Irrigation systems in arid lands divert the discharge of a large river, such as the Nile, Indus, or Colorado, into a distributary system that allows the water to infiltrate the soil of areas under crop cultivation. Ultimately, such irrigation projects suffer from two undesirable side effects-salinization and waterlogging of the soil.
The irrigated area is subject to very heavy water losses through evapotranspiration. Salts contained in the irrigation water remain in the soil and increase in concentration-the process of salinization. Ultimately, when salinity of the soil reaches the limit of tolerance of the plants, the land must be abandoned. Prevention or cure of salinization may be possible by flushing down soil salts to lower levels by use of more water. This remedy requires more water use than for crop growth alone. Furthermore, when salts are flushed down to lower levels, they may enter the ground water body and contaminate it with toxic ions, such as nitrates.
Infiltration of large volumes of water causes a rise in the water table, any may ultimately bring the zone of saturation close to the surface-a phenomenon known as waterlogging. Crops cannot grow in perpetually saturated soils. Furthermore, when the water table rises to the point that upward movement under capillary action can bring water to the surface, evaporation is increased and salinization is intensified.
One of the largest of the modem irrigation project affected adversely by salinization and waterlogging lies within the basin of the lower Indus River in, West Pakistan. Here the annual rate of rise of the water table has averaged about one foot (0.3m), while the annual increased in land areas adversely affected is on the order of 50,000 to 100,000 acres (20,000 to 40,000 hectares).
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