on periphery of dry channels and river valleys, which have non-drained or low drained soils with highly saline ground water. It worth to take into account , that all Aral-Caspian depression, according to famous pedologist V. Kovda and V. Yegorov, is the place of salt accumulation and salt water solution seepage which are accumulated in soils and ground waters.
In conditions of developing irrigation which is imposed on stabilized mass of salts and saline water, consequences of natural water and salt exchange processes violation appeared especially in zones with shortage or absence of ground water outflow leading to ground water salinity changes; secondary ground water head creation, salt accumulation and secondary soil salinization strengthening, and as a result, salt disposal into the rivers and sinks.
Nevertheless, understanding of all these difficulties has led, in the second half of XX century, to engineering construction and drainage system development within the irrigated lands.
Active fight against significance was started; meaningness of reclamation measures as combination of irrigation and drainage parameters; design and implementation of the systems and their background, successful soil leaching of initially saline as well as secondary salinized lands. As a results of all these measures 4,7 from 5,2 mln ha are provided by artificial drainage. By 1.06.1996 there are 174,5 th km (39,4 m/ha) of horizontal drainage (including 145,4 th km on-farm drainage) and 8650 wells of vertical drainage.
Average command area for 1 well is 85,5 ha under maximum 250-300 ha. Total area of close horizontal drainage is around 600 th ha from which 581 th ha belong to Uzbekistan. In other countries of the region subsurface drainage was practiced only in "pilot stage".